Patrice Fivat

Vortex motion in coupled DyBa2Cu3O7/(Y0.6Pr0.4)Ba2Cu3O7 heterostructures

Multilayer structures containing 24 Å thick DyBa2Cu3O7 layers, separated by 96 Å of an (Y0.6Pr0.4)Ba2Cu3O7 alloy, are studied to investigate the effect of coupling on vortex dynamics. With the magnetic field perpendicular to theab plane, and as a function of the number of superconducting layers in the structure, we find that the activation energy for flux motion increases, first linearly, and then saturates. This linear increase is taken as evidence that pancake vortices belonging to different DyBa2Cu3O7 layers are stacked and have a coupled motion. Above a characteristic number of superconducting layers,Nc, shear of the vortex structures becomes important and the thermally activated process only displaces a stack ofNc pancake vortices, meaning that the vortex lattice is turning three dimensional. In these structures we findNc to be 2 to 3.

Possible entanglement of the vortices in DyBa2Cu3O7-(Y1−xPrx)Ba2Cu3O7 coupled hetero-structures

Thermally activated flux motion is studied in proximity coupled multilayers, containing a number N of 24A thick DyBa2Cu3O7 layers separated by 96A (or more) of (Y1−xPrx)Ba2Cu3O7 alloy (x=0.4, x=0.55). In contrast to ultra-thin decoupled layers, for which we find that the activation energy for flux motion U≈logB (Hc), coupled samples show a cross-over to a power law behaviour U≈B−0.5 at a magnetic field H∗. This characteristic field depends on the anisotropy and decreases as N increases, a behaviour which can be understood in terms of the entanglement of the vortices, resulting in a relative increase of U due to the flux line crossing energy.

Superconducting properties of ultrathin Y(Dy)Ba2Cu3O7 layers in Y(Dy)Ba2Cu3O7-PrBa2Cu3O7 heterostructures

Abstract We have measured the superconducting properties of ultra thin layers of YBa2Cu3O7 in c-axis YBa2Cu3O7-PrBa2Cu3O7 (YBCO-PrBCO) heterostructures. These structures contain one or a few layers of YBCO (or in some cases DyBCO). The critical temperature of single layers was investigated through a series of PrBCO-YBCO-PrBCO trilayers. The trilayers consist of d A of YBCO sandwiched between a buffer and a capping PrBCO layer. The YBCO thicknesses were not chosen to be multiples of a unit cell ( ∼12 A ), but equal to n ∗ 1 3 of a unit cell. The series was built with nominal YBCO thicknesses of ∼4 A , ∼8 A , ∼12 A , ⇒ ∼40 A . Superconductivity is observed in all the samples, except in the 4A one which displays a pronounced semiconducting behavior. We discuss these results and the differences between the properties of single and multilayer structures.

Vortex solid-to-liquid transition in DyBa2Cu3O7- delta /(Y0.45Pr0.55)Ba2Cu3O7- delta multilayers.

Current-voltage ({ital I}-{ital V}) characteristics of a coupled multilayer system consisting of thin (24 A) superconducting layers of DyBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}, each separated by 96 A of (Y{sub 0.45}Pr{sub 0.55})Ba{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} have been studied in the mixed state. Analyses of vortex glass scaling and signs of the clean system melting transition in the dynamics of our multilayers both suggest that the vortex system is close to three-dimensional for magnetic fields up to several T. Therefore, the theoretically predicted crossover when in-plane vortex correlations become more important than out-of-plane vortex correlations (occurring at a few tenths of a T in our multilayers) does not correspond to a complete decoupling of the superconducting layers in these materials. {copyright} {ital 1996 The American Physical Society.}

Vortex dynamics and vortex phase diagram in artificial oxide superlattices

Abstract DyBa 2 Cu 3 O 7 /(Y 1 − x Pr x )Ba 2 Cu 3 O 7 multilayers are model materials to study the nature of the vortex system in high-temperature superconductors and its dynamics. Indeed, by changing in such structures the thickness of the superconducting and the alloy layers, and the composition of the alloy, we can control at wish the critical temperature, the anisotropy and the pinning strength. Additionally, finite thickness effects may be studied. We review here our results on the study of the flux motion and phase diagram in coupled multilayers, displaying an intermediate anisotropy between those of YBa 2 Cu 3 O 7 and Bi 2 Sr 2 CaCu 2 O 8 . Transport measurements with an applied field H | c allow to identify the solid and liquid phases in the mixed state, and reveal the presence of a field-induced crossover in the thermally activated dynamics of both phases, which lead us to draw a possible mixed state phase diagram for these artificial superconductors and correlate it to the existing results for other HTS.

Long range coupling in YBa{sub 2}Cu{sub 3}O{sub 7-{delta}}/(Y{sub 1-x}Pr{sub x})Ba{sub 2}Cu{sub 3}O{sub 7-{delta}} multilayers

YBa2Cu3O7−δ/(Y1−xPrx)Ba2Cu3O7−δ multilayers have been used to probe coupling through (Y1−x:Prx)Ba2Cu3O7−δ alloys. We observe that the coupling between ultrathin YBa2Cu3O7−δ layers, 12 or 24 Å thick, survives through several hundred Å of (Y1−xPrx)Ba2Cu3O7−δ with x=0.4 and 0.55. Tc versus the thickness of the spacer-alloy, and activation energies for flux motion, with fields parallel and perpendicular to the c-axis, have been used to probe this long range coupling. All these experiments point to an unusually large coupling length for these two alloy compositions. In the x=0.55 case this result is particularly surprising since the alloy material display a semiconducting behaviour for this composition. Tc measurements, activation energies, and a study of the vortex dynamics in these coupled multilayers is presented along with new results obtained on a series of multilayers built with a more insulating alloy, x=0.7.

Investigations of coupled DyBa2Cu307-(Y1-xPrx)Ba2Cu307 multilayer structures

We report on the behavior of high Tc superconductor coupled multilayer structures in magnetic field perpendicular to the ab plane. In these structures the number N of superconducting layers in the sample has been systematically varied. We extract the activation energy for flux motion from resistive measurements of the superconducting transitions. For samples containing N 24 angstroms thick DyBa2Cu3O7 layers, each separated from the next by 96 angstroms of an (formula available in paper) alloy, we find that the activation energy U increases linearly with the number N of layers for N < 3 - 4, and then saturates. The linear increase of U is taken as an evidence that coupling through (formula available in paper) increases the correlated volume involved in the flux motion and thus U. For N equals 3 stacked pancake vortices move rigidly and the vortex lattice is 2D. For N > 3 - 4 shear of the vortex lattice becomes important, the vortex lattice dimensionally crosses-over to 3D, and the activation energy saturates.

Flux motion in DyBa2Cu3O7/(Y1−xPrx)Ba2Cu3O7 hetero-structures

We investigate the thermally activated flux motion in structures made of DyBa2Cu3O7 layers separated by an alloy of (Y1−xPrx)Ba2Cu3O7. With x=0.4 we have grown a series of samples containing N layers of DyBa2Cu3O7 (24A thick, N between 1 and 10) separated by 96A of (Y0.6Pr0.4)Ba2Cu3O7. For H applied parallel to the c-axis, the activation energy for flux motion U, extracted from the lnϱ versus 1/T curves, increases linearly with N for N<3 and then progressively saturates, demonstrating the coupling of the superconducting layers through the alloy.

Vortex solid-to-liquid transition inDyBa2Cu3O7−δ/(Y0.45Pr0.55)Ba2Cu3O7−δmultilayers

Current-voltage ({ital I}-{ital V}) characteristics of a coupled multilayer system consisting of thin (24 A) superconducting layers of DyBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}, each separated by 96 A of (Y{sub 0.45}Pr{sub 0.55})Ba{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} have been studied in the mixed state. Analyses of vortex glass scaling and signs of the clean system melting transition in the dynamics of our multilayers both suggest that the vortex system is close to three-dimensional for magnetic fields up to several T. Therefore, the theoretically predicted crossover when in-plane vortex correlations become more important than out-of-plane vortex correlations (occurring at a few tenths of a T in our multilayers) does not correspond to a complete decoupling of the superconducting layers in these materials. {copyright} {ital 1996 The American Physical Society.}

Vortex solid-to-liquid transition in DyBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}/(Y{sub 0.45}Pr{sub 0.55})Ba{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} multilayers

Current-voltage ({ital I}-{ital V}) characteristics of a coupled multilayer system consisting of thin (24 A) superconducting layers of DyBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}, each separated by 96 A of (Y{sub 0.45}Pr{sub 0.55})Ba{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} have been studied in the mixed state. Analyses of vortex glass scaling and signs of the clean system melting transition in the dynamics of our multilayers both suggest that the vortex system is close to three-dimensional for magnetic fields up to several T. Therefore, the theoretically predicted crossover when in-plane vortex correlations become more important than out-of-plane vortex correlations (occurring at a few tenths of a T in our multilayers) does not correspond to a complete decoupling of the superconducting layers in these materials. {copyright} {ital 1996 The American Physical Society.}