Stochastic motion of finite-size immiscible impurities in a dilute quantum fluid at finite temperature

The dynamics of an active, finite-size and immiscible impurity in a dilute quantum fluid at finite temperature is characterized by means numerical simulations the projected Gross--Pitaevskii equation. modeled as localized repulsive potential described with classical degrees freedom. It shown that impurities different sizes thermalize undergo stochastic compatible Ornstein--Uhlenbeck process sufficiently large time-lags. velocity correlation function displacement are measured increment friction observed. Such behavior phenomenologically explained scenario where exchanges momentum gas thermal excitations, experiencing Epstein drag.