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We report the low-frequency and tunneling studies of yttrium hexaboridesingle crystal. Ac susceptibility at frequencies 10 - 1500 Hz has been measuredin parallel to the crystal surface DC felds, H0. We found that in the DC feldH0 > Hc2 DC magnetic moment completely disappears while the ac responseexhibited the presence of superconductivity at the surface. Increasing of theDC field from Hc2 revealed the enlarging of losses with a maximum in the feldbetween Hc2 and Hc3. Losses at the maximum were considerably larger than in themixed and in the normal states. The value of the DC field, where loss peak wasobserved, depends on the amplitude and frequency of the ac feld. Close to Tcthis peak shifts below Hc2 which showed the coexistence of surfacesuperconducting states and Abrikosov vortices. We observed a logarithmicfrequency dependence of the in-phase component of the susceptibility. Suchfrequency dispersion of the inphase component resembles the response ofspin-glass systems, but the out-of-phase component also exhibited frequencydispersion that is not a known feature of the classic spin-glass response.Analysis of the experimental data with Kramers-Kronig relations showed thepossible existence of the loss peak at very low frequencies (< 5 Hz). We foundthat the amplitude of the third harmonic was not a cubic function of the acamplitude even at considerably weak ac felds. This does not leave any room fortreating the nonlinear effects on the basis of perturbation theory. We showthat the conception of surface vortices or surface critical currents could notadequately describe the existing experimental data. Consideration of a model ofslow relaxing nonequilibrium order parameter permits one to explain the partialshielding and losses of weak ac field for H0 > Hc2.

Linear and nonlinear low frequency electrodynamics of the surface superconducting states in an yttrium hexaboride a single crystal