Josephson supercurrent in a topological insulator without a bulk shunt

A Josephson supercurrent has been induced into the three-dimensional topological insulator Bi $_{1.5}$ Sb $_{0.5}$ Te $_{1.7}$ Se $_{1.3}$. We show that the transport in Bi $_{1.5}$ Sb $_{0.5}$ Te $_{1.7}$ Se $_{1.3}$ 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 50-80 nm on several Bi $_{1.5}$ Sb $_{0.5}$ Te $_{1.7}$ Se $_{1.3}$ 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.