  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.

Mesoscale and Nanoscale Physics;Other Condensed Matter