Superconducting Proximity Effect in Graphene

Abstract

Graphene is a popular candidate when it comes to the study of the superconductivity proximity effect. However, experiments have mainly been focusing on the transport behavior in two-terminal Josephson junctions. This dissertation presents unconventional transport experiments which study aspects of the proximity effect that were previously unexplored. The first experiment investigates systematically the graphene (G) - superconductor (S) hybrid system with a controllable interfacial barrier strength. We demonstrate how the Andreev process can be modulated by the magnetic field and the carrier density in graphene. The second study focuses on the multi-terminal S-G-S junction with a loop which hosts Andreev bound states (ABS) in higher dimensions. Under specific voltage configurations, we are able to capture the elusive quartet-- a state composed of four entangled electrons that is responsible for nonlocal supercurrents. With the Josephson junction biased, the system becomes periodically driven and is analyzed in the framework of Floquet theory. The associated dynamical energy spectrum can be modified with gate control and probed via a phase difference achieved by flux threaded through the device loop. Altogether, we have opened up new possibilities for the manipulation of Andreev bound states, creating a playground potentially for topological states.