Using scanning susceptibility microscopy, we shed new light on the dynamics of individual superconducting vortices and examine the hypotheses of the phenomenological models traditionally used to explain the macroscopic ac electromagnetic properties o
f superconductors. The measurements, carried out on a 2H-NbSe$_2$ single crystal at relatively high temperature $T=6.8$ K, show a linear amplitude dependence of the global ac-susceptibility for excitation amplitudes between 0.3 and 2.6 Oe. We observe that the low amplitude behavior, typically attributed to the shaking of vortices in a potential well defined by a single, relaxing, Labusch constant, corresponds actually to strongly non-uniform vortex shaking. This is particularly accentuated in the field-cooled disordered phase, which undergoes a dynamic reorganization above 0.8 Oe as evidenced by the healing of lattice defects and a more uniform oscillation of vortices. These observations are corroborated by molecular dynamics simulations when choosing the microscopic input parameters from the experiments. The theoretical simulations allow us to reconstruct the vortex trajectories providing deeper insight in the thermally induced hopping dynamics and the vortex lattice reordering.
We investigate theoretically vortex-antivortex (v-av) matter moving in thin superconducting films with a regular array of in-plane magnetic dipoles. Our model considers v-av pair creation induced by the local current density generated by the magnetic
texture and the transport current and simulates the dynamics of vortices and antivortices by numerical integration of the Langevin equation of motion. Calculations of the transport properties at zero applied field show a strong dependence of the v-av dynamics on the current intensity and direction. The dynamics of the v-av matter is characterized by a series of creation and annihilation processes, which reflect on the time dependence of the electrical field, and by guided motion, resulting in a zero-field transverse resistance.
