Superconductivity near spin and valley orders in graphene multilayers: a systematic study

Abstract

Spin excitations that soften near the onset of magnetic order have long been known to act as `paramagnon' pairing glue that can drive spin-triplet superconductivity. Recent findings of superconductivity in graphene bilayers and trilayers, occurring in the proximity of different itinerant ordered phases polarized in isospin (spin and valley), have motivated us to conduct a comprehensive investigation of an isospin extension of the paramagnon pairing mechanism in the vicinity of spin/isospin orders. In each case, we identify a soft mode, associated with the order parameter fluctuations, that mediates pairing interaction. We develop an approach that relates the soft mode described through summation of the contributions most strongly divergent at the onset of spin/valley isospin orders. This interaction is not always attractive, but if it is, it gives rise to an enhancement of superconducting $T_c$ in an appropriate pairing channel. In the cases when the pairing interaction is attractive, it leads to the formation of a superconducting state which can be either spin-triplet and valley-singlet or vice versa, depending on the specific isospin order type. These findings demonstrate that the occurrence of superconductivity in the vicinity of an itinerant magnetic phase is a generic phenomenon, closely mirroring experimental observations.