Polarons and bipolarons in strongly interacting electron-phonon systems

TL;DR

This paper investigates polaron and bipolaron formation in strongly interacting electron-phonon systems using exact diagonalization, revealing detailed insights into their spectral properties, dispersion, and effects of Coulomb interactions.

Contribution

It provides a comprehensive analysis of polarons and bipolarons in the Holstein Hubbard and t--J models across various parameters using advanced computational methods.

Findings

Identification of a polaron band with non-free dispersion at intermediate phonon frequencies.

Evidence of bipolaron states and their band formation for two electrons or holes.

Demonstration that strong Coulomb correlations favor hole-polaron formation in the 2D Holstein t--J model.

Abstract

The Holstein Hubbard and Holstein t--J models are studied for a wide range of phonon frequencies, electron--electron and electron--phonon interaction strengths on finite lattices with up to ten sites by means of direct Lanczos diagonalization. Previously the necessary truncation of the phononic Hilbert space caused serious limitations to either very small systems (four or even two sites) or to weak electron--phonon coupling, in particular in the adiabatic regime. Using parallel computers we were able to investigate the transition from `large' to `small' polarons in detail. By resolving the low--lying eigenstates of the Hamiltonian and by calculating the spectral function we can identify a polaron band in the strong--coupling case, whose dispersion deviates from the free--particle dispersion at low and intermediate phonon frequencies. For two electrons (holes) we establish the existence of bipolaronic states and discuss the formation of a bipolaron band. For the 2D Holstein t--J model we demonstrate that the formation of hole--polarons is favoured by strong Coulomb correlations. Analyzing the hole--hole correlation functions we find that hole binding is enhanced as a dynamical effect of the electron--phonon interaction.