Isospin magnetism and spin-polarized superconductivity in Bernal bilayer graphene

• Published in: Science (American Association for the Advancement of Science) Vol. 375; no. 6582; pp. 774 - 778
• Main Authors: Zhou, Haoxin, Holleis, Ludwig, Saito, Yu, Cohen, Liam, Huynh, William, Patterson, Caitlin L., Yang, Fangyuan, Taniguchi, Takashi, Watanabe, Kenji, Young, Andrea F.
• Format: Journal Article
• Language: English
• Published: United States The American Association for the Advancement of Science 18.02.2022; AAAS
• Subjects: Bilayers; CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; Electric fields; Graphene; Magnetic fields; Magnetism; Superconductivity; Two dimensional materials
• ISSN: 0036-8075; 1095-9203; 1095-9203
• DOI: 10.1126/science.abm8386

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 a 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 magnetic field B ‖ ≈ 150 milliteslas 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 critical temperature T C ≈ 30 millikelvins, consistent with a spin-triplet order parameter. Superconductivity observed in “twisted” bilayers of the two-dimensional (2D) material graphene requires the layers to be carefully stacked on top of each other at a very precise angle. Zhou et al . found superconductivity in the most common form of bilayer graphene, called Bernal, which does not require such precise manipulation (see the Perspective by Heikkilä). The researchers subjected the samples to an external electric field, but superconductivity emerged only after they also turned on an in-plane magnetic field. This unusual effect of a magnetic field, which normally acts to suppress superconductivity, is suggestive of exotic, spin-triplet pairing in Bernal-stacked bilayer graphene. —JS Superconductivity emerges in a stable form of bilayer graphene upon the application of an in-plane magnetic field.