  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 felds, H0. We found that in the DC feld
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 feld
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 feld. 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 felds. 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.
