Rhombohedral graphenes host van Hove singularities near the band edge which have been found to host magnetism and superconductivity. These systems have no no moire superlattice, and a ballistic mean free path far exceeding device dimensions, allowing precise measurements of the interplay of different symmetry breaking orders, including a cascade of half- and quarter metals with broken spin and/or valley symmetry as well as both spin-singlet and spin-triplet superconducting states. I will provide an overview of the physics of these systems, focusing on some recent results pertaining to the effects of spin orbit coupling.
In one example, I will show how introducing synthetic Ising spin orbit coupling—achieved by supporting a graphene bilayer on a WSe2 substrate—can dramatically increase the domain of superconductivity in temperature and density. Using tilted magnetic field measurements of the transition temperature, we show that the superconducting order parameter changes, with Ising superconductivity favored by the spin orbit coupling.
In the second example, I will describe the discovery of an internally coherent state in rhombohedral trilayer graphene. I will describe experiments in the quarter metal regime, where we find two distinct states separated by a first order phase transition. One is valley polarized, while the second is internally coherent, as shown by comparing their orbital magnetic moments and anomalous Hall signatures. Unexpectedly, the states are distinguished by their spin susceptibility to applied in plane magnetic fields—which we ascribe to intrinsic spin orbit coupling with a strength of 70 micro-electronVolts. I will discuss these findings in the context of possible mechanisms for superconducting pairing, as well as for the magnetic phase diagram of interacting graphene systems.
Please note the time change from our usual seminars.