Stability of fractional Chern insulators in the effective continuum limit of Harper-Hofstadter bands with Chern number $|C|>1$

TL;DR

This study investigates the stability of fractional quantum Hall states in Harper-Hofstadter bands with Chern number greater than one, demonstrating the robustness of composite fermion states and identifying conditions for gapped incompressible phases.

Contribution

It provides the first detailed analysis of composite fermion fractional quantum Hall states in higher Chern number bands, confirming their stability and the existence of gapped states in the effective continuum limit.

Findings

Almost all finite-size spectra match predicted ground-state degeneracy.

States at low ranks in the hierarchy are most robust with clear thermodynamic gaps.

Evidence for gapped fermionic states with large entanglement gaps in |C|>1 bands.

Abstract

We study the stability of composite fermion fractional quantum Hall states in Harper-Hofstadter bands with Chern number $|C|>1$. We analyze the states of the composite fermion series for bosons with contact interactions and (spinless) fermions with nearest-neighbor interactions. We examine the scaling of the many-body gap as the bands are tuned to the effective continuum limit $n_\phi\to 1/|C|$. Near these points, the Hofstadter model realises large magnetic unit cells that yield bands with perfectly flat dispersion and Berry curvature. We exploit the known scaling of energies in the effective continuum limit in order to maintain a fixed square aspect ratio in finite-size calculations. Based on exact diagonalization calculations of the band-projected Hamiltonian, we show that almost all finite-size spectra yield the ground-state degeneracy predicted by composite fermion theory. We confirm that states at low ranks in the composite fermion hierarchy are the most robust and yield a clear gap in the thermodynamic limit. For bosons in $|C|=2$ and $|C|=3$ bands, our data for the composite fermion states are compatible with a finite gap in the thermodynamic limit. We also report new evidence for gapped incompressible states of fermions in $|C|>1$ bands, which have large entanglement gaps. For cases with a clear spectral gap, we confirm that the thermodynamic limit commutes with the effective continuum limit. We analyze the nature of the correlation functions for the Abelian composite fermion states and find that they feature $|C|^2$ smooth sheets. We examine two cases associated with a bosonic integer quantum Hall effect (BIQHE): For $\nu=2$ in $|C|=1$ bands, we find a strong competing state with a higher ground-state degeneracy, so no clear BIQHE is found in the band-projected Hofstadter model; for $\nu=1$ in $|C|=2$ bands, we present additional data confirming the existence of a BIQHE state.