Symmetry-induced formation of antivortices in mesoscopic superconductors

Recent progress in nanotechnology has stimulated interest in mesoscopic sup erconductors as components for quantum computing and cryoelectronics. The c ritical parameters for superconductivity (current and field) of a mesoscopi c sample are determined by the pattern of vortices in it, which in turn is controlled by the symmetry imposed by the shape of the sample (see ref. 1 a nd references therein). Hitherto it has been unclear what happens when the number of vortices is not consistent with the natural symmetry. Here we sho w that additional vortex-antivortex pairs nucleate spontaneously so as to p reserve the symmetry of the sample. For example, in a square with three vor tices, the spontaneously generated pair, along with the original three vort ices, distribute themselves so that the four vortices sit in the four corne rs, with the antivortex in the centre. The measured superconducting phase b oundary (of superconducting transition temperature T-c versus magnetic fiel d strength) is in very good agreement with the calculations, giving direct experimental evidence for these symmetry-induced vortex-antivortex pairs. V ortex entry into the sample is also changed: vortices enter a square in fou rs, with antivortices generated to preserve the imposed vorticity. The symm etry-induced nucleation of antivortices is not restricted to superconductor s, but should also apply to symmetrically confined superfluids and Bose-Ein stein condensates.