Bipolarons from long range interactions: Singlet and triplet pairs in the screened Hubbard-Froehlich model on the chain

TL;DR

This paper develops a quantum Monte Carlo method to study bipolarons with long-range electron-phonon interactions, revealing how interaction range influences bipolaron properties like effective mass, binding energy, and dispersion.

Contribution

It introduces a continuous-time Monte Carlo algorithm for the screened Hubbard-Froehlich bipolaron and explores how interaction range affects bipolaron characteristics.

Findings

Froehlich bipolarons have broader bandwidths than Holstein bipolarons.

Long-range interactions lead to lighter, more mobile bipolarons.

Screening does not significantly alter bipolaron properties for long-range interactions.

Abstract

We present details of a continuous-time quantum Monte-Carlo algorithm for the screened Hubbard-Froehlich bipolaron. We simulate the bipolaron in one dimension with arbitrary interaction range in the presence of Coulomb repulsion, computing the effective mass, binding energy, total number of phonons associated with the bipolaron, mass isotope exponent and bipolaron radius in a comprehensive survey of the parameter space. We discuss the role of the range of the electron-phonon interaction, demonstrating the evolution from Holstein to Froehlich bipolarons and we compare the properties of bipolarons with singlet and triplet pairing. Finally, we present simulations of the bipolaron dispersion. The band width of the Froehlich bipolaron is found to be broad, and the decrease in bandwidth as the two polarons bind into a bipolaron is found to be far less rapid than in the case of the Holstein interaction. The properties of bipolarons formed from long range electron-phonon interactions, such as light strongly bound bipolarons and intersite pairing when Coulomb repulsion is large, are found to be robust against screening, with qualitative differences between Holstein and screened Froehlich bipolarons found even for interactions screened within a single lattice site.