Superconducting Dynamics and Vortex Phenomena in Gate-Tunable Magic-Angle Graphene Josephson Junctions

Experimentally, magic-angle twisted bilayer and multilayer graphene have been found to exhibit gate-tunable superconducting phases, enabling the realization of monolithic superconducting devices controlled purely by electrostatic gating. However, a consensus on the microscopic mechanisms underlying superconductivity in these materials has yet to emerge. In this talk, we discuss our recent experiments on gate-tunable Josephson junctions in magic-angle twisted bilayer (MATBG) and magic-angle twisted quadruple layer graphene (MATQG).

 

In MATBG we probe the Josephson junction using a DC current bias with a superimposed high-frequency modulation in the radio frequency range. Under these conditions, we observe a pronounced frequency dependence of the switching and retrapping currents. We interpret this behavior in terms of electronic quasiparticle thermalization via phonon scattering and the inductive response of the superconducting condensate. A phenomenological model enables us to relate the observed dynamics to electron-phonon coupling and superfluid stiffness.

 

In MATQG we employ a Josephson junction to detect vortices in the superconducting leads, which manifest as abrupt shifts in the Fraunhofer interference pattern. Time-resolved measurements allow us to investigate the dynamics of individual vortices, providing access to the characteristic vortex energy scale and the London penetration depth. Our measurements reveal a high-temperature regime dominated by classical thermal activation over an energy barrier, which crosses over at low temperatures to a regime of macroscopic quantum tunnelling through the barrier.