Light and stable triplet bipolarons on square and triangular lattices

TL;DR

This study uses quantum Monte Carlo simulations to analyze bipolaron properties on 2D lattices, revealing the existence of ultra-light hybrid bipolarons and stability differences between singlet and triplet states.

Contribution

It provides the first detailed comparison of bipolaron properties under long-range and local electron-phonon interactions on square and triangular lattices.

Findings

Existence of extra-light hybrid singlet bipolarons.

Triplet bipolarons are more stable at certain momenta on triangular lattices.

Significant differences between models with long-range and local interactions.

Abstract

We compute the properties of singlet and triplet bipolarons on two-dimensional lattices using the continuous time quantum Monte Carlo algorithm. Properties of the bipolaron including the total energy, inverse mass, bipolaron radius and number of phonons associated with the bipolaron demonstrate the qualitative difference between models of electron phonon interaction with long-range interaction (screened Fr\"ohlich) and those with purely local (Holstein) interaction. A major result of our survey of the parameter space is the existence of extra-light hybrid singlet bipolarons consisting of an on-site and an off-site component on both square and triangular lattices. We also compute triplet properties of the bipolarons and the pair dispersion. For pair momenta on the edge of the Brillouin zone of the triangular lattice, we find that triplet states are more stable than singlets.