Zero-Temperature Phases of the 2D Hubbard-Holstein Model: A Non-Gaussian Exact Diagonalization Study

TL;DR

This paper introduces a numerical method combining variational non-Gaussian wavefunctions with exact diagonalization to study correlated electron-phonon systems, revealing complex phases in the 2D Hubbard-Holstein model.

Contribution

The authors develop a novel embedding of variational non-Gaussian wavefunctions within exact diagonalization, enabling efficient analysis of electron-phonon interactions beyond Gaussian approximations.

Findings

Identification of intervening phases between spin and charge order

Discovery of a phase with strong charge susceptibility and binding energy

Observation of superconductivity at weak couplings

Abstract

We propose a numerical method which embeds the variational non-Gaussian wavefunction approach within exact diagonalization, allowing for efficient treatment of correlated systems with both electron-electron and electron-phonon interactions. Using a generalized polaron transformation, we construct a variational wavefunction that absorbs entanglement between electrons and phonons into a variational non-Gaussian transformation; exact diagonalization is then used to treat the electronic part of the wavefunction exactly, thus taking into account high-order correlation effects beyond the Gaussian level. Keeping the full electronic Hilbert space, the complexity is increased only by a polynomial scaling factor relative to the exact diagonalization calculation for pure electrons. As an example, we use this method to study ground-state properties of the two-dimensional Hubbard-Holstein model, providing evidence for the existence of intervening phases between the spin and charge-ordered states. In particular, we find one of the intervening phases has strong charge susceptibility and binding energy, but is distinct from a charge-density-wave ordered state, while the other intervening phase displays superconductivity at weak couplings. This method, as a general framework, can be extended to treat excited states and dynamics, as well as a wide range of systems with both electron-electron and electron-boson interactions.