Simulation Studies on the Stability of the Vortex-Glass Order

The stability of the three-dimensional vortex-glass order in random type-II superconductors with point disorder is investigated by equilibrium Monte Carlo simulations based on a lattice XY model with a uniform field threading the system. It is found that the vortex-glass order, which stably exists in the absence of screening, is destroyed by the screenng effect, corroborating the previous finding based on the spatially isotropic gauge-glass model. Estimated critical exponents, however, deviate considerably from the values reported for the gauge-glass model.Comment: Minor modifications made, a few referenced added; to appear in J. Phys. Soc. Jpn. Vol.69 No.1 (2000

Longitudinal Current Dissipation in Bose-glass Superconductors

A scaling theory of vortex motion in Bose glass superconductors with currents parallel to the common direction of the magnetic field and columnar defects is presented. Above the Bose-glass transition the longitudinal DC resistivity $\rho_{||}(T)\sim (T-T_{BG})^{\nu' z'}$ vanishes much faster than the corresponding transverse resistivity $\rho_{\perp}(T)\sim (T-T_{BG})^{\nu' (z'-2)}$, thus {\it reversing} the usual anisotropy of electrical transport in the normal state of layered superconductors. In the presence of a current $\bf J$ at an angle $\theta_J$ with the common field and columnar defect axis, the electric field angle $\theta_E$ approaches $\pi/2$ as $T\rightarrow T_{BG}^+$. Scaling also predicts the behavior of penetration depths for the AC currents as $T\rightarrow T_{BG}^-$, and implies a {\it jump discontinuity} at $T_{BG}$ in the superfluid density describing transport parallel to the columns.Comment: 5 pages, revte

First order phase transition of the vortex lattice in twinned YBa2Cu3O7 single crystals in tilted magnetic fields

We present an exhaustive analysis of transport measurements performed in twinned YBa2Cu3O7 single crystals which stablishes that the vortex solid-liquid transition is first order when the magnetic field H is applied at an angle theta away from the direction of the twin planes. We show that the resistive transitions are hysteretic and the V-I curves are non-linear, displaying a characteristic s-shape at the melting line Hm(T), which scales as epsilon(theta)Hm(T,theta). These features are gradually lost when the critical point H*(theta) is approached. Above H*(theta) 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, the available data support the scenario for a vortex-line melting rather than a vortex sublimation as recently suggested [T.Sasagawa et al. PRL 80, 4297 (1998)].Comment: 10 pages, 8 figures, submitted to PR

Plasticity and memory effects in the vortex solid phase of twinned YBa2Cu3O7 single crystals

We report on marked memory effects in the vortex system of twinned YBa2Cu3O7 single crystals observed in ac susceptibility measurements. We show that the vortex system can be trapped in different metastable states with variable degree of order arising in response to different system histories. The pressure exerted by the oscillating ac field assists the vortex system in ordering, locally reducing the critical current density in the penetrated outer zone of the sample. The robustness of the ordered and disordered states together with the spatial profile of the critical current density lead to the observed memory effects

Melting and Dimensionality of the Vortex Lattice in Underdoped YBa2Cu3O6.60

Muon spin rotation measurements of the magnetic field distribution in the vortex state of the oxygen deficient high-Tc superconductor YBa{2}Cu{3}O{6.60} reveal a vortex-lattice melting transition at much lower temperature than that in the fully oxygenated material. The transition is best described by a model in which adjacent layers of ``pancake'' vortices decouple in the liquid phase. Evidence is also found for a pinning-induced crossover from a solid 3D to quasi-2D vortex lattice, similar to that observed in the highly anisotropic superconductor Bi{2+x}Sr{2-x}CaCu{2}O{8+y}.Comment: 8 pages, 4 figures, 5 postscript file

Hall Anomaly and Vortex-Lattice Melting in Superconducting Single Crystal YBa2Cu3O7-d

Sub-nanovolt resolution longitudinal and Hall voltages are measured in an ultra pure YBa2Cu3O7-d single crystal. The Hall anomaly and the first-order vortex-lattice melting transition are observed simultaneously. Changes in the dynamic behavior of the vortex solid and liquid are correlated with features of the Hall conductivity sxy. With the magnetic field oriented at an angle from the twin-boundaries, the Hall conductivity sharply decreases toward large negative values at the vortex-lattice melting transition.Comment: 6 pages, 2 figures included, Postscript, to appear in Phys. Rev. Let

Reliability of self-report of health in juvenile offenders

The aim of the present study was to investigate the accuracy of self-reports of juvenile offenders on physical factors (e.g., sleep difficulties, weight related behaviors and weight perceptions), health risk behaviors (e.g., alcohol use), trauma history (e.g., physical and sexual abuse) and psychological factors (e.g., anxiety, suicidal and self-harm behaviors). Self-reports obtained via a Health Questionnaire from 242 incarcerated juvenile offenders were compared with standardized measures (Body Mass Index, Adolescent Psychopathology Scale and Child Trauma Questionnaire) to investigate the reliability (via construct validity) and veracity of their self-report. Using kappa estimates and receiver operating characteristic curves, results generally showed high agreement across measures, suggesting that self-report questions from the health survey could all be used reliably. The degree of accuracy indicated that young offenders are as reliable as clinical and community samples of adolescents in their self-report. These findings have implications for routine assessments and practice evaluations that rely on self-report as the method of data collection and as the basis for clinical formulation and treatment planning

The phase diagram of high-Tc's: Influence of anisotropy and disorder

We propose a phase diagram for the vortex structure of high temperature superconductors which incorporates the effects of anisotropy and disorder. It is based on numerical simulations using the three-dimensional Josephson junction array model. We support the results with an estimation of the internal energy and configurational entropy of the system. Our results give a unified picture of the behavior of the vortex lattice, covering from the very anysotropic BiSrCaCuO to the less anisotropic YBaCuO, and from the first order melting ocurring in clean samples to the continuous transitions observed in samples with defects.Comment: 8 pages with 7 figure

Time-dependent Ginzburg-Landau Simulations of vortex guidance by twin boundaries

The driven motion of vortices in the presence of a planar defect is simulated by means of the time-dependent Ginzburg-Landau equations. Guided motion of the vortices, both internal and external to the twin boundary, is found to occur over a range of driving forces. Experimental transport data on a single crystal of YBa{sub 2}Cu{sub 3}O{sub 7} are consistent with guided motion

Hysteretic behavior of the vortex lattice at the onset of the second peak for HgBa2_2CuO4+δ_{4+\delta} superconductor

By means of local Hall probe ac and dc permeability measurements we investigated the phase diagram of vortex matter for the HgBa$_2$CuO$_{4+\delta }$ superconductor in the regime near the critical temperature. The second peak line, $H_{\rm sp}$, in contrast to what is usually assumed, doesn't terminate at the critical temperature. Our local ac permeability measurements revealed pronounced hysteretic behavior and thermomagnetic history effects near the onset of the second peak, giving evidence for a phase transition of vortex matter from an ordered qausilattice state to a disordered glass