Interference Effects in Graphene Josephson Junctions Subject to Magnetic Field

Abstract

The coupling of a superconductor to topological states is expected to bring about non-abelian excitations which may enable fault-tolerant quantum computing. The observation of supercurrent in hybrid superconductor / quantum Hall devices was an important step towards the realization of these modes in a gate- and field- tunable platform. However, early studies of quantum Hall supercurrent led to ambiguity regarding the microscopic mechanism at play, leaving doubt as to whether this current was nontrivial.This work sheds light on the mechanisms by which supercurrent is mediated in graphene Josephson junctions at both low and high magnetic fields through interference measurements. Magnetic interference patterns provide information about the spatial distribution of supercurrent and their periodicity can serve as an indication of nontrivial behavior. Anomalous interference patterns observed around zero field hint at undiscovered graphene physics. At higher fields, unchanged patterns in devices with thin trenches provide evidence that quantum Hall supercurrent in traditional graphene Josephson junctions is mediated by trivial states at the vacuum edge. This motivates distancing vacuum edges from superconducting leads and the introduction of efficient, native graphene side gates. These enable the induction of local quantum Hall states, which are shown via interference patterns to independently carry supercurrent - the most persuasive evidence to date of supercurrent mediated by quantum Hall edge states.