Transition from Bose-Einstein Condensate to Berezinskii-Kosterlitz-Thouless Phase

TL;DR

This paper maps the phase diagram of a 2D ultra-cold Bose gas, identifying a transition from a Bose-Einstein condensate to a Berezinskii-Kosterlitz-Thouless phase characterized by vortex pairs and thermal behavior.

Contribution

It provides the first detailed phase diagram for a trapped 2D Bose gas, highlighting the vortex pair phase and its experimental accessibility.

Findings

Identifies a critical temperature for vortex pair formation.

Defines three distinct phases: BEC, vortex pair plasma, and thermal gas.

Suggests experimental verification of the vortex pair phase.

Abstract

We obtain a phase diagram for a trapped two-dimensional ultra-cold Bose gas. We find a critical temperature above which the free energy of a state with a pair of vortices of opposite circulation is lower than the one for a vortex-free Bose-Einstein condensed ground state. We identify three distinct phases which are, in order of increasing temperature; a phase coherent Bose-Einstein condensate, a vortex pair plasma with a fluctuating condensate phase, and a thermal Bose gas. The existence of the vortex pair phase could be verified using current experimental setups.