Classical Rotons in Cold Atomic Traps

TL;DR

This paper predicts the formation of a roton minimum in the excitation spectrum of cold atomic gases caused by diffusive light and multiple scattering, revealing a new polariton mode linked to long-range order.

Contribution

It introduces the concept of roton minima arising from diffusive light and multiple scattering in cold atomic traps, a novel mechanism for collective excitations.

Findings

Roton minimum appears in the dispersion relation of atomic excitations.

The roton is associated with long-range order in the system.

A new polariton mode emerges due to the interplay of forces.

Abstract

We predict the emergence of a roton minimum in the dispersion relation of elementary excitations in cold atomic gases in the presence of diffusive light. In large magneto-topical traps, multiple-scattering of light is responsible for the collective behavior of the system, which is associated to an effective Coulomb-like interaction between the atoms. In optically thick clouds, the re-scattered light undergoes diffusive propagation, which is responsible for a stochastic short-range force acting on the atoms. We show that the dynamical competition between these two forces results on a new polariton mode, which exhibits a roton minimum. Making use of Feynman's formula for the static structure factor, we show that the roton minimum is related to the appearance of long-range order in the system.