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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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Massa, N. E., I. Rasines, G. Nieva, J. A. Campá, and C. Fainstein. "Far infrared transmission spectra of Bi2Sr2Ca1(Cu1−x,Znx)2O8+δ, 0.0≤x≤0.05, crystallites in the normal state: Detection of superconducting gap precursors." Physica C: Superconductivity 235-240, no. 2 (1994): 1065–1066.
Abstract: We report transmission spectra of crystallites, taken from single crystal specimens of two plane Bi2Sr2Ca1(Cu1−x,Znx)2O8+δ, 0.0≤x≤0.05, diluted in CsI or polyethylene matrices In addition to weaker features assigned to unscreened phonons and a continuous background probably due to carriers, we found that the ratios of room temperature spectra to those taken lowering the temperature in 50 degrees intervals signal the presence of superconducting gap behavior with 2Δ/kBTc6. We verified this by down shifting frequency as a function of Zn increment and the lowering of Tc induced by this strong superconductivity suppressor. Our preliminary search for phonons self-energy effects close to the superconducting transition temperature gave negative results.
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de la Cruz, F., D. López, and G. Nieva. "Thermally induced change in the vortex dimensionality of YBa2Cu3O7 single crystals." Philosophical Magazine Part B 70, no. 3 (1994): 773–786.
Abstract: Transport measurements in YBa2Cu3O7 single crystals using a modification of the d.c. flux transformer configuration are reported. We show that, for applied magnetic fields in the crystallographic c direction, the vortex correlation length in this direction is reduced to values smaller than the thickness of the sample. The reduction can be induced either by thermal disorder (linear response) or by flux cutting after the vortices reach a critical velocity (nonlinear response). The results show that, in the transition from a vortex-glass state to a vortex-liquid state, the vortices are effectively three dimensional in character. A two-dimensional regime is induced at higher temperatures well inside the reversibility region.
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Gao, Z. X., I. Heyvaert, B. Wuyts, E. Osquiguil, C. V. Haesendonck, and Y. Bruynseraede. "Evidence for a “brick wall” microstructure in epitaxial YBa 2Cu3Ox films." Applied Physics Letters 65, no. 6 (1994): 770–772.
Abstract: We have measured the in-plane (ab plane) critical current density J c in sputtered, c-axis oriented YBa2Cu3O x films as a function of the oxygen content x. The important reduction of Jc with decreasing x can be explained in terms of a “brick wall” microstructure, where the critical current density is dominated by the coupling strength (along the c axis) between the superconducting CuO 2 planes. Scanning tunneling microscopy images of the film surfaces are consistent with the presence of the brick wall structure, which apparently results from the overlap between terraces belonging to adjacent spiral-shaped islands.
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Lederman, D., J. Hasen, I. K. Schuller, E. Osquiguil, and Y. Bruynseraede. "Photoinduced superconductivity and structural changes in high temperature superconducting films." Applied Physics Letters 64, no. 5 (1994): 652–654.
Abstract: The illumination of PryGd1-yBa2Cu 3Ox semiconducting and superconducting thin films increases their critical temperatures and decreases their normal state resistivities if and only if the films are oxygen deficient. Moreover, these changes are enhanced near the Pr-induced metal-insulator transition. Light also causes a contraction of the c-axis in YBa2Cu3Ox which is correlated with the observed photoinduced resistivity changes. These changes are similar to those observed when oxygen-deficient YBa 2Cu3Ox is enriched with oxygen or annealed at room temperature after quenching from high temperatures.
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F. de la Cruz, E. Rodríguez, H. Pastoriza, A. Arrib Ìere, and M. F. Goffman. "Effects of reduced dimensionality on the energy dissipation of superconducting Bi$2$Sr$2$CaCu$2Ο8$ compounds." Physica B 197 (1994): 596–600.
Abstract: Strong experimental evidence of the important role of the reduced dimensionality of the vortices in the c direction in BSCCO (2 2 1 2) crystals is provided. As a result of this, it is shown that the long-range order of the vortex in the c direction is destroyed by the disorder induced by thermal fluctuations as well as by the nonuniform distribution of the pancake vortices in low-temperature ZFC experiments.
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SERENI, J. G., O. TROVARELLI, J. P. KAPPLER, C. PASCHKE, T. TRAPPMANN, and H. VONLOHNEYSEN. "LOW-TEMPERATURE MAGNETIC-BEHAVIOR OF CE7NI3." PHYSICA B 199 (1994): 567–569.
Abstract: Low-temperature specific heat (C(P)) (70 mK < T < 30 K) and AC magnetic susceptibility (chi) (70 mK < T < 3 K) measurements on Ce7Ni3 (under applied magnetic fields up to B = 6 T) are presented. The maxima in C(P) (at 1.55 K) and in chi (at 1.85 K) are related to an antiferromagnetic (AF) type transition. The experimental results are interpreted in terms of three Ce sublattices (1Ce(I), 3Ce(II) and 3Ce(III) atoms per formula unit) of the Th7Ni3-type structure. These sublattices correspond to the three different Ce local environments. According to the entropy gain the AF order is related to the Ce(I) sublattice and the Ce(II) atoms behave as heavy fermions, with gamma(LT) almost-equal-to 2.7 J/Ce(II)-at. K2 and a characteristic temperature of T(K) almost-equal-to 4 K. The Ce(III) atoms are responsible for an intermediate valence behavior with Y(HT) almost-equal-to 0.3 J/Ce(III)-at. K2 and a characteristic temperature of T0 almost-equal-to 70 K, which is confirmed by the ln T dependence of the electrical resistivity.
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Trovarelli, O., P. Stickar, J. G. Sereni, G. Schmerber, and J. P. Kappler. "Coexistence of magnetic and non magnetic states in Ce14X51 (X = Au, Ag and Cu)." Solid State Communications 89, no. 5 (1994): 421–424.
Abstract: Magnetic (chi), thermal(C-p), and transport (p) measurements on Ce(l4)X(51) (X = Au, Ag and Cu) are presented. All these compounds show antiferromagnetic type order at low temperatures, with T-N = 0.9 K (Au), 1.1 K (Ag) and 3.2 K (Cu). Within the Gd14Ag51-type structure the Ce atoms sit in different sublattices denoted by I (2 atoms), II (6 atoms) and III (6 atoms), therefore the results can be interpreted in terms of three different behaviors of Ce depending on its local environment. From the entropy involved in the magnetically ordered (MO) phase it can be established that the Ce-II and Ce-III atoms order in Cel4Au51 and Ce14Ag51, while the Ce-I order in Cel4Cu51. There is a large linear contribution to Cp at T > T-N,T-gamma HT = 0.81 (Au), 1.24 (Ag) and 3.1 (Cu) J/mol K-2, which indicates the coexistence of MO and heavy fermions. In the case of Cel4Cu51, the magnetic contribution to Cp from sublattice Ce-I around T-N,T- resembles that of a modulated system and the characteristic temperatures for Ce-II and Ce-III atoms are estimated from p(T) as:T-K=16 K and >120 K respectively.
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Wuyts, B., E. Osquiguil, M. Maenhoudt, S. Libbrecht, Z. X. Gao, and Y. Bruynseraede. "Relation between the Hall angle slope and the carrier density in oxygen-depleted YBa2Cu3Ox films." Physica C: Superconductivity and its applications 222, no. 3-4 (1994): 341–348.
Abstract: A detailed study of the Hall angle cot?H in YBa2Cu3Ox epitaxial thin films with various oxygen contents clearly shows the existence of a linear relation between the slope of cot?H versus T2 and the carrier density. The zero-temperature intercept C=cot?H(T=0 K) and the mobility at room temperature change substantially for an oxygen content x?6.6, indicating that both effects are closely related.
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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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