Pairing, Ferromagnetism, and Condensation of a normal spin-1 Bose gas

TL;DR

This paper uses a theoretical approach to analyze the stability of a spin-1 Bose gas, revealing how interactions influence phase transitions and the absence of nematic order instabilities.

Contribution

It introduces a Random Phase Approximation with exchange (RPA-X) to study phase stability and transitions in a spin-1 Bose gas, highlighting the effects of interactions and external fields.

Findings

Repulsive spin-independent interactions stabilize the normal state.

Transition from normal to single particle condensate is direct under typical interactions.

No instability towards nematic order within RPA-X.

Abstract

We theoretically study the stability of a normal, spin disordered, homogenous spin-1 Bose gas against ferromagnetism, pairing, and condensation through a Random Phase Approximation which includes exchange (RPA-X). Repulsive spin-independent interactions stabilize the normal state against both ferromagnetism and pairing, and for typical interaction strengths leads to a direct transition from an unordered normal state to a fully ordered single particle condensate. Atoms with much larger spin-dependent interaction may experience a transition to a ferromagnetic normal state or a paired superfluid, but, within the RPA-X, there is no instability towards a normal state with spontaneous nematic order. We analyze the role of the quadratic Zeeman effect and finite system size.