Quantum Cafe Webinar: Andrea Young

Title: Superconductivity at magnetic phase transitions in crystalline graphene allotropes

Abstract: The interplay between superconductivity and magnetism is thought to play a role in a variety of unconventional superconductors, including cuprates, heavy fermions, and moire graphene. Here, I will describe a new venue for examining this interplay by tuning the chemical potential through a van Hove singularity in simple allotropes of graphene, in particular rhombohedral trilayer and Bernal bilayer. In both systems, applying a perpendicular electric field gaps out a series of low-energy Dirac nodes, leading to large divergences in the density of states at low densities. Using both transport and compressibility measurements, we find that this regime is characterized by a cascade of phase transitions between states of differing fermi surface degeneracy. These include quarter- and half-metals with only one or two occupied (out of a possible four) combined spin- and valley flavors, as well as a variety of states showing partial polarization within the spin- and valley- isospin space. Most surprisingly, superconductivity arises near a number of phase boundaries. In the trilayer, we observe two superconducting states for hole doping; one arises from a normal state that preserves the spin and valley symmetry, and is suppressed by in-plane magnetic fields in accordance with the Clogston-Chandrasekhar limit, while the other arises from a full spin polarized half metallic state and is not affected by in plane magnetic fields. In bilayer graphene, superconductivity is not observed at B=0, but emerges only above a critical field in plane field, consistent with a magnetic field induced transition into a spin polarized ferromagnetic state with a superconducting ground state. I will lay out the many outstanding theoretical puzzles in these systems, as well as experimental opportunities enabled by the exceptionally high sample quality.

For Zoom details please contact Mary Kate Hennelly at [email protected]