Fate of the Bose polaron at finite temperature

TL;DR

This paper investigates the finite-temperature behavior of the Bose polaron, revealing that the number of quasiparticle branches correlates with the number of hole excitations, and provides a detailed theoretical analysis beyond previous methods.

Contribution

The study introduces a variational approach that includes multi-body correlations to accurately predict the Bose polaron spectrum at finite temperature, extending beyond prior finite-temperature techniques.

Findings

Number of quasiparticle branches equals the number of hole excitations.

Polaron energy matches quantum Monte Carlo results at zero temperature.

Broadening of the quasiparticle peak scales as T^{3/4} at low temperatures.

Abstract

We consider an impurity immersed in a Bose-Einstein condensate with tunable boson-impurity interactions. Such a Bose polaron has recently been predicted to exhibit an intriguing energy spectrum at finite temperature, where the ground-state quasiparticle evenly splits into two branches as the temperature is increased from zero [Guenther et al., Phys. Rev. Lett. 120, 050405 (2018)]. To investigate this theoretical prediction, we employ a recently developed variational approach that systematically includes multi-body correlations between the impurity and the finite-temperature medium, thus allowing us to go beyond previous finite-temperature methods. Crucially, we find that the number of quasiparticle branches is simply set by the number of hole excitations of the thermal cloud, such that including up to one hole yields one splitting, two holes yields two splittings, and so on. Moreover, this effect is independent of the impurity mass. We thus expect that the exact ground-state quasiparticle will evolve into a single broad peak for temperatures $T>0$, with a broadening that scales as $T^{3/4}$ at low temperatures and sufficiently weak boson-boson interactions. In the zero-temperature limit, we show that our calculated ground-state polaron energy is in excellent agreement with recent quantum Monte Carlo results and with experiments.