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

Abstract

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 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 induced by a magnetic field. We describe an attraction-from-repulsion mechanism for pairing mediated by a quantum-critical mode, which fully explains the observed phenomenology. While it is usually notoriously difficult to infer the pairing mechanism from the observed superconducting phases, 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 superconducting phase at the onset of 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 a triplet superconductivity driven by quantum-critical fluctuations.

Inducing Superconductivity in Bilayer Graphene by Alleviation of the Stoner Blockade

“…By virtue of allowing the Fermi energy to come closer to the vHS, the reduced repulsion will lead to an appreciable increase in T c , and to lowering of the critical Zeeman energy required for superconductivity -up to a point where superconductivity may be detected without a magnetic field at all. This prediction is in contrast to previous works discussing BLG superconductivity, where changing the distance to a metallic gate either weakly influences T c [23], or has the opposite effect altogether [40,41].…”

“…Several noteworthy issues related to the experimental phenomenology of intrinsic superconductivity in BLG, are not addressed by the unusual mechanism presented here. The precise nature of the electron pairing glue, be it phonons [23,43] or a Kohn-Luttinger-like mechanism [40,41,44], is intentionally kept ambiguous. Various possibilities may comfortably fit within our framework, which requires only that the pairing glue itself is weak enough such that there is an appreciable lever effect with regards to small Fermi-level DOS modifications [Eq.…”

Spontaneous momentum polarization and diodicity in Bernal bilayer graphene

“…This competition is particularly intriguing since the PVP/PMP phases are directly linked to the superconducting phase, which is stabilized in the presence of an in-plane magnetic field or proximity with a tungsten diselenide crystal [9,10]. Our observation raises the possibility of an interesting interplay between the stability of the superconducting phase and the momentum-space instability [40][41][42][43]. Furthermore, the universal presence of momentum polarization across the phase space of BLG suggests that the Coulomb-driven instability in the momentum space is fundamental to the electronic order in multilayer graphene systems.…”

Superconductivity induced by the inter-valley Coulomb scattering in a few layers of graphene

“…Furthermore, the Refs. [48][49][50][51][52] explored other unconventional mechanisms, in which the pairing is mediated by soft electronic collective modes. Remarkably, it has not yet been clarified whether the superconductivity observed in TBG and in TTG has the same origin as in the untwisted RTG and BBG.…”