Bipolarons bound by repulsive phonon-mediated interactions

TL;DR

This paper demonstrates that under certain conditions, phonon-mediated interactions can be repulsive yet still bind particles into bipolarons with unique properties, revealing a new type of quasiparticle.

Contribution

It introduces a novel mechanism for bipolaron formation via repulsive phonon-mediated interactions in systems with hard-core particles.

Findings

Bipolarons appear in the gap above the two-polaron continuum.

The bipolaron has a large dispersion and negative effective mass.

Repulsive phonon-mediated interactions can bind particles into bipolarons.

Abstract

When dressed particles (polarons) exchange quantum phonons, the resulting interactions are generally attractive. If the particles have hard-core statistics and the coupling to phonons is through the kinetic energy terms, phonon-mediated interactions are repulsive. Here, we show that such repulsive phonon-mediated interactions bind dressed particles into bipolarons with unique properties. These bipolaron states appear in the gap between phonon excitations, above the two-polaron continuum. While thermodynamically unstable, the bipolaron is protected by energy and momentum conservation and represents a novel quasiparticle with a large dispersion and a negative effective mass near zero momentum. We discuss possible experimental implementation of the conditions for the formation of such repulsively bound bipolarons.