Pearl-Vortex Tunneling in Magic-Angle Twisted Graphene

TL;DR

This study uses a novel single-vortex sensor in twisted graphene to observe how vortices enter superconductors via thermal activation at higher temperatures and quantum tunneling at lower temperatures, revealing a quantum-to-classical transition.

Contribution

It introduces a gate-defined Josephson junction as a single-vortex sensor to directly study vortex dynamics in twisted graphene superconductors.

Findings

Vortices enter via thermal activation above 100 mK.

Below 90 mK, vortices tunnel quantum mechanically.

Energy barriers are on the order of a few Kelvin.

Abstract

Twisted graphene provides a tunable platform for studying superconductivity in two dimensions. In the presence of electric currents and magnetic fields, vortices determine the phenomenological properties of the material. Related studies usually address bulk properties averaging over ensembles of vortices. Here, we employ a gate-defined Josephson junction as a single-vortex sensor, enabling direct access to individual vortex dynamical events. Our measurements reveal that, at elevated temperatures (T > 100 mK), vortices enter the superconducting leads via classical thermal activation over energy barriers. At lower temperatures (T < 90 mK), we observe macroscopic quantum tunneling through these barriers. The data are consistent with a sharp, first-order type quantum-to-classical transition. From our measurements, we extract vortex entry and exit energy barriers on the order of a few Kelvin and estimate the barrier thickness to be approximately 100 nm, corresponding to about one tenth of the device width.