Magnetic-Field-Driven Insulator-Superconductor Transition in Rhombohedral Graphene

Recent studies of rhombohedral multilayer graphene have revealed a variety of superconducting states that can be induced or enhanced by magnetic fields, reinforcing rhombohedral multilayer graphene as a powerful platform for investigating novel superconductivity. Here, we report an insulator-superconductor transition driven by in-plane magnetic fields B_{∥} in rhombohedral hexalayer graphene. The upper critical field of B_{∥} can reach 2 T and an analysis based on isospin symmetry breaking supports a spin-polarized superconductor. At B_{∥}=0, such spin-polarized superconductor transitions into an insulator, exhibiting a thermally activated gap of Δ≈0.14 meV. In addition, we observe four superconducting states in the hole-doped regime, which violate the Pauli limit, as well as phases with magnetoelectric hysteresis near charge neutrality point. These findings substantially enrich the phase diagram of rhombohedral graphene and provide new insight into the microscopic mechanisms of superconductivity.