Thermally induced rotons in two-dimensional dilute Bose gases

TL;DR

This paper demonstrates that thermal fluctuations induce roton-like excitations in two-dimensional dilute Bose gases, affecting their phonon spectrum and relating to the Kosterlitz-Thouless transition, with implications for the normal state.

Contribution

It reveals the emergence of thermally induced roton-like excitations in 2D Bose gases and connects their effects to the Kosterlitz-Thouless transition within an effective vortex theory.

Findings

Roton-like excitations are thermally induced in 2D Bose gases.

The phonon spectrum exhibits a temperature-dependent anomalous exponent.

An anomalous normal state may exist at specific temperatures.

Abstract

We show that roton-like excitations are thermally induced in a two-dimensional dilute Bose gas as a consequence of the strong phase fluctuations in two dimensions. At low momentum, the roton-like excitations lead for small enough temperatures to an anomalous phonon spectrum with a temperature dependent exponent reminiscent of the Kosterlitz-Thouless transition. Despite the anomalous form of the energy spectrum, it is shown that the corresponding effective theory of vortices describes the usual Kosterlitz-Thouless transition. The possible existence of an anomalous normal state in a small temperature interval is also discussed.