Direct Evidence of Superconductivity with Twofold Symmetry in Bi2Te3 Thin Film Deposited on FeTe0.55Se0.45

Topological superconductor is a timely and frontier topic in condensed matter physics. In superconducting state, an order parameter will be established with the basic or subsidiary symmetry of the crystalline lattice. In doped Bi2Se3 with a basic threefold symmetry, it was predicted however that superconductivity may have a twofold symmetry of odd parity. Here we report the proximity effect induced superconductivity in Bi2Te3 thin film on top of an iron-based superconductor FeTe0.55Se0.45. By using the quasiparticle interference technique, we demonstrate clear evidence of twofold symmetry of the superconducting gap. The gap minimum is along one of the main crystalline axis following the so-called Delta_4y notation. This is also accompanied by the elongated vortex shape mapped out by the density of states within the superconducting gap, with probably the Majorana mode within the vortex core. Our results reveal the direct evidence of superconductivity with odd parity in Bi2Te3 thin film.