Causa, M. T., S. M. Dutrús, C. Fainstein, G. Nieva, H. R. Salva, R. D. Sánchez, L. B. Steren, M. Tovar, and Zysler R. "MAGNETIC AND STRUCTURAL PROPERTIES OF SOME ABa2Cu3O7?? SUPERCONDUCTORS." International Journal of Modern Physics B 1, no. 3-4 (1987): 989–992.
Abstract: We report here normal and superconducting properties of ABa2Cu3O7?? (with A=Y, Gd, Dy, and Er) and of Fe doped YBa2Cu3O7??. Results from X-ray powder diffraction, electrical resistivity, magnetic susceptibility, ESR, and specific heat measurements are presented, leading to a characterization of the magnetic properties of these materials. The effect of structural modifications of the lattice on the superconducting properties and the relative insensitivity of Tc to the presence of magnetic moments is discussed.
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Civale, L., and F. de la Cruz. "Anomalous behavior of superconducting samples with a fixed number of vortices." Physical Review B 36, no. 7 (1987): 3560–3564.
Abstract: We report an anomalous reversible temperature dependence of the magnetization of amorphous type-II superconductors. The experimental results were obtained studying the vortex pinning in samples with very low vortex concentration. The anomaly is shown to be a consequence of the lack of flux quantization in finite samples.
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De La Cruz, F., J. Luzuriaga, E. N. Martinez, and E. J. Osquiguil. "Current-voltage characteristics in collective pinning." Physical Review B 36, no. 13 (1987): 6850–6852.
Abstract: The critical-current curves of amorphous samples which exhibit collective flux pinning have been analyzed, and it was found that the nonlinear part can be fitted by an expression of the form V(I)[(I-IK)/IK]. This form, which could be due to critical behavior, is consistent with an interpretation of the unpinning of vortices in the collective regime as a phase transition between the static flux-line lattice (FLL) and the FLL in steady-state movement as has been proposed by Fisher.
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Durán, C., P. Esquinazi, J. Luzuriaga, and E. H. Brandt. "Bc1 of high-Tc La1.8Sr0.2CuO4 and amorphous Zr70Cu30 superconductors measured by a vibrating reed." Physics Letters A 123, no. 9 (1987): 485–488.
Abstract: The lower critical field Bc1 of the high-Tc superconductor La1.8Sr0.2CuO4 (Tc=40 K, Bcless-than-or-equals, slant7.7 mT) and of amorphous Zr70Cu30 (Tc=2.6K, Bc1less-than-or-equals, slant3.8 mT) is obtained from the resonance frequency ?/2? of a vibrating reed made of these materials. At applied magnetic fields Ba<Bc1, ?(Ba)=?i(Ba) as predicted for an ideally diamagnetic reed. Above Bc1 we observe a reversible linear law ?2??2inot, vert, similarB2a?B2c1. This allows a simple definition of Bc1 even in ceramic superconductors into which flux starts to penetrate at Bamuch less-thanBc1 due to their granular structure.
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Esparza, D. A., C. A. D'Ovidio, J. Guimpel, E. Osquiguil, L. Civale, and F. de la Cruz. "The granular nature of bulk superconductivity at 40K in La1.8Sr0.2CuO4." Solid State Communications 63, no. 2 (1987): 137–140.
Abstract: We have measured the electrical resistivity, magnetic flux expulsion and critical fields of the new high temperature superconductor La1.8Sr0.2CuO4 after different heat treatments. Our data show that the complete flux expulsion in these samples is consistent with the presence of shielding currents induced in multiple connected structures. The magnetization measurements indicate that the lowest field where flux penetrates into the sample should not be taken to be the bulk Hcl. It is shown that the absolute value of the electrical resistivity and its temperature dependence are not correlated with the superconducting critical field and temperature. We discuss the behaviour of the superconducting parameters and their relation with the electrical resistivity.
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Esquinazi, P., J. Luzuriaga, C. Durán, D. A. Esparza, and C. D'Ovidio. "Anomalies in the internal friction and sound velocity in the high-temperature superconductor La1.8Sr0.2CuO4." Physical Review B 36, no. 4 (1987): 2316–2318.
Abstract: The internal friction Q-1 and the Young's-modulus sound velocity vE have been measured in La1.8Sr0.2CuO4 using the vibrating-reed technique. Measurements were performed at a frequency of 700 Hz at temperatures between 0.2 and 100 K. A drastic change of slope in the internal friction versus temperature curve is observed at T=44 K just above the superconducting transition temperature Tc=40 K. A plateau in Q-1 is observed between 44 and 5 K. The sound velocity shows a decrease with decreasing temperature between 100 and 20 K and remains almost constant between 2 and 0.2 K.
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Guimpel, J., F. de la Cruz, J. Murduck, and I. K. Schuller. "Penetration depth of a superconducting superlattice." Physical Review B 35, no. 7 (1987): 3655–3656.
Abstract: We have measured the magnetic-flux expulsion of a superconducting superlattice and related it to the Meissner penetration depth. The temperature dependence of the penetration depth is in quantitative agreement with theoretical expectation based on mean-field theory. The zero-temperature penetration depth is found in excellent, quantitative agreement without adjustable parameters with theoretical predictions for dirty superconductors.
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Guimpel, Julio. "Efectos de tamaño en superconductores inhomogeneos." Ph.D. thesis, , 1987.
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Luzuriaga, J. "Pinning forces and defects as measured by critical currents in amorphous superconducting materials." Physical Review B 35, no. 7 (1987): 3625–3628.
Abstract: Analysis of the available data on critical currents in amorphous superconductors shows a striking similarity in the defects responsible for the pinning force in all samples in which two-dimensional flux pinning can be observed. Order-of-magnitude calculations show that this is consistent with the existence of disclinations in glasses as has been proposed in different theoretical papers by Rivier, Nelson, Kleman, and Sadoc. The evolution of pinning forces with annealing also shows a similar behavior in all the published data, and the trend is towards a state with fewer or smaller disclinations. If disclinations are identified with two-level systems this trend is consistent with a decrease of the scattering of phonons by TLS which is observed upon annealing in thermal conductivity measurements.
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Núñez Regueiro, M., D. Castello, M. A. Izbizky, D. Esparza, and C. D'Ovidio. "Anomalous thermal conductivity of La2-xSrxCuO4: Possibility of a polaronic glass." Physical Review B 36, no. 16 (1987): 8813–8815.
Abstract: e present measurements of the thermal conductivity of La2CuO4 and La1.85Sr0.15CuO4. The results on the pure sample show that lattice conduction is predominant with a T1.8 dependence at low temperatures and a strong T-independent scattering above 30 K. We observe a similar behavior for the superconductor but with a T1.4 law at low temperature and a small electronic contribution in the normal state. These results together with the behavior of the specific heat and sound attenuation are consistent with the existence of a two-level tunneling system for an amorphous state.
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