Dynamic regimes in the ac response of YBa2Cu3O7 with columnar defects: Intra- and inter-valley vortex motion

We use ac susceptibility measurements to explore the dynamic response of vo rtices in YBCO single crystals pinned by aligned columnar defects at low de fields. Using a combination of techniques for the analysis of the data, we investigate the vortex motion as a function of the amplitude and frequency of the applied ac field h(ac), identifying the influence of both intra- an d inter-valley motion. We build up a dynamic diagram in the h(ac)-T plane i ndicating the crossover lines among several regimes in the solid phase. At low h(ac), a linear response with very low dissipation arises from the osci llation of pinned vortices inside the tracks (Campbell regime). At high h(a c) a critical state develops. The large influence of thermal fluctuations i n this limit is indicated by the frequency dependence, which allows us to o btain the current density dependence of the activation energy and the glass y exponent. We also discuss the origin of the large nonlinear transition re gime observed at intermediate amplitudes. [S0163-1829(99)02613-2].