Superconductivity in quantum Hall edge states

One can induce a supercurrent in a normal metal N by placing it between two superconducting electrodes S. It consists in transferring Cooper pairs through simultaneous conversion of electrons and holes at the NS interface. In a magnetic field, the electrons' and holes' trajectories bend and can no longer merge at the NS interface, rapidly destroying the proximity supercurrent. In ballistic systems however, electrons' and holes' trajectories can occasionally return to the same position after multiple bounces on mesoscopic edges [1]. In the QH regime, electrons and holes propagate in the same direction on the same edge, which does not support a supercurrent unless one manages to couple oppositely propagating edges. This can be achieved via chiral Andreev edge states in the superconducting sheath, resulting in critical current of ~1nA at 1.5T [2]. Narrow devices would show better coupling but increased backscattering.

Here we suggest a new geometry where edge states are carried along the valley-polarised domain walls (DWs) of minimally twisted bilayer graphene [2]. At high magnetic fields, individual domains are gapped by cyclotron motion and the DWs form topologically-protected conducting edges. We induced proximity superconductivity in individual DWs and observed supercurrent >50 nA at 5T. This system may allow for the observation of exotic excitations with non-trivial braiding statistics [4].

1 Nat Phys 12, 318 (2016)

2 Science 352, 966 (2016)

3 Nat. Commun. 10 (2019)

4 Phys. Rev. Lett. 110, 186805 (2013).