Light polarons with electron-phonon coupling

TL;DR

This paper uncovers the existence of light polarons in strong electron-phonon coupling regimes, analyzing how bond and Holstein couplings influence polaron properties using a novel Monte Carlo method.

Contribution

It introduces a new Monte Carlo approach to study the impact of bond electron-phonon coupling on Holstein polarons, revealing non-monotonic effective mass behavior in strong coupling regimes.

Findings

Light polarons can exist even in strong coupling regimes.

Effective mass shows non-monotonic dependence on bond coupling g_B.

Minimal change in effective mass when Holstein coupling g_H is in the light mass regime.

Abstract

In most cases, as the strength of electron-phonon coupling increases, the effective mass of polarons typically increases. However, in this paper, we uncover a fascinating phenomenon: the presence of light polarons even within the strong coupling regime, where electron-phonon coupling includes both Holstein and bond types (electron-phonon coupling occurring on the hopping bonds). To investigate this, we employ a novel Diagrammatic Monte Carlo method based on the path-integral formulation of the particle sector and the Fock-state representation of the phonon sector. Our study centers on analyzing the impact of bond electron-phonon coupling on the Holstein polaron's essential properties, including its effective mass, ground state energy, and the average number of phonons. We examine two distinct scenarios: one where the phonon frequency of both Holstein and bond types is the same, and another where the phonon frequency of the Holstein type is twice as large as that of the bond type. In both of these two cases, when the Holstein coupling falls within the light mass regime, we observe minimal changes in the effective mass as a function of bond coupling, compared to the case of the bare bond polaron. However, the behavior of the effective mass undergoes a significant change in the heavy mass regime. Most intriguingly, we discover a non-monotonic dependence of the effective mass on the bond coupling $g_B$ when the Holstein coupling $g_H$ falls within the heavy mass regime. This finding holds promise, particularly in the context of bipolaron, where achieving a light effective mass and a large binding energy to get the compact size, is crucial.