Correlated singlet phase in the one-dimensional Hubbard-Holstein model

TL;DR

This paper investigates a correlated singlet phase in the one-dimensional Hubbard-Holstein model under strong interactions and non-adiabatic conditions, revealing mutually exclusive superfluid and charge-density-wave orders and analyzing BEC occupation scaling.

Contribution

It introduces an effective Hamiltonian for the singlet phase and maps it to a hard-core-boson model, providing new insights into the phase behavior and order coexistence.

Findings

Superfluidity and CDW occur mutually exclusively.

Diagonal long-range order appears only at one-third filling.

BEC occupation number scales as √N with a smaller coefficient in the interacting phase.

Abstract

We show that a nearest-neighbor singlet phase results (from an effective Hamiltonian) for the one-dimensional Hubbard-Holstein model in the regime of strong electron-electron and electron-phonon interactions and under non-adiabatic conditions ($t/\omega_0 \leq 1$). By mapping the system of nearest-neighbor singlets at a filling $N_p/N$ onto a hard-core-boson (HCB) $t$-$V$ model at a filling $N_p/(N-N_p)$, we demonstrate explicitly that superfluidity and charge-density-wave (CDW) occur mutually exclusively with the diagonal long range order manifesting itself only at one-third filling. Furthermore, we also show that the Bose-Einstein condensate (BEC) occupation number $n_0$ for the singlet phase, similar to the $n_0$ for a HCB tight binding model, scales as $\sqrt N$; however, the coefficient of $\sqrt N$ in the $n_0$ for the interacting singlet phase is numerically demonstrated to be smaller.