Particle-hole asymmetric ferromagnetism and spin textures in the triangular Hubbard-Hofstadter model

TL;DR

This paper explores the complex quantum phases in the triangular Hubbard-Hofstadter model under strong magnetic fields, revealing ferromagnetic regions, particle-hole asymmetry, and skyrmion formation through advanced numerical simulations.

Contribution

It provides the first detailed numerical phase diagram of the Hubbard-Hofstadter model on a triangular lattice, highlighting ferromagnetism and particle-hole asymmetry in the Hofstadter regime.

Findings

Broad ferromagnetic wedge in the phase diagram for filling factor ν ≤ 1.

Signatures of SU(2) quantum Hall ferromagnetism at ν=1 and ν=3.

Particle-hole asymmetry and skyrmion formation on the electron-doped side.

Abstract

In a lattice model subject to a perpendicular magnetic field, when the lattice constant is comparable to the magnetic length, one enters the "Hofstadter regime," where continuum Landau levels become fractal magnetic Bloch bands. Strong mixing between bands alters the nature of the resulting quantum phases compared to the continuum limit; lattice potential, magnetic field, and Coulomb interaction must be treated on equal footing. Using determinant quantum Monte Carlo (DQMC) and density matrix renormalization group (DMRG) techniques, we study this regime numerically in the context of the Hubbard-Hofstadter model on a triangular lattice. In the field-filling phase diagram, we find a broad wedge-shaped region of ferromagnetic ground states for filling factor $\nu \leq 1$, bounded below by filling factor $\nu = 1$ and bounded above by half-filling the lowest Hofstadter subband. We observe signatures of SU(2) quantum Hall ferromagnetism at filling factors $\nu=1$ and $\nu=3$. The phases near $\nu=1$ are particle-hole asymmetric, and we observe a rapid decrease in ground state spin polarization consistent with the formation of skyrmions only on the electron doped side. At large fields, above the ferromagnetic wedge, we observe a low-spin metallic region with spin correlations peaked at small momenta. We argue that the phenomenology of this region likely results from exchange interaction mixing fractal Hofstadter subbands. The phase diagram derived beyond the continuum limit points to a rich landscape to explore interaction effects in magnetic Bloch bands.