Structure factor of Bose-condensed systems

TL;DR

This paper analyzes the structure factor of Bose-condensed systems using a self-consistent mean-field approach, emphasizing the importance of approximation order, anomalous averages, and gauge symmetry breaking to avoid unphysical divergences.

Contribution

It highlights the critical conditions needed in theoretical calculations to accurately describe Bose-condensed systems without unphysical artifacts.

Findings

Proper approximation order prevents divergences.

Including anomalous averages is essential.

Gauge symmetry breaking avoids fictitious instabilities.

Abstract

The structure factor for a Bose system with Bose-Einstein condensate is considered in the frame of the self-consistent mean-field approximation. The accomplished analysis demonstrates the principal importance of the following three points: the necessity of preserving the approximation order, the necessity of taking into account anomalous averages, and the necessity of gauge symmetry breaking. If any one of these necessary conditions is not satisfied, calculations yield the appearance of unphysical divergences of the structure factor, which implies the occurrence of fictitious instability, which contradicts experiments.