Vortex dynamics is studied in coupled multilayer structures, containing N 24-A-thick DyBa&Cu307 layers, each separated from the next by 96 A of (Y& "Pr")Ba2Cu307 (x =0. 4, 0.55). When the magnetic field is parallel to the c axis, we find that the activation energy U for 6ux motion increases linearly with the number of superconducting layers N in the structure for N & 3. This linear increase is the result of the coupled motion of pancake vortices belonging to de'erent DyBa2Cu307 layers. For larger N samples the activation energy saturates meaning that the vortex lattice is turning three dimensional. In contrast to samples whose vortex lattice is purely two dimensional and for which we find the activation energy for flux motion proportional to log, oE, samples in these series show a crossover to a power-law behavior, U=8, at a magnetic field H which decreases as N increases. I. IN. rRODUc:rxONThe H-T phase diagram of high-T, superconductors is one of the most fascinating and discussed aspects of these materials. Questions related to, for instance, thermal fiuctuations, the irreversibility line, or the existence of a vortex glass and/or a melting transition are still under debate. ' For several of these problems the dimensionality of the vortex lattice is a crucial parameter and predictions concerning the low-temperature phase of the vortex lattice have been made depending on its dimensionality. High-T, superlattices may be a useful tool to address some of these questions. The main advantage of these arti6cial structures is to provide a way to control and modify the vortex lattice parameters and, in particular, its dimensionality, allowing to track the effects of such changes on specific physical properties. Recently, s s we studied extensively the flux dynamics in YBaxCu307/PrBa2Cu307 (YBCO/PrBCO) superlattices.In these superlattices the thickness of "insulating" PrBCO, 96 A, is sufficien to essentially decouple the superconducting layers (as will be discussed below, magnetic coupling is irrelevant to this study}. The main result was that the activation energy for Aux motion U, in the regime of linear I-V characteristics, is proportional to the YBCO thickness in the multilayers for d-YBCO & 300 A, and has a logarithmic field dependence, for H parallel to the c axis. The scaling of the activation energy with the YBCO thickness refiects the absence of interaction between pancake vortices of difFerent superconducting layers. The correlation length L, involved in this activated motion of vortices is found to be about 400 A implying a two-dimensional (2D) vortex lattice in YBCO layers thinner than 400 A. The logarithmic field dependence of the activation energy found in YBCO/PrBCO multilayers as well as in thin YBCO layers has its origin in the 2D nature of the vortex lattice as discussed below. Notice here that the correlation length L"as de6ned and measured in this work is not identical to the Larkin-Ovchinnikov collective pinning correlation length, although certainly related to it. Ongoing measurements of creep at l...

We report on the observation of a change in the field dependence of the critical current density J,. of DyBa&Cu&07/(Yp4&Prp&5)Ba2Cu307 superlattices at a characteristic magnetic field 0, " This. change shows up as an abrupt transition when analyzing the current-voltage (I V) characteristics in the framework of the vortex glass and collective Aux creep theories. This behavior can be traced back to a sudden reduction in the collective pinning energy U,. above 0, *, and may reflect a softening transition of the vortex lines in the vortex solid. H, *. is very similar to the characteristic field H&" in the vortex liquid phase, which we interpreted to be due to entanglement.

From dynamic relaxation measurements of the decay of supercurrents in YBa 2 Cu 3 O 7Ϫ␦ films with thicknesses between 1.2 and 150 nm we determine the correlation length L c ͑0͒ ͑the length of the tunneling vortex segment at Tϭ0 K͒ to be 2.2 nm and show that quantum creep occurs in a transition regime where Hall tunneling is as important as dissipative tunneling.Soon after the discovery of high-T c superconductivity Yeshurun and Malozemoff 1 reported on the existence of giant flux creep in high-temperature superconductors. Giant flux creep which has been found in all layered superconductors arises from the thermally activated motion of vortices from one metastable configuration to a neighboring one. The probability for such a hopping process is proportional to exp͓ϪU( j,T,B e )/kT͔, where U( j,T,B e ) is the activation energy which depends on the current j, external field B e ϭ 0 H e and temperature T. At low temperature U( j,T,B e )/kT diverges and the hopping probability vanishes. However, several experiments demonstrated that substantial relaxation was still present in the millikelvin regime. One of the most convincing proofs of the existence of this quantum creep was given by Fruchter et al.2 who found a temperature independent relaxation rate below 1 K in an YBa 2 Cu 3 O 7Ϫ␦ single crystal. Evidence for the existence of quantum creep has also been found in many other investigations on high-T c superconductors, 3-7 heavy fermions, 3 organic superconductors, 4 and Chevrel phases. 8In contrast to thermally activated flux motion which depends essentially on the height of the activation energy barrier, quantum creep depends on the time spent under the energy barrier during the tunneling of a vortex segment of length L c .9 Quantum creep is, therefore, inherently related to the dynamics of a vortex segment, which according to Kopnin et al. 10,11 is described by the following equation of motion:where v ជ v is the vortex velocity, ⌽ 0 the flux quantum, j s the current density, and ẑ a unit vector parallel to the vortex. The first term on the left-hand side of Eq. ͑1͒ represents the viscous drag and the second term is the Hall contribution. Until recently quantum creep experiments have always been interpreted by assuming that dissipative effects were dominant, i.e., ӷ␣. However, Feigel'man et al. 12 proposed recently that high-T c superconductors might be in the superclean limit and that quantum creep is essentially determined by the Hall term in Eq. ͑1͒. Their proposition was based on an estimate of n ͑0͒ by using a linear extrapolation of n (TуT c ) data which leads to a low-temperature resistivity n ͑0͒Ϸ10 ⍀ cm and a mean free path lϷ70 nm, which is indeed much larger than E F /⌬, where is the coherence length, E F the Fermi energy, and ⌬ the superconducting gap.The purpose of this paper is to show that at low temperatures the high-T c superconductor YBa 2 Cu 3 O 7 is ͑i͒ neither in the purely dissipative regime nor in the superclean limit, but in an intermediate regime where both Hall and dissipative terms contr...

We report on the behavior of high T superconductors coupled multilayer structures in magnetic field parallel and perpendicular to the ab plane. These structures consist of N 24A thick DyBa2Cu3O7 layers, each separated from the next by 96A of an (Y045Pr055)Ba2Cu3O7 alloy. This alloy allows a coupling of the superconducting layers as demonstrated by measurements of the activation energies for flux motion U. For a N=15 sample one finds, in perpendicular fields, that a correlated motion of pancake vortices occurs with a characteristic length of about 480A. Additionally, U is found to be proportional to logB at low fields while at higher fields a power law behavior is observed. In parallel field we find U to be field independent up to iT, suggesting that the vortices are trapped at low fields in the material where weak superconductivity is induced by proximity effect.

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