Superconductivity and spin canting in spin-orbit-coupled trilayer graphene.

Abstract

Graphene and transition metal dichalcogenide flat-band systems show similar phase diagrams, replete with magnetic^1-5 and superconducting^6-11 phases. An abiding question has been whether magnetic ordering competes with superconductivity or facilitates pairing. For example, recent studies of Bernal bilayer graphene in the presence of enhanced spin-orbit coupling show a substantial increase in the observed domain and critical temperature T_c of superconducting states^12-14; however, the mechanism for this enhancement remains unknown. Here we show that introducing spin-orbit coupling in rhombohedral trilayer graphene (RTG) by substrate proximity effect generates new superconducting pockets for both electron and hole doping, with maximal T_c ≈ 300 mK, which is three times larger than in RTG encapsulated by hexagonal boron nitride. Using local magnetometry, we show that superconductivity straddles a transition between a spin-canted state with a finite in-plane magnetic moment and a state with complete spin-valley locking. This transition is reproduced in our Hartree-Fock calculations, in which this transition is driven by the competition between spin-orbit coupling and the carrier-density-tuned Hund's interaction. Our experiment suggests that the enhancement of superconductivity by spin-orbit coupling is driven by a quantitative change in the canting angle rather than a change in the ground state symmetry. These results align with a recently proposed mechanism for the enhancement of superconductivity¹⁵, in which fluctuations in the spin-canting order contribute to the pairing interaction.