Condensed Matter Zoom Seminar | Andrea Young, UCSB | "Orbital magnetism in graphene heterostructures"

Wed, May 19, 2021, 2:00 pm

I will describe experiments probing magnetic states based on the spontaneous alignment of electron orbitals. Such orbital ferromagnetism may be a generic phenomena, but has, to date, found its fullest expression in graphene heterostructures in which the two dimensional orbits of electrons in distinct momentum space valleys provide the underlying degree of freedom. As an elementary example I will show data from rhombohedral trilayer graphene, where band edge van Hove singularities lead to a cascade of transitions between metallic ferromagnetic states distinguished by different broken valley and spin symmetries.  Adding a moire potential to the trilayer by hBN alignment allows for energy gaps at finite density when the underlying degeneracy of the Fermi surface matches the superlattice filling factor.   Because orbital degrees of freedom arise directly from the band wavefunctions, they are uniquely susceptible to experimental control via materials design and in situ knobs. I will show examples from a variety of heterostructures where magnetic moments, and in some cases the resulting quantized anomalous Hall effects, can be tuned using electric currents and the electric field effect. Finally, I will conclude with an outlook for realizing more exotic topological phases of matter based on orbital magnetism.