Phases of a Bose-Einstein condensate of microwave-shielded dipolar molecules

TL;DR

This paper investigates the phases and stability of microwave-shielded dipolar molecular Bose-Einstein condensates, revealing a phase transition from a homogeneous gas to layered structures depending on density and microwave shielding parameters.

Contribution

It introduces a many-body variational approach to analyze the phase behavior of dipolar molecules with microwave shielding, predicting novel layered and self-bound liquid phases.

Findings

Homogeneous gas phase becomes unstable at low density.

Critical density for phase transition depends on microwave-induced repulsion.

Layered and self-bound liquid phases can form at certain densities.

Abstract

Bose-Einstein condensation of dipolar molecules can be achieved by shielding loss channels with microwave fields. The microwave coupling can be approximated by effective dipole-dipole interactions with a short-range repulsion. We study properties and stability of these molecular Bose gases with a many-body variational method, the hypernetted-chain Euler-Lagrange method for a wide range of densities and repulsion strengths of the microwave shield. We find a homogeneous gas-like phase which, however, is unstable at low density against density waves: at a critical density, which depends on the repulsion strength, the dipolar fluid undergoes a phase transition to a layer phase. Thus, if the molecular condensate is expanded adiabatically by decreasing the confinement strength, it will spontaneously form layers at the critical density. These quasi-two-dimensional layers can be self-bound, hence form two-dimensional liquids. By varying the microwave shield, the predicted equilibrium densities span more than an order of magnitude.