  Topological superconductivity is the quantum condensate of paired electrons
with an odd parity of the pairing function. One of the candidates is the
triplet pairing superconductor derived from topological insulator Bi2Se3 by
chemical doping. Theoretically it was predicted that a two-fold superconducting
order parameter may exist in this kind of bulk topological superconductor.
Earlier experiments by nuclear magnetic resonance and angle-resolved
magnetocaloric measurements seem to support this picture. By using a
Corbino-shape like electrode configuration, we measure the c-axis resistivity
without the influence of the vortex motion and find clear evidence of two-fold
superconductivity in the recently discovered topological superconductor
SrxBi2Se3. The Laue diffraction measurements on these samples show that the
maximum gap direction is either parallel or perpendicular to the main
crystallographic axis.
