  A Josephson supercurrent has been induced into the three-dimensional
topological insulator Bi1.5Sb0.5Te1.7Se1.3. We show that the transport in
Bi1.5Sb0.5Te1.7Se1.3 exfoliated flakes is dominated by surface states and that
the bulk conductivity can be neglected at the temperatures where we study the
proximity induced superconductivity. We prepared Josephson junctions with
widths in the order of 40 nm and lengths in the order of 50 to 80 nm on several
Bi1.5Sb0.5Te1.7Se1.3 flakes and measured down to 30 mK. The Fraunhofer patterns
unequivocally reveal that the supercurrent is a Josephson supercurrent. The
measured critical currents are reproducibly observed on different devices and
upon multiple cooldowns, and the critical current dependence on temperature as
well as magnetic field can be well explained by diffusive transport models and
geometric effects.
