  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.
