Transformer spin-triplet superconductivity at the onset of isospin order in bilayer graphene

We consider the origin of superconductivity found recently in Bernal bilayer graphene at the onset of isospin-polarized order, trying to infer the pairing mechanism and superconducting order from the measurements available to date. The superconductivity is induced by a parallel magnetic field and persists well above the Pauli limit, indicating an unconventional scenario of quantum-critical pairing, where soft fluctuations of isospin give rise to spin-triplet superconductivity. We consider the scenario in which the pairing interaction is entirely repulsive, which stands in contrast to the typical quantum-critical pairing mechanisms. Superconductivity emerges through a transformer mechanism where, in the presence of an in-plane magnetic field, the incipient valley polarization converts a frequency-independent repulsion into one with a strong nonmonotonic frequency dependence. Such an interaction enables a nonzero solution for the pairing gap function that changes sign as a function of frequency. The same mechanism holds at zero field in the presence of spin-orbit coupling, providing a likely explanation for the recently observed superconductivity in bilayer graphene on the WSe2 monolayer.

Automated summary

We investigate the origin of superconductivity in Bernal bilayer graphene and propose an unconventional scenario of quantum-critical pairing driven by isospin fluctuations. The superconductivity, induced by a parallel magnetic field, persists above the Pauli limit, suggesting the presence of spin-triplet superconductivity. We demonstrate that an entirely repulsive interaction can lead to the emergence of superconductivity through a transformer mechanism involving frequency-dependent pairing effects.