Phase diagram, confining strings, and a new universality class in nematopolar matter

TL;DR

This paper investigates a minimal model of nematopolar matter with coexisting nematic and polar orders, revealing a new universality class, unique critical scaling, and defect confinement phenomena through analytical and numerical methods.

Contribution

It introduces a novel universality class for the nematopolar-nematic transition and characterizes defect confinement due to frustration in the system.

Findings

Distinct phase diagram with three phases and multiple transition types

Unique critical scaling at the nematopolar-nematic transition

Confined nematic +1/2 defect pairs connected by tensioned strings

Abstract

We study a minimal model of a system with coexisting nematic and polar orientational orders, where one field tends to order and the other prefers isotropy. For strong coupling, the ordered field aligns the isotropic one, locking their orientations. The phase diagram reveals three distinct phases--nematopolar (aligned orders), nematic (independent orders), and isotropic (vanishing orders)--separated by continuous and discontinuous transitions, including a triple and a tricritical point. We find unique critical scaling for the nematopolar-nematic transition, distinct from standard nematic or polar universality classes. Additionally, in the locked nematopolar phase, we show nematic $+1/2$ topological defect pairs are connected and confined by strings with constant tension. These strings arise from frustration in locking the orientational orders and can be interpreted as elongated cores of $+1$ polar topological defects. When a sufficiently strong background field couples to the polar order, all topological defects are expelled from the region. Analytical predictions are quantitatively confirmed by numerical simulations.