Strong Coupling Limit of the Holstein-Hubbard Model

TL;DR

This paper explores the phase diagram of the Holstein-Hubbard model in the strong-coupling limit, revealing diverse phases such as unconventional superconductors, charge-density waves, and phase separation through an asymptotically exact expansion.

Contribution

It provides a comprehensive analysis of the Holstein-Hubbard model's phases in the strong-coupling regime using an exact expansion, uncovering novel phases and crossovers.

Findings

Identification of pair-density wave (PDW) phase.

Discovery of charge 4e and 6e superconducting regimes.

Mapping of phase transitions and crossovers as a function of retardation.

Abstract

We analyze the quantum phase diagram of the Holstein-Hubbard model using an asymptotically exact strong-coupling expansion. We find all sorts of interesting phases including a pair-density wave (PDW), a charge 4e (and even a charge 6e) superconductor, regimes of phase separation, and a variety of distinct charge-density-wave (CDW), spin-density-wave (SDW) and superconducting regimes. We chart the crossovers that occur as a function of the degree of retardation, i.e. the ratio of characteristic phonon frequency to the strength of interactions.