***ATTENTION Indico Users***

Important changes to user logins are coming to Indico at BNL.

Please see the News section for more information.

BNL Physics Colloquia

Superconductivity and magnetism in crystalline graphene allotropes

by Prof. Andrea Young (UC Santa Barbara)

US/Eastern
Online

Online

Description

Magnetism---in particular magnetic fluctuations---are thought thought to play a role in a variety of unconventional superconductors, including cuprates, heavy fermions, and moire graphene.  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 [1-2] and Bernal bilayer [3] graphene.  In both systems, applying a perpendicular electric field gaps out a series of low-energy Dirac nodes, leading to a divergence in the density of states at densities easily accessible by the field effect.  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 spin-triplet superconducting ground state.  I will lay out the many outstanding theoretical puzzles in these systems, as well as  experimental opportunities enabled by the exceptionally good reproducibility and and high sample quality.

[1] H. Zhou, T. Xie, A. Ghazaryan, T. Holder, J. R. Ehrets, E. M. Spanton,

T. Taniguchi, K. Watanabe, E. Berg, M. Serbyn, & A. F. Young. “Half and quarter metals in rhombohedral trilayer graphene.” 598: 429-433 Nature (2021).

[2] H. Zhou, T. Xie, T. Taniguchi, K. Watanabe & A. F. Young. “Superconductivity in rhombohedral trilayer graphene,” Nature 598: 434-438 (2021).

[3] H. Zhou, Y.  Saito, L. Cohen, W.  Huynh, C. L. Patterson, F.  Yang, T. Taniguchi, K. Watanabe, "Isospin magnetism and spin-triplet superconductivity in Bernal bilayer graphene." Science, eabm8386 (2022).

Zoom link:  https://bnl.zoomgov.com/j/1605020278?pwd=cHJ1bDRuK1FDNnZLSnpxVkZhcDQ3QT09