Nematic Order by Disorder in Spin-2 BECs

TL;DR

This paper investigates how quantum and thermal fluctuations influence the phase diagram of spin-2 Bose-Einstein condensates, revealing fluctuation-induced order that favors specific nematic states and predicting observable phase transitions.

Contribution

It demonstrates that fluctuations resolve degeneracies in spin-2 BECs, favoring uniaxial or biaxial nematic states depending on scattering lengths, and predicts a continuous phase transition induced by magnetic fields.

Findings

Fluctuations select uniaxial nematic state for a_4 > a_2.

Fluctuations favor biaxial nematic state for a_4 < a_2.

A continuous Ising transition from uniaxial to biaxial order is predicted.

Abstract

The effect of quantum and thermal fluctuations on the phase diagram of spin-2 BECs is examined. They are found to play an important role in the nematic part of the phase diagram, where a mean-field treatment of two-body interactions is unable to lift the accidental degeneracy between nematic states. Quantum and thermal fluctuations resolve this degeneracy, selecting the uniaxial nematic state, for scattering lengths $a_4>a_2$, and the square biaxial nematic state for $a_4<a_2$. Paradoxically, the fluctuation induced order is stronger at higher temperatures, for a range of temperatures below $T_c$. For the experimentally relevant cases of spin-2 $^{87}$Rb and $^{23}$Na, we argue that such fluctuations could successfully compete against other effects like the quadratic Zeeman field, and stabilize the uniaxial phase for experimentally realistic conditions. A continuous transition of the Ising type from uniaxial to square biaxial order is predicted on raising the magnetic field. These systems present a promising experimental opportunity to realize the `order by disorder' phenomenon.