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Jaime, M., R. Movshovich, G. R. Stewart, W. P. Beyermann, M. G. Berisso, M. F. Hundley, P. C. Canfield, and J. L. Sarrao. "Closing the spin gap in the Kondo insulator Ce3Bi4Pt3 at high magnetic fields." Nature 405, no. 6783 (2000): 160–163.
Abstract: Kondo insulator materials(1)-such as CeRhAs, CeRhSb, YbB12, Ce3Bi4Pt3 and SmB6-are 3d, 4f and 5f intermetallic compounds that have attracted considerable interest in recent years(2-5). At high temperatures, they behave like metals. But as temperature is reduced, an energy gap opens in the conduction band at the Fermi energy and the materials become insulating. This contrasts with other f-electron compounds, which are metallic at all temperatures. The formation of the gap in Kondo insulators has been proposed to be a consequence of hybridization between the conduction band and the f-electron levels(6,7), giving a 'spin' gap. If this is indeed the case, metallic behaviour should be recovered when the gap is closed by changing external parameters, such as magnetic field or pressure. Some experimental evidence suggests that the gap can be closed in SmB6 (refs 5, 8) and YbB12 (ref. 9). Here we present specific-heat measurements of Ce3Bi4Pt3 in d.c. and pulsed magnetic fields up to 60 tesla. Numerical results and the analysis of our data using the Coqblin-Schrieffer model demonstrate unambiguously a field-induced insulator-to-metal transition.
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Sirena, M., L. Steren, and J. Guimpel. "Thickness dependence of the properties of La0.6Sr0.4MnO3 thin films." Thin Solid Films 373, no. 1-2 (2000): 102–106.
Abstract: We have studied the thickness dependence of the transport and magnetic properties of strongly textured La0.6Sr0.4MnO3 thin films grown on MgO and SrTiO3, for thickness between 5 and 500 nm. While the resistivity and the magnetoresistance are strongly affected, the magnetization is only moderately changed. The effect on the metal-insulator transition, the low field magnetoresistance and carrier localization is dependent on the substrate, evidencing the effect of the induced defects and strains. (C) 2000 Elsevier Science S.A. All rights reserved.
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Silhanek, K., J. Guimpel, L. Civale, H. Lanza, and P. Levy. "Thickness effects on the pinning by columnar defects." PHYSICA C 341 (2000): 1217–1218.
Abstract: We compare the angular dependence of the pinning of equivalent columnar defects in GdBa2Cu2O7 thin films and a thicker YBa2Cu3O7 single crystal. The tracks' related enhancement of the critical currents is much smaller for the case of the thin films. We interpret this thickness dependence as resulting from a competition between the pinning of the correlated structures, related to total sample thickness, and surface boundary conditions, which force the vortices to depin from the columnar defects near the sample surface.
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Herbsommer, J. A., G. Nieva, and J. Luzuriaga. "Reproducibility of the disordered vortex structure in twinned single crystals of YBCO." Physica C 341 (2000): 1087–1088.
Abstract: Bitter decorations of the vortices in twinned single crystal YBCO have been obtained to study the disordered structure formed on successive cooldowns of the sample under the same conditions of field and temperature. Between succesive decorations, the iron dots could be removed completely, giving a surface clean enough for further decoration. It was found that successive realizations of the disordered vortex state were very similar at long range, and differed mostly on short scales, compared to the average vortex separation. We have quantified the correlation between successive structures, and double sided decorations have been also performed to compare with the extent of spatial correlation in the applied field direction. The coincidence of the position of the vortices in successive realizations of the structure, is unexpected, and indicates an unusual behavior for a glassy system. Our technique makes it possible to image the position of the vortices in detail and repeatedly, providing us with a model amorphous system in which to study the reproducibility of the disordered structure.
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Correa, V. F., J. A. Herbsommer, E. E. Kaul, F. de la Cruz, and G. Nieva. "Onset of the 0-dimensional vortex pinning behavior in the optimally doped and overdoped Bi2Sr2CaCu2O8+delta system." PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 341 (2000): 1285–1286.
Abstract: We report results of transverse AC magnetic permeability in the linear regime and DC magnetization loops for optimally doped and oxygen overdoped samples of Bi2Sr2CaCu2O8+delta The data cover the low temperature (T < 25K) and low held (H-a < 1000 Oe) region of the magnetic (H-a-T) phase diagram. In optimally doped as well as in overdoped samples a crossover to a O-dimensional pinning regime (Larkin length, L-C(C), shorter than the inter-plane CuO2 distance) is observed at a characteristic temperature Too for all investigated fields. The order-disorder transition, identified by the second peak in magnetization and a non monotonous field dependence of the AC penetration depth, is quenched below T-0D.
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Kaul, E. E., and G. Nieva. "Oxygen doping effects on the magnetization of Bi2Sr2CaCu2O8+delta single crystalline system in the mixed state." PHYSICA C 341 (2000): 1343–1344.
Abstract: We present a systematic study of the oxygen overdoping effects on the de magnetization loops of Bi2Sr2CaCu2O8+delta single crystalline samples. The overdoping is produced by high oxygen pressure (from 1 to 190 bar) annealing at 500 degreesC. The super conducting critical temperature and the c-axis lattice parameter are used to identify the oxygen content of the samples. The dc magnetization loop measurements are performed with the applied magnetic field parallel to the c-axis of the crystals. The characteristic second peak in the magnetization is present far all the investigated oxygen contents. We show that the position of this second peak and the irreversibility field increase monotonically as a function of oxygen doping.
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Roa Rojas, J., A. R. Jurelo, R. Menegotto Costa, L. Mendonca Ferreira, P. Pureur, M. T. D. Orlando, P. Prieto, and G. Nieva. "Fluctuation conductivity and the dynamical universality class of the superconducting transition in the high-Tc cuprates." Physica C: Superconductivity 341-348, no. 3 (2000): 1911–1912.
Abstract: Systematic measurements of the fluctuation conductivity in several high-temperature superconducting cuprates show the occurrence of a genuine critical regime. The conductivity exponent is consistent with the predictions of the 3D-XY universality class and yields a dynamical critical exponent z = 1.5.
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André, M. - O., M. Polichetti, H. Pastoriza, and P. H. Kes. "Vortex dynamics in 2H-NbSe$_2$ containing a periodic channel structure." Physica C 338 (2000): 179–188.
Abstract: We have measured the resistivity and magnetic AC susceptibility of 2H-NbSe single crystals containing alternating 2 stripes of irradiated and non-irradiated regions of columnar defects created by heavy-ion bombardment. Without applied magnetic field, the sample undergoes a double-step transition into the superconducting state, each step corresponding to the transitions in the irradiated and non-irradiated regions, respectively. For fields smaller than half of the matching field and upon increasing the temperature, the onset of flux motion in the non-irradiated channels occurs, when the applied stress due to the electrical current equals the shear stress at the channel edges, while depinning in the irradiated stripes occurs at higher temperature. The weak amplitude dependence of the shear process suggests that it takes place at the melting transition. We observed only a single-step transition at DC magnetic fields larger than half the matching field, because pinning by the columnar defects in the irradiated stripes is much less effective, and consequently the shear stress at the channel edges is strongly reduced. The comparison between the shear stress deduced from I curves and the theoretical value $?max = AC66$ yields a value for the constant Α, which is in good agreement with theoretical predictions.
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Pedrazzini, P., G. Schmerber, M. G. Berisso, J. P. Kappler, and J. G. Sereni. "Magnetic and superconducting properties of A(7)B(3) compounds (A = Th or La and B=Ni, Co, Fe or Pd, Rh, Ru)." PHYSICA C 336, no. 1-2 (2000): 10–18.
Abstract: High-temperature magnetization, low temperature specific heat, and ac-susceptibility measurements on Th7X3 (X = Ni, Co, and Fe) and La(7)Z(3) (Z = Pd, Rh, and Ru) superconductors are presented. The magnetic behaviour of the Th7X3 group has the characteristics of Pauli paramagnets. The La(7)Z(3) group also shows Pauli paramagnetism for Z = Pd and Ru, and signs of spin fluctuations for La7Rh3. Specific heat (C-p) measurements indicate a conventional BCS behaviour for La7Pd3 and La7Rh3, like that of the previously measured Th7X3 superconductors. in the case of La7Ru3, the reduced specific heat jump at the transition, the anomalous temperature dependence of C-p under magnetic field and a large upper critical field slope are discussed. (C) 2000 Elsevier Science B.V. All rights reserved.
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Pastoriza, H., S. Candia, and G. Nieva. "Anisotropic vortex dynamics in twined YBaCuO single crystals." Physica B 284-288, no. 2 (2000): 763–764.
Abstract: Rotating continuously the applied vortex force and measuring the vortex velocity vector, we were able to directly determine the role of twin boundaries in the vortex dynamics in YBa$2$Cu$3Ο_7$ single crystals with unidirectional twins. We show the different effect of these correlated pinning defects into the various vortex phases. In the entangled vortex liquid state, we observe a linear response but with an anisotropic viscosity. On the other hand, an anomalous guided motion by twin planes is found in the vortex solid.
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