Glasslike low-frequency ac response of ZrB12 and Nb single crystals in the surface superconducting state

We report experimental studies of the low-frequency electrodynamics of ZrB12 and Nb single crystals. The ac susceptibility at frequencies 3-1000 Hz has 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 ultralow 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.