Marconi, V. I., S. Candia, P. Balenzuela, H. Pastoriza, D. Domínguez, and P. Martinoli. "Orientational pinning and transverse voltage: Simulations and experiments in square Josephson junction arrays." Phys. Rev. B 62, no. 6 (2000): 4096–4104.
Abstract: We study the dependence of the transport properties of square Josephson Junctions arrays with the direction of the applied dc current, both experimentally and numerically. We present computational simulations of currentvoltage curves at finite temperatures for a single vortex in an array of $L\times L$ junctions ($Ha^2/?_0=f=1/L^2$), and experimental measurements in $100\times1000$ arrays under a low magnetic field corresponding to $f\approx0.02$. We find that the transverse voltage vanishes only in the directions of maximum symmetry of the square lattice: the [10] and [01] direction (parallel bias) and the [11] direction (diagonal bias). For orientations different than the symmetry directions, we find a finite transverse voltage which depends strongly on the angle φ of the current. We find that vortex motion is pinned in the [10] direction ($?=0$), meaning that the voltage response is insensitive to small changes in the orientation of the current near $?=0$. We call this phenomenon orientational pinning. This leads to a finite transverse critical current for a bias at $?=0$ and to a transverse voltage for a bias at $?\not=0$. On the other hand, for diagonal bias in the [11] direction the behavior is highly unstable against small variations of φ, leading to a rapid change from zero transverse voltage to a large transverse voltage within a few degrees. This last behavior is in good agreement with our measurements in arrays with a quasidiagonal current drive.

Correa, V. F., J. A. Herbsommer, E. E. Kaul, F. de la Cruz, and G. Nieva. "Onset of the 0dimensional vortex pinning behavior in the optimally doped and overdoped Bi2Sr2CaCu2O8+delta system." PHYSICA CSUPERCONDUCTIVITY 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 (Ha < 1000 Oe) region of the magnetic (HaT) phase diagram. In optimally doped as well as in overdoped samples a crossover to a Odimensional pinning regime (Larkin length, LC(C), shorter than the interplane CuO2 distance) is observed at a characteristic temperature Too for all investigated fields. The orderdisorder transition, identified by the second peak in magnetization and a non monotonous field dependence of the AC penetration depth, is quenched below T0D.

Niebieskikwiat, D., L. Civale, C. A. Balseiro, and G. Nieva. "Nonglassy relaxation by double kinks in YBa2Cu3O7delta with columnar defects." PHYSICAL REVIEW B 61, no. 10 (2000): 7135–7141.
Abstract: We present longterm relaxation measurements of the irreversible magnetization in YBa2Cu3O7delta Single crystals with columnar defects. Within the single vortex pinning regime we find a nonglassy response at intermediate currents, in between the half loops and superkinks glassy regimes. We demonstrate that such behavior is due to double kink excitations whose critical nucleus has a size independent of current, and occur even in the presence of pinning energy dispersion.

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. 12 (2000): 10–18.
Abstract: Hightemperature magnetization, low temperature specific heat, and acsusceptibility 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 (Cp) 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 Cp under magnetic field and a large upper critical field slope are discussed. (C) 2000 Elsevier Science B.V. All rights reserved.

Herbsommer, J. A., G. Nieva, and J. Luzuriaga. "Interplay between pinning energy and vortex interaction in YBa2Cu3O7delta with oriented twin boundaries in tilted magnetic fields: Bitter decoration and tiltmodulus measurements." Physical Review B 62, no. 5 (2000): 3534–3541.
Abstract: We have performed Bitter decoration and ac susceptibility measurements in singlecrystal YBa2Cu3O7delta with oriented twin boundaries. The twin boundaries (TB's) pin vortices over approximately 65% of the sample. The pinned areas are unevenly distributed and some relatively large TBfree regions are present. The Bitter decorations were performed in a 52 Oe de magnetic field rotated off the c axis so that the plane defined by the field direction and the c axis is perpendicular to the TB's. Several decorations were performed in the same sample. Additional dynamical information was obtained from ac susceptibility measurements. Results show that for small tilts the vortices remain locked to the caxis direction, for angles greater than 12 degrees they form a staircase pattern, and in this case pinning by the twin boundaries remains effective up to 75 degrees. We observe vortex chains in twinfreezones of the sample for tilted fields at 65 degrees and 40 degrees. Due to our particular experimental arrangement, in the twinned regions the interplay of the potentials giving rise to the chains and the pinning potential produces a structure with a disordered square symmetry. The data allow us to estimate the dominant energy of the vortex system for some inclinations of the applied magnetic field.

Herbsommer, J. A., G. Nieva, and J. Luzuriaga. "Interaction of vortices with oriented twin boundaries in singlecrystal YBa2Cu3O7delta." Physical Review B 61, no. 17 (2000): 11745–11751.
Abstract: We have studied the response of the vortex lattice (VL) in a YBa2Cu3O7delta sample with oriented twin boundaries (TB's) and in an untwinned sample. In the twinned sample the density of twin boundaries is estimated from decoration experiments. The TB's affect the liquidtosolid transition in the vortex structure, which is of first order in the twin free crystal and to a Bose glass in the twinned sample. We measure the ac susceptibility, and find that the response strongly depends on the orientation of the probing ac field with respect to the TB's. The ac field produces a tilting stress on the vortices. With the applied de magnetic field Hdc parallel to the c axis, the response of the VL is weaker when the tilt is parallel to the TB's than when it is perpendicular or at 45 degrees to the TB's. The results are explained by the fact that the VL is locked to the twin boundaries for small deviations of Hdc from the c axis and is partially pinned to them for larger angles. By measuring over the whole angular range we estimate the angles over which each regime is realized for different temperatures. The results of the detwinned sample are used for comparison.

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 highTc cuprates." Physica C: Superconductivity 341348, no. 3 (2000): 1911–1912.
Abstract: Systematic measurements of the fluctuation conductivity in several hightemperature superconducting cuprates show the occurrence of a genuine critical regime. The conductivity exponent is consistent with the predictions of the 3DXY universality class and yields a dynamical critical exponent z = 1.5.

Maiorov, B., G. Nieva, and E. Osquiguil. "Firstorder phase transition of the vortex lattice in twinned YBa2Cu3O7 single crystals in tilted magnetic fields." Physical Review B – Condensed Matter and Materials Physics 61, no. 18 (2000): 12427–12432.
Abstract: We present an exhaustive analysis of transport measurements performed in twinned YBa2Cu3O7 single crystals which establishes that the vortex solidliquid transition is first order when the magnetic field H is applied at an angle ? away from the direction of the twin planes. We show that the resistive transitions are hysteretic and the VI curves are nonlinear, displaying a characteristic S shape at the melting line Hm(T), which scales as ?(?)Hm(T,?), where ?(?) is the anisotropy factor. These features are gradually lost when the critical point H*(?) is approached. Above H*(?) the VI characteristics show a linear response in the experimentally accessible VI window, and the transition becomes reversible. Finally we show that the firstorder phase transition takes place between a highly correlated vortex liquid in the field direction and a solid state of unknown symmetry. As a consequence, our data support the scenario for a vortexline melting in twinned YBa2Cu3O7 crystals in contrast to a vortex sublimation as recently suggested for untwinned La2xSrxCuO4, YBa2Cu3O7 and Bi2Sr2CaCu2O8 [T. Sasagawa et al., Phys. Rev. Lett. 80, 4297 (1998)].

Sereni, J. G. "Evanescence of magnetic transitions in Ce systems." Physica B – Condensed Matter 281282 (2000): 337–339.
Abstract: Three types of magnetic phase diagrams can be identified in Ce systems: (I) with ordering temperature Tord > 0 continuously, followed by a narrow nonFermiliquid (NFL) region, (II) with Tord vanishing at finite temperature, followed by an extended NFL region, and (III) where the degrees of freedom condensed into the ordered phase decreases, without a concomitant decrease of Tord. Intrinsic differences are found in the way that the degrees of freedom condense at Tord and from the rising of the specific heat tails above Tord. (C) 2000 Elsevier Science B.V. All rights reserved.

Herbsommer, Juan A. "Dinámica y estática de vórtices en superconductores de alta temperatura crítica con defectos correlacionados." Ph.D. thesis, , 2000.
