  We used the asymmetric superconducting quantum interference device (SQUID)
technique to extract the current phase relation (CPR) of a Josephson junction
with a 3D-topological insulator (TI) Bi2Se3 nanobelt as the barrier. The
obtained CPR shows deviations from the standard sinusoidal CPR with a
pronounced forward skewness. At temperatures below 200 mK, the junction
skewness values are above the zero temperature limit for short diffusive
junctions. Fitting of the extracted CPR shows that most of the supercurrent is
carried by ballistic topological surface states (TSSs), with a small
contribution of diffusive channels primarily due to the bulk. These findings
are instrumental in engineering devices that can fully exploit the properties
of the topologically protected surface states of 3D TIs.
