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Bruynseraede, Y., E. Osquiguil, M. Maenhoudt, B. Wuyts, G. Nieva, J. Guimpel, D. Lederman, and I. K. Schuller. "Enhancement of Superconductivity by Photoexcitation." The Vortex State (NATO Science Series C: Mathematical and Physical Sciences) 438 (1994): 303–312.
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de la Cruz, F., D. López, and G. Nieva. "Thermally induced change in the vortex dimensionality of YBa2Cu3O7 single crystals." Philosophical Magazine Part B 70, no. 3 (1994): 773–786.
Abstract: Transport measurements in YBa2Cu3O7 single crystals using a modification of the d.c. flux transformer configuration are reported. We show that, for applied magnetic fields in the crystallographic c direction, the vortex correlation length in this direction is reduced to values smaller than the thickness of the sample. The reduction can be induced either by thermal disorder (linear response) or by flux cutting after the vortices reach a critical velocity (nonlinear response). The results show that, in the transition from a vortex-glass state to a vortex-liquid state, the vortices are effectively three dimensional in character. A two-dimensional regime is induced at higher temperatures well inside the reversibility region.
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de la Cruz, F., H. Pastoriza, D. López, M. F. Goffman, A. Arribére, and G. Nieva. "Consequences of 3D-2D vortex decoupling on the phase diagram of HTS." Physica C: Superconductivity 235-240, no. 1 (1994): 83–86.
Abstract: We discuss the quench of the long range order in the direction parallel and perpendicular to the field, in the mixed state of BSCCO(2212) and YBCO(123) single crystals. The experimental data show that the high temperature vortex phase in both systems is an uncorrelated liquid phase in all directions.
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Decca, Ricardo. "Procesos disipativos en el superconductor granular La1.8Sr0.2Cu4-δ : contribución de las barreras y los granos." Ph.D. thesis, , 1994.
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El Azrak, A., L. A. de Vaulchier, N. Bontemps, C. Thivet, M. Guilloux-Viry, A. Perrin, B. Wuyts, and E. Osquiguil. "Optical and transport measurements of YBa2Cu3O6+x thin films (0.38?x?1)." Physica C: Superconductivity and its applications 235-240, no. PART 2 (1994): 1431–1432.
Abstract: We report optical reflectivity and transmission measurements at room temperature on a set of five YBa2Cu3O6+x films for 0.38?x?, and ?{variant}(T) resistivity data down to 10K, prepared from a single YBa2Cu3O7 thin film. In contrast with x=1, as x decreases, the scattering rate deduced from the optical conductivity departs from its linear variation with ?, in correlation with the absence of linearity of ?{variant}(T) versus temperature.
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F. de la Cruz, E. Rodríguez, H. Pastoriza, A. Arrib Ìere, and M. F. Goffman. "Effects of reduced dimensionality on the energy dissipation of superconducting Bi$2$Sr$2$CaCu$2Ο8$ compounds." Physica B 197 (1994): 596–600.
Abstract: Strong experimental evidence of the important role of the reduced dimensionality of the vortices in the c direction in BSCCO (2 2 1 2) crystals is provided. As a result of this, it is shown that the long-range order of the vortex in the c direction is destroyed by the disorder induced by thermal fluctuations as well as by the nonuniform distribution of the pancake vortices in low-temperature ZFC experiments.
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Galván, D. H., M. Avalos-Borja, M. H. Farías, L. Cota-Araiza, A. Reyes, J. Cruz-Reyes, E. A. Early, G. Nieva, M. De Andrade, and M. B. Maple. "Effect of silver in Y1Ba2Cu3O7−x samples." Journal of Materials Science 29, no. 10 (1994): 2713–2718.
Abstract: Two techniques for preparing Y-Ba-Cu-O/Ag specimens are presented: (a) solid-state reaction, and (b) normal precursors dissolved in hot concentrated nitric acid and dried in air. The samples are characterized by scanning electron microscopy (SEM), scanning Auger, X-ray diffraction, optical microscopy and electrical resistivity measurements. In comparison with a normal 1-2-3 superconductor it was found that the normalized resistance [R(T)/R(280 K)] decreases for both preparation methods. The material retained a high initial temperature of superconductivity, T c, for a maximum of 6% w/o Ag2O during solid-state reaction and 6.5% w/o Ag2O for samples dissolved in hot concentrated nitric acid and dried in air.
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Gao, Z. X., I. Heyvaert, B. Wuyts, E. Osquiguil, C. V. Haesendonck, and Y. Bruynseraede. "Evidence for a “brick wall” microstructure in epitaxial YBa 2Cu3Ox films." Applied Physics Letters 65, no. 6 (1994): 770–772.
Abstract: We have measured the in-plane (ab plane) critical current density J c in sputtered, c-axis oriented YBa2Cu3O x films as a function of the oxygen content x. The important reduction of Jc with decreasing x can be explained in terms of a “brick wall” microstructure, where the critical current density is dominated by the coupling strength (along the c axis) between the superconducting CuO 2 planes. Scanning tunneling microscopy images of the film surfaces are consistent with the presence of the brick wall structure, which apparently results from the overlap between terraces belonging to adjacent spiral-shaped islands.
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Griessen, R., Wen Hai-Hu, A. J. J. Van Dalen, B. Dam, J. Rector, H. G. Schnack, S. Libbrecht, E. Osquiguil, and Y. Bruynseraede. "Evidence for mean free path fluctuation induced pinning in YBa2Cu3O7 and YBa2Cu4O8 films." Physical Review Letters 72, no. 12 (1994): 1910–1913.
Abstract: The critical current jc and the pinning energy Uc have been determined for three types of yttrium-based superconducting films from current js and dynamic relaxation Q=d lnjs/d ln(dB/dt) data by means of the generalized inversion scheme. For B<2 T and T<80 K the temperature dependence of jc and Uc for all films is found to be in excellent agreement with a model of single vortices pinned by randomly distributed weak pinning centers via spatial fluctuations of the charge carrier mean free path. Pinning due to spatial fluctuations of Tc is not observed.
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H. Pastoriza, A. Arribére, M. F. Goffman, F. de la Cruz, D. B. Mitzi, and A. Kapitulnik. "Evidence for the quasi-two dimensional behavior of the vortex structure in Bi$2$Sr$2$CaCu$_2$O8-? single crystals." Physica B 194-196 (1994): 2227–2228.
Abstract: AC susceptibility and dc magnetization measurements on Bi$2$Sr$2$CaCu$2Ο8$ (BSCCO) single crystals in a wide range of temperatures clearly show that below the dc irreversibility line the vortex system loss the long range order in the c direction. The susceptibility data taken at 7 Hz show the different nature of two dissipation peaks: One related to the interplane currents at temperatures well below the dc irreversibility line and the other associated with the intraplane ones at temperatures above that line. In this sense the irreversibility line corresponds to the temperature where quasi-two dimensional vortices are depinned.
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