Spin textures in rotating two-component Bose-Einstein condensates

TL;DR

This paper explores complex spin textures like skyrmions and meron-pairs in rotating two-component Bose-Einstein condensates, deriving a generalized model and analyzing how system parameters influence their stability and configurations.

Contribution

It introduces a nonlinear sigma model for two-component BECs, linking vortex textures to pseudospin representations and analyzing effects of external fields and interactions.

Findings

Skyrmion and meron-pair textures are degenerate in SU(2) symmetry.

External rf-fields favor meron-pair textures by breaking degeneracy.

Interaction differences lead to anisotropic vorticity distributions.

Abstract

We investigate two kinds of coreless vortices with axisymmetric and nonaxisymmetric configurations in rotating two-component Bose-Einstein condensates. Starting from the Gross-Pitaevskii energy functional in a rotating frame, we derive a nonlinear sigma model generalized to the two-component condensates. In terms of a pseudospin representation, an axisymmetric vortex and a nonaxisymmetric one correspond to spin textures referred to as a "skyrmion" and a "meron-pair", respectively. A variational method is used to investigate the dependence of the sizes of the stable spin textures on system parameters, and the optimized variational function is found to reproduce well the numerical solution. In the SU(2) symmetric case, the optimal skyrmion and meron-pair are degenerate and transform to each other by a rotation of the pseudospin. An external rf-field that couples coherently the hyperfine states of two components breaks the degeneracy in favor of the meron-pair texture due to an effective transverse pseudomagnetic field. The difference between the intracomponent and intercomponent interactions yields a longitudinal pseudomagnetic field and a ferromagnetic or an antiferromagnetic pseudospin interaction, leading to a meron-pair texture with an anisotropic distribution of vorticity.