Moir\'e Fractional Chern Insulators III: Hartree-Fock Phase Diagram, Magic Angle Regime for Chern Insulator States, the Role of the Moir\'e Potential and Goldstone Gaps in Rhombohedral Graphene Superlattices

TL;DR

This paper explores the phase diagram of rhombohedral graphene aligned with hBN, revealing how interaction schemes, moiré potentials, and stacking influence topological phases and collective excitations in the large displacement field regime.

Contribution

It provides a detailed Hartree-Fock analysis of the influence of moiré potentials and stacking on topological phases in rhombohedral graphene/hBN, highlighting the importance of interaction modeling.

Findings

Moiré potential effects can break translation symmetry and alter electronic topology.

The phase diagram is highly sensitive to the choice of interaction scheme.

Collective mode spectrum reveals competing pseudophonon and magnon excitations.

Abstract

We investigate in detail the $\nu=+1$ displacement-field-tuned interacting phase diagram of $L=3,4,5,6,7$ layer rhombohedral graphene aligned to hBN (R$L$G/hBN). Our calculations account for the 3D nature of the Coulomb interaction, the inequivalent stacking orientations $\xi=0,1$, the effects of the filled valence bands, and the choice of `interaction scheme' for specifying the many-body Hamiltonian. We show that the latter has a dramatic impact on the Hartree-Fock phase boundaries and the properties of the phases, including for pentalayers (R5G/hBN) with large displacement field $D$ where recent experiments observed a Chern insulator at $\nu=+1$ and fractional Chern insulators for $\nu<1$. In this large $D$ regime, the low-energy conduction bands are polarized away from the aligned hBN layer, and are hence well-described by the folded bands of moir\'eless rhombohedral graphene at the non-interacting level. Despite this, the filled valence bands develop moir\'e-periodic charge density variations which can generate an effective moir\'e potential, thereby explicitly breaking the approximate continuous translation symmetry in the conduction bands, and leading to contrasting electronic topology in the ground state for the two stacking arrangements. Within time-dependent Hartree-Fock theory, we further characterize the strength of the moir\'e pinning potential in the Chern insulator phase by computing the low-energy $\mathbf{q}=0$ collective mode spectrum, where we identify competing gapped pseudophonon and valley magnon excitations. Our results emphasize the importance of careful examination of both the single-particle and interaction model for a proper understanding of the correlated phases in R$L$G/hBN.