How to realise a homogeneous dipolar Bose gas in the roton regime

TL;DR

This paper investigates how to create a near-homogeneous dipolar Bose gas in the roton regime, revealing that soft-walled trapping potentials are essential for achieving the critical density in such long-range interacting systems.

Contribution

It demonstrates that in dipolar gases, soft-walled (inhomogeneous) traps are necessary to reach the roton regime, contrasting with short-range systems that use homogeneous box potentials.

Findings

Intermediate power-law potentials maximize the roton regime proximity.

Higher power-law potentials induce density oscillations near the trap wall.

Optimal density distribution depends on trap wall steepness.

Abstract

Homogeneous quantum gases open up new possibilities for studying many-body phenomena and have now been realised for a variety of systems. For gases with short-range interactions the way to make the cloud homogeneous is, predictably, to trap it in an ideal (homogeneous) box potential. We show that creating a close to homogeneous dipolar gas in the roton regime, when long-range interactions are important, actually requires trapping particles in soft-walled (inhomogeneous) box-like potentials. In particular, we numerically explore a dipolar gas confined in a pancake trap which is harmonic along the polarisation axis and a cylindrically symmetric power-law potential $r^p$ radially. We find that intermediate $p$'s maximise the proportion of the sample that can be brought close to the critical density required to reach the roton regime, whereas higher $p$'s trigger density oscillations near the wall even when the bulk of the system is not in the roton regime. We characterise how the optimum density distribution depends on the shape of the trapping potential and find it is controlled by the trap wall steepness.