Superconductivity with twofold symmetry in Bi₂Te₃/FeTe_0.55Se_0.45 heterostructures

Topological superconductors are an interesting and frontier topic in condensed matter physics. In the superconducting state, an order parameter will be established with the basic or subsidiary symmetry of the crystalline lattice. In doped Bi₂Se₃ or Bi₂Te₃ with a basic threefold symmetry, it was predicted, however, that bulk superconductivity with order parameters of twofold symmetry may exist because of the presence of odd parity. We report the proximity effect-induced superconductivity in the Bi₂Te₃ thin film on top of the iron-based superconductor FeTe_0.55Se_0.45. 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 axes following the so-called Δ₄ notation. This is also accompanied by the elongated vortex shape mapped out by the density of states within the superconducting gap. Our results provide an easily accessible platform for investigating possible topological superconductivity in Bi₂Te₃/FeTe_0.55Se_0.45 heterostructures.