Low-frequency response in the surface superconducting state of single-crystal ZrB ₁₂

A large nonlinear response of a single crystal of ZrB ₁₂ to an ac field (frequency 40-2500 Hz) for H ₀ >H _c2 has been observed. Direct measurements of the ac wave form and the exact numerical solution of the Ginzburg-Landau equations, as well as the phenomenological relaxation equation, permit the study of the surface superconducting state dynamics. It is shown that the low-frequency response is defined by transitions between the metastable superconducting states under the action of an ac field. The relaxation rate that determines such transition dynamics is found.