Libbrecht, S., E. Osquiguil, and Y. Bruynseraede. "Influence of field inhomogeneity on the magnetization of YBCO films." Physica C: Superconductivity and its applications 225, no. 3-4 (1994): 337–345.
Abstract: DC magnetization data of superconducting thin YBCO films, measured by means of a magnetometer in which the sample is pulled through a set of pick-up coils, are often calculated from pick-up signals which clearly deviate from the ideal response. We show how these signal distortions influence the values obtained for the magnetization, and present a simple model that enables one to simulate the temperature dependence of these distorted pick-up signals, in the case of low-field magnetization measurements. The model is based on the magnetic field inhomogeneity of the magnetometer, in combination with the hysteretic magnetization of the superconducting sample. Optimum values for the model parameters are derived and discussed.
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Luzuriaga, J., M. - O. Andre, and W. Benoit. "Frequency and amplitude response of the flux-line lattice to mechanical perturbation in ceramic YBa2Cu3O7." Physica C: Superconductivity and its applications 201, no. 3-4 (1992): 257–262.
Abstract: The mechanical response of the flux-line lattice has been measured with a low-frequency forced pendulum in ceramic YBa2Cu3O7. A dissipation peak observed in temperature sweeps is frequency-independent between 1 mHz and 5 Hz. Dissipation depends strongly on applied torque, and for fixed temperatures this dependence is well fitted by a rheological model of extended dry friction. If the model is extended to take account of thermal activation, however, it does not agree with the measured frequency independence, which is hard to explain within simple models of thermal activation.
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D'Anna, G., and Andr. "Flux-line response in 2H-NbSe2 investigated by means of the vibrating superconductor method." Physica C: Superconductivity and its applications 218, no. 1-2 (1993): 238–244.
Abstract: We measure transverse AC losses in the low- and high-amplitude regime of 2H-NbSe2 single crystals using vibrating superconductor methods. The measurements are sensitive to small deviations of the critical state. The data constitute evidence for a peak effect of the critical current as a function of the temperature in this compound. We construct in the H-T phase diagram the “peak-effect” line which is supposed to mark an abrupt cross-over in the vortex-pinning regime.
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L. "Dynamic softening of vortex lines in YBa2Cu3O7-? single crystals." Physica C: Superconductivity and its Applications 260, no. 3-4 (1996): 211–216.
Abstract: Transport measurements in the mixed state of oxygen-deficient YBa2Cu3O7-? single crystals using the flux transformer configuration show that the flux liquid changes with increasing anisotropy from strongly correlated to uncorrelated in the field direction. For intermediate coupling, the current inducing loss of vortex correlation has a maximum near the irreversibility temperature. Thus, an effective softening of vortex lines with decreasing temperature is detected. We propose a simple model that accounts for this behavior by including the effects of the pinning potential on the dynamics of vortices.
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Herbsommer, J. A., J. Luzuriaga, and S. - W. Cheong. "Vortex glass melting in single crystal La1.825 Sr0.075CuO4." Physica C 258, no. 1-2 (1996): 169–174.
Abstract: The vortex phase diagram in single crystalline La1.85Sr0.075CuO4 has been studied using an AC-susceptibility technique. A peak in the out-of-phase (x?) component of the susceptibility indicates a transition from a pinned flux lattice (FLL) to an unpinned one. This peak is frequency dependent for all the values of the magnetic field measured (0.01 to 4 T), and this, as well as the general behavior found in the cuprates, has prompted us to interpret our data as evidence for a vortex-glass to liquid transition in the FLL. The activation energies obtained can be fitted to a theory developed by Vinokur et al. Measurements with the magnetic field at an angle with the Cu-O planes may also be understood qualitatively within this framework.
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Pedrazzini, P., D. Jaccard, G. Lapertot, J. Flouquet, Y. Inada, H. Kohara, and Y. Onuki. "Probing the extended non-Fermi liquid regimes of MnSi and Fe." Physica B – Condensed Matter 378-380 (2006): 165–166.
Abstract: Recent studies show that the non-Fermi liquid (NFL) behavior of MnSi and Fe spans over an unexpectedly broad pressure range, between the critical pressure p and around 2p. In order to determine the extension of their NFL regions, we analyze the evolution of the resistivity ρ(T)˜A(p)T at higher pressures. We find that in MnSi the n=32 exponent holds below 4.8GPa≈3p, but it increases above that pressure. At 7.2 GPa we observe the low-temperature Fermi liquid exponent n=2 whereas for T>1.5K, n=53. Our measurements in Fe show that the NFL behavior ρ˜T extends at least up to 30.5 GPa, above the entire superconducting (SC) region. In the studied pressure range, the onset of the SC transition reduces by a factor 10 down to Tconset(30.5GPa)=0.23K, while the A—coefficient diminishes monotonically by around 50%.
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Vanacken, J., E. Osquiguil, and Y. Bruynseraede. "Irreversibility line and critical currents in oxygen deficient YBa2Cu3Ox ceramics." Physica C: Superconductivity and its applications 183, no. 1-3 (1991): 163–166.
Abstract: The influence of oxygen concentration on the irreversibility line and critical current of polycrystalline YBa2Cu3Ox was determined using field and zero-field cooled magnetic measurements and magnetic hysteresis data. Although the field dependence of Tirr(H) is unchanged, the irreversibility line shifts to lower temperatures for decreasing oxygen content and the critical current density is strongly reduced.
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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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Watt. "Photoinduced changes in Raman spectra of YBa2Cu3O6.4 films." Journal of Superconductivity 7, no. 1 (1994): 131–136.
Abstract: The evolution of Raman spectra with illumination has been studied in YBa2Cu3O6.4 films at temperatures between 5-300 K. Low laser power has always been used to avoid local overheating, which was controlled by measuring the local temperature by the Stokes/anti-Stokes ratio. Three important photoinduced effects have been found: (i) the enhancement of the intensity of the observed phonon modes: (Cu(2) at 141 cm-1, O(2)-O(3) at 338 cm-1, and O(4) at 488 cm-1), which may be related to the ordering of oxygen vacancies, (ii) the increase of the electronic scattering background for low Raman frequencies ?, which is in agreement with the enhancement of the static conductivity ?(??0) after illumination, and (iii) the suppression of the intensity of the two-magnon band, which may be caused by the increase of charge carriers due to photodoping.
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Lederman, D., E. Osquiguil, G. Nieva, J. Guimpel, J. Hasen, Y. Bruynseraede, and I. K. Schuller. "Photoinduced enhancement of superconductivity." Journal of Superconductivity 7, no. 1 (1994): 127–130.
Abstract: The photoinduced enhancement of superconductivity in RBa2Cu3Ox (R=rare earth or yttrium) and PryR1-yBa2Cu3Ox was explored through temperature-dependent resistivity, Hall coefficient and mobility, and x-ray diffraction measurements. The increases in Tc are enhanced near the metal-insulator transition, although photoinduced changes always exist in oxygendeficient samples. Several explanations, including intergrain Josephson coupling, photoassisted oxygen ordering, and the trapping of photogenerated electrons in oxygen vacancies, are discussed.
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