Laurent Mieville

Artificially prepared YBa2Cu3O7 PrBa2Cu3O7 superlattices: Growth and superconducting properties

Abstract We report on the growth and superconducting properties of artificially prepared YBa 2 Cu 3 O 7 PrBa 2 Cu 3 O 7 ( YBCO PrBCO ) superlattices. We have grown these superlattices by single target dc planar magnetron sputtering using the high sputtering pressure technique to obtain the in situ growth with the correct stoichiometry. We find in these modulated structures that the superconducting critical temperature, Tc, depends markedly on the individual YBCO thickness in the multilayer and on the insulating PrBCO separation thickness. For series of samples with constant 12A or 24A layers of YBCO (one or two unit cells), Tc initially decreases with increasing thickness of PrBCO, d-PrBCO, and then saturates for d-PrBCO larger than 60–70A. We discuss this behavior in terms of a progressive decoupling of the ultrathin YBCO layers in the multilayer. Resistively measured parallel critical fields show, for structures with thick PrBCO layers, a transition from a flux flow regime for thick YBCO layers to a behavior with only very weak magnetic field induced broadening of the resistive transitions for thin YBCO layers. For multilayers with individual YBCO layers of 48A or 96A thickness we find a linear behavior of the resistively determined critical field with a slope of ≅7.5T/K. When the individual YBCO layer thickness is reduced to 24A practically no effect of a 9T magnetic field could be observed. These different regimes are discussed in terms of the particular vortex structures in the multilayers.

Epitaxial high Tc cuprate superlattices: A study of the thermal flux motion

Abstract Epitaxial c-axis YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 (YBCO/PrBCO) superlattices grown by single target dc planar magnetron sputtering are used as a model system to investigate the nature of flux motion in high T c superconductors (HTS). When a magnetic field H is applied parallel to the c-axis we find that, for sufficiently thick PrBCO layers, the activation energy for flux motion U is proportional to the YBCO thickness. This allows us to extract a lower limit, about 500A, for the correlation lenght along the vortices in pure YBCO. We find also for thin YBCO layers that U∼lnH which could be the signature that the dissipation process is related to the creation in the 2-D vortex lattice of dislocations pairs. When the field is applied in the a-b plane we find for thin YBCO layers separated by thick enough PrBCO layers that the resistive transition becomes field independent. Measurements on a 24A/96A YBCO/PrBCO multilayer up to 20T do not show any broadening of the resistive transition. We also find that the activation energy for flux motion decreases with decreasing the YBCO thickness and is insensitive to magnetic field below the H c1 of the individual YBCO layers.

Artificially prepared superlattices: Growth and superconducting properties

Abstract We report on the growth and superconducting properties of artificially prepared YBa 2 Cu 3 O 7 PrBa 2 Cu 3 O 7 ( YBCO PrBCO ) superlattices. We have grown these superlattices by single target dc planar magnetron sputtering using the high sputtering pressure technique to obtain the in situ growth with the correct stoichiometry. We find in these modulated structures that the superconducting critical temperature, Tc, depends markedly on the individual YBCO thickness in the multilayer and on the insulating PrBCO separation thickness. For series of samples with constant 12A or 24A layers of YBCO (one or two unit cells), Tc initially decreases with increasing thickness of PrBCO, d-PrBCO, and then saturates for d-PrBCO larger than 60–70A. We discuss this behavior in terms of a progressive decoupling of the ultrathin YBCO layers in the multilayer. Resistively measured parallel critical fields show, for structures with thick PrBCO layers, a transition from a flux flow regime for thick YBCO layers to a behavior with only very weak magnetic field induced broadening of the resistive transitions for thin YBCO layers. For multilayers with individual YBCO layers of 48A or 96A thickness we find a linear behavior of the resistively determined critical field with a slope of ≅7.5T/K. When the individual YBCO layer thickness is reduced to 24A practically no effect of a 9T magnetic field could be observed. These different regimes are discussed in terms of the particular vortex structures in the multilayers.

Anisotropic magnetotransport in high temperature superconductor multilayers

The high temperature superconducting cuprates are layered materials in which the CuO2 layers are found to play an important role for the superconducting properties. When produced in thin film form, these materials can be grown in a layer by layer sequence and this has allowed us to make superlattices of various combinations of these compounds. In particular, in the YBa2Cu3O7/PrBa2Cu3O7 system it is possible to produce superlattices where the individual layers are as thin as one c-axis unit cell. This allows us to modify the anisotropy of these materials in a controlled manner. In this contribution we present a study of the resistive transition in a magnetic field perpendicular or parallel to the layers, and how the results can be used to gain insight into the role of the anisotropy for the dissipative behaviour of these high temperature superconductors.

Superconductivity in artificially grown copper-oxide superlattices

Abstract The high-temperature superconductors are layered materials whose properties are related to the stacking sequence of the various metal-oxide layers. This paper describes recent efforts to modify artificially this stacking sequence and thus to produce new superlattice structures. In particular, we have grown YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 superlattices with modulation wave lengths as small as 24 A. We review here recent experiments on this system and we demonstrate how these superlattices can be used to study the role of anisotropy in the thermally activated flux creep behaviour of the high-temperature superconductors.

Layer by layer preparation of (Y1 − xPrx)Ba2Cu3O7 alloy thin films

Abstract We report on a new way to prepare highly homogeneous high Tc alloy thin films. We use dc magnetron sputtering with two stoichiometric targets (YBa2Cu3O7 (YBCO) and PrBa2Cu3O7 (PrBCO)) to prepare (Y1 − xPrx)BCO alloys by sequentially depositing ultrathin layers (less than a unit cell) of both materials. With this technique we fill each rare earth (R) plane with the desired amount of Y and Pr. Using MgO substrates we get highly oriented alloys with the c-axis parameter perpendicular to the substrate. The superconducting transition can be adjusted between 90K and OK with transition widths as narrow as 2K. Resistively measured critical fields Hcr display a behavior rather similar to the one found in YBCO, but indicative of an activation energy for flux flow smaller than in YBCO. We find also in the alloy series that Hcr is proportional to ( 1−T T c ) α with an exponent different from the 3 2 observed in YBCO.

Growth and properties of SrCuO2 and (Sr1-xNdx)CuO2 epitaxial thin films

Thin films of the infinite layer compound (Sr1−x Ndx)CuO2 have been grown on (100) SrTiO3 substrates by RF reactive magnetron sputtering. Four-circle x-ray diffractometry reveals that (Sr,Nd)CuO2 thin films can be epitaxially grown with (001) orientation. Four-point transport measurements show resistance drops in some as-grown films of Sr1−xNdxCuO2 with x=0.11, 0.12 at temperatures ranging from 27 K to 35 K.

Investigation of superlattices and ultrathin layers made from HTS cuprates

The fabrication of epitaxially grown superlattices and ultrathin films made from high temperature superconductors has allowed a systematic study of certain properties of this class of materials. It is, for instance, possible to modify the anisotropy in a controlled manner and thus investigate the role of the reduced dimensionality of the high temperature superconductors on the superconducting properties. In particular, we have investigated the thermally activated flux flow as observed in the resistive tails of the superconducting transition. We find in YBa2Cu3O7/PrBa2Cu3O7 (YBCO/PrBCO) superlattices that the activation energy varies proportional to the thickness of the individual YBCO layers when the PrBCO thickness is large enough to decouple the superconducting layers (d-PrBCO > 36 A). We thus find that the pancake vortices in the individual CuO2 layers in YBCO are strongly coupled and that the flux line lattice is 2D in the thin individual YBCO layers up to layer thicknesses of more than 200 A. An analysis of the zero field resistive transition in terms of Coulomb gas scaling confirms this result.

Effect of a magnetic field on the resistive transitions of artificial YBa2Cu3O7PrBa2Cu3O7 superlattices

Abstract We report on a study of the behavior of the resistive transitions of a series of YBa 2 Cu 3 O 7 PrBa 2 Cu 3 O 7 multilayers in a magnetic field parallel to the ab-plane. The multilayers studied here were built with constant 144A thickness of the PrBCO layers and increasing thickness of the superconducting YBCO layers (from 48A to 262A). The detailed behavior of these modulated structures in parallel magnetic field, which is presented in this work, suggests a progressive transition from a flux flow regime for thick individual YBCO layers, to a regime where apparently no fluxlines are present in the thin YBCO layers and the flux is centered in the normal PrBCO layers. In this case the resistively measured critical field Hcr show an almost linear temperature dependence with a slope of −7.5T/K.

Investigation of thermally activated flux flow of YBa2Cu3O7/ PrBa2Cu3O7 superlattices in magnetic fields parallel to the a,b plane

Abstract YBa 2 Cu 3 O 7 is a strongly anisotropic 3D material. Our observations of thermally activated flux flow in magnetic fields perpendicular to the a,b plane show that the vortices are 3D-like in this material. Making multilayers of the type YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 (YBCO/PrBCO) allows to increase the anisotropy and turn the material into a 2D system. We explore this 3D to 2D transition by investigating the thermally activated flux flow behavior in parallel applied fields. We find the parallel field dependence to be markedly different from the perpendicular dependence. At low fields and in multilayers with relatively thick individual layers we find that the activation is largely field independent. At higher fields we observe a crossover to a power law behaviour similar to what is observed in thick YBCO films. In multilayers with 24A YBCO layers decoupled by thick PrBCO layers (>> 24A) we find no broadening of the resistive transition up to 20 Tesla.