A van der Waal hetero-junction approach to probing proximity-induced superconductivity in topological insulator thin films

Topological insulator/superconductor heterostructures are a potential platform for topological quantum computing as they may support Majorana zero modes. In this talk, I will present our work in probing the proximity-induced superconductivity in (Bi,Sb)₂Te₃/graphene/Ga thin films by constructing a clean, lithography-free van der Waals tunnel junction consisting of graphite electrode and thin h-BN tunnel barriers¹. This method is particularly suited for air sensitive surfaces and surfaces prone to deposition damage. Transport tunneling spectroscopy measurements reveal clear signatures of the proximity-induced superconducting gaps in the Dirac surface states of the BST film and discrete tunneling conductance jumps due to the addition of a single vortex. Further studies combining tunneling spectroscopy measurements and Josephson junction experiments are needed to explore potential topological superconductivity and phase transitions. In the second part of my talk, I will also describe our development of a transfer-based approach to make smooth and van der Waals-like interfaces between a 3D superconductor and a 2D material. We constructed clean, lithography-free NbN-graphene-NbN Josephson junctions and observed a gate-tunable supercurrent and multiple Andreev reflections. The performance of the device is on par with those made by conventional techniques. Our approach may be generalized to other van der Waals materials and open up opportunities for engineering novel superconducting heterostructures.



[1] C. Li, Y.-F. Zhao, A. Vera, O. Lesser, H. Yi, S. Kumari, Z. Yan, C. Dong, T. Bowen, K. Wang, H. Wang, J. L. Thompson, K. Watanabe, T. Taniguchi, D. Reifsnyder Hickey, Y. Oreg, J. A. Robinson, C.-Z. Chang, J. Zhu, “Proximity-induced superconductivity in epitaxial topological insulator/graphene/gallium heterostructures”, Nat. Mater. 22, 570–575 (2023).