  Topological superconductivity is central to a variety of novel phenomena
involving the interplay between topologically ordered phases and
broken-symmetry states. The key ingredient is an unconventional order
parameter, with an orbital component containing a chiral $p_x$ + i$p_y$ wave
term. Here we present phase-sensitive measurements, based on the quantum
interference in nanoscale Josephson junctions, realized by using Bi$_2$Te$_3$
topological insulator. We demonstrate that the induced superconductivity is
unconventional and consistent with a sign-changing order parameter, such as a
chiral $p_x$ + i$p_y$ component. The magnetic field pattern of the junctions
shows a dip at zero externally applied magnetic field, which is an
incontrovertible signature of the simultaneous existence of 0 and $\pi$
coupling within the junction, inherent to a non trivial order parameter phase.
The nano-textured morphology of the Bi$_2$Te$_3$ flakes, and the dramatic role
played by thermal strain are the surprising key factors for the display of an
unconventional induced order parameter.
