Isospin magnetism and spin-polarized superconductivity in Bernal bilayer graphene

Abstract

In conventional superconductors, Cooper pairing occurs between electrons of opposite spin. We observe spin-polarized superconductivity in Bernal bilayer graphene when doped to a saddle-point van Hove singularity generated by large applied perpendicular electric field. We observe a cascade of electrostatic gate-tuned transitions between electronic phases distinguished by their polarization within the isospin space defined by the combination of the spin and momentum-space valley degrees of freedom. Although all of these phases are metallic at zero magnetic field, we observe a transition to a superconducting state at finite B ‖ ≈ 150mT applied parallel to the two-dimensional sheet. Superconductivity occurs near a symmetry breaking transition, and exists exclusively above the B ‖ -limit expected of a paramagnetic superconductor with the observed transition temperature T C ≈ 30mK, consistent with a spin-triplet order parameter.

Acoustic-phonon-mediated superconductivity in moiréless graphene multilayers

“…Within mean field approximation, we reproduce the recently experimentally-observed superconductivity phenomenology in BBG and RTG, and we further predict the existence of superconductivity in ABCA tetralayer graphene, which should be experimentally investigated. Our theory captures the qualitative and semiquantitative features of the experiments [18,19], suggesting that superconductivity in graphene untwisted multilayers is likely due to acoustic phonons.…”

“…In the BBG experiment, a sufficiently large in-plane magnetic field, which suppresses a competing order, is required to observe superconductivity [19]. Based on our theory, observable superconductivity (i.e., T > 20mK) can happen only near VHS.…”

“…To understand why and how the acoustic-phonon mechanism can explain the BBG [19] and RTG [18] experiments, one has to take into account the Coulomb repulsion which causes a suppression of the predicted T c leading to agreement with experiments. Because the BBG band generates a smaller DOS resulting int weaker screening, the Coulomb repulsion suppresses superconductivity for most doping densities except near VHS.…”

“…Twisting or moiré flatband or magic angle, however, is not an essential condition for superconductivity in graphene-based materials as rhombohedral trilayer graphene (RTG) [17,18] and Bernal Bilayer graphene (BBG) [19] also demonstrate robust superconducting be- * yzchou@umd.edu havior in recent experimental studies. There are two distinct superconducting phases in RTG, termed SC1 and SC2.…”

Universal suppression of superfluid weight by disorder independent of quantum geometry and band dispersion

“…The observations of superconductivity in twisted bilayer graphene [5][6][7][8][9] and other graphene multilayer systems [10][11][12][13][14] have intensified the theoretical interest in the study of systems with flat energy bands and superconductivity [15][16][17][18][19][20][21][22][23]. In a flat band the effective mass m * diverges and one would expect the superfluid weight to vanish.…”