Rotons in interacting ultracold Bose gases

TL;DR

This paper demonstrates that a roton minimum appears in the excitation spectrum of interacting ultracold Bose gases like helium-4 and rubidium-87, linking it to shifts in critical temperature and proposing experimental observation methods.

Contribution

It provides a microscopic explanation for the emergence of rotons in short-range interacting Bose gases and relates this to critical temperature shifts using Path-Integral Monte-Carlo simulations.

Findings

Roton minimum appears above a threshold gas parameter.

Roton minimum correlates with maximal upward critical temperature shift.

Provides experimental suggestions for observing rotons.

Abstract

In three dimensions, non-interacting bosons undergo Bose-Einstein condensation at a critical temperature, $T_{c}$, which is slightly shifted by $\Delta T_{\mathrm{c}}$, if the particles interact. We calculate the excitation spectrum of interacting Bose-systems, \sup{4}He and \sup{87}Rb, and show that a roton minimum emerges in the spectrum above a threshold value of the gas parameter. We provide a general theoretical argument for why the roton minimum and the maximal upward critical temperature shift are related. We also suggest two experimental avenues to observe rotons in condensates. These results, based upon a Path-Integral Monte-Carlo approach, provide a microscopic explanation of the shift in the critical temperature and also show that a roton minimum does emerge in the excitation spectrum of particles with a structureless, short-range, two-body interaction.