dc and ac magnetic properties of two thin-walled superconducting Nb cylinders with a rectangular cross-section are reported. Magnetization curves and the ac response were studied on as-prepared and patterned samples in magnetic fields parallel to the
cylinder axis. A row of micron-sized antidots (holes) was made in the film along the cylinder axis. Avalanche-like jumps of the magnetization are observed for both samples at low temperatures for magnetic fields not only above $H_{c1}$, but in fields lower than $H_{c1}$ in the vortex-free region. The positions of the jumps are not reproducible and they change from one experiment to another, resembling vortex lattice instabilities usually observed for magnetic fields larger than $H_{c1}$. At temperatures above $0.66T_c$ and $0.78T_c$ the magnetization curves become smooth for the patterned and the as-prepared samples, respectively. The magnetization curve of a reference planar Nb film in the parallel field geometry does not exhibit jumps in the entire range of accessible temperatures. The ac response was measured in constant and swept dc magnetic field modes. Experiment shows that ac losses at low magnetic fields in a swept field mode are smaller for the patterned sample. For both samples the shapes of the field dependences of losses and the amplitude of the third harmonic are the same in constant and swept field near $H_{c3}$. This similarity does not exist at low fields in a swept mode.
We report results of experimental studies of ac susceptibility of Nb single crystal at low frequencies in swept magnetic fields applied parallel to the surface. Analysis of the experimental data shows that the swept magnetic field significantly chang
es the vortex conductivity in the bulk. It becomes dissipative with unexpected large frequency dispersion. At the surface, one observes a layer with enhanced conductivity in comparison to the bulk. This layer provides a considerably large contribution to the shielding and absorption of an ac field even in dc fields below $H_{c2}$. We have also demonstrated that the swept magnetic field apparently affects an ac response of the surface superconducting states.
We present the results of an experimental study of the nucleation of superconductivity at the surface of a single crystal YB$_6$ in a tilted dc magnetic field. A recently developed experimental technique allowed us to determine $H_{c3}$ at each side
of the sample as a function of the angle between the dc magnetic field and the surface. Experiment shows that the ratio $H_{c3}/ H_{c2}approx 1.28 $ in the direction perpendicular to the surface dc field while according to the theory this ratio should be equal to 1. This sharp distinction cannot be ascribed to the surface roughness.
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.
