Phase Diagrams of Antiferromagnetic Spin-1 Bosons on Square Optical Lattice with the Quadratic Zeeman Effect

TL;DR

This paper investigates how the quadratic Zeeman effect influences the phase diagrams of antiferromagnetic spin-1 bosons on a square lattice, revealing control over Mott-superfluid transitions and nematic order through QZE.

Contribution

It provides a detailed analysis of the impact of quadratic Zeeman effect on phase transitions and nematic order in spin-1 bosons, combining quantum Monte Carlo and mean field methods.

Findings

QZE destroys the tip of even Mott lobes, favoring superfluid phases.

Odd Mott lobes' tips remain unaffected by QZE.

QZE can control the nature of the Mott-superfluid transition, changing it from first to second order.

Abstract

We study the quadratic Zeeman effect (QZE) in a system of antiferromagnetic spin-1 bosons on a square lattice and derive the ground-state phase diagrams by means of quantum Monte Carlo simulations and mean field treatment. The QZE imbalances the populations of the magnetic sublevels $\sigma=\pm1$ and $\sigma=0$, and therefore affects the magnetic and mobility properties of the phases. Both methods show that the tip of the even Mott lobes, stabilized by singlet state, is destroyed when turning on the QZE, thus leaving the space to the superfluid phase. Contrariwise, the tips of odd Mott lobes remain unaffected. Therefore, the Mott-superfluid transition with even filling strongly depends on the strength of the QZE, and we show that the QZE can act as a control parameter for this transition at fixed hopping. Using quantum Monte Carlo simulations, we elucidate the nature of the phase transitions and examine in detail the nematic order: the first-order Mott-superfluid transition with even filling observed in the absence of QZE becomes second order for weak QZE, in contradistinction to our mean-field results which predict a first-order transition in a larger range of QZE. Furthermore, a spin nematic order with director along the $z$ axis is found in the odd Mott lobes and in the superfluid phase for energetically favored $\sigma=\pm1$ states. In the superfluid phase with even filling, the $xy$ components of the nematic director remain finite only for moderate QZE.