Temperature-dependence of small harmonically trapped atom systems with Bose, Fermi and Boltzmann statistics

TL;DR

This paper investigates the finite-temperature properties of small harmonically trapped atomic systems with Bose, Fermi, and Boltzmann statistics, using analytical and numerical methods to explore energetics, structural features, and superfluid behavior.

Contribution

It provides a comprehensive analysis of temperature-dependent properties of small trapped atoms, highlighting differences between condensate and superfluid fractions and demonstrating the effectiveness of path integral Monte Carlo methods.

Findings

Bosonic systems form droplets at low temperature in the unitary regime.

Path integral Monte Carlo yields reliable results across a wide temperature range.

The transition from droplet to gas occurs over a narrow temperature window.

Abstract

While the zero-temperature properties of harmonically trapped cold few-atom systems have been discussed fairly extensively over the past decade, much less is known about the finite-temperature properties. Working in the canonical ensemble, we characterize small harmonically trapped atomic systems as a function of the temperature using analytical and numerical techniques. We present results for the energetics, structural properties, condensate fraction, superfluid fraction, and superfluid density. Our calculations for the two-body system underline that the condensate and superfluid fractions are distinctly different quantities. Our work demonstrates that the path integral Monte Carlo method yields reliable results for bosonic and fermionic systems over a wide temperature range, including the regime where the de Broglie wave length is large, i.e., where the statistics plays an important role. The regime where the Fermi sign problem leads to reasonably large signal to noise ratios is mapped out for selected parameter combinations. Our calculations for bosons focus on the unitary regime, where the physics is expected to be governed by the three-body parameter. If the three-body parameter is large compared to the inverse of the harmonic oscillator length, we find that the bosons form a droplet at low temperature and behave approximately like a non-interacting Bose and eventually Boltzmann gas at high temperature. The change of the behavior occurs over a fairly narrow temperature range. A simple model that reproduces the key aspects of the phase transition like feature, which can potentially be observed in cold atom Bose gas experiments, is presented.