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The large nonlinear response of a single crystal ZrB12 to an ac field (frequency 40 -2500 Hz) for H0 > Hc2 has been observed. Direct measurements of the ac wave form and the exact numerical solution of the Ginzburg-Landau equations, as well as phenomenological relaxation equation, permit the study of the surface superconducting states dynamics. It is shown, that the low frequency response is defined by transitions between the metastable superconducting states under the action of an ac field. The relaxation rate which determines such transitions dynamics, is found.

We report experimental studies of the low frequency electrodynamics of ZrB12 and Nb single crystals. AC susceptibility at frequencies 3 -1000 Hz have been measured under a dc magnetic field, H0, applied parallel to the sample surface. In the surface superconducting state for several H0 the real part of the ac magnetic susceptibility exhibits a logarithmic frequency dependence as for spin-glass systems. Kramers-Kronig analysis of the experimental data, shows large losses at ultra low frequencies (< 3 Hz). The wave function slope at the surface was found. The linear response of the order parameter to the ac excitation was extracted from the experimental data.

We report the low-frequency and tunneling studies of yttrium hexaboride single crystal. Ac susceptibility at frequencies 10 -1500 Hz has been measured in parallel to the crystal surface DC fields, H0. We found that in the DC field H0 > Hc2 DC magnetic moment completely disappears while the ac response exhibited the presence of superconductivity at the surface. Increasing of the DC field from Hc2 revealed the enlarging of losses with a maximum in the field between Hc2 and Hc3. Losses at the maximum were considerably larger than in the mixed and in the normal states. The value of the DC field, where loss peak was observed, depends on the amplitude and frequency of the ac field. Close to Tc this peak shifts below Hc2 which showed the coexistence of surface superconducting states and Abrikosov vortices. We observed a logarithmic frequency dependence of the in-phase component of the susceptibility. Such frequency dispersion of the inphase component resembles the response of spin-glass systems, but the out-of-phase component also exhibited frequency dispersion that is not a known feature of the classic spin-glass response. Analysis of the experimental data with Kramers-Kronig relations showed the possible existence of the loss peak at very low frequencies (< 5 Hz). We found that the amplitude of the third harmonic was not a cubic function of the ac amplitude even at considerably weak ac fields. This does not leave any room for treating the nonlinear effects on the basis of perturbation theory. We show that the conception of surface vortices or surface critical currents could not adequately describe the existing experimental data. Consideration of a model of slow relaxing nonequilibrium order parameter permits one to explain the partial shielding and losses of weak ac field for H0 > Hc2.

We report the results of the measurement the ac conductivity of a Nb superconducting thin film in a swept dc magnetic field. In the mixed state the swept dc field creates vortices at the film surface which pass through the film and form the observed ac conductivity. Vortex rate generation does not depend on the value of the dc field and there is a large plateau-like region of dc magnetic fields where the dissipation is approximately constant. A proposed phenomenological model describes quite well the main features of the ac response in these fields including its dependency on the sweep rate, ac amplitude, frequency, and value of the second and third harmonics.