Nematicity and Orbital Depairing in Superconducting Bernal Bilayer Graphene with Strong Spin Orbit Coupling

TL;DR

This paper reports the discovery of two distinct superconducting states in Bernal bilayer graphene with strong spin-orbit coupling, revealing complex normal states and robustness to magnetic fields, with implications for understanding unconventional pairing mechanisms.

Contribution

It identifies and characterizes two superconducting states in Bernal bilayer graphene with strong proximity-induced spin-orbit coupling, highlighting their different normal states and magnetic field responses.

Findings

Two distinct superconducting states (SC1 and SC2) identified.

SC1's normal state aligns with single-particle band structure.

SC2 emerges from a nematic normal state with broken rotational symmetry.

Abstract

Superconductivity (SC) is a ubiquitous feature of graphite allotropes, having been observed in Bernal bilayers[1], rhombohedral trilayers[2], and a wide variety of angle-misaligned multilayers[3-6]. Despite significant differences in the electronic structure across these systems, supporting the graphite layer on a WSe$_2$ substrate has been consistently observed to expand the range of SC in carrier density and temperature[7-10]. Here, we report the observation of two distinct superconducting states (denoted SC$_1$ and SC$_2$) in Bernal bilayer graphene with strong proximity-induced Ising spin-orbit coupling. Quantum oscillations show that while the normal state of SC$_1$ is consistent with the single-particle band structure, SC$_2$ emerges from a nematic normal state with broken rotational symmetry. Both superconductors are robust to in-plane magnetic fields, violating the paramagnetic limit; however, neither reach fields expected for spin-valley locked Ising superconductors. We use our knowledge of the Fermi surface geometry of SC$_1$ to argue that superconductivity is limited by orbital depairing arising from the imperfect layer polarization of the electron wavefunctions. Finally, a comparative analysis of transport and thermodynamic compressibility measurements in SC$_2$ shows that the proximity to the observed isospin phase boundaries, observed in other rhombohedral graphene allotropes, is likely coincidental, constraining theories of unconventional superconducting pairing mechanisms in theses systems.