Spin polarons and skyrmion superconductivity in topological bands: application to graphene moire heterostructures
Eslam Khalaf
Harvard University
Wednesday, January 31, 2024
10:30 a.m.
In-person: QNC 1101
Abstract: Understanding the phase diagram of twisted bilayer graphene and related moir ́e systems is a central theoretical challenge. While the ground states at inte- ger fillings have been shown in many cases to be simple flavor ferromagnets, the charge excitations above such states can be non-trivial due to band topology. Conventional approaches to understand such excitations as real space topologi- cal textures fail to account for the distinct momentum space features of Chern bands and obscures their comparison to single particle excitations. Here, we present a general fully momentum space formulation for the problem of charge excitations in Chern bands. In the limit of (normal-ordered) contact interac- tions in an ideal flat bands, we construct exact analytical wavefunctions for the lowest energy excitation with charge ±e and spin n+1/2, a spin polaron. Away from this ideal limit, we show that these analytical wavefunctions are excellent variational states describing a bound state of an electron/hole with n spin flips. We apply our formalism to study charge excitations in twisted bilayer graphene and find that (i) in the chiral limit, multispin flip polarons are the lowest en- ergy charge excitations at charge neutrality and at non-zero integer fillings when doping towards neutrality. In the realistic limit, we find that the multispin flip states are the lowest charged excitations at ν = ±(1 − ε) for any strong cou- pling state and at ν = ±(2 − ε) for the time-reversal intervalley coherent state (TIVC) but not the Krammers intervalley coherent state (KIVC). We discuss the experimental implications of these results for low strain devices.