Fasano, Y., M. Menghini, F. de la Cruz, and G. Nieva. "Weak interaction and matching conditions for replicas of vortex lattices." PHYSICAL REVIEW B 62, no. 22 (2000): 15183–15189.
Abstract: We present experiments demonstrating the conditions under which weak interaction between replicas of vortex structures break their translational and rotational symmetry. The Fe clumps introduced by Bitter decorations of vortex structures are used to induce an extremely weak interaction fur subsequent vortex structures in low and high temperature superconducting crystals. The matching between the Fe pattern acid the vortex configuration require the structure to be prepared under quite stringent conditions: The field creating the structure has to be the same in magnitude although it can differ in orientation from that used for the preparation of the Fe pattern. The interaction between vortex replicas as represented by the Fe clumps of Bitter decorations makes it possible to differentiate the vortex nucleation and propagation mechanisms in low and high temperature superconductors.
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Maiorov, B., G. Nieva, and E. Osquiguil. "First-order 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 solid-liquid 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 V-I 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 V-I characteristics show a linear response in the experimentally accessible V-I window, and the transition becomes reversible. Finally we show that the first-order 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 vortex-line melting in twinned YBa2Cu3O7 crystals in contrast to a vortex sublimation as recently suggested for untwinned La2-xSrxCuO4, YBa2Cu3O7 and Bi2Sr2CaCu2O8 [T. Sasagawa et al., Phys. Rev. Lett. 80, 4297 (1998)].
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Herbsommer, J. A., G. Nieva, and J. Luzuriaga. "Interaction of vortices with oriented twin boundaries in single-crystal YBa2Cu3O7-delta." Physical Review B 61, no. 17 (2000): 11745–11751.
Abstract: We have studied the response of the vortex lattice (VL) in a YBa2Cu3O7-delta 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 liquid-to-solid 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 H-dc 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 H-dc 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.
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Niebieskikwiat, D., L. Civale, C. A. Balseiro, and G. Nieva. "Nonglassy relaxation by double kinks in YBa2Cu3O7-delta with columnar defects." PHYSICAL REVIEW B 61, no. 10 (2000): 7135–7141.
Abstract: We present long-term relaxation measurements of the irreversible magnetization in YBa2Cu3O7-delta 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.
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Steren, L. B., M. Sirena, and J. Guimpel. "Substrate effect on the magnetic behavior of manganite films." JOURNAL OF APPLIED PHYSICS 87, no. 9 (2000): 6755–6757.
Abstract: The film thickness (t) dependence of the magnetic properties of La0.6Sr0.4MnO3 (LSMO) films grown on (001) MgO and (001) SrTiO3 substrates has been studied. Hysteresis loops measured at low temperature show a smooth increase of the retentivity accompanied by a decrease of the coercitive field as the film thickness increases. The increase of coercitivity with decreasing t can be interpreted in terms of a change in the domain structure of the films mainly due to an augmentation of domain pinning defects. The magnetic anisotropy has been measured using ferromagnetic resonance (FMR). A volume (VA) and a surface (SA) anisotropy contribution have been deduced from FMR angular dependence studies for both series of samples. In the LSMO films grown on MgO a VA component that corresponds to an easy-axis perpendicular to the plane of the films has been found while in contrast, the LSMO films grown on SrTiO3 present an easy-plane anisotropy. The SA is positive for both series favoring a perpendicular magnetic anisotropy. The measured magnetic anisotropy has been assigned to substrate-induced effects. (C) 2000 American Institute of Physics. [S0021-8979(00)55708-9].
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Fainstein, A., B. Maiorov, J. Guimpel, G. Nieva, and E. Osquiguil. "Annealing disorder and photoinduced order of oxygen chains in detwinned YBa2Cu3O6.65 single crystals probed by Raman scattering." Physical Review B – Condensed Matter and Materials Physics 61, no. 6 (2000): 4298–4304.
Abstract: Raman scattering in detwinned YBa2Cu3O6.65 single crystals is studied as a function of photoexcitation and annealing. Copper-oxygen chain-related forbidden Raman bands that are known to strongly bleach with illumination at low temperatures, increase their intensity with chain fragmentation induced by annealing at high temperature. This contrasting behavior proves the conjunction of short Cu-O fragments into longer chains on photoexcitation. We interpret the Raman modes as due to vibrations at the end of CuO chain fragments and Cu-O-Cu monomers, and use their evolution with illumination and annealing as anisotropic sensitive markers of oxygen reordering processes. The identification of the “forbidden” Raman bands is discussed in the context of our results and recent literature in the subject. We also present absorption measurements performed on GdBa2Cu3Ox thin films with varying oxygen content. These experiments show that the 2.2-eV absorption and the chain-related Raman peaks have different dependencies with oxygen content and illumination, ruling out an explanation that suggests that the Raman intensity reduction of these modes is due to a photobleaching of intermediate defect states. These results highlight the potentialities of Raman scattering for oxygen dynamics studies and demonstrate the presence of photoinduced oxygen ordering in these high-Tc superconductor compounds.
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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 current-voltage 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 quasi-diagonal current drive.
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Decca, R. S., H. D. Drew, E. Osquiguil, B. Maiorov, and J. Guimpel. "Anomalous proximity effect in underdoped YBa2Cu3O6+x Josephson junctions." Physical Review Letters 85, no. 17 (2000): 3708–3711.
Abstract: Experiments were carried out to probe the underdoped insulating material by the Josephson effect. Junctions were fabricated by exploiting the capability of locally photodoping insulating RBa2Cu3O6+x (R = rare earth) material. The existence of an anomalously large proximity effect was confirmed. The critical current of the junctions was consisted with the conventional Josephson relationship.
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Herbsommer, J. A., G. Nieva, and J. Luzuriaga. "Interplay between pinning energy and vortex interaction in YBa2Cu3O7-delta with oriented twin boundaries in tilted magnetic fields: Bitter decoration and tilt-modulus measurements." Physical Review B 62, no. 5 (2000): 3534–3541.
Abstract: We have performed Bitter decoration and ac susceptibility measurements in single-crystal YBa2Cu3O7-delta 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 TB-free 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 c-axis 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 twin-free-zones 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.
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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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