cond-mat.str-el

Top Pith

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cond-mat.str-el 2026-05-20 2 theorems

Small s squared controls expansion for correlated electron transport

by Pavel A Nosov, Eslam Khalaf +1 more

Controlled expansion for correlated electrons with concentrated kinematics

In the limit of concentrated momentum space, long self-avoiding paths dominate to enable analytic calculations of spectra and finite DC

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We introduce a systematic expansion tailored to systems with strong local interactions and capable of computing response functions, including finite DC transport, analytically. The expansion is controlled by a small parameter $s^2$ that measures the area of the momentum space region where kinematics of the theory is concentrated. In real space, this corresponds to single-particle or correlated hopping terms with amplitudes that decay over a length scale $1/s$ and scale in magnitude as $s^2$ in two dimensions. In the limit $s^2\ll 1$, long, self-avoiding tunneling paths dominate over paths revisiting the same site. This enables systematic controlled calculations of various physical quantities. We illustrate the method with three applications. (i) A Hubbard model with concentrated dispersion: we analytically obtain spectral broadening which scales as $s^2$ and identify a high-temperature bad metal with $T$-linear resistivity coexisting with parametrically long-lived quasiparticles, as well as an intermediate-temperature "thermal FL*" with a small hole pocket that coexists with thermally disordered fluctuating local moments, all within a single controlled framework. (ii) A correlated-hopping model with interesting electron-trion dynamics. (iii) A model of Chern bands with concentrated Berry curvature, motivated by twisted bilayer graphene, which realizes a Mott semimetal where we compute the broadening for the electron and trion spectral functions. At the end, we discuss how our approach paves the way to addressing various challenging questions in strongly correlated systems and outline its various generalizations.

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cond-mat.str-el 2026-05-19 2 theorems

Nickelate shows four-unit-cell stripes with 66 meV gap and switchable dynamics

by Uladzislau Mikhailau, Luke Rhodes +6 more

Imaging stripe dynamics in trilayer nickelate La₄Ni₃O₁₀

Tunneling electrons above 20 meV trigger phase slips for atomic-scale imaging and highlight cuprate-like correlations.

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Since the discovery of high-temperature superconductivity in nickelate superconductors, it is an open question how closely the superconducting state resembles that of cuprate superconductors. One salient feature of the phase diagram of the high-temperature cuprate superconductors is stripe order. Despite their prevalence, real-space imaging has been limited to the charge sector. Here we use spin-polarised scanning tunnelling microscopy to visualize the local magnetic and charge distribution emerging due to a stripe order in the trilayer nickelate La$_4$Ni$_3$O$_{10}$. The stripe order exhibits a four unit cell periodicity, closely resembling that seen in cuprates, and opens a near-complete $\sim66\mathrm{meV}$ gap at the Fermi level. Crucially, discrete phase slips can be triggered by tunneling electrons above a $\sim 20\mathrm{meV}$ threshold, allowing imaging of stripe dynamics at the atomic scale. These results highlight the importance of correlation physics driving stripe-like orders in lanthanum nickelates with striking similarities to the cuprates.

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cond-mat.str-el 2026-05-18 2 theorems

Spin kernel computation shows warm-dense LSDA mismatch

by Pengcheng Hou, Zhiyi Li +2 more

Finite-Temperature Spin Exchange-Correlation Kernel of the Uniform Electron Gas

Long-wavelength limit of the finite-temperature spin XC kernel agrees with LSDA spin stiffness at low T but exposes a residual when warm and

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The finite-temperature spin response of the uniform electron gas (UEG) is a fundamental reference for spin-polarized and magnetized electron liquids, including warm dense matter (WDM), yet it remains far less constrained than charge response. Using variational diagrammatic Monte Carlo, we compute the static spin exchange--correlation (XC) kernel $K_{xc}(q;T)$ of the unpolarized UEG at metallic densities across the quantum-degenerate, warm-dense, and classical regimes. The kernel connects smoothly to zero-temperature spin-response parametrizations at low temperature, while heating suppresses the Fermi-surface-scale spin-correlation structure and weakens the XC-driven Stoner enhancement. Its long-wavelength limit provides a direct response test of the spin stiffness implied by thermal local-spin-density-approximation (LSDA) parametrizations, showing low-temperature consistency while exposing a resolved warm-dense residual in current LSDA parametrizations. In the classical regime, the spin XC kernel becomes nearly local on the Fermi-momentum scale, in sharp contrast to the corresponding charge XC kernel. These results provide a first-principles basis for finite-temperature spin-response theory and magnetized WDM modeling.

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cond-mat.stat-mech 2026-07-03

Integrability-breaking gates trigger chaos via OTOC hotspots

by Sounak Biswas, Sthitadhi Roy +1 more

On the emergence of quantum many-body chaos for tunably-broken integrability

In a tunable free-fermion circuit, local amplifications accumulate to set explicit crossover scales and velocity dependence.

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We develop a quantitative theory for the emergence of quantum many-body chaos as integrability is broken via a tunable parameter. In a circuit model of free fermions, 'doped' with a tunable density of integrability-breaking gates, we uncover the microscopic mechanisms underpinning the crossover from early-time integrable behaviour to late-time chaos through the lens of the out-of-time-ordered correlators (OTOCs). The integrability-breaking gates act as local, in spacetime, hotspots which locally amplify the OTOCs such that an accumulation of them eventually leads to fully-developed chaos. We identify the explicit characteristic time and length scales governing this crossover, as well as the dependence of the chaotic OTOC characteristics -- such as the butterfly velocity and front broadening -- on the integrability-breaking parameter.

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quant-ph 2026-07-03

Total correlation fluctuations distinguish integrable from chaotic dynamics

by Nirupam Sen, Keshav Das Agarwal +1 more

Quantum mutual information as a robust probe of integrability in open quantum systems

The long-time average and temporal variations of the Haar-averaged sum of total correlations serve as size-independent probes that persist u

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The dynamics of a quantum system encode signatures of whether the underlying Hamiltonian is integrable or chaotic, giving rise to the concept of quantum information scrambling through the properties of the resulting dynamical states or operators. We introduce an information-theoretic framework based on the Haar-averaged sum of total correlations (aSTC), together with average genuine multipartite entanglement generated dynamically from initially fully separable states, as robust probes of quantum information scrambling. Using the long-range quantum XYZ spin model in transverse and longitudinal magnetic fields, whose integrable limit is the nearest-neighbor transverse XY model, we demonstrate that the long-time average and, more importantly, the temporal fluctuations of the aSTC provide a faithful and system-size-independent signature of integrable and chaotic dynamics, similar to the conventional measure of scrambling, out-of-time-ordered correlator (OTOC). When the system is in contact with the thermal reservoir and system-bath coupling follows Markovianity, we find that the fluctuations of the aSTC and OTOC continue to distinguish integrable and chaotic dynamics only at intermediate times. However, we observe that in the non-Markovian domain, information backflow restores the scrambling dynamics, enabling the aSTC to retain its distinguishing power even at long times. Interestingly, we exhibit that, under Markovian amplitude damping and non-Markovian dephasing noise, the temporal fluctuations of the aSTC can discriminate between integrability and non-integrability in the weak Markovian regime, even when OTOC fails to do so.

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cond-mat.str-el 2026-07-03

Overdamped modes drag electrons to yield thermopower log anomalies

by G. Mirarchi, G. Seibold +2 more

Anomalous thermopower from the drag of overdamped collective modes

A phonon-drag analog for charge fluctuations with rising low-T damping explains Seebeck divergences in cuprates.

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Inspired by the observation of a Seebeck coefficient ratio that exhibits a seemingly logarithmic divergence at low temperature in high-temperature superconducting cuprates, we show that a mechanisms similar to the standard phonon drag can give rise to anomalies in the thermopower of a metal, if the dragged collective mode is overdamped, with a damping coefficient that increases with lowering the temperature. Our finding adds a piece to the puzzle of the strange-metal behavior observed in many different systems and supports our proposal that overdamped charge density fluctuations can be responsible of such a behavior in high-temperature superconducting cuprates.

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cond-mat.str-el 2026-07-03

Green's function invariants miss interacting topology

by Emile Pangburn, Olivier Gingras

Fundamental limitations of single-particle Green's-function zeroes as probes of many-body topology

Single-particle Green's functions leave the many-body ground state topology unchanged under interactions that trivialize it in coupled SSH c

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We show that topological invariants constructed from single-particle Green's functions (GFs) cannot reliably diagnose the topology of interacting many-body states. Using coupled interacting SSH chains as a minimal example, we demonstrate that a spin-spin interaction can trivialize the many-body ground state without affecting the GF topological invariant. This breakdown originates from the GF's inability to probe electronic excitations in the Fock sectors responsible for the topological degeneracy. Consequently, GF zeroes are not associated with physical topological quasiparticles and cannot generally characterize interacting topological phases.

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quant-ph 2026-07-03

Bockstein braiding appears for Z_N excitations with p+q=d-1

by Po-Shen Hsin, Yu-An Chen

Bockstein braiding statistics

A unitary process on staggered operators measures statistics that block simultaneous condensation and symmetric gapped phases.

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Braiding statistics, from the Aharonov-Bohm phase to anyons in fractional quantum Hall systems, play a central role in quantum physics. For $p$- and $q$-dimensional excitations in $d$ spatial dimensions, ordinary braiding requires $p+q=d-2$. In a field-theoretic description of $\mathbb Z_N$ excitations, ordinary braiding is described by the linking response $(2\pi i/N)\int A_{d-p}\cup B_{d-q}$, where $A_{d-p}$ and $B_{d-q}$ are background fields coupled to the two excitation types. In this work, we identify new mutual statistics in the adjacent case $p+q=d-1$. For two invertible excitations obeying $\mathbb Z_N$ fusion, one can choose local creation operators $X$ and $Y$ whose supports have a staggered one-dimensional overlap. The closed unitary process $W_N(X,Y)=(Y^{-1}X^{-1})^N(YX)^N$ measures the resulting mutual statistic. Its field-theory description is $(2\pi i/N)\int A_{d-p}\cup\beta_N B_{d-q}$, where $\beta_N$ is the Bockstein operation; we therefore call the invariant Bockstein braiding statistics. The construction yields particle-particle statistics in one dimension, particle-loop statistics in two dimensions, and loop-loop or particle-membrane statistics in three dimensions. Nontrivial Bockstein braiding statistics obstructs simultaneous condensation of the two $\mathbb Z_N$ excitations. It also rules out a fully symmetric gapped phase for systems with the corresponding mixed anomaly and implies symmetry fractionalization when one of the $\mathbb Z_N$ symmetries is broken.

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cond-mat.mtrl-sci 2026-07-03

FePd films shift from mixed to pure Néel domain walls with depth

by Annika Stellhorn, Alicia Backs +12 more

From Bloch to N\'eel: Anisotropy-dependent Domain-Wall Character in FePd Thin Films

Higher anisotropy produces a depth transition linked to L10 order, while lower anisotropy yields only Néel walls throughout.

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We report an experimental investigation of the depth-dependent domain wall formation in L1$_0$-FePd thin films with high perpendicular magnetic anisotropy. Using circular dichroism X-ray resonant magnetic scattering (CD-XRMS) as a function of the incident X-ray angle, we explore the depth evolution of chiral spin textures in two samples with different strengths of magnetocrystalline anisotropy. Combined with CD-STXM, CD-ptychography, and macroscopic characterization of the structural order, magnetic properties, and surface morphology, we relate these observations to differences in the long-range order of the L1$_0$ phase of FePd. One FePd thin film with very high magnetocrystalline anisotropy, characterized by $Q_{PMA}=1.8$, exhibits an unexpectedly large N\'eel contribution. Angular-dependent CD-XRMS directly reveals a smooth transition from a hybrid Bloch-N\'eel chirality within the upper FePd layer towards a purely N\'eel-type structure at the lower FePd interface. In the second FePd sample, despite a still relatively large $Q_{PMA}=1.45$, the domain walls were found to be purely N\'eel type. These results indicate a crucial role of the long-range structural order in determining the formation of the magnetic structure.

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quant-ph 2026-07-03

Tsallis entropy of occupation numbers bounds fermionic non-Gaussianity

by Poetri Sonya Tarabunga, Bernhard Jobst +5 more

Computable measures of fermionic non-Gaussianity from the covariance matrix

One member is monotonic under Gaussian protocols and lower-bounds the non-Gaussian gates needed to prepare pure fermionic states.

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Fermionic non-Gaussianity, or fermionic magic, is a key resource underlying the computational complexity of fermionic quantum systems, yet tractable and operationally meaningful ways to quantify it remain limited. We address this challenge by developing a convex resource theory of fermionic non-Gaussianity and introducing two families of computable measures for pure fermionic states, both derived from the Williamson normal form of the covariance matrix. The first family, occupation number entropies, is defined as the Tsallis-$\alpha$ entropy of the occupation numbers. We prove that one member of this family is monotonic under Gaussian protocols, establishing it as a computable convex resource monotone. It consequently lower bounds the number of non-Gaussian gates needed for state preparation. The second family, natural-orbital participation entropies, is given by the R\'enyi-$\alpha$ entropy of the squared amplitudes of the state in the natural-orbital basis, defined by the eigenvectors of the covariance matrix. These measures quantify state compressibility in this basis and thus upper bound the classical simulation cost in an orthonormal Gaussian basis. We analyze both families for stabilizer and translation-invariant states, where they simplify and reveal additional structure. We further study representative examples, including random SWAP-doped matchgate circuits and the bond-modulated XXZ model, highlighting the role of non-Gaussianity in many-body phenomena. Our work establishes a resource-theoretic framework for computable fermionic non-Gaussianity that unifies notions arising across quantum information, condensed-matter physics, and quantum chemistry, opening new directions for studying the complexity of quantum many-body systems and providing practical tools to assess the classical simulability of fermionic states relevant for quantum advantage.

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cond-mat.str-el 2026-07-03

Strong repulsion erases confinement signatures in 1D spectra

by Nair Aucar Boidi, Mikhail Kiselev

A density matrix renormalization group approach to quantum point contacts

DMRG calculations show that correlations override harmonic potential effects on the local density of states once interactions become strong.

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Using the density matrix renormalization group (DMRG) combined with the correction-vector method, we investigate the competition between an harmonic potential and repulsive interactions in a one-dimensional fermionic system. The parabolic confinement induces spatial inhomogeneity, and by tuning its curvature one can continuously interpolate between a potential well--relevant for cold-atom setups--and a quantum barrier, as realized in mesoscopic systems such as quantum point contacts. We analyze how the ground-state particle distribution evolves with the strength and sign of the confining potential and how the confinement reshapes the spectral weight of the local density of states (LDOS) at the center of the chain. In the barrier regime, a localized peak emerges in the electron part of the spectrum ($\omega >0$) as a direct consequence of the potential. In contrast, in the well configuration and for weak interactions, a localized feature persists but shifts to the hole sector ($\omega <0$). However, for stronger interactions, the LDOS no longer displays clear signatures of the external potential, indicating that correlations dominate over single-particle confinement.

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cond-mat.str-el 2026-07-03

Self-energy suppresses polaron phonon weight near zone boundary

by Lawrence Rai, Sudhakar Pandey

Phonon spectral function of Holstein polaron: Investigation of many-body effects with self-energy and vertex correction

Vertex corrections nearly cancel the effect at small wave vectors but the net result is suppression at large q in the antiadiabatic regime

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We study the impact of the many-body effects on the phonon spectral function of Holstein polaron in one-dimension in the antiadiabatic regime by incorporating the contributions from the electron self-energy and vertex corrections within a weak-coupling approach that respects the charge-conserving Ward identity. We find that while the polaronic spectral weight is suppressed due to contribution from the electron self-energy, on the other hand, the same is enhanced due to contribution from the vertex corrections. While strength of both the contributions increases with increasing the wave vector ($\q$) of phonons, they nearly cancel each other for the small-$\q$ modes so that the polaronic spectral weight is weakly affected due to the many-body effects. For the large-$\q$ modes near the zone boundary, the net many-body correction is dominated by the contribution of the electron self-energy which increases faster in comparison to that of the vertex corrections with increasing the wave vector thereby resulting in a significant suppression of the polaronic spectral weight. We find that while the weak-coupling perturbative approach provides a reliable estimation of the impact of the many-body effects deep inside the antiadibatic regime, the renormalization of quasiparticle spectrum must be taken into account for an accurate estimation when the phonon energy approaches the electronic bandwidth.

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cond-mat.str-el 2026-07-03

TbB4 non-coplanar order mixes odd- and even-parity spin textures

by Dong-Choon Ryu, Jae-Ho Han +2 more

Unconventional Mixed-Parity Magnetism in Rare-Earth Tetraborides

Scalar spin chirality produces p- and f-wave in-plane components with d-wave out-of-plane character in this compensated magnet.

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Altermagnetism has advanced the study of compensated magnets by revealing non-relativistic spin splitting, traditionally classified into strictly even- or odd-parity spin textures. Here, we unveil a fundamentally different regime: component-resolved mixed-parity spin splitting in a fully three-dimensional compensated magnet. Using first-principles calculations, tight-binding and $\mathbf{k} \cdot \mathbf{p}$ models, along with spin-group symmetry analysis, we demonstrate that the non-coplanar ground state of $\mathrm{TbB}_4$ enforces a unique momentum-space spin texture. The in-plane spin components exhibit odd-parity $p$- and $f$-wave-like textures, whereas the out-of-plane component retains an even-parity $d$-wave altermagnetic character. Crucially, the coexistence of the in-plane odd-parity textures is driven not by relativistic spin-orbit coupling, but by a staggered Berry phase arising from the inherent scalar spin chirality. This mixed-parity structure dictates distinct transport fingerprints, including bulk non-relativistic Edelstein and spin Hall responses, as well as a symmetry-allowed Berry curvature dipole. These results establish the rare-earth tetraborides as a robust platform for engineering complex spin-charge conversion phenomena.

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cond-mat.str-el 2026-07-03

Raman detects chiral magnons with plaid splitting in MnTe2

by Dirk Wulferding, Daehyeon An +9 more

Plaid-Like Spin Splitting and Chirality of Magnon Bands in Antiferromagnetic MnTe₂

Helicity imbalance and reduced symmetry show momentum-dependent magnon handedness that matches altermagnetic patterns.

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Altermagnets constitute an emerging class of magnetic materials that combine compensated antiferromagnetic order with spin-split excitations arising from crystalline symmetries. Despite strong theoretical interest, their experimental identification remains challenging. Here, we demonstrate that helicity- and angle-resolved Raman scattering measurements reveal reduced rotational symmetries of magnons and a pronounced imbalance between left- and right-circular polarization channels, indicating momentum-dependent magnon handedness. First-principles DFT+$U$ calculations combined with linear spin-wave theory uncover a characteristic plaid-like spin-splitting structure in momentum space. The resulting magnon spin textures are dictated by the unconventional sublattice symmetries of MnTe$_2$ and closely emulate those of altermagnetic electronic bands. Our work provides evidence of chiral spin-wave excitations unique to this non-coplanar antiferromagnet.

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cond-mat.supr-con 2026-07-03

Fermi arcs in PtBi₂ shown singly degenerate and spin-polarized

by Anders Christian Mathisen, Xin Liang Tan +13 more

Fermiology and spin polarization of topological surface states in PtBi₂

This establishes their nontrivial topology, a necessary condition for topological superconductivity.

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Layered PtBi$_2$ is a candidate for topological superconductivity arising in Fermi-arc surface states. Using spin- and angle-resolved photoemission spectroscopy, we demonstrate that the Fermi arcs in PtBi$_2$ are singly degenerate and spin-polarized, which establishes their nontrivial topology and constitutes a necessary condition for topological superconductivity. We further uncover a pronounced surface-termination dependence of the Fermi-arc dispersion, yielding either nearly flat or approximately linear bands in agreement with first-principles calculations. Together, the observed spin polarization and termination-dependent bandwidth of the Fermi-arc surface states identify key ingredients underlying the potential emergence of topological superconductivity in PtBi$_2$.

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cond-mat.mtrl-sci 2026-07-03

Oxygen ions carry 90% of circular phonon angular momentum in STO

by Roman Mankowsky, Serhane Zerdane +10 more

Quantifying angular momentum of coherently driven circular phonons

X-ray diffraction quantifies ionic motions and shows charge imbalance that produces THz-induced magnetism

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The use of intense terahertz (THz) pulses to manipulate low-energy excitations offers a powerful approach for ultrafast control of electronic and magnetic properties in materials. Theory suggests that circular ionic motions driven by THz fields carry angular momentum, potentially generating internal magnetic fields. Recent experiments in nonmagnetic SrTiO3 (STO) have hinted at such THz-induced fields, but their origin remains debated. Here, we employ ultrafast x-ray diffraction to resolve the time-dependent ionic trajectories in STO following excitation by circularly polarized THz pulses. Our analysis reveals that oxygen ions, despite their lower mass, contribute around 90% of the phonon angular momentum. The resulting imbalance between the negatively and positively charged ions provides a clear explanation for the mechanism behind induced magnetism in STO. This work further provides the first quantitative measurement of circular ionic motions and their angular momentum and establishes a general methodology for the investigation of angular momentum transfer in solids, paving the way for new strategies to control topological phonon transport and phonon-driven magnetism in quantum materials.

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cond-mat.str-el 2026-07-03

Zigzag order coexists with Z2 topology in Kitaev-J3 model

by Jiucai Wang, Chuan Chen

Evidence for Deconfined Magnetic Order in the Kitaev-J₃ Model

Variational wave functions retain multiple torus sectors and gapless Majorana cones when vison pairs condense but single visons stay gapped.

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We investigate the Kitaev-$J_3$ honeycomb model using variational Monte Carlo calculations combined with a vison-quasiparticle analysis of the parent Kitaev spin liquid (KSL). We provide evidence for deconfined magnetic phases in which zigzag or antiferromagnetic order coexists with remnant $\mathbb{Z}_2$ topological structure inherited from the KSL. The optimized variational wave functions retain multiple linearly independent topological sectors on a torus, whereas those of conventional ordered phases collapse to a single sector. The vison-quasiparticle analysis shows that magnetic order naturally arises from vison-pair condensation while single visons remain gapped, yielding a microscopic mechanism for magnetic ordering without immediate confinement. The resulting phases further host gapless spinons with multiple Majorana cones, offering a possible microscopic scenario for the anomalous low-temperature longitudinal thermal transport reported in magnetically ordered Kitaev materials such as Na$_2$Co$_2$TeO$_6$. Our results reveal a microscopic route to fractionalized magnetism beyond the conventional dichotomy between magnetic order and spin-liquid behavior.

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cond-mat.str-el 2026-07-03

Ultrasound shows slowing hexadecapole fluctuations in Sr2RuO4

by Ryosuke Kurihara, Mitsuhiro Akatsu +6 more

Ultrasonic Observation of Slowing Down of Multipole Fluctuations in Sr₂RuO₄

Attenuation rises toward 1.4 K and keeps rising under 10 T field that suppresses superconductivity.

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We performed ultrasonic measurements on the unconventional superconductor Sr$_2$RuO$_4$ to investigate the dynamical properties of the electronic states near its superconducting transition temperature, $T_\mathrm{c} = 1.4$ K. We observed an increase in the in-plane transverse ultrasonic attenuation coefficient as the temperature approached $T_\mathrm{c}$. The ultrasonic attenuation exhibited a Landau-Khalatnikov-type ultrasonic frequency dependence with a typical relaxation time of approximately $10^{-10}$ s. Under an applied magnetic field of 10 T, the superconducting transition was suppressed. However, the ultrasonic attenuation coefficient exhibited an increase down to low temperatures, indicating the slowing down of fluctuations associated with multipole degrees of freedom. Based on group-theoretical considerations, we propose that the electric hexadecapole plays a crucial role in the slowing down. Furthermore, we discuss the relationship between multi-component superconducting order parameters and multipole degrees of freedom.

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quant-ph 2026-07-03

Log-depth circuits implement exact KW dualities

by Yanting Cheng, Shang Liu

Shallow Unitary Circuits for Kramers-Wannier Dualities

Maps arbitrary short-range entangled states to long-range entangled duals in logarithmic depth within the symmetric sector.

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The quantum Kramers-Wannier (KW) duality is a fundamental transformation mapping short-range entangled (SRE) states to long-range entangled (LRE) states. While spatially local unitary circuits require linear-in-system-size depth to implement this duality, the ultimate speed limit for purely unitary circuits equipped with nonlocal connectivity remains an open question. Here, we explicitly construct logarithmic depth, spatially nonlocal unitary circuits that realize the exact $\mathbb{Z}_2$ KW dualities in both one and two spatial dimensions. We further generalize the construction to arbitrary $\mathbb{Z}_n$ KW dualities. Unlike algorithms tailored to prepare specific target states, our circuits implement complete duality maps. Within the symmetric (charge-neutral) sector, these dualities exactly transform arbitrary non-fixed-point SRE states into their corresponding LRE duals. Consequently, our results establish an efficient, purely coherent pathway for exploring phase transitions and topological dualities on modern quantum platforms.

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cond-mat.str-el 2026-07-02

Green's functions reach nonlinear Hall response past perturbative crossover

by Lei Geng, Martin Eckstein +5 more

Nonperturbative Nonlinear Hall Effect in Nonequilibrium Steady States

New nonequilibrium method handles interactions where constant-relaxation-time and dipole approximations fail

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The nonlinear Hall effect in quantum materials has attracted broad interest, yet most existing studies focus on the weak-field, perturbative regime. Here we develop a nonperturbative approach based on nonequilibrium steady-state Green's functions for dc-field-driven lattice systems, with dissipation and interactions incorporated through self-energies beyond the constant relaxation-time approximation and interband transitions treated alongside their intraband counterparts. Applied to a two-band semimetal model, our approach provides direct access to the strong-field Hall response beyond the nonperturbative crossover where the edge of the nonequilibrium distribution reaches Berry-curvature hot spots, a regime in which constant relaxation-time estimates and Berry curvature dipole calculations become unreliable. We further demonstrate that interaction and electron-phonon self-energies within dynamical mean-field theory can substantially change the Hall signal. Our framework enables quantitative simulations of nonequilibrium nonlinear Hall phenomena and provides guidance for strong-field transport experiments.

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cond-mat.str-el 2026-07-02

Vertex correlations encode generalized quantum geometry

by Alejandro S. Miñarro, Gervasi Herranz

Generalized quantum geometry formulated through interacting vertex correlations

The tensor is expressed through interacting vertices conjugate to deformation parameters instead of Bloch momentum alone.

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Quantum geometry characterizes the variation of electron wavefunctions in solids along a parameter space. Conventionally, crystal momentum is chosen as the parameter, since it couples to electromagnetic fields, offering an interpretation of quantum geometry in terms of dipole matrix elements, polarization fluctuations, and optical responses. However, Bloch momentum is not the only possible parameter space in which a wavefunction can evolve. In this work, we show that quantum geometry can be extended beyond the bare Bloch-band geometry to manifolds whose adiabatic parameters represent deformations of the ground state, including collective bosonic fluctuations, external fields, or structural distortions. We show that the generalized quantum geometric tensor is encoded by correlations of interacting vertices, conjugate to the deformation parameters. By way of illustration, we briefly discuss the application of these extended geometric concepts to manifolds generated by Hubbard-Stratonovich bosonic fields, or Jahn-Teller configurational spaces. The formulation presented here is framed by general manifolds, which extend quantum geometry to generic structural, collective, and interactive many-body systems.

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cond-mat.str-el 2026-07-02

Tunneling splits Jackiw-Rebbi modes and drives coherent oscillations

by Surajit Mandal

Quantum Tunneling-induced Hybridization and Coherent Dynamics of Jackiw-Rebbi Zero Modes in a Modified Su-Schrieffer-Heeger Chain

Finite overlap in a modified SSH chain opens a hybridization gap that sets the period of probability oscillations between the zero modes.

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We investigate analytically and numerically the tunneling-induced hybridization and coherent dynamics of Jackiw-Rebbi (JR) zero modes in a modified Su-Schrieffer-Heeger (SSH) model. Unlike the conventional SSH model, this modified system possess two bulk gap closing points, namely, the quadratic-type gap closing point at $k=0$ and the Dirac-type gap closing point at $k=\pm\pi/4a$. While the quadratic point does not support a topological domain wall due to the absence of mass inversion, the low-energy Dirac theory around $k=\pm\pi/4a$ predicts an effective mass that changes sign at two spatially separated interfaces under a kink profile, generating a pair of JR bound states localized at those interfaces. We show that finite overlap between the JR zero modes lifts the zero-energy degeneracy through quantum tunneling, producing symmetric-antisymmetric hybridized states analogous to a quantum mechanical double-well system. An effective two-level description reveals coherent oscillations of the occupation probability between the two JR modes, accompanied by periodic transfer of sublattice polarization between the (A,C) and (B,D) sectors. The oscillation period is governed by the hybridization gap, providing a tunable route for controlling topological bound states. Our results establish a unified framework connecting JR zero modes, quantum tunneling, and coherent dynamics in modified SSH systems, offering a promising platform for controllable topological quantum-state transfer in engineered lattice structures.

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cond-mat.str-el 2026-07-02

Spin Hall response in chiral spin liquids has only exponential finite-size corrections

by Kumar Ghosh

From Dirac Cones to Semions: An Exact Finite-Size Theory of Parity-Anomaly Transport in Chiral Spin Liquids

Exact cylinder determinant maps spinon Chern number to fractional conductance with no 1/L term, confirmed by kagome DMRG at -0.5.

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Chiral spin liquids carry a hidden bookkeeping problem: the integer Chern number of their fractionalized spinons, the level of the emergent Chern--Simons gauge field, and the fractional spin response actually measured in experiment or simulation are related but distinct quantities, and the literature routinely conflates them. Here we resolve this by deriving the exact parity-odd determinant of a gapped Dirac cone on a spatial cylinder, resummed to all orders in the compact holonomy rather than truncated at leading order. The result proves that finite-circumference corrections to the topological response are strictly exponential, with no universal $1/L$ term, and fixes the precise map from microscopic spinon Chern number to physical spin Hall conductance. We validate this chain of reasoning on the kagome lattice at three independent levels: an exact parton band-structure calculation ($C=-1$, converging exponentially over cylinders four to twelve sites wide), and an interacting density-matrix renormalization group flux pump ($\nu_s=-0.500\pm0.011$) that agrees with the analytic prediction without any adjustable parameter. Together, these results turn a one-loop anomaly calculation into a quantitatively verified bridge between microscopic topology and observable fractional response.

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quant-ph 2026-07-02

One feature edit steers a quantum observable in neural states

by Zihao Qi, Christopher Earls

Mechanistic Interpretability and Causal Feature Steering of Neural Quantum States via Sparse Autoencoders

Sparse autoencoders uncover unsupervised internal features in NQS whose targeted intervention adjusts magnetization and correlators while en

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Neural Quantum States (NQS) are a remarkably expressive class of variational ans\"atze for quantum many-body wavefunctions, yet little is understood about their internal mechanisms: trained on variational objectives alone, how do NQS accurately capture physical observables that they have never been explicitly optimized for? In this work, we present a systematic approach to analyze the internal activations of NQS using sparse autoencoders. We extract features from the residual stream and demonstrate that these features strongly correlate with physical observables such as order parameters, staggered magnetization, and half-chain correlators, across both ground state representation and real-time dynamics. Remarkably, the discovery of these features is entirely unsupervised, with no physical labels provided. We further establish that such features causally affect the corresponding observables predicted by NQS, by showing that targeted, post-training intervention on a \textit{single} feature smoothly and monotonically steers the corresponding observable, while leaving the variational energy nearly unchanged. These results demonstrate that NQS are not merely functional approximators, but encode rich, interpretable internal representations of physical information. Our approach provides both a diagnostic and an intervention tool for NQS, and serves as a foundation for using mechanistic interpretability towards more reliable, transparent NQS.

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quant-ph 2026-07-02

Negativity equals entanglement cost for random mixed states

by Bowen Ouyang, Jonah Kudler-Flam

Logarithmic negativity typically equals exact entanglement cost

In large random induced states, this computable measure matches the exact cost under PPT-preserving operations.

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Quantum entanglement plays a leading role in modern understanding of physical systems, from quantum phases of matter to quantum gravity. In quantum information theory, one seeks operationally meaningful quantifiers of entanglement, which for large quantum systems are notoriously difficult to evaluate due to the lack of computationally efficient algorithms. In this Letter, we show that for large random induced mixed states the logarithmic negativity, an efficiently computable entanglement measure, generically coincides with the exact entanglement cost under positive-partial-transpose-preserving operations, thereby acquiring a precise operational interpretation. Our results establish logarithmic negativity as an exact characterization of entanglement in generic many-body states and provide a tractable route for quantifying entanglement in complex quantum systems.

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cond-mat.mtrl-sci 2026-07-02

Flat bands observed near chemical potential in TaRhTe4

by Harry Rankin, Tyler J. Slade +8 more

Observation of Flat Bands in Type-II Weyl Semimetal TaRhTe₄

ARPES data shows dispersionless bands in bulk type-II Weyl semimetal not predicted by DFT, enabling study of topology-flat band coexistence.

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Flat bands have been theoretically predicted for decades but have only recently been realized in quantum materials such as magic-angle twisted bilayer graphene, kagome and Lieb lattices, and rare-earth metal compounds. To date, only twisted layered materials have enabled tuning of flat-band energies near the electronic chemical potential, thereby influencing transport and thermodynamic properties. Here, we report the presence of flat bands near the chemical potential in bulk TaRhTe$_{4}$, a noncentrosymmetric van-der Waals type-II Weyl semimetal. Flat bands are rarely observed in Weyl semimetals, particularly in nonmagnetic bulk systems, and the observed flat bands were not predicted by density functional theory calculations. TaRhTe$_{4}$ therefore provides a platform in which nontrivial topology coexists with flat bands near the Fermi level, as evidenced by our angle-resolved photoemission spectroscopy measurements.

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cond-mat.stat-mech 2026-07-02

Fuzzy sphere extracts extensive 3D CFT data at low cost

by Yin-Chen He, W. Zhu

A Fuzzy Sphere Journey in Critical Phenomena

The regularization links critical phenomena to noncommutative geometry and the quantum Hall effect via state-operator correspondence on S^2

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This review discusses the recently proposed fuzzy sphere regularization for studying $2+1$D critical phenomena, particularly three-dimensional (3D) conformal field theory (CFT). The fuzzy sphere scheme not only offers remarkable efficiency in extracting extensive CFT data at low computational cost but also reveals unexpected connections among 3D CFT (critical phenomena), noncommutative geometry, and the quantum Hall effect. We introduce the fundamental ideas of fuzzy sphere regularization, emphasizing its role in demonstrating the state-operator correspondence of 3D CFTs on the $S^2 \times \mathbb{R}$ geometry. Additionally, we review key developments in this approach across various directions and outline potential future applications.

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cond-mat.str-el 2026-07-02

Cavity field creates strange Luttinger liquid breaking velocity rule

by Danh-Phuong Nguyen, Christophe Mora +1 more

Strange Luttinger liquids in a cavity-embedded one-dimensional electronic chain

Light-matter coupling alters low-energy sector of 1D electrons and shifts interaction phase boundaries including Majorana modes

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We study a one-dimensional electronic chain coupled to a homogeneous quantized vacuum field and electron-electron interactions. In the absence of the latter, we derive a low-energy effective description in the presence of light-matter coupling, which we identify as a strange Luttinger liquid. Although it retains a formal resemblance to conventional Luttinger liquid theory, the coupling to the quantum field qualitatively modifies the low-energy sector and breaks the standard velocity relation underlying Luttinger universality. For finite electron-electron interactions, we recover a phase diagram featuring several phases as a function of interaction strength and hopping amplitude, including a phase hosting Majorana-like zero modes. Using exact diagonalization, we compute observables that characterize the phase boundaries and show that the cavity field significantly shifts them. We also study the fate of Majorana-like states under the influence of the cavity field, highlighting their modification by light-matter coupling. Finally, we investigate whether the strange Luttinger liquid description identified in the noninteracting regime continues to hold when electron-electron interactions are introduced.

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cond-mat.str-el 2026-07-02

Conditions guarantee dimer ground states for long-range spin chains

by Jędrzej Wardyn, Mi{l}osz Panfil

Exact dimer ground states of long-range spin chains and ladders

Explicit rules on couplings make a dimer product state the exact ground state in generalized Majumdar-Ghosh models.

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Interacting spin chains and ladders are known to support a plethora of quantum phases with complex ground-state phase diagrams. In this work, we study a large family of such models and determine precise, explicit conditions under which an exact dimer state is guaranteed to be the ground state. These general conditions are validated for various generalizations of the Majumdar-Ghosh model using exact diagonalization. Our results provide exact reference points in the phase diagrams of a wide class of spin chains and ladders, including those with anisotropic and arbitrary-range interactions.

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cond-mat.str-el 2026-07-02

Scalar chirality strengthens two-magnon bound states without shifting one-magnon spectrum

by László Rudner, Karlo Penc

Chiral enhancement of two-magnon bound states in an S=1/2 triangular-lattice magnet

The term cancels for single magnons but splits opposite chiralities in the E2 channel, enhancing one bound-state channel at the Gamma point.

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We study one- and two-magnon excitations above the fully polarized state of the spin-$1/2$ triangular-lattice $J_1$-$J_2$-$J_3$ Heisenberg model with an additional uniform scalar-chirality interaction. In the one-magnon sector of the Heisenberg model, we identify two special minimum manifolds by rewriting the dispersion in complete-square form. The scalar-chirality term cancels exactly in this sector, leaving the one-magnon dispersion and the single-magnon instability unchanged. In contrast, it survives in the two-magnon sector as an oriented interaction between neighboring flipped spins. Using symmetry-adapted triangular-lattice harmonics, we derive finite-dimensional gap equations at the $\Gamma$ point in the symmetry-resolved $\mathsf{A_1}$ and $\mathsf{E_2}$-type partial-wave channels. The chirality coupling splits the two opposite relative-motion chiralities in the $\mathsf{E_2}$-type sector, thereby selectively enhancing one two-magnon bound-state channel. Exact diagonalization confirms this mechanism and reveals enhanced binding, as well as additional bound states at $M$ and at incommensurate total momenta. Our results identify scalar chirality as an efficient microscopic mechanism for strengthening two-magnon binding without shifting the one-magnon spectrum, and provide a route toward high-field spin-nematic and multipolar instabilities.

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cond-mat.mes-hall 2026-07-02

Critical current scales with miniband width in tunable moiré graphene

by Riccardo Bertini, Xueqiao Wang +17 more

Bandwidth-Limited Critical Currents in Electrically Tunable Moir\'e Bands

Displacement field narrows the valence band and lowers the nonequilibrium threshold, with identical scaling across graphene superlattices.

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Moir\'e superlattices host narrow minibands whose bandwidth governs correlated and topological phases. Here, we demonstrate that the bandwidth also sets the critical current for the onset of out-of-equilibrium transport. In bilayer graphene aligned to hexagonal boron nitride, we explore the high-current transport regime as we continuously flatten the valence miniband using an out-of-plane displacement field. We observe a significant reduction in the critical current, which is captured by a minimal analytical model and corresponds to the calculated narrowing of the miniband. Moreover, by comparing distinct moir\'e platforms, we show that the scaling between critical current and bandwidth is a universal feature of graphene superlattices. Our results reveal a direct link between miniband dispersion and high-current transport, and establish this regime as a fast and accessible electrical probe of bandwidth evolution.

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cond-mat.supr-con 2026-07-02

Kagome breathing mode turns s-wave into odd-frequency Cooper pairs

by Debmalya Chakraborty, Anushree Datta +1 more

Breathing mode inducing dynamical pairing in Kagome materials

Controlling this lattice distortion realizes time-nonlocal spin-singlet pairs on top of conventional superconductivity.

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The breathing mode in Kagome materials is a structural modulation that breaks inversion symmetry and has been shown to be a crucial source for intriguing phases in the normal state. In this work, we carry out a full classification of superconducting symmetries in kagome superconductors and demonstrate the emergence of odd-frequency dynamical Cooper pairs entirely driven by the breathing mode. We then show that odd-frequency spin-singlet Cooper pairs can be realized by controlling the breathing mode in kagome lattices with conventional spin-singlet $s$-wave superconductivity. Since odd-frequency pairing is intrinsically nonlocal in time, our results put forward the breathing mode for designing dynamical Cooper pairs in kagome materials.

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cond-mat.str-el 2026-07-02

Triplet pairs create exact eigenstates in bilayer Hubbard models

by F. X. Liu, Z. Song

Exact interlayer triplet-pairing eigenstates in the extended Hubbard model

Interlayer interactions enable states with long-range order in two- and three-layer systems, preserving singlet pairing symmetry.

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$\eta$-pairing symmetry generalizes the pairing mechanisms in superconductivity but is broken in the presence of interlayer interactions. In this work, we extend this approach to triplet pairs. We propose interlayer triplet-pairing operators for the multi-layer extended Hubbard model. We find that a set of exact condensate-pair eigenstates can be constructed, which exhibit off-diagonal long-range order. In contrast to the $\eta$-pairing mechanism, this originates from restricted spectrum generating algebra and is only available for bilayer and trilayer systems in the presence of interlayer Hubbard interactions. Nevertheless, the system also retains the original on-site $\eta$-pairing symmetry in the absence of interlayer interactions. Consequently, both singlet and triplet pairs coexist in the eigenstates of the multi-layer Hubbard model. We employ quench dynamics to demonstrate the results through numerical simulations. Our findings open avenues for the study of exact condensate-pair states in strongly correlated systems.

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cond-mat.str-el 2026-07-02

Tensor networks solve billion-site tight-binding problems

by Tiago V. C. Antão, Anouar Moustaj +2 more

Tensor Network Solvers for Ultra-large Tight-binding Hamiltonians: Algorithms and Applications

Mapping to pseudospin chains keeps bond dimensions small and fixed for compressible real-space Hamiltonians.

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Understanding quantum materials at meso and even macroscopic scales requires tight-binding calculations on system sizes where explicit matrix representations become prohibitively costly. This represents a major bottleneck to rationalize phenomena in moir\'e and super-moir\'e heterostructures and quasicrystals. Here, we present a unified tensor-network methodology to solve tight-binding problems at exceptionally large scales, by mapping a system of $N = 2^L$ sites onto a many-body problem of $L$ pseudospin sites, which is subsequently solved with tensor network algorithms. For Hamiltonians with compressible real-space structure, the tensor network bond dimension remains modest, typically of order a few tens, independent of $N$.Tensor network representations of arbitrary hopping functions including long-range, spatially modulated, and twisted-layer couplings are built with quantics tensor cross interpolation, and all physical observables are evaluated entirely with tensor network algebra without explicit matrix storage or diagonalization. We demonstrate applications to spectral functions, momentum-space spectra via the tensor-network quantum Fourier transform, real-space topological invariants, real-time dynamics, correlation induced symmetry breaking with self-consistent mean-field calculations, non-Hermitian phenomena, and excitonic many-body physics. Our methodology enables routinely solving systems with billions of sites, by leveraging the tensor network compressibility of real-space structures, and establishing a flexible framework to study quantum matter at ultra-large length scales. The methodology is implemented in the open-source Julia package TensorBinding.jl.

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cond-mat.supr-con 2026-07-02

The critical temperature of superconductivity in YPtBi shows little variation despite…

by Prathum Saraf, Nicholas A. Crombie +12 more

Resilient j=3/2 superconductivity in topological semimetal YPtBi

Critical temperature shows little change across two orders of disorder and three of carrier density, pointing to phase stiffness as the limi

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Cooper pairing in most of the known fermionic superfluids occurs via spin-1/2 quasiparticle interactions that lead to spin-singlet or spin-triplet pairing. In the topological semimetal YPtBi, strong spin-orbit coupling results in a band inversion between highly symmetric $s$- and $p$-like electronic bands and a degeneracy at the $\Gamma$ point that ensures the manifold of $j$=3/2 quasiparticle states thrive near the Fermi level, where superconducting pairing occurs. Here we study the effects of magnetic and nonmagnetic disorder and carrier density on this exotic superconducting pairing state. By varying levels of disorder and carrier densities by nearly two and three orders of magnitude, respectively, we show that the superconducting critical temperature of YPtBi has a remarkable robustness, with little variation across this span. Our results suggest that superconductivity in YPtBi may reside in a regime where phase stiffness, rather than pair formation, governs the transition temperature. The insensitivity of Cooper pairing to dramatic changes in quasiparticle environment in a $j$=3/2 superconductor highlights a new form of protection realized in topological high-spin superconductors.

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cond-mat.str-el 2026-07-02

Continuous deconfined transition links antiferromagnet to d-wave superconductor

by Chuang Chen, Subir Sachdev +1 more

Deconfined criticality between an antiferromagnetic insulator and a nodal d-wave superconductor: a quantum Monte Carlo study

Parton Monte Carlo simulation shows both orders vanish together at half filling on the square lattice.

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We present a quantum Monte Carlo study of the transition between the insulating N\'eel state and the nodal $d$-wave superconductor on the square lattice at half-filling. We access a regime of frustrated magnetic order without a sign problem using a parton representation of the electron in terms of fermionic spinons and bosonic chargons. Both partons move in a background $\pi$-flux (so the electron experiences no net flux) and are coupled to a quantum fluctuating SU(2) lattice gauge field. In contrast to earlier studies directly on the electronic degrees of freedom, we find evidence for a second-order deconfined quantum phase transition at which both the N\'eel and $d$-wave superconductivity orders vanish continuously. We compute correlators of the spinon-chargon composite with the same quantum numbers as the electron: we find a gapless Dirac dispersion inside the $d$-wave superconductor, turning into a gapped dispersion in the antiferromagnet.

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cond-mat.str-el 2026-07-02

Tensor method solves nonequilibrium DMFT impurity problem

by Shuta Matsuura, Hiroshi Shinaoka +2 more

Weak-coupling tensor cross interpolation impurity solver for nonequilibrium dynamical mean-field theory

Approximates high-dimensional integrals in low-rank tensor-train form to avoid Monte Carlo sampling and sign issues.

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Simulating nonequilibrium quantum many-body systems remains a major challenge due to the exponential growth of the computational complexity with real time. Here we implement a nonequilibrium impurity solver based on the weak-coupling expansion and the tensor cross interpolation (TCI), and apply it to nonequilibrium dynamical mean-field theory (DMFT). The method approximates the integrands of the high-dimensional integrals arising in the weak-coupling expansion in a tensor-train form, enabling efficient evaluations without stochastic sampling and thereby mitigating the sign problem affecting continuous-time quantum Monte Carlo (CT-QMC) methods. Benchmark calculations for an exactly solvable nonequilibrium impurity model agree well with the exact results and reveal a low-rank structure of the integrands. When applied to interaction-quench problems in the half-filled Hubbard model, the method reproduces fast thermalization at a critical interaction strength with accuracy comparable to CT-QMC. Away from half filling, where the sign problem becomes even more severe, the present approach remains well controlled, revealing a crossover instead of a sharply defined fast thermalization point in the 3/4-filled case. The solver can also be applied to steady-state DMFT problems, yielding accurate spectral functions in the metallic regime without analytic continuation.

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physics.ins-det 2026-07-02

Collimated setup enables in-situ polarization tuning for magneto-IR

by Zeping Shi, Wenbin Wu +15 more

In-Situ Polarimetry in Collimated Magneto-Infrared Spectroscopy System

Achieves 0.0033% noise and 40:1 extinction while switching between linear, circular, and elliptical states without breaking vacuum.

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Magneto-infrared spectroscopy under strong magnetic fields provides a powerful probe of Landau quantization and field-induced collective excitations, yet its full potential has long been constrained by the lack of in-situ polarization control, because the highly divergent infrared beam propagating through narrow light tubes undergoes multiple wall reflections, leading to severe polarization degradation. Here we report a collimated magneto-infrared spectroscopy system that integrates continuous in-situ polarimetry. The system employs incident and exit collimation chambers forming a Kepler type optical architecture, which converts the large-aperture FTIR output into a low-divergence beam and strongly suppresses multi-reflection trajectories inside long gold-plated light tubes, thereby enhancing both optical throughput and polarization fidelity. A remotely controlled polarization module, consisting of an automated linear polarizer and a switchable Fresnel rhomb positioned entirely outside the high-field region, enables continuous in-situ tuning between linear, circular, and arbitrary elliptical polarization states without thermal cycling, manual realignment, or breaking vacuum. Interchangeable compact focusing modules further support Faraday and Voigt geometries in both transmission and reflection experiments within a 50 mm magnet bore, providing efficient beam focusing and signal collection while maintaining polarization fidelity. The setup achieves a minimum root-mean-square noise of 0.0033%, an average noise of 0.0082%, and a linear polarization extinction ratio up to 40:1. We demonstrate the capability through continuous in-situ linear polarimetry and broadband circular polarimetry in the magneto-infrared spectroscopy of various single crystals. This platform establishes a robust experimental framework for in-situ polarization-resolved magneto-infrared spectroscopy.

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cond-mat.str-el 2026-07-02

C-axis stress tunes Sr2RuO4 Fermi sheets by charge transfer

by Fabian Jerzembeck, Maximilian T. Pelly +8 more

Quantum Oscillations of Sr₂RuO₄ under c-Axis Uniaxial Stress

Quantum oscillations show α and β areas move oppositely while γ grows, driving the system toward a Lifshitz transition unlike in-plane stres

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Uniaxial stress has now been widely used to study correlated electron materials. However, Fermi surface-resolved experimental data on the evolution of the electronic structure under piezoelectrically applied stress are sparse, with no reports of de Haas-van Alphen (dHvA) effects under uniaxial stress. Here we present dHvA measurements under $c$-axis uniaxial stress on the unconventional superconductor $\mathrm{Sr}_2\mathrm{RuO}_4$. This allows us to study the evolution of the electronic structure directly and to gain insight into the contradicting behavior of the predicted enhancement of the electronic density of states and the observed suppression of $T_\text{c}$. We are able to follow all Fermi surfaces for stress up to $-1.8$~GPa and find that the cross-sectional areas of the hole-like $\alpha$ sheet increase and electron-like $\beta$ sheet decrease. At the same time, the area of the electron-like $\gamma$ sheet increases. Therefore, in contrast to in-plane uniaxial stress, charge transfer is the mechanism for approaching the electron-to-hole Lifshitz transition and the associated Van Hove singularity. Additionally, we find that the effective masses on all three Fermi sheets are slightly enhanced as the Lifshitz transition is approached. We compare the dHvA results with quantum oscillations in the magnetostriction and band structure calculations, and find good agreement. At a more general level, our findings show that quantum oscillation measurements under uniaxial stress, combined with band-structure calculations, offer a promising new route for studying quantum materials.

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cond-mat.mtrl-sci 2026-07-02

Phonon and spin entropy drive 90 K transition in Nb3Cl8

by Chenjie Zhu, Shuai Zhang +4 more

Entropy-Driven Structural Phase Transition in Nb₃Cl₈ via Density Functional Theory and an Effective Model

Free-energy calculations show softer vibrations plus paramagnetic disorder stabilize the high-temperature alpha phase while dimerization que

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As a prototypical flat-band cluster Mott insulator on an effective triangular lattice, Nb$_3$Cl$_8$ is a potential candidate for hosting a quantum spin liquid (QSL) state. Nevertheless, a first-order structural phase transition around 90K transforms the high-temperature paramagnetic $\alpha$ phase into the low-temperature nonmagnetic $\beta$ phase, suppressing the candidate QSL regime of the $\alpha$ phase. To clarify the microscopic origin of this transition, we combine first-principles calculations with an extended Hubbard model to construct a unified free-energy framework. This framework reveals that the transition is jointly driven by phonon and spin entropy: the $\alpha$ phase is stabilized by softer phonons and larger paramagnetic spin entropy, whereas the $\beta$ phase is favored by interlayer dimerization, which hardens the phonons and quenches the spin entropy through singlet formation. Furthermore, by evaluating the pressure-dependent generalized enthalpy, we provide a thermodynamic explanation for the suppression of the transition under c-axis uniaxial pressure, where stabilizing the $\alpha$ phase may allow the candidate QSL regime of the $\alpha$ phase to be explored at low temperatures.

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cond-mat.str-el 2026-07-02

2D magnetic order appears in 3D perovskite at 4.4 K

by Bocheng Yu, Otkur Omar +13 more

Strongly frustrated 2D magnetism in a 3D hexagonal perovskite

Manganese moments lock into 120° pattern within planes but remain disordered along the stacking axis, yielding high frustration.

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Exotic quantum phenomena are often found to occur in spin systems that exhibit low-dimensional magnetism. By combining nuclear magnetic resonance, neutron scattering, and muon-spin spectroscopy ($\mu$SR) techniques, we report a rare instance of strongly frustrated two-dimensional (2D) magnetism in a three-dimensional (3D) hexagonal perovskite. Here, Ba$_2$La$_2$MnTe$_2$O$_{12}$, a triangular-lattice magnet, is shown to undergo a magnetic transition at $T_\mathrm{N} \approx$ 4.4 K, below which the manganese moments form a 120$^{\circ}$ AFM order within the $ab$-plane, while staying disordered along the $c$-axis. This exotic ground state, which exhibits ideal 2D magnetism, is highly consistent with the persistently strong spin fluctuations and the large internal field distributions revealed by zero-field $\mu$SR. Further, the 2D magnetism also leads to a significant frustration, much larger than that of most known magnetically-ordered frustrated systems. Our work on Ba$_2$La$_2$MnTe$_2$O$_{12}$ not only challenges the interpretations of magnetic order in other 3D hexagonal perovskites, but it also provides insight into how the dimensionality affects the exotic magnetic states.

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cond-mat.str-el 2026-07-02

NiS2 nanocrystals favor domain walls over canting for weak ferromagnetism

by Hayato Miyazaki, Tomohiko Yoshinaga +8 more

Weak Ferromagnetism in NiS₂ under Nanocrystallization

27-nm crystals show surface moments alone explain the low-temperature response and leave no room for a bulk-like weak-FM component.

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Structurally well-ordered NiS$_2$ nanocrystals with an average diameter of $27.0 \pm 6.5$ nm retain the bulk-like two-step antiferromagnetic transitions, as shown by magnetization and heat-capacity measurements. Below the lower transition, the nanocrystals exhibit a hysteretic ferromagnetic response with large coercivity, exchange bias, and a vertical loop shift after field cooling, whereas the $M$-$H$ response just above the transition is nearly linear. These features are best explained by uncompensated surface moments generated where the low-temperature antiferromagnetic order terminates at the nanocrystal surface. The absence of a clear additional bulk-like weak-ferromagnetic component constrains homogeneous-canting models and indirectly favors a domain-wall scenario for the weak ferromagnetism of bulk NiS$_2$.

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cond-mat.str-el 2026-07-02

Transformer projects 8x8 training directly onto 10x10 quantum lattices

by Xingran Guo, Tiaojie Xiao +2 more

Holographic Quantum Transformer: A Generalist Neuro-Symbolic Architecture for Solving Frustrated Systems via Generative Attention

Continuous positional embedding interpolation yields energies matching variational benchmarks on larger frustrated systems with no retrainin

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Simulating two-dimensional frustrated quantum matter is a grand challenge due to the sign problem and exponential Hilbert space complexity. In this work, we introduce the Holographic Quantum Transformer (HQT), a physics-inspired generative architecture that leverages global self-attention to resolve non-local entanglement patterns. We validate HQT on the square lattice $J_1-J_2$ Heisenberg model. On the heavily frustrated $8 \times 8$ lattice at the quantum critical point ($J_2=0.5$), HQT reaches a ground-state energy per site ($E/N$) of $\mathbf{-0.5001(1)}$, consistent with the expected finite-size scaling trend. Beyond numerical accuracy, HQT exhibits intrinsic physical awareness, autonomously recovering the underlying $J_2$ interaction geometry through interpretable attention maps. Our central contribution is ``Holographic Transfer", a zero-shot size-extrapolation protocol with rapid alignment: a model trained on $8 \times 8$ systems is directly projected onto larger $10 \times 10$ lattices via continuous positional-embedding interpolation and head re-initialization, achieving high-fidelity initialization and rapid convergence. This zero-shot protocol yields an energy of $E/N = \mathbf{-0.49782(3)}$, statistically consistent with the variational state of the art while requiring no from-scratch training on the target lattice. Our results establish generative attention as a scalable paradigm for transferable quantum simulation.

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cond-mat.str-el 2026-07-01

Quantized entropy persists in toric code despite broken symmetry

by Haruki Watanabe

Phase distinction of Gibbs states without symmetry breaking: topological invariants of the 3D toric code

The 3D Z2 toric code keeps topological entanglement entropy at ln 2 at finite temperature, protected geometrically, and a new Wilson loop me

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We study the finite-temperature topological order of the three-dimensional $\mathbb{Z}_2$ toric code in a generic magnetic field, where every higher-form symmetry is explicitly broken and can at most be emergent. We show perturbatively, and confirm by large-scale quantum Monte Carlo, that the topological entanglement entropy stays quantized at $\gamma = \ln 2$ throughout the topological phase -- at finite temperature and under the symmetry-breaking field alike -- and collapses to $0$ across the thermal transition, a quantization protected geometrically by the Bianchi identity rather than by any exact symmetry of the system. The plateau $\gamma = \ln 2$ is, however, not invariant under quasi-local channels: a constant-depth channel can generate this identical quantized value from a trivial product state. We therefore introduce the decoded Wilson-loop correlation $f_W$, which quantizes to $1$ in the topological phase and $0$ in the trivial phase as $L\to\infty$ and, unlike $\gamma$, is a quasi-local-channel invariant -- a robust topological invariant of the mixed state.

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cond-mat.str-el 2026-07-01

Dissipation splits the Mott transition into two critical points

by Oscar Bouverot-Dupuis, Alberto Rosso +1 more

Dissipation splits the Mott transition in one dimension

An intermediate dissipative phase appears between the Luttinger liquid and Mott insulator for bath exponents below 3/2

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Understanding how dissipation modifies quantum phase transitions is a central challenge in many-body physics. A paradigmatic example is the one-dimensional Mott transition, which in isolated systems separates a conducting Luttinger liquid (LL) from a Mott insulator (MI). Here, we study the fate of this transition in the presence of dissipative baths locally coupled to the density. Using bosonisation and an exact integration of the bath degrees of freedom, we show that dissipation fundamentally reshapes the phase diagram for bath exponents $s<3/2$, where $s$ characterises the low-energy bath spectrum. Rather than undergoing a direct LL-MI transition, the system develops an intermediate dissipative phase (DP) that is compressible and gapless, yet has zero superfluid stiffness. As a result, the conventional Mott transition splits into two distinct critical phenomena: a Berezinskii-Kosterlitz-Thouless transition from the LL to the DP, followed by a new commensurate-incommensurate transition from the DP to the MI. We derive an effective field theory for the latter transition and characterize its universality. For $1<s<3/2$, the critical exponents vary continuously with the bath exponent as $\beta=\nu=1/z=s-1$, while for $s<1$ the transition is governed by $\beta=\nu=1/z=0$ and the doping vanishes sharper than any power law. State-of-the-art Monte Carlo simulations quantitatively support our predictions. These results demonstrate that dissipation can qualitatively alter the nature of the Mott transition and generate novel critical behaviour in strongly correlated one-dimensional systems.

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hep-th 2026-07-01

Lattice discretization matches MCS degeneracy exactly at commensurable sizes

by Andrea Bulgarelli, Maria Cristina Diamantini +7 more

Toward Hamiltonian simulations of Maxwell-Chern-Simons theory: constant modes and gauge field truncation

Constant mode sector on the torus maps to a Harper-Hofstadter model that preserves the magnetic translation algebra when lattice sizes satis

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Maxwell-Chern-Simons (MCS) theory in $2+1$ dimensions provides a paradigmatic example of a topological gauge theory with both dynamical and topological degrees of freedom. Its Euclidean formulation suffers from a sign problem, making Hamiltonian numerical approaches particularly attractive. As a first step toward the non-perturbative Hamiltonian study of MCS theory, we investigate the constant mode sector on a spatial torus. Being analytically solvable in the continuum, it provides an ideal benchmark for understanding how the topological properties of the theory are encoded in a finite-dimensional lattice Hilbert space. We construct a finite-dimensional discretization of the torus of flat connections and show that the resulting lattice problem maps onto a generalized Harper-Hofstadter model with twisted boundary conditions. We identify the commensurability conditions under which the finite lattice exactly reproduces the magnetic translation algebra and the topological degeneracy of the continuum theory. A systematic analysis of gauge field truncation and its convergence toward the continuum limit is then presented.

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cond-mat.mtrl-sci 2026-07-01

Interface widens skyrmion stability range in heterostructure

by Sina Mehboodi, Victor Ukleev +8 more

Proximity-Induced Skyrmion Stabilization at the Cu2OSeO3/Bi2Se3 Interface

Proximity coupling produces a distinct interfacial skyrmion phase stable across broader fields than bulk, detected via split resonances and

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We investigate proximity-induced magnetic interactions at the interface between the topological insulator Bi2Se3 and the chiral magnetic insulator Cu2OSeO3, with particular focus on the low temperature skyrmion phase. Broadband ferromagnetic resonance spectroscopy reveals enhanced stability of noncollinear spin textures in the Cu2OSeO3/Bi2Se3 heterostructure compared with bare Cu2OSeO3. In addition to an extra resonance mode in the tilted conical phase that is absent in bare Cu2OSeO3, field cycling resolves two counterclockwise skyrmion resonance branches separated by approximately 238 MHz, consistent with the coexistence of a bulk skyrmion lattice and an interfacial skyrmion phase stabilized by proximity-induced exchange coupling and enhanced interfacial Dzyaloshinskii-Moriya interactions. The finite frequency separation indicates that the two skyrmion phases occupy distinct magnetic energy landscapes while retaining similar resonance character. Resonant elastic x-ray scattering measurements further confirm that the interfacial skyrmion phase spans a broader magnetic-field range than the bulk phase, demonstrating enhanced stability and ordering of topological spin textures at the interface. These findings establish interface engineering as a promising route for extending the stability regime of skyrmion and tilted-conical phases in topological-magnetic heterostructures.

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cond-mat.mtrl-sci 2026-07-01

Bichromatic drive yields giant perpendicular polarization in 2D magnets

by Mohsen Yarmohammadi, Daegeun Jo +4 more

Giant perpendicular Edelstein polarization in 2D compensated magnets via bichromatic Floquet driving

Two-frequency light breaks rotational symmetry to produce 0.5-1.5 μ_B Edelstein responses usable for memory writing.

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While unconventional $p$-wave magnets can generate nonrelativistic Edelstein polarizations, spin-group symmetries strictly forbid these responses in unconventional magnets with higher-order harmonics, such as $d$-wave altermagnets. Here, we demonstrate that combining Rashba spin-orbit coupling with bichromatic Floquet driving activates giant perpendicular Edelstein polarizations (PEPs) across 2D altermagnets and broader classes of unconventional spin-polarized magnets -- a feat monochromatic driving cannot achieve. By dynamically breaking two-fold rotational symmetry, the two-frequency drive (including bilinear, bicircular, and circular-linear configurations) induces a stray-field-free in-plane Zeeman-like field that generates orbitally dominated PEPs (0.5--1.5 $\mu_{\rm B}$). This massive response is governed by universal selection rules tied to the system's magnetic parity and the second beam's harmonics. These emergent PEPs provide a powerful mechanism for perpendicular memory writing.

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cond-mat.str-el 2026-07-01

Algebra contractions link Sommerfeld coefficient to spin and charge responses

by Eoin Quinn

The exceptional origin of the strange metal and the LFL-HFL transition

A parameter-free relation 4π²γ^{-1} = χ_s^{-1} + χ_c^{-1} follows from competition between Landau-Fermi and Hubbard-Fermi liquids in the str

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We propose an algebraic framework for the strange metal regime of strongly correlated electrons. We show that the exceptional superconformal algebra $D(2,1;\alpha)$ admits two distinct contractions of its conformal sector: one to a pair of canonical fermions, the underlying degrees of freedom of the Landau-Fermi liquid (LFL), and one to the algebra of Hubbard operators, which characterise a distinct metallic regime, the Hubbard-Fermi liquid (HFL). We argue that competition between these two metallic states drives the emergence of the strange metal as a $0+1$D superconformal bath. We analyse the resulting thermodynamics, and obtain a parameter-free prediction, $4\pi^2\gamma^{-1} =\chi_s^{-1} + \chi_c^{-1}$, relating the Sommerfeld coefficient to the static spin and charge susceptibilities. We further show that the LFL-HFL transition is discontinuous at low temperature, owing to a degeneracy at the emergence of the HFL, and map out the resulting phase diagram. We connect the framework to microscopic lattice models and to the phenomenology of correlated insulators.

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cond-mat.str-el 2026-07-01

Fourier transform of neutron data maps local magnon modes in real space

by Shin-ichi Shamoto, Yukio Yasui +5 more

Local magnon modes studied by dynamic magnetic pair-density function analysis

Reveals energy-dependent sign changes in spin-pair correlations even without periodicity

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The dynamic magnetic pair-density function (DymPDF) $D_{\rm M}(r, E)$ is obtained via the Fourier transform of the dynamic magnetic structure factor, $S_{\rm M}(Q, E)$, which is measured using nonpolarized inelastic neutron scattering. While there is a long history of magnetic excitation studies with $S_{\rm M}(Q, E)$, there are no reports on $D_{\rm M}(r, E)$. In this study, we examine simple magnet models and representative magnet examples, such as FeTiO$_{3}$ and YBa$_{2}$Cu$_{3}$O$_{6}$, to investigate the real-space dynamics of $D_{\rm M}(r, E)$. We derive the $D_{\rm M}(r, E)$ equations for simple magnet models in a low energy limit. By comparing these equations to the simulations, we demonstrate the characteristic energy dependence of real-space local magnon modes, including the transition of the magnon mode from acoustic to optical. Our novel analysis reveals the local magnon modes accompanied by a sign change in each spin-pair correlation at a given energy in nanoscale real space even under non-periodic conditions. This method is unique for studying local magnetic dynamics.

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cond-mat.str-el 2026-07-01

Ground doublet hopping yields isotropic exchange in rare-earth magnets

by Kotaro Shimizu, Esteban Agustin Ghioldi +2 more

Design Principles for Quasi-Isotropic Exchange in Rare-Earth Quantum Magnets

Design principle selects maximal angular momentum character perpendicular to the ligand plane to suppress anisotropy.

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Rare-earth quantum materials provide a promising platform for emergent phenomena ranging from quantum spin liquids with long-range entanglement to topological magnetic textures. However, the strong spin-orbit coupling that stabilizes their low-energy pseudospin degrees of freedom also tends to generate strongly anisotropic exchange interactions, complicating the realization of quasi-isotropic Heisenberg magnetism. Here we investigate the microscopic origin of superexchange in $\mathrm{Ce}^{3+}$- and $\mathrm{Yb}^{3+}$-based insulators with edge-sharing octahedral geometry. Using degenerate perturbation theory for a multiorbital Hubbard model, we show that isotropic exchange originates predominantly from virtual hopping within the ground-state Kramers doublet, whereas anisotropic interactions arise primarily from processes involving excited multiplets. This leads to a simple orbital design principle: quasi-isotropic exchange is promoted when the ground-state doublet has a strong maximal-angular-momentum character with respect to the quantization axis perpendicular to the superexchange plane spanned by rare-earth and ligand ions. We demonstrate this mechanism for both ideal and distorted geometries and show that it is broadly consistent with experimentally studied Yb-based insulators. Our results establish a practical framework for engineering quasi-isotropic interactions in rare-earth quantum materials.

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cond-mat.mes-hall 2026-07-01

Driving doubles Cooper-pair symmetries in magnet-superconductor interface

by Subhendu Kumar Patra, Gaurab Kumar Dash +1 more

Floquet Majorana flat bands and emergent Cooper pair symmetries in p-wave magnet-superconductor heterostructure

Periodic drive in p-wave magnet and s-wave superconductor setup creates odd-Floquet pairing channels and extra Majorana bands with no static

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We investigate the emergence of topological superconductivity at the two-dimensional heterostructure interface between a $p$-wave magnet (pWM) and an $s$-wave superconductor. By analyzing nodal gap closings, we identify seven distinct nodal topological phases, each characterized by the presence of Majorana zero-energy flat bands and quantized zero-bias conductance peaks. We demonstrate that the effective $p$-wave nature of the system gives rise to spin-triplet pairing correlations with even-frequency, odd-parity and odd-frequency, even-parity symmetries. Notably, the introduction of inter-orbital hopping induces an exotic orbital-singlet term characterized by simultaneous odd-parity and odd-frequency. Furthermore, we explore the transition from static phases to Floquet topological regimes through periodic driving. These driven phases host both zero and $\pi$ Majorana flat bands, with transport signatures governed by the Floquet sum rule. Most significantly, we show that periodic driving fundamentally reshapes the topological and superconducting landscape by generating multiple nodal points that support higher winding numbers and multiple Majorana flat bands, while the emergent Floquet degree of freedom doubles the number of symmetry-allowed Cooper-pair correlations. The first class of correlations is hosted by the even-Floquet sectors and has a direct counterpart in the static limit. In contrast, the second is a distinct Floquet-generated class that confines to the odd-Floquet sectors, representing a fundamentally nonequilibrium pairing channel that cannot exist in static systems. Finally, we demonstrate the robustness of these topological modes against strong disorder, confirming their potential for stable fault-tolerant applications.

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cond-mat.str-el 2026-07-01

Fluence reversal fingerprints fragile Kondo hybridization

by Xin-Yi Tian, Qi-Yi Wu +14 more

Ultrafast Fluence-Reversal Fingerprint of Fragile Kondo Hybridization in CePt₂In₇

Pump-probe reflectivity in CePt2In7 shows high-fluence enhancement at low T that a fixed-gap model cannot produce.

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The emergence of heavy quasiparticles in a Kondo lattice is usually viewed as the formation of a low-energy hybridization gap. Whether this gap represents a rigid electronic structure or a fragile many-body state that can be dynamically reconfigured remains a central question for heavy-fermion systems near magnetic order, quantum criticality, and unconventional superconductivity. Here we use femtosecond pump-probe reflectivity to interrogate this problem in the weakly hybridized Kondo-lattice compound CePt$_2$In$_7$. At low fluence, a slow quasiparticle relaxation channel emerges below $T^* \sim$ 40 K and follows a Rothwarf-Taylor bottleneck response with a low-energy recombination scale 2$\Delta \approx$ 7.4 meV. Coherent optical phonons, independently identified by Raman spectroscopy, act as an internal lattice thermometer and rule out large quasi-equilibrium lattice heating as the origin of the nonlinear electronic response. The phonon-free electronic amplitude $A_{\rm elec}$ reveals a fluence-reversal fingerprint: with cooling from the hybridization-crossover regime, the response evolves from weak-linear behavior to Rothwarf-Taylor-like bottleneck suppression and finally to anomalous high-fluence enhancement at the lowest temperatures. This reversal cannot be accounted for by a rigid fixed-gap bottleneck alone and instead identifies an ultrafast optical signature of photoinduced redistribution of a fragile Kondo-hybridized electronic response.

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cond-mat.str-el 2026-07-01

Soft phonons open pseudogaps up to 1 eV in TiSe₂ normal state

by Sotirios Fragkos, Nina Girotto Erhardt +10 more

Phonon-induced pseudogap phase in TiSe₂

Strong electron-phonon coupling suppresses spectral weight in a momentum-dependent way above the Fermi level.

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To comprehend quantum ordered states, such as charge density waves (CDW), in layered transition metal dichalcogenides (TMDCs), it is essential to uncover their underlying normal states. Here, we use time- and angle-resolved extreme ultraviolet photoemission spectroscopy and ab initio electron-phonon calculations to perform excited state band mapping of three prototypical 1T TMDCs, i.e., TiSe$_2$, HfTe$_2$, and ZrTe$_2$, at room temperature. The results reveal the profound impact of strong electron-phonon-induced thermal fluctuations on the normal-phase electronic structure. Specifically, in the moderate electron-phonon coupling regime, as in HfTe$_2$ and ZrTe$_2$, thermal fluctuations only lead to small spectral broadening and band renormalization. In the strongly coupled case, exemplified by TiSe$_2$, we observe soft-phonon-induced, momentum-dependent suppression of spectral weight, i.e., pseudogaps - extending up to 1 eV above the Fermi level. Our work establishes the normal phase of TiSe$_2$ as a phonon-induced pseudogap phase governed by strong CDW fluctuations, thereby uncovering previously missing aspects of the TiSe$_2$ phase diagram, with broader implications for other TMDCs in the strong electron-phonon coupling regime.

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cond-mat.str-el 2026-07-01

Topology sets stable lasing mode in nonlinear laser

by Zi-Yuan Li, Zi-Xiang Hu +1 more

Nonlinear topological laser based on multipole insulators

In the BBH model, diagnostic ratios extracted from long-time evolution jump at phase transitions and confirm that topology dictates the outp

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Two-dimensional higher-order topological insulators (HOTIs), characterized by distinctive one-dimensional edge states and zero-dimensional corner states, provide an ideal platform for developing higher-order topological lasers. In this work, we systematically investigate the two-dimensional Benalcazar-Bernevig-Hughes (BBH) model, which hosts quantized quadrupole moments and topologically protected corner and edge states. By confining the lasing mode to selected topological corner or edge states under controlled gain, we demonstrate that the stable light excitation achieved after long-time evolution is predominantly determined by the topological properties of the model Hamiltonian. To characterize the system's topological features, we introduce several diagnostic ratios: the corner decay ratio $\tau_{1}$ and edge-to-corner ratio $\tau_{2}$ quantify the localization degree and spatial extent of corner states, respectively, while the inter-corner transfer ratio $\chi$ measures the intensity transfer efficiency mediated by coherent edge-state dynamics. The abrupt changes in $\tau_{1}$ and $\tau_{2}$ as functions of the hopping parameter $\gamma/\lambda$ directly reveal topological phase transitions, providing a comprehensive toolkit for extracting topological signatures from the system's dynamical evolution. Additionally, modulating the lattice site parity enables flexible tuning of corner state localization positions, offering insights for device engineering. Our calculations reveal that achieving bistability between corner states and edge states is relatively challenging.

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cond-mat.str-el 2026-07-01

J1-J2 triangle model transitions directly to gapless QSL at J2/J1=0.08

by Litao Ma, Wei-Lin Tu +2 more

Investigation of the J₁-J₂ Heisenberg model on the triangular lattice: A study with projected entangled-pair states

PEPS calculations show spinons condense and visons confine, ruling out Z2 order in favor of a U(1) Dirac scenario.

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The nature of the quantum spin liquid (QSL) phase in the frustrated $J_1$-$J_2$ Heisenberg model on the triangular lattice remains an open and actively debated problem. In this work, we employ the infinite projected entangled-pair state (PEPS) to systematically investigate the model under different symmetry constraints. Our simulations reveal a direct transition from the $120^\circ$ N\'eel state to a putative QSL at $J_2/J_1\approx 0.08$, signaled by the collapse of magnetic order. We further show that, through either an appropriate unitary rotation or spontaneous spin long-range order, the stripe antiferromagnetic phase can also be accurately captured within the infinite PEPS framework. A central focus of our study is the role played by the PEPS symmetry in approximating the QSL ground-state sandwiched between the two magnetic phases. We first found that a fully-symmetric topological $\mathbb{Z}_2$ Resonating Valence Bond state, which can be written as a simple PEPS with bond dimension $D=3$, exhibits a reasonably good variational energy. Motivated by this finding, we have further constructed generic $\mathbb{Z}_2$-symmetric PEPS of larger bond dimension (up to $D=7$). We found that, under wavefunction optimization, spinons condense and, simultaneously, topological vison excitations get confined, hence precluding $\mathbb{Z}_2$ topological order. This strongly indicates the gapless (or critical) nature of the QSL phase, which is most naturally consistent with a U(1) Dirac spin liquid scenario.

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cond-mat.str-el 2026-07-01

Magnons govern ordered phases

by P. N. Bibikov

The competition between low-temperature kinks and magnons at the vicinity of the deconfinement transition point in 1D easy-axis XXZ ferromagnet

Near deconfinement in the 1D XXZ ferromagnet, lowest-energy excitations set the character of each low-temperature regime

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Studying the ordered phases and quantum supercritical low-temperature regime at the vicinity of the deconfinement transition point in 1D easy-axis XXZ ferromagnet, we suggest their interpretations according to the corresponding dominant lowest-energy excitations. We show, that the two ordered phases are governed by magnons, while the quantum supercritical regime is governed by kinks. Within this framework the Ising model is treated in detail.

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cond-mat.mes-hall 2026-06-30

NV sensors detect nanoscale phase segregation in doped oxide

by Izidor Benedičič, J. Paul Attfield +1 more

Quantum sensing of nanoscale electronic phase segregation

ODMR splitting grows 15 MHz and 1/T1 jumps an order of magnitude at Tc, showing charge-ordered domains at nanometer scale.

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Doping of transition metal oxides such as CaFe$_3$O$_5$ offers a controlled way to tune the interplay of charge, spin, and lattice degrees of freedom, yet local-probe studies remain difficult because strong correlations and dynamic charge-spin fluctuations obscure fine spectroscopic features in powder samples. Here, we employ quantum magnetometry based on nitrogen-vacancy (NV) centers in nanodiamonds impressed into an Mn-doped CaFe$_3$O$_5$ powder pellet to probe static and dynamic magnetic fields at the nanoscale across the weak ferromagnetic transition. The splitting and broadening of the optically detected magnetic resonance (ODMR) spectra exhibit an order-parameter-like increase by ~ 15 MHz upon cooling below the critical temperature, T$_{\rm c}$. Concomitantly, the spin-lattice relaxation rate, 1/T$_1$, exhibits a pronounced, divergence-like enhancement at T$_{\rm c}$, increasing by about one order of magnitude from its high-temperature value. Moreover, detailed lineshape fits of ODMR spectra together with the stretched-exponential NV magnetization recovery curves corroborate the proposed electronic phase segregation in charge-ordered and charge-averaged phases at the nanometric scales. The presented study demonstrates the viability of using nanodiamonds as a platform for nanoscale magnetic probing of strongly correlated matter, including phenomena such as electronic phase separation.

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cond-mat.supr-con 2026-06-30

Residual states in 4Hb-TaS₂ carry no heat at zero field

by M. Gillig, I. Mangel +5 more

Thermal transport and low-temperature specific heat in 4Hb-TaS₂

Specific heat detects leftover low-energy states but thermal conductivity shows none, while fields rapidly increase transport more steeply i

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We investigate the low-energy excitation spectrum of the van der Waals heterostructure superconductor 4Hb-TaS$_2$ using ultra-low-temperature specific-heat and thermal-conductivity measurements with magnetic fields applied parallel and perpendicular to the crystallographic $c$ axis. The specific heat is broadly consistent with a nodeless superconducting gap, but retains a finite residual linear contribution, indicating a small residual low-energy density of states in the superconducting state. In addition, a pronounced upturn appears below approximately 0.3K. Its weak magnetic-field dependence, together with the absence of a corresponding feature in thermal transport, supports an interpretation in terms of localized degrees of freedom, most likely a nuclear Schottky contribution. In contrast to the finite residual thermodynamic density of states, the thermal conductivity extrapolates to a vanishing zero-field electronic linear term within experimental uncertainty for both field orientations. Thus, the residual low-energy states do not form a detectable itinerant heat-conduction channel. In finite magnetic field, the electronic heat transport grows rapidly. For out-of-plane fields, this response is broadly consistent with previous thermal-conductivity measurements and with the behavior commonly associated with multigap nodeless superconductivity. The even steeper increase observed for in-plane fields suggests that the field-induced quasiparticle response of 4Hb-TaS$_2$ is more complicated than the standard multigap picture alone.

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cond-mat.supr-con 2026-06-30

Triplet superconductivity dominates in AV2X2O surface altermagnets

by M. Franz

Unconventional superconductivity in AV₂X₂O family of surface altermagnets

Magnetic and sublattice structure blocks spin-singlet pairing, yielding topologically nontrivial phases with spin currents.

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Motivated by the recent discovery of superconductivity at 16.3 K in layered oxychalcogenide Na$_{2-x}$V$_2$Se$_2$O we investigate pairing instabilities in the broader family of layered materials composed of V$_2$O planes, believed to exhibit altermagnetic order in their monolayer form. Even though the bulk family members KV$_2$Se$_2$O and Rb$_{1-\delta}$V$_{2}$Te$_{2}$O are likely conventional antiferromagnets that show only surface altermagnetism, our analysis predicts exotic equal-spin triplet superconductivity as the dominant pairing instability in these materials. This is a consequence of their unique magnetic and sublattice structure that renders electron bands incompatible with conventional spin-singlet pairing. The predicted triplet superconducting phases are topologically non-trivial and capable of supporting spin-polarized persistent currents, properties potentially useful in technological applications.

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quant-ph 2026-06-30

Disorder boosts fermionic superradiance scaling

by David Pascual Solis, Andrea Legramandi +2 more

Disorder-Induced Enhancement of Fermionic Superradiance

Random couplings let many grey fermionic states join coherently, producing faster growth of the condensate with system size than uniform cou

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Collective light-matter phenomena such as Dicke superradiance are often described as a collection of effective spins coupled homogeneously to a bosonic mode, giving rise to a collective bright mode with enhanced light-matter coupling. In fermionic systems, Pauli exclusion and Fermi-surface structure can significantly modify this picture, while randomness in the atom-light couplings raises the question of whether disorder promotes or suppresses collective behavior. Here, we study a cavity model in which fermionic particles couple to a photonic mode through a random all-to-all interaction matrix with tunable mean and variance. Combining numerical mean-field methods, analytic stability analysis and random-matrix predictions, and benchmarks against exact diagonalization, we characterize both the onset and structure of the superradiant phase. While mean coupling and disorder variance contribute in the same way to the onset, they lead to drastically different behavior within the condensed phase. Uniform coupling supports a single bright collective fermionic mode with conventional Dicke-like scaling of the cavity field. Disorder, instead, gives rise to a qualitatively different collective regime in which many grey fermionic states participate coherently, producing a parametrically enhanced scaling of the condensate with system size. Our results reveal a mechanism through which disorder can, perhaps counterintuitively, promote collective light-matter phenomena.

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cond-mat.str-el 2026-06-30

Magnetization oscillates in odd-wave magnets before Edelstein steady state

by Jonas Habel, Johannes Knolle

Magnetic Bloch Oscillations in Odd-Wave Magnets and the Nonlinear Edelstein Effect

Bloch oscillations appear in the magnetic response and precede the nonlinear Edelstein value, visible in THz higher harmonics.

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Bloch oscillations (BOs) are a quantum phenomenon in which electrons subjected to an electric field in a periodic potential exhibit an oscillating current without a net drift. In real conductors, scattering reduces the coherence required for BOs driving the system toward a steady state with a DC current. While previous studies have focused on charge transport, charge carriers also possess spin, raising the question of whether BOs can emerge in magnetic observables. Here, we show that the magnetization of odd-wave magnets can undergo BOs before relaxing to the steady-state Edelstein value, a phenomenon we term $\textit{magnetic}$ BOs. Using analytical and numerical methods, we demonstrate this effect in a minimal one-dimensional model of a p-wave magnet and generalize it to two dimensions. Our analysis further reveals that the Edelstein magnetization is generically nonlinear in the applied electric field. Finally, we argue that magnetic BOs can be detected in materials through higher-harmonic generation in THz sub-cycle lightwave spectroscopy. Magnetic BOs provide a genuine non-equilibrium signature of spin-charge coupling in unconventional magnets.

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hep-th 2026-06-30

Rational CFTs stay real under complex continuation

by Yuma Furuta, Wataru Harada +2 more

Complex Conformal Manifolds

Analytic continuation of marginal couplings produces complex spectra but keeps rational points on the real axis, checked in free boson and I

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Complex conformal field theories (CFTs) have recently emerged as essential frameworks for understanding non-Hermitian criticality, weakly first-order phase transitions, and walking renormalization group flows, while their general structures remain largely unknown. In this work, we propose a systematic construction of complex CFTs by analytically continuing exactly marginal couplings into the complex plane. This procedure applies uniformly to bulk, boundary, and defect deformations, preserving conformal symmetry while generically complexifying operator spectra and other universal data. Using the compact free boson as a solvable laboratory, we uncover the global structure of the complexified Gaussian conformal manifold. More generally, we demonstrate that genuinely complex rational CFTs do not exist: rational points remain confined to the real regime, providing a sharp distinction between real and complex theories. In the defect case, we investigate the one-parameter family of conformal defects in the Ising CFT and derive exact expressions for the defect spectrum, energy transmission coefficient, and effective central charge from analytic continuation. The theoretical predictions are precisely verified in non-Hermitian critical Ising and free fermion chains using bulk-defect correlators, entanglement entropy, and complex energy transport, providing concrete evidence for the complex defect conformal manifold. Finally, we study complex boundary renormalization-group flows through the AdS/BCFT correspondence. Our results establish complex conformal manifolds as a controlled bridge between solvable lattice models, complex CFTs, and holography, while providing stringent analytic benchmarks for the nonunitary conformal bootstrap.

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cond-mat.str-el 2026-06-30

Hubbard models on frustrated lattices are ferromagnetic at low densities

by Wojciech Niedzió{l}ka, Jacek Wojtkiewicz

Ferromagnetic ordering in Hubbard models

Density expansion shows fully polarized states are favored on five lattices including FCC even at moderate electron fillings.

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One of the long-standing and only partially solved problems of theoretical condensed matter physics and mathematical physics is to demonstrate that ground states of some of the versions of the Hubbard model can exhibit a ferromagnetic ordering. It has long been speculated that the opportunity crucial for the occurrence of ferromagnetism is the structure of the lattice on which the Hubbard model is formulated \cite{TasakiMB}. As a consequence, while on simple cubic lattices no ferromagnetic ordering seems to be possible, it can naturally arise, even for low densities of magnetic moment carriers, on so-called frustrated lattices. We investigate the problem of ground state ferromagnetic ordering with the use of the formula for ground-state energy of interacting fermions as the first term of `density expansion', proven rigorously by Lieb, Seiringer and Solovej \cite{fermi exact} in continuum and by Giuliani \cite{hub exact} for the simple cubic lattice. Assuming that analogous expansion holds also for certain another lattices we apply this formula to five frustrated lattices -- among them to the face-centered cubic one. The hypothesis is confirmed: most of examined models formulated on frustrated lattices do indeed have ferromagnetic ground states already for densities being moderate or even low. Although the approach adopted cannot be treated as a rigorous proof that the ground state is ferromagnetic, the results obtained here strongly indicate that it can be the case. Moreover, as in some cases FM occurs at low densities, one can hope that it would be possible to prove convergence of the density expansion and prove rigorously the occurrence of `wealthy ferromagnetism' in these cases.

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cond-mat.quant-gas 2026-06-30

Finite-size effects expose anyonic signatures in 1D spin chains

by B. Perković, M. Bonkhoff +1 more

Finite-size effects in Schulz-Shastry-Luttinger models for determining anyonic signatures in 1d spin chains

Schulz-Shastry-Luttinger liquids in zigzag chains produce low-energy anyonic excitations detectable in boundary currents and spin correlatio

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We study finite-size properties of Schulz-Shastry-Luttinger liquids to reveal anyonic signatures, realized as low-energy excitations on top of the helical ground state in saturated spin-1/2 zigzag chains. The model features asymmetric and marginal couplings of density and phase gradients and belongs to the Schulz-Shastry class. We investigate periodic and Dirichlet boundary conditions and discuss its diagonalization as well as its stability. Although Dirichlet boundary conditions require a fine-tuning of coupling constants and universal parameters, only their magnitude is restricted for cyclic systems. We derive boundary characteristic quantities like Friedel oscillations and persistent currents. Finally, we discuss the bulk and boundary behavior of the longitudinal spin correlations including subleading corrections.

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cond-mat.supr-con 2026-06-30

Ultrasound detects 1 K anomaly inside TaS2 superconductor

by Yongwei Li, Dmitri V. Efremov +8 more

Ultrasound Evidence for a Low-Temperature Anomaly Inside the Superconducting State of 4Hb-TaS₂

Attenuation stays high below 2.9 K transition and drops only near 1 K, suppressed by field and Se doping.

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We report low-temperature ultrasound measurements on single crystals of the layered van der Waals superconductor 4Hb-TaS$_2$. Specific heat and ac magnetic susceptibility show a sharp bulk superconducting transition at $T_{\rm c}\approx 2.9$~K. Ultrasound measurements reveal an additional anomaly deep inside the superconducting state near $T^{*}\approx 1$~K. The most direct signature is observed in the relative ultrasonic attenuation change $\Delta\alpha$: instead of being rapidly suppressed at $T_{\rm c}$, $\Delta\alpha$ remains large throughout the intermediate superconducting regime and drops strongly only near $T^{*}$. This loss of acoustic dissipation is accompanied by a pronounced anomaly in the relative sound velocity change $\Delta v/v$, indicating strong coupling to the lattice. The low-temperature anomaly is rapidly suppressed by magnetic field and by Se substitution, suggesting a possible superconducting origin of the anomaly. We speculate that this feature may be related to induced superconductivity in the 1T layers.

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cond-mat.mes-hall 2026-06-30

TRS breaking changes transport differently in long-range Kitaev chains

by Averi Banerjee, Syeda Rafisa Rahaman +1 more

Transport in extended Kitaev chain with time reversal symmetry breaking and long-range interaction

The symmetry-breaking term alters density of states and localization in ways that differ from the short-range case, yielding distinct conduc

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We consider a junction consisting of an extended one-dimensional Kitaev chain which incorporates both time-reversal symmetry (TRS) breaking and long-range interaction, sandwiched between two metallic leads from two sides. In this hybrid device, we study electrical transport under voltage bias for varying strength of the TRS breaking phase. We compare the transport characteristics of long-range type Kitaev chain with that of the short-range Kitaev chain as the strength of the TRS breaking phase varies. We find that the TRS breaking modifies the density of states and localisation/delocalisation property of the eigenstates which in turn affect the transport characteristics. Moreover, we find that the impact of the TRS breaking is not identical for the long-range Kitaev chain and its short-range counterpart. Therefore, noticeable differences in the transport properties can be observed due to the interplay between the TRS breaking and the range of interaction.

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cond-mat.str-el 2026-06-30

Health-aware tuning beats human baselines for neural quantum states

by Jia-Qi Wang, Xiao-Qi Han +3 more

NQS-Agent: Health-Aware Agentic Hyperparameter Optimization for Neural-Network Quantum States

NQS-Agent monitors energy trajectories to stop unstable runs and rank architectures, yielding better results on the Heisenberg model.

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Neural-network quantum states (NQS) provide expressive variational representations for strongly correlated quantum many-body systems, but their practical accuracy depends sensitively on architecture-level hyperparameters and optimization schedules. Here we develop NQS-Agent, an implemented open-source software framework for health-aware hyperparameter optimization (HPO) in NQS calculations. Its workflow monitors energy trajectories, detects destructive optimization events, stops unstable calculations, modifies the learning-rate schedule, resumes optimization from safe checkpoints, and ranks candidates with an anomaly-aware score. We demonstrate the approach on a residual convolutional NQS for the square-lattice Heisenberg $J_1$-$J_2$ model, using architectures with parameter counts comparable to aCNN, a convolutional NQS architecture used here as a reference. The results show that NQS-Agent improves over the reported human-tuned aCNN baseline for the aCNN reference architecture and identifies a structurally distinct wide-and-shallow competitive candidate within the parameter-count-matched residual-CNN search space. These results show that the stability and recovery history of an optimization trajectory should be considered when assessing an NQS result. Health-aware HPO therefore provides a reproducible tuning protocol that goes beyond selecting a single lowest-energy calculation.

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quant-ph 2026-06-30

DMS tracks biexciton entanglement via entropy difference

by Yusuke Masaki, Takashi Otaki +1 more

Connecting Density Matrix Spectroscopy to Biexciton Entanglement Dynamics

Analytical link and simulations show the technique captures S_bi minus S_k dynamics in two-dimensional electron-hole systems.

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Quantum entanglement is one of the most intriguing features of quantum mechanics. To investigate the entanglement between two excitons in a biexciton, an experimental technique called density matrix spectroscopy (DMS) has recently been developed. DMS combines stimulated emission tomography and pump-probe techniques to obtain a time-resolved density matrix of the polarization state of a photon pair emitted from the biexciton. The reconstructed density matrix is expected to encode information about the biexciton state and its entanglement dynamics, but the precise nature of this connection has remained unclear. In this paper, we derive an analytical relationship between the density matrix obtained by DMS and the biexciton state. In addition, we perform numerical simulations to compare the entanglement dynamics obtained by DMS with the biexciton's entanglement dynamics in a two-dimensional electron-hole system using an extended ionic Hubbard model. We find that DMS can partially capture the entanglement in the biexciton, in particular, the dynamics of the difference $S_{\mathrm{bi}} - S_k$, where $S_{\mathrm{bi}}$ is the entanglement entropy of the biexciton and $S_k$ is the entanglement in terms of the wavevectors of the excitons that constitute the biexciton. These results demonstrate the validity of DMS for obtaining information about the entanglement dynamics of the biexciton.

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cond-mat.str-el 2026-06-30

Ferrimagnetic phase stable until V reaches U/4

by R. R. Montenegro-Filho, D. R. B. Silva +2 more

Trimers in the Extended Hubbard Model

The Lieb phase in the trimer extended Hubbard model holds despite rising doublon density until phase separation at V ≳ U/4

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The Lieb theorem is a cornerstone of quantum magnetism theory in condensed matter. In this work, we investigate the instability of the Lieb insulating ferrimagnetic phase in the extended Hubbard model on a trimer chain at half-filling, with one electron per site, under increasing the nearest-neighbor Coulomb coupling $V$. Our results show that despite a noticeable increase in doublon density with $V$, the ferrimagnetic insulating phase remains robust up to the phase separation (PS) line, which is observed at $V \gtrsim U/4$, where $U$ is the local Coulomb repulsion. Above the PS line, one of the coexisting phases is primarily populated by doublons on one of the two sublattices of the chain. This phase coexists with a metallic, unsaturated ferromagnetic phase for $U \gtrsim t$, and with a singlet phase for $U \lesssim t$, where $t$ is the intra-trimer hopping amplitude. We estimate the PS and the crossover lines with the help of density matrix renormalization group calculations.

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cond-mat.mes-hall 2026-06-30

Elliptical probe reads quantum Hall droplet metric

by Bruno Mera, Alberto Nardin +4 more

Perfect elliptic dichroism: Probing the metric of anisotropic quantum Hall droplets

Perfect elliptic dichroism occurs when probe polarization matches the droplet's intrinsic geometry, giving a direct measurement in quantum H

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Understanding the geometry of quantum Hall systems is a central challenge in modern condensed matter physics. We introduce a framework for probing the geometric structure of quantum Hall droplets by engineering the geometry of a dichroic probe and identifying the onset of "perfect elliptic dichroism", a regime in which the system responds exclusively to an elliptically polarized drive of a given chirality. This phenomenon provides a direct diagnostic of the droplet's intrinsic metric, and we show that it extends naturally to ideal Chern bands, where holomorphicity of the occupied states guarantees the vanishing of one chiral absorption rate with a quantized response for the other. In lattice realizations, such as the Harper-Hofstadter model, finite lattice-spacing corrections break the exact continuum metric description and give rise to a renormalized, emergent Landau-orbit metric; the probe ellipticity at which perfect dichroism is achieved then shifts accordingly, offering a direct spectroscopic window onto this lattice-induced geometric renormalization. Our results illuminate the rich geometric structure of quantum Hall phases and offer concrete pathways for observing these effects in quantum-engineered platforms.

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cond-mat.mes-hall 2026-06-30

Rotating saddle drives measure Hall viscosity via dichroism

by Alberto Nardin, Bruno Mera +4 more

Hall viscosity from metric-sensitive dichroic probes

Chiral metric perturbations produce a dichroic signal that isolates the geometric response in quantum Hall droplets.

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Hall viscosity characterizes the geometric response of a quantum Hall droplet to deformations of the underlying metric, yet it has remained difficult to measure directly. We propose a spectroscopic probe based on circular dichroism, using chiral metric-sensitive drives -- implemented as rotating quadrupolar ("saddle") perturbations -- that effectively modulate the metric and couple to the generators of area-preserving deformations. The resulting dichroic signal directly measures the Hall viscosity, while frequency-resolved spectroscopy disentangles it from other excitations. A local formulation further enables spatially resolved markers of Hall viscosity applicable to both continuum and lattice systems. Our results open a direct route to measuring Hall viscosity in quantum-engineered platforms such as cold atoms in optical lattices.

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cond-mat.supr-con 2026-06-30

Cs termination less inhomogeneous than Sb in CsV3Sb5 surfaces

by T. Mizokawa, G. Tomassucci +7 more

Electronic inhomogeneity in Cs- and Sb-terminated surfaces of CsV₃Sb₅ probed by scanning photoemission spectromicroscopy

SPEM maps show Cs termination is preferable for interfaces while its inhomogeneity still affects hybridization at Γ/A and K/H points.

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Electronic structures of Cs- and Sb-terminated surfaces of a kagome superconductor CsV$_3$Sb$_5$ have been elucidated by means of scanning photoemission microscopy (SPEM). The observed band structure of the Cs-terminated surface is rather close to that of the bulk while that of the Sb-terminated one is substantially modified around K/H point of the Brillouin zone. While the contrast between the Cs- and Sb-terminated regions is reduced below the charge density wave transition temperature, the Sb 5$p$ band of Cs-terminated region exhibits electronic inhomogeneity which slightly increases below it. The inhomogeneity of the Sb 5$p$ band would be related to disorders of the out-of-plane Sb and relevant for the band folding along $\Gamma$-A with the charge density wave. The SPEM results suggest that the less inhomogeneous Cs termination is more suitable for interface of kagome superconductors. However, the inhomogeneity of Cs termination, which is significant at $\Gamma$/A, noticeable at K/H, and negligible at M/L, is expected to affect the Sb 5$p$-V 3$d$ hybridization at the interface.

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cond-mat.str-el 2026-06-29

Double-stripe spin wave in nickelate unstable to charge order

by Lauro B. Braz, Steffen Bötzel +5 more

Density waves in low-pressure bilayer nickelates

Calculations show the 130 K transition arises when a pure spin-density wave at Q=(0,π) develops a commensurate charge modulation

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The low-pressure phase diagram of La$_3$Ni$_2$O$_7$ provides an important reference for understanding its pressure-induced high-temperature superconductivity. While the spin-density-wave transition at $T_{\text{SDW}}\approx150$ K is increasingly well established, the origin of the second density-wave transition at $T_{\text{DW}}\approx130$ K has remained unresolved. Here, we perform unrestricted Hartree-Fock calculations to investigate the potential origin of the second transition. {Within the orthorhombic phase, the degeneracy between possible ordering wavevectors at $\boldsymbol{Q}_{Y}=(0,\pi)$ and at $\boldsymbol{Q}_{X}=(\pi,0)$ is lifted and the electronic system} develops a double-stripe spin-density wave with ordering vector $\boldsymbol{Q}_{Y}=(0,\pi)$. We identify that the pure double stripe spin state is unstable in La$_3$Ni$_2$O$_7$ towards a commensurate charge-density wave instability, which favors a spin-modulated double stripe order with intertwined charge and spin instabilities and establish the hierarchy of ordered states in La$_3$Ni$_2$O$_7$, providing an important link between its ambient-pressure and superconducting high-pressure phases. We further discuss our results in the context of available experimental literature and propose further experimental tests to elucidate the origin of the SDW/DW states in this system.

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cond-mat.str-el 2026-06-29

Hund's rule pairing fits full gap in La3Ni2O7

by Yu-Bo Liu, Zhi-Yan Shao +3 more

What Does the Single-Particle Spectrum Imply on the Pairing Nature and Pairing Mechanism in La₃Ni₂O₇?

Symmetry shows hybridization vanishes on BZ diagonal, ruling out d_z2-dominated mechanisms that predict nodes

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The pairing mechanism of the bilayer nickelates La$_3$Ni$_2$O$_7$ remains a hotly-debated open question. Existing strong-coupling theories are divided into class favoring intralayer d-wave pairing and that favoring interlayer s-wave pairing, with the latter further divided into $d_{z^2}$ orbital dominated mechanism driven by orbital hybridization and $d_{x^2-y^2}$ orbital dominated mechanism driven by Hund's rule. Recent angle-resolved-photoemission-spectrum (ARPES) and scanning-tunneling-microscope (STM) combinedly reveal a nodeless full pairing gap with low anisotropy, supporting the s-wave pairing. Here we propose that the pairing gap along the Brillouin zone (BZ) diagonal can serve as a useful probe of pairing mechanism. Symmetry analysis suggests that orbital hybridization vanishes along the BZ diagonal, rendering that the pairing gaps on the $\gamma$- and $\alpha/\beta$- pockets reflect the $d_{z^2}$- and $d_{x^2-y^2}$- orbital pairing strength respectively. Under the $d_{z^2}$ orbital dominated pairing mechanism driven by orbital hybridization, gap nodes are inevitable on the $\alpha$- and $\beta$- pockets along the BZ diagonal, which conflicts with the full gap revealed by ARPES and the U-shaped dI/dV curve observed by STM. The Hund's rule driven pairing mechanism instead leads to a full pairing gap, which well fits the ARPES and STM results. Furthermore, through a random-phase-approximation based calculation, we show that the weak-coupling theory, which tends to yield a $d_{z^2}$-orbital dominated pairing, also leads to nodes or near-nodes on the $\alpha$- and $\beta$- pockets along the BZ diagonal, conflicting with experiments. This analysis clarifies the dominant role of $d_{x^2-y^2}$ orbital in the pairing and establishes the Hund's rule driven pairing mechanism as the most relevant one in La$_3$Ni$_2$O$_7$.

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cond-mat.str-el 2026-06-29

Theory matches measured 1/3 quantum Hall gap using thickness and disorder

by Yi-Han Zhou, Zi-ang Wang +2 more

Energy Gap in Weakly Disordered Fractional Quantum Hall Liquids: Quantitative Comparison to GaAs Quantum Well Experiments at ν= 1/3

Charge and mobility gaps from device wave functions and experimental disorder inputs agree with activation data in narrow wells.

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Based on a recent experiment in high-quality GaAs quantum wells [Phys. Rev. Lett. 127, 056801 (2021)], we present a microscopic study of the energy gap in two-dimensional electron gases at filling factor $\nu=1/3$, explicitly incorporating both finite layer thickness and disorder effects. The finite layer thickness is modeled by solving the Poisson-Schr\"odinger equations for the experimental devices, yielding the electron wave functions in the perpendicular direction. Using these and the disorder energy extracted from the experiment, we estimate the charge gap and the mobility gap at $\nu=1/3$ in the weakly disordered lowest Landau level. Remarkably, both gaps show good quantitative agreement with the activation gap measured from the experiment in narrow quantum wells. Our results also indicate the potential need of incorporating higher subbands to make accurate theoretical predictions of the energy gap in wide quantum wells.

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cond-mat.supr-con 2026-06-29

Repulsive forces may drive cuprate superconductivity

by Navinder Singh Bathinda

40 years of cuprate high-Tc superconductors: a perspective on theories

After 40 years, theories converge on this idea with three schools, a key 2022 result, and refreshed experimental dogmas.

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An attempt is made to give a brief but coherent account of the situation of the theoretical ideas in addressing the mechanism of superconductivity in cuprate high-Tc superconductors. Specifically, the idea of superconductivity from repulsive interactions is discussed as it is gaining ground since the `consensus' paper was written in 2015\cite{kei}. The challenges it faces is also discussed. Three main schools of thought are presented, and an experimental result of 2022 pertaining to Anderson's super-exchange mechanism is also discussed. An updated list of Anderson's ``dogmas" is also presented, as after year 2000, many other universally applicable experimental facts has been discovered. The ``dogmas" are universal facts which are distilled from a variety of complex experimental results, and highlights the key findings that seems to be central to the mechanism of superconductivity in cuprates. These are discussed as a commemoration of 40 years of high-Tc cuprate research.

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cond-mat.str-el 2026-06-29

Helical valence bond phase stays incommensurate at large scales

by Yan Liu, Jie Lou +1 more

Winding-Sector Transitions and Thermodynamic Incommensurability in Helical Valence Bond Phase under Tilted Boundary Conditions

Tilted boundaries reveal continuous wavevector change without lock-in, confirming genuine 2D incommensurability in the thermodynamic limit.

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We investigate the ground states of the $S = 1/2$ staircase $J$-$Q_3$ model in the maximally anisotropic limit by employing projector quantum Monte Carlo simulations. To overcome boundary-induced finite-size ambiguities inherent in the study of spatially modulated structures, we implement a $45^{\circ}$ tilted periodic boundary condition that eliminates intermediate phases and provides direct access to winding-sector transitions of the system. By defining a domain wall density to quantify the spatial modulation of the helical valence bond phase, we perform thermodynamic extrapolations and demonstrate that both the domain wall density and the characteristic wavevector evolve continuously with the coupling ratio, exhibiting no commensurate lock-in behavior. Our results establish that the helical valence bond phase is a genuine two-dimensional incommensurate phase with long-range bond-bond order in the thermodynamic limit, clarifying that winding-sector transitions are finite-size effects enforced by boundary commensurability. Furthermore, we determine the phase transition point between columnar valence bond solid phase and helical valence bond phase to be $g_c = 0.046(2)$.

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cond-mat.mtrl-sci 2026-06-29

STM reveals two magnetic orders in KV2Se2O

by Jin-Cheng Gu, Mingzhe Hu +12 more

Real-space identification of distinct magnetic configurations in a candidate d-wave altermagnet

C-type altermagnetic and G-type antiferromagnetic configurations coexist with nearly identical spin-split bands.

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Altermagnetism is an emerging class of magnetic order characterized by momentum-dependent spin-split electronic structures despite vanishing net magnetization. Although momentum-space signatures consistent with altermagnetism have been reported in a growing number of materials, their relationship to the underlying real-space magnetic configurations remains incompletely understood, because similar spin-split electronic structures can arise from distinct magnetic orders. In the candidate d-wave altermagnet KV2Se2O, the magnetic origin of the observed momentum-dependent spin splitting has remained controversial. Here, we employ spin-polarized scanning tunnelling microscopy combined with magnetic-field-dependent quasiparticle interference imaging to determine the magnetic configuration of KV2Se2O at the atomic scale. Spin-resolved quasiparticle interference reveals a checkerboard-like antiparallel spin texture within the V2O layer and determines its interlayer spin arrangement across unit-cell step edges. Remarkably, we identify both C-type and G-type magnetic configurations, both of which generate similar spin-split electronic structures at the single-layer level but correspond to d-wave altermagnetic and conventional antiferromagnetic orders, respectively. These observations reveal a complex magnetic landscape arising from nearly degenerate magnetic states. Our results establish a direct connection between momentum-space spin splitting and real-space magnetic order, providing a framework for identifying the microscopic origin of spin-split electronic structures in altermagnetic materials.

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quant-ph 2026-06-29

Shadow tomography gives O(1) samples for long-range tensor network Hamiltonians

by Jiace Sun, Garnet Kin-Lic Chan

Shadow tomography for classical tensor network simulations

Adapting the estimators to classical tensor network contractions yields constant sample needs for fixed error on large systems.

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Shadow tomography has appeared as a powerful tool for estimating observables on quantum computers from a small number of samples. We show that shadow-tomography-inspired ideas can offer similarly improved sample scaling for estimating observables on tensor network states on classical computers after proper adaptation. We develop strategies for both spin (bosonic) and fermionic systems, tailored to the contraction requirements of tensor networks, and generate scaling improvements of factors of $O(N)$ to $O(N^{3})$ (where $N$ is system size), depending on the specific task and system type. For the important and difficult task of evaluating the expectation value of long-range interacting Hamiltonians, we achieve the optimal $O(1)$ overall scaling (up to logarithmic factors) for an arbitrarily fixed relative Monte Carlo error in both spin and fermionic systems. Additionally, we show that shadow estimators offer more stable gradients of observables in variational optimization tasks than standard Monte Carlo estimators. We demonstrate practical advantage by simulating systems with long-range interactions, including the 2D long-range Heisenberg model and an ab-initio quantum chemistry Hamiltonian.

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