Unconventional superconductivity in systems with annular Fermi surfaces: Application to rhombohedral trilayer graphene

TL;DR

This paper demonstrates that in 2D electron gases with annular Fermi surfaces, Coulomb interactions can induce unconventional superconductivity, especially in rhombohedral trilayer graphene, explaining experimental observations and trends.

Contribution

It introduces a mechanism for unconventional superconductivity in systems with annular Fermi surfaces, applying it to rhombohedral trilayer graphene and explaining key experimental features.

Findings

Superconductivity is enhanced when inner and outer Fermi surfaces are close.

Chiral p-wave pairing is the most prevalent state.

The mechanism accounts for observed $T_c$ magnitudes and trends.

Abstract

We show that in a two-dimensional electron gas with an annular Fermi surface, long-range Coulomb interactions can lead to unconventional superconductivity by the Kohn-Luttinger mechanism. Superconductivity is strongly enhanced when the inner and outer Fermi surfaces are close to each other. The most prevalent state has chiral $p$-wave symmetry, but $d$-wave and extended $s$-wave pairing are also possible. We discuss these results in the context of rhombohedral trilayer graphene, where superconductivity was recently discovered in regimes where the normal state has an annular Fermi surface. Using realistic parameters, our mechanism can account for the order of magnitude of $T_c$, as well as its trends as a function of electron density and perpendicular displacement field. Moreover, it naturally explains some of the outstanding puzzles in this material, that include the weak temperature dependence of the resistivity above $T_c$, and the proximity of spin singlet superconductivity to the ferromagnetic phase.