Glass-like low frequency ac response of ZrB$_{12}$ and Nb single   crystals in the surface superconducting state
  We report experimental studies of the low frequency electrodynamics of ZrB$_{12}$ and Nb single crystals. AC susceptibility at frequencies 3 - 1000 Hz have been measured under a dc magnetic field, $H_0$, applied parallel to the sample surface. In the surface superconducting state, for several $H_0$, 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. 
Menachem I. Tsindlekht, Grigory I. Leviev, Valery M. Genkin, Israel   Felner, Yurii B. Paderno and Vladimir B. Filippov