Radial and angular rotons in trapped dipolar gases

TL;DR

This paper investigates the stability and shape of dipolar Bose-Einstein condensates in pancake traps, revealing roton-like instabilities and biconcave density profiles that depend on particle number and trap geometry.

Contribution

It introduces the concept of angular rotons and analyzes the conditions for biconcave condensate formation and their instabilities in trapped dipolar gases.

Findings

Condensates become unstable to collapse at high particle numbers.

Identification of angular roton excitations causing instability.

Observation of biconcave density profiles in certain conditions.

Abstract

We study Bose-Einstein condensates with purely dipolar interactions in oblate (pancake) traps. We find that the condensate always becomes unstable to collapse when the number of particles is sufficiently large. We analyze the instability, and find that it is the trapped-gas analogue of the ``roton-maxon'' instability previously reported for a gas that is unconfined in two dimensions. In addition, we find that under certain circumstances, the condensate wave function attains a biconcave shape, with its maximum density away from the center of the gas. These biconcave condensates become unstable due to azimuthl excitation - an angular roton.