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

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.

Skyrmions in twisted bilayer graphene: stability, pairing, and crystallization

“…This latter observation can be explained by the increased dispersion due to the interaction renormalization from the extra filled bands (Fig. 10a), and has been argued to be consistent with the asymmetry of the Landau fans 85 .…”

“…Second, this additional interaction renormalization enters in a particlehole asymmetric way. Generally the bands away from neutrality have a significantly enhanced bandwidth with a prominent extremum at Γ M 85,86 (Fig. 10a), which disfavors skyrmions because they are built from momentum states throughout the mBZ.…”

“…Unlike in QHFM, there is no explicit Zeeman field, meaning that the expands to fill the available system size to minimize the Coulomb energy. This reflected in the slow convergence of ∆E with L. There is a clear asymmetry between adding holes and electrons because the interaction-renormalized band structure [85][86][87][88][89][90] at finite integer fillings strongly breaks particle-hole symmetry (see Fig. 10a).…”

Spectroscopy of Twisted Bilayer Graphene Correlated Insulators

“…This will then used for analytic approximations of the LDOS. Part of this parametrization was first derived in [67].…”

“…In other words, we assume that for small doping away from integer fillings, the charge-one excitations can be approximated as non-interacting fermions and, as such, the chargeone excitation bands can be filled (or depleted) according to the Pauli exclusion principle. Such an approximation can be justified by noting that the steep dispersion of the charge-one excitations above |ϕ is enough to stabilize a Fermi liquid for small doping away from integer filling [67].…”