Experimental detection of vortices in magic-angle graphene: Part 1

Magic-angle twisted-layer graphenes are promising candidates for superconducting electronic devices, thanks to the possibility of tuning them into and out of the superconducting state through electrostatic gating. Here, we implement a gate-tuned Josephson junction in a magic-angle twisted four-layer graphene film. We measure the magnetic field-dependence of the junction’s critical current and observe a Fraunhofer-like pattern consistent with the film's weak transverse screening properties. The pattern exhibits both an unconventional magnetic field periodicity and a slow decay, which markedly differ from the behavior expected in a standard Josephson junction. In addition, we observe sudden shifts in the measured pattern, which we attribute to vortices jumping into and out of the leads. When tuning the carrier density in the leads to the edge of the superconducting dome, we measure fast switching between the superconducting and the normal state within a finite interval of current bias. We associate this effect with thermally activated vortex dynamics. Operating the junction as a vortex sensor, we analyze the vortex jumps and their dynamics and extract fundamental properties of the two-dimensional superconductor, such as superfluid stiffness and London penetration depth.

Perego, Marta, et al. "Experimental detection of vortices in magic-angle graphene." arXiv preprint arXiv:2410.03508 (2024).