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Surface-Barrier Effects in the Microwave Second-Harmonic Response of Superconductors in the Mixed State

We report on transient effects in the microwave second-order response of different type of superconductors in the mixed state. The samples have contemporarily been exposed to a dc magnetic field, varying with a constant rate of 60 Oe/s, and a pulsed microwave magnetic field. The time evolution of the signal radiated at the second-harmonic frequency of the driving field has been measured for about 500 s from the instant in which the dc-field sweep has been stopped, with sampling time of about 0.3 s. We show that the second-harmonic signal exhibits two relaxation regimes; an initial exponential decay, which endures roughly 10 s, and a logarithmic decay in the time scale of minutes. Evidence is given that the decay in the time scale of minutes is ruled by magnetic relaxation over the surface barrier.