Ferromagnetic phase transition and Bose-Einstein condensation in spinor Bose gases

TL;DR

This paper investigates how ferromagnetic interactions influence phase transitions in spinor Bose gases, revealing that even weak couplings can induce ferromagnetism above the Bose-Einstein condensation temperature, unlike in Fermi gases.

Contribution

It demonstrates that infinitesimal ferromagnetic couplings can trigger ferromagnetic phase transitions in spinor Bose gases at finite temperatures, a behavior distinct from Fermi gases.

Findings

Infinitesimal FM coupling induces FM transition above BEC temperature.

FM coupling raises critical temperatures of FM transition and BEC.

Contrast with Fermi gases where a threshold coupling is needed.

Abstract

Phase transitions in spinor Bose gases with ferromagnetic (FM) couplings are studied via mean-field theory. We show that an infinitesimal value of the coupling can induce a FM phase transition at a finite temperature always above the critical temperature of Bose-Einstein condensation. This contrasts sharply with the case of Fermi gases, in which the Stoner coupling $I_s$ can not lead to a FM phase transition unless it is larger than a threshold value $I_0$. The FM coupling also increases the critical temperatures of both the ferromagnetic transition and the Bose-Einstein condensation.