Two-dimensional dipolar Bose gas with the roton-maxon excitation spectrum

TL;DR

This paper analyzes fluctuations in a two-dimensional dipolar Bose gas with a roton-maxon spectrum, revealing how the roton influences density fluctuations, condensate depletion, and superfluid properties at various temperatures.

Contribution

It provides a detailed calculation of fluctuations and establishes the validity of the Bogoliubov approach for this system, highlighting the impact of the roton minimum.

Findings

Roton presence enhances density fluctuations.

Condensate depletion becomes significant near the roton minimum.

Thermal fluctuations increase the normal fraction at finite temperatures.

Abstract

We discuss fluctuations in a dilute two-dimensional Bose-condensed dipolar gas, which has a roton-maxon character of the excitation spectrum. We calculate the density-density correlation function, fluctuation corrections to the chemical potential, compressibility, and the normal (superfluid) fraction. It is shown that the presence of the roton strongly enhances fluctuations of the density, and we establish the validity criterion of the Bogoliubov approach. At T=0 the condensate depletion becomes significant if the roton minimum is sufficiently close to zero. At finite temperatures exceeding the roton energy, the effect of thermal fluctuations is stronger and it may lead to a large normal fraction of the gas and compressibility.