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H. Pastoriza, M. F. Goffman, A. Arribére, and F. de la Cruz. "Phase diagram of the mixed state of Bi$2$Sr$2$CaCu$_2$O8-? single crystals: first order phase transition at the irreversibility line." Physica C 235-240 (1994): 2579–2580.
Abstract: Magnetization and susceptibility measurements show a first order phase transition in the magnetic flux structure at the irreversibility line of Bi$2$Sr$2$CaCu$2Ο8$ single crystals, in the field range $0 < H < 360$ Oe. The resistivity in the c direction drops six orders of magnitude in less that 0.1 K, and there is a discontinuous change in the magnetization at the same temperature. The change in magnetization together with the Clausius-Clapeyron equation gives a variation of entropy of 0.06kB per vortex layer, at the transition. Above 360 Oe the previous $H-T$ phase diagram is determined by the laminar nature of superconductivity and the irreversibility line is frequency dependent.
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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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Pastoriza, H., H. Safar, E. F. Righi, and F. de la Cruz. "Flux creep in YBa$2$Cu$3Ο7$ single crystals near $TC$." Physica B 194-196 (1994): 2237–2238.
Abstract: We present magnetic flux relaxation measurements which show that the flux creep behavior in the field and temperature range in which the lower critical field $Hc1$ is quenched to zero differs from that of the mixed state region above the mean field previous $Hc1(T)$ line. This is taken as evidence for the presence of magnetic excitations that cause the Meissner state to break down below the mean field $T_C$ at previous $H=0$.
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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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López, D., E. Rodríguez, G. Nieva, F. de la Cruz, and S. W. Cheong. "Three-dimensional vortex flux flow in LSCO and YBCO through pseudo dc flux transformer measurements." Physica B: Condensed Matter 194-196, no. 2 (1994): 1977–1978.
Abstract: A pseudo dc flux transformer configuration shows that the flux flow in La2−xSrxCuO4 (x=0.075) and YBa2Cu3O7 single crystals is due to the movement of 3-D vortices across the sample. We show that the loss of c-axis long range vortex correlation is achieved either by thermal fluctuations or by an inhomogeneus Lorentz force acting on the vortices.
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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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Martin, J. I., B. Wuyts, M. Maenhoudt, E. Osquiguil, J. L. Vicent, V. V. Moshchalkov, and Y. Bruynseraede. "Sign reversal of the flux flow Hall effect in oxygen deficient YBa2Cu3Ox films." Physica C: Superconductivity and its applications 235-240, no. PART 2 (1994): 1451–1452.
Abstract: The Hall effect of high temperature superconductors has been reported to slow in some cases a striking sign reversal near Tc, the origin of which is not yet fully understood. We report on measurements of the Hall effect in the mixed state of oxygen deficient YBa2Cu3Ox thin films, for 7?
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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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Wuyts, B., M. Maenhoudt, S. Libbrecht, Z. X. Gao, E. Osquiguil, V. V. Moshchalkov, and Y. Bruynseraede. "Linear relation between the Hall angle slope and the carrier density in YBa2Cu3Ox films." Physica C: Superconductivity and its applications 235-240, no. PART 2 (1994): 1369–1370.
Abstract: A universal relation cot?H??{variant}/RHB = ?T2 + C for the Hall angle ?H has been experimentally observed served in a wide variety of high-Tc systems. In contrast to the linear temperature dependence of the resistivity ?{variant} or the inverse Hall coefficient R-1H, the T2-dependence of cot?H remains valid in under- and overdoped systems. We present a systematic study of cot?H(T) in oxygen deficient epitaxial YBa2Cu3Ox thin films. We find evidence for a linear relation between ?, the slope of cot?H versus T2, and the carrier concentration. Moreover, we find a steep increase in the intercept C = cot?H (T = 0) below x ? 6.6.
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Pastoriza, H., M. F. Goffman, and F. de la Cruz. "Comment on ``Irreversibility line of monocrystaline {Bi$2$Sr$2$CaCu$2$O$8$}: Experimental evidence for a dimensional crossover of the vortex ensemble''." Phys. Rev. Lett. 72, no. 8 (1994): 1303.
Abstract: A Comment on the Letter by A. Schilling et al., Phys. Rev. Lett. \bf 71, 899 (1993).
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