EQuAL Student Seminar: Ruoxi Zhang

Imaging the Meissner effect and local superfluid stiffness in a graphene superconductor

We report the observation of the Meissner effect in a rhombohedral graphene superconductor, realized via direct imaging of the static fringe magnetic field. In our few-micron sample, the onset of superconductivity manifests as a diamagnetic response that screens only ∼100 ppm of the applied magnetic field. Tracking the evolution of the resulting nanotesla- scale fields in real space allows us to observe the entry of superconducting vortices and map the local superfluid stiffness, ρ_s. Correlating fringe field signals from both Meissner screening and magnetically ordered states, we show that superconductivity onsets during a continuous quantum phase transition to a canted spin ferromagnet. Within the superconducting state, we find the temperature dependence of ρ_s to be incompatible with isotropic Bardeen-Cooper- Schrieffer theory and the zero-temperature stiffness ρ_s0 to be linearly proportional to T_c, constraining future theoretical models of superconductivity in this system.