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^1,2,3,4,5 and superconducting^{6,7,8,9,10,11} 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^12,13,14; 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.