Modulation of Superconductivity across a Lifshitz Transition in Alternating-Angle Twisted Quadrilayer Graphene

We report electric field-controlled modulation of the Fermi surface topology and explore its effects on the superconducting state in alternating-angle twisted quadrilayer graphene (TQG). The unique combination of flat and dispersive bands in TQG allows us to simultaneously tune the band structure through carrier density, n, and displacement field, D. From density-dependent Shubnikov-de Haas quantum oscillations and Hall measurements, we quantify the D-dependent bandwidth of the flat and dispersive bands and their hybridization. In the high |D| regime, the increased bandwidth favors the single particle bands, which coincides exactly with the vanishing of the superconducting transition temperature T_{c}, showing that superconductivity in TQG is strongly bound to the symmetry-broken state. For a range of lower |D| values, a Lifshitz transition occurs when the flat and dispersive band Fermi surfaces merge within the ν=+2 symmetry-broken state. The superconducting state correspondingly shows an enhanced T_{c}, suggesting that the superconducting condensate is strongly dependent on the Fermi surface topology and density of states within this symmetry-broken state.