Bose polarons at finite temperature and strong coupling

TL;DR

This paper investigates how finite temperature affects Bose polarons, revealing phase transition effects, quasiparticle fragmentation, and non-monotonic energy behavior across the Bose-Einstein condensation temperature.

Contribution

It introduces a diagrammatic resummation scheme to analyze impurity behavior at finite temperature, capturing effects at strong coupling and near the phase transition.

Findings

At finite temperature, the attractive polaron fragments into two quasiparticles.

The higher energy quasiparticle disappears at the critical temperature $T_c$.

The ground state quasiparticle remains well-defined across $T_c$ with non-monotonic energy dependence.

Abstract

The temperature dependence of a mobile impurity in a dilute Bose gas, the Bose polaron, is investigated for wide a range of impurity-bath interactions. Using a diagrammatic resummation scheme designed to include scattering processes important at finite temperature $T$, we show that the phase transition of the environment to a Bose-Einstein condensate at the critical temperature $T_c$ leads to several non-trivial effects. The attractive polaron present at $T=0$ fragments into two quasiparticle states for finite temperature whenever $|a|\gtrsim a_B$, with $a$ and $a_B$ the impurity-boson and boson-boson scattering lengths. While the quasiparticle with higher energy disappears at $T_c$, the ground state quasiparticle remains well-defined across $T_c$. Its energy depends non-monotonically on temperature, featuring a minimum at $T_c$, after which it increases towards zero, and at $T\gg T_c$ the polaron eventually becomes overdamped due to strong scattering with thermally excited bosons.