Safar, H., P. L. Gammel, D. A. Huse, S. N. Majumdar, L. F. Schneemeyer, D. J. Bishop, D. Lopez, G. L. Nieva, and F. de la Cruz. "Observation of a nonlocal conductivity in the mixed state of YBa2Cu3O7-d: Experimental evidence for a vortex line liquid." Physical Review Letters 72, no. 8 (1994): 1272.
Abstract: We report on transport measurements in the mixed state of YBa2Cu3O7 single crystals using the dc flux transformer geometry. In the vortex fluid regime we have observed the onset of a nonlocal conductivity. At lower temperatures, close to but above the phase transition temperature the vortices become effectively three dimensional in character. This is in sharp contrast to recent flux transformer experiments on Bi2Sr2CaCu2O which have failed to see a nonlocal conductivity, confirming the two-dimensional character at high temperatures of the vortices in that system. These observations suggest that in a finite temperature interval, the vortex state in YBa2Cu3O7 is a line liquid.
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López, D., G. Nieva, and F. de la Cruz. "In- and out-of-plane vortex correlations in YBa2Cu3O7." Physical Review B 50 (1994): 7219.
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López, D., G. Nieva, F. de la Cruz, Henrik Jeldtoft Jensen, and Dominic O’Kane. "Flux cutting in YBa2Cu3O7-δ single crystals: Experiment and phenomenological model." Physical Review B 50 (1994): 9684.
Abstract: We measured the current-induced loss of vortex correlation in YBa2Cu3O7-δ crystals using the pseudo-dc-flux-transformer geometry. We find that the current density Jcut at which the top and bottom voltage drops differ is a linear function of temperature, is independent of pinning, and depends on the thickness of the sample. This thickness dependence strongly suggests that the electrical current flows at the surface of the sample. The observed behavior with temperature, magnetic field, and thickness is described by a phenomenological model.
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López, D., H. Safar, P. L. Gammel, D. A. Huse, S. N. Majumdar, L. F. Schneemeyer, D. J. Bishop, G. Nieva, and F. de la Cruz. "Reply to the Comment: Local versus Non-Local Conductivity in YBa2Cu3O7-d." Physical Review Letters 75 (1995): 2447.
Abstract: A Reply to the Comment by Yu. Eltsev and O. Rapp.
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Lopez, D., E. F. Righi, G. Nieva, F. de la Cruz, W. K. Kwok, J. A. Fendrich, G. W. Crabtree, and L. Paulius. "Amplification of the c-axis vortex correlation by twin-boundary pinning at the vortex liquid-solid phase transition." PHYSICAL REVIEW B 53 (1996): R8895.
Abstract: Transport measurements using the dc flux transformer configuration together with c-axis resistance data in clean and twinned YBa2Cu3O7-δ single crystals show that vortices are localized by the twin-boundary potential. The results support the existence of a divergent tilt modulus [D. R. Nelson and V. M. Vinokur, Phys. Rev. B 48, 13 060 (1993)] in a liquid that freezes into a Bose glass
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López, D., E. F. Righi, G. Nieva, and F. de la Cruz. "Coincidence of Vortex-Lattice Melting and Loss of Vortex Correlation along the c Direction in Untwinned YBa2Cu3O7-δ Single Crystals." Physical Review Letters 76 (1996): 4034.
Abstract: We report on transport measurements using the dc flux transformer configuration in clean and twinned single crystals of YBa2Cu3O7-δ. We show that correlated disorder changes not only the order of the solid-liquid phase transition but also the nature of the vortices in the liquid state. In clean crystals, melting is shown to take place between a solid of vortex lines and a liquid with no correlation in the direction of the magnetic field.
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Safar, H., D. Lopez, P. L. Gammel, D. A. Huse, S. N. Majumdar, L. F. Schneemeyer, D. J. Bishop, G. Nieva, and F. de la Cruz. "Experimental evidence of a non-local resistivity in a vortex line liquid." Physica C: Superconductivity 235-240, no. 4 (1994): 2581–2582.
Abstract: We present transport measurements on YBCO single crystals in the vortex liquid regime using the DC flux transformer geometry. We show that the data are inconsistent with the material being an anisotropic, local conductor. The results provided evidence for non-local transport which indicate that there can be correlated vortex motion over macroscopic distances in this system. We have shown that this correlated vortex motion can be destroyed by either thermal fluctuations or the applications of large driving forces.
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López, D., G. Nieva, and F. de la Cruz. "Long range correlations of the vortex structure in YBCO single crystals." Physica C: Superconductivity 235-240, no. 4 (1994): 2575–2576.
Abstract: Transport measurements in the mixed state of YBa2Cu3O7 single crystals using the “DC flux transformer” configuration show that the glass-liquid phase transition at the irreversibility line, TI(H), behaves as a three dimensional vortex phase transition. These results together with resistance measurements in the c direction show that the vortex correlation across the sample is destroyed at a temperature Tth(H), higher than TI(H). The sample thickness dependence of Tth(H) suggest that in the real thermodynamic phase transition (infinite sample), the long range correlation in the ab plane as well in the c direction is lost at the single temperature TI(H).
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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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López, D., E. A. Jagla, E. F. Righi, E. Osquiguil, G. Nieva, E. Morré, F. de la Cruz, and C. A. Balseiro. "Dynamic softening of vortex lines in YBa2Cu3O7−δ single crystals." Physica C: Superconductivity 260, no. 3-4 (1996): 211–216.
Abstract: Transport measurements in the mixed state of oxygen-deficient YBa2Cu3O7−δ 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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