Phases and exotic phase transitions of a two-dimensional Su-Schrieffer-Heeger model
Phys. Rev. B, 109 (19):
195154 (May 16, 2024)DOI: 10.1103/PhysRevB.109.195154
Abstract
We study a Su-Schrieffer-Heeger electron-phonon model on a square lattice by means of auxiliary-field quantum Monte Carlo simulations. The addition of a symmetry-allowed interaction permits analytical integration over the phonons at the expense of discrete Hubbard-Stratonovich fields with imaginary-time correlations. Using single-spin-flip and global updates, we investigate the phase diagram at the O(4)-symmetric point as a function of hopping t and phonon frequency ω0. For t=0, where electron hopping is boson assisted, the model maps onto an unconstrained Z2 gauge theory. A key quantity is the emergent effective flux per plaquette, which equals π in the assisted-hopping regime and vanishes for large t. Phases in the former regime can be understood in terms of instabilities of emergent Dirac fermions. Our results support a direct and continuous transition between a (π,0) valence bond solid (VBS) and an antiferromagnetic (AFM) phase with increasing ω0. For large t and small ω0, we find finite-temperature signatures, a disordered pseudogap phase, of a previously reported (π,π) VBS ground state related to a nesting instability. With increasing ω0, AFM order again emerges.