Cascades between light and heavy fermions in the normal state of magic angle twisted bilayer graphene

TL;DR

This paper develops a theoretical framework to understand cascade transitions, Landau level degeneracies, and charge excitations in twisted bilayer graphene, explaining experimental observations and setting the stage for superconductivity.

Contribution

It introduces a model describing how Coulomb interactions cause band dispersion of charged excitations, elucidating the normal state properties of twisted bilayer graphene.

Findings

Charged excitations form new bands with different masses and degeneracies.

The system exhibits Fermi liquid behavior on the small mass side.

Narrow bandwidth and negative compressibility indicate potential instability.

Abstract

We present a framework for understanding the recently observed cascade transitions and the Landau level degeneracies at every integer filling of twisted bilayer graphene. The Coulomb interaction projected onto narrow bands causes the charged excitations at an integer filling to disperse, forming new bands. If the excitation moves the filling away from the charge neutrality point, then it has a band minimum at the moire Brillouin zone center with a small mass that compares well with the experiment; if towards the charge neutrality point, then it has a much larger mass and a higher degeneracy. At a non-zero density away from an integer filling the excitations interact. The system on the small mass side has a large bandwidth and forms a Fermi liquid. On the large mass side the bandwidth is narrow, the compressibility is negative and the Fermi liquid is likely unstable. This explains the observed sawtooth features in compressibility, the Landau fans pointing away from charge neutrality as well as their degeneracies. By providing a description of the charge itineracy in the normal state this framework sets the stage for superconductivity at lower temperatures.