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 <i>B</i>_‖ ≈ 150mT applied parallel to the two-dimensional sheet. Superconductivity occurs near a symmetry breaking transition, and exists exclusively above the <i>B</i>_‖-limit expected of a paramagnetic superconductor with the observed transition temperature <i>T_C</i> ≈ 30mK, consistent with a spin-triplet order parameter.