Symmetry-induced vortex-antivortex configurations in superconducting squares and triangles were predicted earlier; yet, they have not been resolved in experiment up to date. Namely, with vortex-antivortex states being highly unstable with respect to
defects and temperature fluctuations, it is very unlikely that samples can be fabricated with the needed quality. Here we show how these drawbacks can be overcome by strategically placed nanoholes in the sample. As a result, (i) the actual shape of the sample becomes far less important, (ii) the stability of the vortex-antivortex configurations in general is substantially enhanced, and (iii) states comprising novel giant-antivortices (with higher winding numbers) become energetically favorable in perforated disks. In the analysis, we stress the potent of strong screening to destabilize the vortex-antivortex states. In turn, the screening-symmetry competition favors stabilization of new asymmetric ground states, which arise for small values of the effective Ginzburg-Landau parameter kappa.
In submicron superconducting squares in a homogeneous magnetic field, Ginzburg-Landau theory may admit solutions of the vortex-antivortex type, conforming with the symmetry of the sample [Chibotaru et al., Nature 408, 833 (2000)]. Here we show that t
hese fascinating, but never experimentally observed states, can be enforced by artificial fourfold pinning, with their diagnostic features enhanced by orders of magnitude. The second-order nucleation of vortex-antivortex molecules can be driven either by temperature or applied magnetic field, with stable asymmetric vortex-antivortex equilibria found on its path.
In a numerical experiment based on Gross-Pitaevskii formalism, we demonstrate unique topological quantum coherence in optically trapped Bose-Einstein condensates (BECs). Exploring the fact that vortices in rotating BEC can be pinned by a geometric ar
rangement of laser beams, we show the parameter range in which vortex-antivortex molecules or multiquantum vortices are formed as a consequence of the optically imposed symmetry. Being low-energy states, we discuss the conditions for spontaneous nucleation of these unique molecules and their direct experimental observation, and provoke the potential use of the phase print of an antivortex or a multiquantum vortex when realized in unconventional circumstances.
