Disorder Effects on Superconducting Island Arrays Proximity-Coupled to Graphene

Two-dimensional superconductors have been extensively studied due to the presence of quantum phase transitions (QPT) in these systems at millikelvin temperatures. One of the most investigated QPT is the Superconductor-(Metal)-Insulator Transition (SMIT). However, in the presence of disorder, the nature of transport across the SMIT is not well understood, largely because disorder is difficult to controllably tune in 2D superconducting films. To overcome this difficulty, we have developed arrays of superconducting Sn islands proximity-coupled to graphene with controllable island point disorder and graphene disorder. We studied the SMIT in these devices as a function of magnetic field and analyzed the transport signatures using scaling. Depending on the amount of point disorder, graphene disorder, and device geometry, we observed an evolution in the critical exponents from diffusive to percolative transport. We additionally performed Hall measurements, in which we observed peaks in the transverse resistance corresponding to pinning and depinning of magnetic field-induced vortices. These transport signatures suggest unusual behavior as a function of disorder near the Quantum Critical Point (QCP).