Bipolarons and polarons in the Holstein-Hubbard model: Analogies and differences

TL;DR

This paper investigates the properties of bipolarons in the 1D Holstein-Hubbard model, revealing their band structure, stability, and crossover behaviors, and discusses their condensation conditions at low temperatures.

Contribution

It provides a detailed analysis of bipolaron physics in the Holstein-Hubbard model, highlighting differences and similarities with polaron behavior and exploring various effects influencing bipolaron stability.

Findings

Bipolaron band structure below phonon threshold identified

Large bipolarons can be stable with significant binding energy

Bipolaron condensation occurs only at very low temperatures in dilute limit

Abstract

The single bipolaron problem is examined in the context of the 1D Holstein-Hubbard model, emphasizing analogies and differences with respect to the complementary single polaron physics. The bipolaron band structure below the phonon threshold is revealed, showing a complex relationship between numerous excited bands as the adiabatic limit is approached. Light bipolarons with significant binding energy, the stability of large bipolarons, the small to large bipolaron crossover as a function of the Hubbard repulsion, as well as the bipolaron dissociation, are investigated in detail, disentangling adiabatic, nonadiabatic and lattice coarsening effects. It is emphasized that condensation of bipolarons occurs in the dilute limit only at very low temperatures.