Topological Andreev bands in three-terminal graphene Josephson junctions

Multiply connected electronic networks threaded by flux tubes was proposed as a potential platform for studying adiabatic quantum transport properties, which are closely linked to topological invariants [1]. This concept could be realized in multi-terminal Josephson junctions (MTJJs) [2]. However, manifestations of topology in MTJJs are still an open case of experimental study.

In this work, we investigate the artificial topological band structure of three-terminal graphene Josephson junctions by using tunneling spectroscopy. Differential tunnelling conductance shows the characteristic features of Andreev bound states (ABS) formed in the graphene. We independently controlled the superconducting phase configurations of the three superconducting leads connected to the junction by applying the flux gates and obtained the tomography of the ABS energy spectrum as a function of two independent phase differences. Such quasi-momentum v.s. energy map of ABS unveils the transition between gapped and gapless states, which corresponds to the topological band structure of nodal line semimetals. Our results show the potential of graphene based MTJJs for engineering band topology.

[1] J.E Avron Rev. Mod. Phys., 60, 873 (1988).

[2] Roman-Pascal Riwar, Manuel Houzet, Julia S.Meyer & Yuli V. Nazarov. Nature Comm. 7, 11167 (2016)