Dissipation spots generated by vortex nucleation points in mesoscopic superconductors driven by microwave magnetic fields

Abstract

The dynamics of vortex nucleation in superconducting materials in the presence of ac magnetic fields is numerically investigated using the time-dependent Ginzburg--Landau equations (TDGL) with long-range magnetic interactions. We study the spatial distribution of the local ac dissipation in mesoscopic samples at microwave frequencies for different values of the dc magnetic field ${H}_{\text{dc}}$. We find that the main contribution to the ac dissipation in mesoscopic superconductors is localized in space around the vortex nucleation centers. The presence of these dissipation spots dominates the ac losses, and their contribution generates the previously reported jumps in the imaginary part of the ac susceptibility ${\ensuremath{\chi}}^{\ensuremath{''}}$ as a function of ${H}_{\text{dc}}$, ${\ensuremath{\chi}}^{\ensuremath{''}}({H}_{\text{dc}})$. Moreover, the information provided by the dissipation maps shows that vortices located inside mesoscopic samples play a secondary role in the dissipation. We show that in samples without surface defects, the vortex nucleation points develop spontaneously following the symmetry of the sample, while, on the contrary, the presence of surface defects breaks the symmetry and favors the development of new nucleation points with a higher dissipation.