Lourdes Fabrega

Epitaxial growth and properties of (CO3) stabilised BaCuO2 superconducting thin films

We report on the epitaxial growth of BaCuO2 infinite layer (stabilised with CO3 groups) thin films by rf magnetron sputtering on (100) SrTiO3 and (100) LaAlO3. Excellent (001) films with c∼4Å are obtained at 540 °C in a sputtering atmosphere of 1–10 mTorr of Ar+O2 with some addition of CO2. Off axis x-ray analysis reveal perfect in plane ordering and a tetragonal structure with a=b∼3.96 Å. For these deposition conditions the resistivity at room temperature is of the order of several Ωcm and exhibits a strong semiconductor-like behaviour. When the deposition temperature is increased to 660 °C, a supercell along the c axis appears with a period of about 8Å. After an in-situ annealing of these films in 7 Torr of oxygen at 500 °C, ϱ(300K) falls to 800μΩcm. The resistance versus temperature curve reveals then a metallic behaviour and a superconducting transitions with onsets between 10–40K.

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.

Epitaxial growth of “infinite layer” thin films and multilayers by rf magnetron sputtering

We report on the preparation and characterization of epitaxial ACuO2 (A = Sr, Ca, Ba) thin films and multilayers with the so- called infinite layer (IL) structure, by rf magnetron sputtering. Films and multilayers without Ba have a remarkable crystal quality, whereas those containing this large ion are often multiphased and unstable. In spite of the excellent crystalline quality of these samples, obtaining thin films having both IL structure and displaying superconducting properties has not succeeded; our pure IL samples display semiconducting behavior, and the different procedures tried in order to dope them—annealings, introduction of disorder or cation vacancies, artificial layering—have failed. These results support that the pure IL structure ACuO2 (A = alkaline earth) cannot superconduct.

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.

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.}

Vortex system in coupled DyBa2Cu3O7−δ/(Y0.45Pr0.55)Ba2Cu3O7−δ heterostructures

Transport measurement have been used to determine the vortex phase diagram (H‖c) of coupled heterostructures made of superconducting ultra-thin DyBa2Cu3O7−δ layers separated by semiconducting (Y0.45Pr0.55)Ba2Cu3O7−δ layers. These structures have an intermediate anisotropy between those of YBa2Cu3O7−δ and Bi2Sr2CaCu2O8+d. The solid-to-liquid transition and the flux dynamics have been investigated as a function of the total sample thickness d. Analyses of the parameters describing the thermally activated dissipation in the solid and liquid phases reveal a field induced crossover characterized by two fields, Hon(T) and H*>Hon. We discuss the meaning of these features and their relationship with results in bulk high temperature superconductors.

Direct determination of the vortex jump attempt frequency in high temperature superconductors

We show that the frequency dependence of the effective critical current, resulting from thermally activated flux motion, may be used to evaluates the jump attempt frequency fo of over vortices over their pinning potential wells. We apply this analysis to estimate fo for a Nd1.85Ce0.15CuO4−y single crystal. The obtained value, fo≈109Hz, is in agreement with the theoretical estimates; furthermore, this high value, together with the experimental activation energies U(T,H,J), allow to explain the important thermally activated flux motion effects in these superconductors, observed at low temperatures or high frequencies.

Critical current properties of DyBa2Cu3O7/(Y1−xPrx)Ba2Cu3O7 multilayers

Transport critical currents of DyBa2Cu3O7/(Y1−xPrx)Ba2Cu3O7 (DyBCO/YPrBCO) multilayers have been measured. Using careful meausurements of the temperature and magnetic field dependence of critical ...