Resistivity and magnetoresistance of FeSe single crystals under Helium-gas pressure

We present temperature-dependent in-plane resistivity measurements on FeSe single crystals under He-gas pressure up to 800 MPa and magnetic fields $B \leq$ 10 T. A sharp phase transition anomaly is revealed at the tetragonal-to-orthorhombic transition at $T_s$ slightly below 90 K. $T_s$ becomes reduced with increasing pressure in a linear fashion at a rate d$T_{s}$/d$P$ $\simeq$ -31 K/GPa. This is accompanied by a $P$-linear increase of the superconducting transition temperature at $T_c \sim$ 8.6 K with d$T_{c}$/d$P$ $\simeq$ +5.8 K/GPa. Pressure studies of the normal-state resistivity highlight two distinctly different regimes: for $T > T_s$, i.e., in the tetragonal phase, the in-plane resistivity changes strongly with pressure. This contrasts with the state deep in the orthorhombic phase at $T \ll T_s$, preceding the superconducting transition. Here a $T$-linear resistivity is observed the slope of which does not change with pressure. Resistivity studies in varying magnetic fields both at ambient and finite pressure reveal clear changes of the magnetoresistance, $\Delta \rho \propto B^{2}$, upon cooling through $T_s$. Our data are consistent with a reconstruction of the Fermi surface accompanying the structural transition.