Gate-Tunable Ambipolar Josephson Current in a Topological Insulator

Dirac surface states in a topological insulator (TI) with proximity-induced superconductivity offer a promising platform for realizing Majorana physics. In this work, we demonstrate gate-tunable ambipolar Josephson current in lateral Josephson junction (JJ) devices based on bulk-insulating (Bi,Sb)₂Te₃ thin films grown by molecular beam epitaxy (MBE). For thinner films, the supercurrent exhibits pronounced gate-tunable ambipolar behavior and is significantly suppressed as the chemical potential approaches the Dirac point yet persists across it. In contrast, thicker films exhibit a much weaker ambipolar response. Moreover, we find that the supercurrent becomes significantly less resilient to external magnetic fields when the chemical potential is tuned near the Dirac point. By performing numerical simulations, we attribute the asymmetric supercurrent observed in thicker TI films to the coexistence of Dirac surface states and bulk conduction channels. The demonstration of gate-tunable ambipolar Josephson transport establishes a foundation for the future exploration of electrically tunable Majorana modes.