Modification of roton instability due to the presence of a second dipolar Bose-Einstein condensate

TL;DR

This study investigates how the presence of a second dipolar Bose-Einstein condensate influences roton instability and system stability, revealing that coupling can significantly alter collapse dynamics in pancake-shaped traps.

Contribution

It provides a detailed analysis of coupled dipolar BECs, showing how the second condensate modifies roton instability and collapse mechanisms, which was not previously understood.

Findings

Coupling destabilizes the system at small trap separations.

Collapse mode differs from the uncoupled roton mode.

Neglecting z-direction dynamics can be inaccurate for pancake-shaped BECs.

Abstract

We study the behavior of two coupled purely dipolar Bose-Einstein condensates, each located in a cylindrically symmetric pancake-shaped external confining potential, as the separation b between the traps along the tight confining direction is varied. The solutions of the coupled Gross-Pitaevskii and Bogoliubov-de Gennes equations, which account for the full dynamics, show that the system behavior is modified by the presence of the second dipolar BEC. For sufficiently small b, the presence of the second dipolar BEC destabilizes the system dramatically. In this regime, the coupled system collapses through a mode that is notably different from the radial roton mode that induces the collapse of the uncoupled system. Finally, we comment on the shortcomings of an approach that neglects the dynamics in the z-direction, which is assumed to be a good approximation for highly pancake-shaped dipolar BECs in the literature.