Spin-triplet superconductivity at the onset of isospin order in biased bilayer graphene

The quest for unconventional superconductivity governed by Coulomb repulsion between electrons rather than phonon attraction received new momentum with the advent of moiré graphene. Initially, delineating the phonon and Coulomb-repulsion-based pairing mechanisms has proven to be a challenging task, however, the situation has changed after the recent discovery of superconductivity in non-twisted graphene bilayers and trilayers. Superconductivity occurring at the phase boundaries of spin and valley polarized orders calls for non-phonon scenarios, yet the specific pairing mechanisms remain to be understood. Here we analyze a striking example --- superconductivity in graphene bilayers occurring at the onset of valley-polarized order. We describe the phenomenon of field-induced pairing by a nominally repulsive interaction mediated by a quantum-critical mode and argue that it fully explains the observed phenomenology. While it is usually notoriously difficult to infer the pairing mechanism without probing superconducting order, this case presents a rare exception, allowing for a fairly unambiguous identification of the origin of the pairing glue. A combination of factors such as the location of the superconducting phase at the onset of the isospin-polarized phase, a threshold in a magnetic field, above which superconductivity occurs, and its resilience at high magnetic fields paints a clear picture of triplet superconductivity driven by quantum-critical fluctuations. We show that the same mechanism holds at zero field in the presence of spin-orbit coupling and argue that it explains recently observed superconductivity in bilayer graphene on WSe₂ monolayer