Quasiparticle nature of the Bose polaron at finite temperature

TL;DR

This study uses ab-initio Path Integral Monte Carlo simulations to investigate how finite temperature affects the Bose polaron's quasiparticle properties, revealing the disappearance of its quasiparticle nature near the critical temperature.

Contribution

It provides the first ab-initio analysis of the Bose polaron at finite temperature, capturing the transition of its quasiparticle behavior close to the superfluid-normal phase transition.

Findings

Polaron energy decreases with temperature for repulsive interactions.

Polaron energy increases with temperature for attractive interactions.

Quasiparticle nature vanishes near the critical temperature.

Abstract

The Bose polaron has attracted theoretical and experimental interest because the mobile impurity is surrounded by a bath that undergoes a superfluid-to-normal phase transition. Although many theoretical works have studied this system in its ground state, only few analyze its behavior at finite temperature. We have studied the effect of temperature on a Bose polaron system performing ab-initio Path Integral Monte Carlo simulations. This method is able to approach the critical temperature without losing accuracy, in contrast with perturbative approximations. We have calculated the polaron energy for the repulsive and attractive branches and we have observed an asymmetric behavior between the two branches. When the potential is repulsive, the polaron energy decreases when the temperature increases, and contrariwise for the attractive branch. Our results for the effective mass and the dynamical structure factor of the polaron show unambiguously that its quasiparticle nature disappears close to the critical temperature, in agreement with recent experimental findings. Finally, we have also estimated the fraction of bosons in the condensate as well as the superfluid fraction, and we have concluded that the impurity hinders the condensation of the rest of bosons.