Excitations and number fluctuations in an elongated dipolar Bose-Einstein condensate

TL;DR

This paper investigates the properties of elongated dipolar Bose-Einstein condensates, revealing how dipole interactions influence excitations and local number fluctuations, including the discovery of an 'anti-roton' effect.

Contribution

It provides a detailed analysis of density fluctuations and excitations in elongated dipolar BECs, introducing a simplified variational approach and identifying the anti-roton phenomenon.

Findings

Density fluctuations are affected by transverse confinement and polarization.

Identification of an anti-roton effect with repulsive interactions at short wavelengths.

Quantitative predictions using numerical and variational methods.

Abstract

We study the properties of a magnetic dipolar Bose-Einstein condensate (BEC) in an elongated (cigar shaped) confining potential in the beyond quasi-one-dimensional (quasi-1D) regime. In this system the dipole-dipole interactions (DDIs) develop a momentum-dependence related to the transverse confinement and the polarization direction of the dipoles. This leads to density fluctuations being enhanced or suppressed at a length scale related to the transverse confinement length, with local atom number measurements being a practical method to observe these effects in experiments. We use meanfield theory to describe the ground state, excitations and the local number fluctuations. Quantitative predictions are presented based on full numerical solutions and a simplified variational approach that we develop. In addition to the well-known roton excitation, occurring when the dipoles are polarized along a tightly confined direction, we find an "anti-roton" effect for the case of dipoles polarized along the long axis: a nearly non-interacting ground state that experiences strongly repulsive interactions with excitations of sufficiently short wavelength.