Current-Phase Relation of Ballistic Graphene Josephson Junctions

Abstract

The current-phase relation (CPR) of a Josephson junction (JJ) determines how the supercurrent evolves with the superconducting phase difference across the junction. Knowledge of the CPR is essential in order to understand the response of a JJ to various external parameters. Despite the rising interest in ultraclean encapsulated graphene JJs, the CPR of such junctions remains unknown. Here, we use a fully gate-tunable graphene superconducting quantum intereference device (SQUID) to determine the CPR of ballistic graphene JJs. Each of the two JJs in the SQUID is made with graphene encapsulated in hexagonal boron nitride. By independently controlling the critical current of the JJs, we can operate the SQUID either in a symmetric or asymmetric configuration. The highly asymmetric SQUID allows us to phase-bias one of the JJs and thereby directly obtain its CPR. The CPR is found to be skewed, deviating significantly from a sinusoidal form. The skewness can be tuned with the gate voltage and oscillates in antiphase with Fabry-Pérot resistance oscillations of the ballistic graphene cavity. We compare our experiments with tight-binding calculations that include realistic graphene–superconductor interfaces and find a good qualitative agreement.

Coherent control of a hybrid superconducting circuit made with graphene-based van der Waals heterostructures

“…Graphene, an atomic layer of carbon atoms in a hexagonal lattice 23 , can inherit superconductivity through the proximity effect 3,6 . When encapsulated in hexagonal boron nitride (hBN), the superconductor-graphene-superconductor (S-G-S) junction supports a voltage-tunable, bipolar Josephson current in the ballistic regime 4,5,24 . In our experiment, the S-G-S junctions are built into a voltage-controlled transmon circuit that enables us to perform both frequency-resolved spectroscopy and coherent quantum control using cQED techniques (Fig.1a).…”

1/f critical current noise in short ballistic graphene Josephson junctions

“…This mechanism results in a nonsinusoidal CPR [7,8,9,10]. Recent measurements of the CPR in the ballistic regime revealed a gate-tunable skewness, sensitive to the junction length and to the nature of the superconducting-graphene interface [13,14]. The maximal supercurrent, I c , depends on the doping and it is nonvanishing even at the Dirac point, despite of the zero carrier concentration resulting from the linear dispersion of graphene.…”

Josephson penetration depth in coplanar junctions based on 2D materials

“…These represent limiting cases: in Geometry A, the width of the superconducting electrodes is much larger than the width of the 2D material; while in Geometry B, the width of the superconducting electrodes is the same as that of the 2D material. Most of the experiments in the literature adopted Geometry A [1,3,6,8,9,12,14,17,18,26,27], while a few of them are better described by Geometry B (or somewhere between Geometry A and Geometry B [2,4,10,11,13,15,16,19,20]). When the superconducting electrodes are thicker than the London penetration depth , the supercurrent tends to flow in a narrow region within ~ from the edge of the superconducting electrodes as shown.…”