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Nieva, Gladys L. "Efectos de presión química por inclusión de intersticiales en CePd3 : valencia intermedia, efecto Kondo y orden magnético." Ph.D. thesis, , 1988.
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Causa, M. T., C. Fainstein, G. Nieva, R. Sánchez, L. B. Steren, M. Tovar, R. Zysler, D. C. Vier, S. Schultz, S. B. Oseroff et al. "Crystal Field Interaction and Magnetic Order in GdBa2Cu3O7." In Progress in High Temperature Superconductivity, edited by J. L. Heiras. Vol. 5. N.J.: World Scientific, 1988.
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Núñez Regueiro, M., P. Esquinazi, M. A. Izbizky, C. Durán, D. Castello, and J. Luzuriaga. "Tunneling systems in high temperature superconductors." Annales de Physique 13, no. 5 (1988): 401–406.
Abstract: We have measured the thermal conductivity, sound attenuation and relative variation of sound velocity of compounds of the La-Sr-Cu-O and RBa2Cu3O7 families, where R is a rare-earth. We have found that all these properties can be quantitatively correlated within a tunneling system model similar to that used in amorphous materials. This interpretation would naturally explain the anomalous linear term of the low temperature specific heat observed in these materials.
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Esquinazi, P., and J. Luzuriaga. "Annealing effects on the sound velocity and internal friction in the superconducting and normal states of the Cu30Zr70 amorphous alloy." Physical Review B 37, no. 13 (1988): 7819–7831.
Abstract: By means of the vibrating-reed technique we have measured the evolution upon low-temperature annealing of the internal friction Q-1 and sound velocity v of the Cu30Zr70 amorphous alloy between 0.1 and 10 K. With an applied magnetic field of 5.6 T we measured also the normal-state behavior of both properties. Upon annealing, the internal friction and the slope of the logarithmic temperature dependence of v in the superconducting state decrease, in agreement with the standard tunneling model. Our results indicate that with thermal treatments, the coupling constant between tunneling systems (TSs) and phonons decreases, and the density of states of the TS remains constant or decreases, at most 25%, in the fully relaxed state, which is in agreement with published specific-heat results in these materials. From our data we conclude that the coupling constant between the TS and phonons should be very sensitive to the relaxed state of the disordered structure. The sound-velocity behavior strongly indicates that relaxation processes up to 10 K involve the interaction with the TS. A well-defined change of slope in v at the superconducting critical temperature Tc is observed in the as-quenched and the first relaxed state, which cannot be explained with the standard tunneling model and the Korringa-like relaxation rate between the TS and electrons. Several features observed in the superconducting and normal states lead to the conclusion that the TS-electron interaction problem remains unsolved.
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Esquinazi, P., J. Luzuriaga, and C. Durán. "Vibrating-reed studies of flux pinning in the superconducting metallic glass Zr70Cu30." Physical Review B 37, no. 7 (1988): 3689–3692.
Abstract: With the vibrating-reed technique we have measured the resonance frequency and damping of superconducting ribbons of a Zr70Cu30 amorphous alloy as a function of magnetic field and temperature for different heat treatments of the same sample. Through these measurements we obtain the Labusch parameter in the reduced-field (b) range 5
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Esquinazi, P., C. Durán, C. Fainstein, and M. Núñez Regueiro. "Evidence of low-energy tunneling excitations in the high-Tc superconductor YBa2Cu3O7-x." Physical Review B 37, no. 1 (1988): 545–547.
Abstract: We have measured the internal friction Q-1 and the Young's-modulus sound velocity vE at 1 kHz for YBa2Cu3O7-x. Below 20 K the relative change in the sound velocity and the temperature dependence of the internal friction perfectly resemble the behavior obtained for Suprasil W. Below 1 K we observe a logarithmic temperature dependence for the sound velocity and between 1 and 5 K a well-defined plateau in the internal friction is obtained. We find quantitative agreement with the predictions of the standard tunneling model which is widely used to explain the low-temperature properties of amorphous materials. The implications of the existence of tunneling systems in these materials are discussed.
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Osquiguil, E. J., L. Civale, R. Decca, and F. De La Cruz. "Metallic to variable-range-hopping transition controlled by oxygen content in La-Sr-Cu-O." Physical Review B 38, no. 4 (1988): 2840–2842.
Abstract: We have performed electrical resistivity measurements on the superconductor La1.8Sr0.2CuO4- over a wide range of temperatures and oxygen concentration. The results provide experimental evidence for a transition from a metallic behavior at high temperatures to a variable-range hopping regime at low temperatures. The transition is controlled by the oxygen content. Correlation between both regimes is also shown.
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Causa, M. T., C. Fainstein, G. Nieva, R. S\'anchez, L. B. Steren, M. Tovar, R. Zysler, D. C. Vier, S. Schultz, S. B. Oseroff et al. "Crystal-field interaction in the GdxEu1-xBa2Cu3O7-? superconductors." Physical Review B 38 (1988): 257–261.
Abstract: We report here experimental data on the crystal-field interaction of gadolinium ions with their environment in GdxEu1-xBa2Cu3O7-?. The ESR spectrum and a Schottky anomaly in the specific heat of dilute samples (x?1) indicate the existence of a crystal-field splitting of the 8S7/2 ground state of Gd3+ ions of about 1.5 K. We discuss the parametrization of the measured splitting in terms of an effective Hamiltonian, and the microscopic origin of the interaction within the framework of a superposition model. Since the single-ion energies involved are of the same order of magnitude as the energies associated with the magnetic ordering of the Gd3+ moments in GdBa2Cu3O7-? (TN?2.24 K), we analyze the effects of the crystal-field interaction on the magnetic transition.
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Guimpel, J., L. Civale, F. de la Cruz, J. M. Murduck, and I. K. Schuller. "Dimensional phase transition in superconductors with short coherence length." Physical Review B 38, no. 4 (1988): 2342–2344.
Abstract: We found a novel dimensional transition in the vortex lattice of a finite-sized superconductor. The low-field dependence (with field applied parallel to the film plane) of the magnetization exhibits two maxima, which signal the transition from one to two dimensions. In one dimension the vortices line up along the film, and with increasing field their arrangement changes into a two-dimensional array due to the competition between vortex-surface and vortex-vortex repulsion. Agreement is found with no adjustable parameter calculations using a priori known surface-barrier and vortex-repulsion effects.
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Izbizky, M. A., M. Núñez Regueiro, P. Esquinazi, and C. Fainstein. "Thermal conductivity and superconductivity in EuBa2Cu3O7-x." Physical Review B 38, no. 13 (1988): 9220–9223.
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