Bipolar Supercurrent and Phase Coherent Transport in Topological Insulators

Superconductivity mediated by Dirac surface states in topological insulators (TIs) offers a promising platform for exploring topological superconductivity and Majorana physics. In this work, we employed molecular beam epitaxy (MBE) to synthesize high-quality (Bi,Sb)₂Te₃ thin films with varying thicknesses and fabricated gate-tunable Josephson junction devices and superconducting quantum interference devices (SQUIDs). These devices exhibit bipolar supercurrent transport behaviors enabled by a SrTiO₃ substrate-assisted back gate. We observed a significant suppression of the Josephson current when the chemical potential crosses solely with Dirac surface states. In addition, we found that the supercurrent shows weak resilience to the external magnetic field within this regime but persists under a finite magnetic field. The realization of bipolar supercurrent and phase-coherent transport in TI paves the way for further exploration of topological superconductivity and Majorana physics in MBE-grown undoped and doped TI samples.