Stabilized vortex-antivortex molecules in a superconducting microdisk with a magnetic nanodot on top

Symmetry-induced vortex-antivortex molecules in submicrometer superconducting polygons in homogeneous magnetic field became of general interest following the prediction of Chibotaru [Nature (London) 408, 833 (2000)]. Recently, Carballeira [Phys. Rev. Lett. 95, 237003 (2005)] found that these fascinating structures can be enforced by a magnetic dot placed on top of the sample. Here, we show that vortex-antivortex configurations can actually be induced in a superconducting disk by the above magnet with perpendicular magnetization, in spite of the nonzero net flux penetrating the sample and the absence of polygonal geometrical constraints. Our study is done within the Ginzburg-Landau formalism and shows that confinement makes vortex-antivortex states metastable (i.e., with higher energy) compared to the conventional vortex states; nevertheless, these states can be experimentally observed, and we propose a procedure for their realization, under a magnet with tilted magnetic moment.