D. Arias

Ferromagnetic/superconducting proximity effect in La0.7Ca0.3MnO3/YBa2Cu3O7-δ superlattices

We study the interplay between magnetism and superconductivity in high-quality YBa_(2)Cu_(3)O_(7)(YBCO)/La_(0.7)Ca_(0.3)MnO_(3)(LCMO) superlattices. We find evidence for the YBCO superconductivity depression in the presence of the LCMO layers. We show that due to its short coherence length, superconductivity survives in the YBCO down to a much smaller thickness in the presence of the magnetic layer than in low Tc superconductors. We also find that for a fixed thickness of the superconducting layer, superconductivity is depressed over a thickness interval of the magnetic layer in the 100 nm range. This is a much longer length scale than that predicted by the theory of ferromagnetic/superconducting proximity effect.

Superconductivity depression in ultrathin YBa2Cu3O7−δ layers in La0.7Ca0.3MnO3/YBa2Cu3O7−δ superlattices

We report on the depression of the superconducting critical temperature of ultrathin YBa2Cu3O7 (YBCO) layers, when their thickness is reduced in the presence of La0.7Ca0.3MnO3 (LCMO) magnetic layers in [LCMO (15 unit cells)/YBCO (N unit cells)] superlattices. The thickness of the manganite layer is kept at 15 unit cells and the YBCO thickness is varied between N=12 and N=1 unit cells. The structural analysis, using x-ray diffraction and electron microscopy, shows sharp interfaces with little structural disorder. While a critical temperature, TC=85 K, is found for 12 YBCO unit cells, superconductivity is completely suppressed for YBCO layer thickness below 3 unit cells. The possible interaction between superconductivity and magnetism is investigated.

Magnetism and superconductivity in La0.7Ca0.3MnO3/YBa2Cu3O7−δ superlattices

We report on the magnetic and superconducting properties of La0.7Ca0.3MnO3/YBa2Cu3O7 (LCMO/YBCO) superlattices. For a constant LCMO layer thickness of 6 unit cells (u.c.), resistance and susceptibility measurements show superconductivity for YBCO layer thickness in excess of 4 unit cells. The critical temperature increases with YBCO thickness, and a Tc of 58 K is found for a YBCO thickness of 10 unit cells. Magnetization measurements show a ferromagnetic transition at 100 K in a (LCMO6 u.c./YBCO5 u.c.)15 bilayer superlattice, and a depressed value of the saturation magnetization of 20 emu cm−3. These results are discussed in terms of interface disorder (analyzed by x-ray diffraction and transmission electron microscopy) and of the possible interaction between magnetism and superconductivity.

Effects of epitaxial strain on the growth mechanism in YBa 2 Cu 3 O 7 − x thin films in YBa 2 Cu 3 O 7 − x / PrBa 2 Cu 3 O 7 superlattices

We report on the growth mechanism of YBa_(2)Cu_(3)O_(7-x). Our study is based on an analysis of ultrathin YBa_(2)Cu_(3)O_(7-x) (YBCO) layers in c-axis-oriented YBa_(2)Cu_(3)O_(7-x)/PrBa_(2)Cu_(3)O_(7)superlattices. We have found that the release of epitaxial strain in very thin YBCO layers triggers a change in the dimensionality of the growth mode. Ultrathin epitaxially strained YBCO layers with thickness below 3 unit cells grow in a blockby-block two -dimensional mode that is coherent over large lateral distances. Meanwhile, when the thickness increases and the strain relaxes, layer growth turns three-dimensional, resulting in rougher layers and interfaces.

Critical temperature depression and persistent photoconductivity in ion irradiated YBa2Cu3O7-x films and YBa2Cu3O7-x/PrBa2Cu3O7 superlattices

We have studied the effect of He+ irradiation with doses in the range 1012–2×1015 cm−2 on two high-temperature superconducting structures: YBa2Cu3O7−x (YBCO) films and YBa2Cu3O7−x/PrBa2Cu3O7 (YBCO/PBCO) superlattices. In particular we have focused on superlattices [YBCON/PBCOM]1000 A with N=1, 8 unit cells of YBCO, and M=5 unit cells of PBCO, with a total thickness of 1000 A. The analysis is presented in terms of depression of the critical temperature (Tc) and modification of the crystalline structure using X ray refinement technique. Single films show a systematic increase in the c-lattice parameter upon irradiation, which is not observed in thin one unit cell YBCO layers in superlattice structures. However, Tc depression resulting from irradiation is deeper in superlattices. These results are explained considering the strained nature of the as-grown [YBCON/PBCO5]1000 A superlattices with low values of N. Both structures show persistent photoconductivity, indicating that defects are related to oxygen dis...

Direct correlation between T c and CuO 2 bilayer spacing in YBa 2 Cu 3 O 7 − x

We report the effects of epitaxial strain and deoxygenation on high quality [YBa2Cu3O7-x (YBCO)_N / PrBa2Cu3O7 (PBCO)_5] (1000 A thick) superlattices, with 1 <= N <= 12 unit cells. High spatial resolution electron energy loss spectroscopy (EELS) shows that strained, fully oxygenated YBCO layers are underdoped. Irrespective of whether underdoping is induced by strain or deoxygenation, X-ray diffraction analysis shows that Tc correlates directly with the separation of the CuO2 bilayers.

Chain oxygen disorder in deoxygenated YBa2Cu3O7-δ thin films induced by light ion irradiation

Abstract We report on the effects of light ion (He + ) irradiation on the superconducting properties of epitaxial YBa 2 Cu 3 O 7− δ thin films. Films were grown by a pure oxygen dc sputtering system operated at high pressure (3.4 mbar). Irradiation at 80 keV and doses ranging between 10 14 and 10 15 cm −2 shows a systematic degradation of the superconducting transition temperature. We have studied the effects of annealing on the recovery of the superconducting properties of films irradiated at different fluences. Two sets of irradiated samples were evaluated, fully oxidized and deoxygenated samples. Irradiated fresh samples do not show significant recovery upon room temperature annealing for times as long as 6 months. Irradiated deoxygenated samples, however, show an up to 10 K increase in the critical temperature after 2 months room temperature annealing. The different behavior is discussed in terms of the two effects caused by irradiation: oxygen displacements from the CuO chains into stable interstitial positions along the a -axis, and oxygen disorder in the deoxygenated samples.

Effect of anisotropy on the vortex liquid dissipation in YBa2Cu3O7−δ thin films

Abstract Transport measurements have been used to investigate the vortex dynamics in the mixed state on high quality deoxygenated YBa 2 Cu 3 O 7− δ thin films, in magnetic fields up to 8 T. Epitaxial YBa 2 Cu 3 O 7− δ thin films, grown by high pressure dc sputtering, were deoxygenated in situ following a stability line of the pressure–temperature phase diagram. We show that varying the oxygen content, the dynamics of the vortex liquid phase reveal a crossover from a densely entangled liquid to a decoupled state. At small oxygen deficiencies the activation energy for vortex motion is found to depend as 1/ H on the applied magnetic field, suggesting a densely entangled liquid. Strongly deoxygenated samples however, show a crossover from an entangled liquid, with a 1/ H 0.5 dependence of the activation energy in magnetic fields up to 2 T, to a decoupled state, characterized by a logarithmic dependence for higher fields.

High-resolution and energy-filtered transmission electron microscopy of YBa2Cu3O7−x/PrBa2Cu3O7 superlattices

Abstract Structural disorder induced by epitaxial strain relaxation in c -axis oriented [YBa 2 Cu 3 O 7− x n u.c /PrBa 2 Cu 3 O 7 5 u.c. ] superlattices on (100) SrTiO 3 substrates has been analyzed by high-resolution and energy-filtered transmission electron microscopy. Epitaxial strain shows up in ultrathin YBCO layers with thickness below 4 unit cells. Strain relaxation takes place when layer thickness increases above this value. Microscopy observations for epitaxially strained superlattices show a good morphology of the layers, with sharp and flat interfaces. However, in relaxed samples rougher interfaces are observed, together with microdomains showing the c -axis parallel to the substrate plane which preserve the superlattice compositional profile. Such microdomains of c -parallel growth provide a path for reduced lattice mismatch, conforming a very efficient mechanism for strain relaxation.

Effect of light ion irradiation on the flux dynamics of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films

Thin films of 123 cuprates (YBa/sub 2/Cu/sub 3/O/sub 7/) grown by DC sputtering are irradiated with light ion (He/sup +/) at 80 keV and with doses in the range 10/sup 14/-10/sup 15/ cm/sup -2/. The point disorder defects, due to light ion irradiation, cause a systematic decrease of the critical temperature. The effect of light ion irradiation on the vortex phase behavior is explored by critical currents from transport and magnetization loops (solid vortex phase) and resistivity (liquid state) in perpendicular fields. Although point disorder does not significantly affect dissipation in the solid state, the dissipation in the liquid is governed by plastic deformation of vortices favored by point disorder pinning.