Vortex glass transition in a random pinning model

We study the vortex glass transition in disordered high temperature superconductors using Monte Carlo simulations. We use a random pinning model with strong point-correlated quenched disorder, a net applied magnetic field, longrange vortex interactions, and periodic boundary conditions. From a finite size scaling study of the helicity modulus, the RMS current, and the resistivity, we obtain critical exponents at the phase transition. The new exponents differ substantially from those of the gauge glass model, but are consistent with those of the pure three-dimensional XY model.Comment: 7 pages RevTeX, 4 eps figure

Vortex Flow and Transverse Flux Screening at the Bose Glass Transition

We investigate the vortex phase diagram in untwinned YBaCuO single crystals with columnar defects. These randomly distributed defects, produced by heavy ion irradiation, are expected to induce a ``Bose Glass'' phase of localized vortices characterized by a vanishing resistance and a Meissner effect for magnetic fields transverse to the defect axis. We directly observe the transverse Meissner effect using an array of Hall probe magnetometers. As predicted, the Meissner state breaks down at temperatures Ts that decrease linearly with increasing transverse magnetic field. However, Ts falls well below the conventional melting temperature Tm determined by a vanishing resistivity, suggesting an intermediate regime where flux lines are effectively localized even when rotated off the columnar defects.Comment: 15 pages, 5 figure

High current-carrying capability in c-axis-oriented superconducting MgB2 thin films

In high-quality c-axis-oriented MgB2 thin films, we observed high critical current densities (Jc) of 16 MA/cm^2 at 15 K under self fields comparable to those of cuprate high-temperature superconductors. The extrapolated value of Jc at 5 K was estimated to be 40 MA/cm^2. For a magnetic field of 5 T, a Jc of 0.1 MA/cm^2 was detected at 15 K, suggesting that this compound would be a very promising candidate for practical applications at high temperature and lower power consumption. The vortex-glass phase is considered to be a possible explanation for the observed high current-carrying capability.Comment: 3 pages and 4 figures, to be published in Physical Review Letter

Disorder Driven Melting of the Vortex Line Lattice

We use Monte Carlo simulations of the 3D uniformly frustrated XY model, with uncorrelated quenched randomness in the in-plane couplings, to model the effect of random point pins on the vortex line phases of a type II superconductor. We map out the phase diagram as a function of temperature T and randomness strength p for fixed applied magnetic field. We find that, as p increases to a critical value p_c, the first order vortex lattice melting line turns parallel to the T axis, and continues smoothly down to low temperature, rather than ending at a critical point. The entropy jump across this line at p_c vanishes, but the transition remains first order. Above this disorder driven transition line, we find that the helicity modulus parallel to the applied field vanishes, and so no true phase coherent vortex glass exists.Comment: 4 pages, 6 eps figure

Structure of vortex liquid phase in irradiated BSCCO(2212) crystals

The c-axis resistivity in irradiated and in pristine BSCCO(2212) crystals is measured as a function of the in-plane magnetic field component at fixed out-of-plane component B_\perp in the vortex liquid phase at T=67 K. From this data we extract the dependence of the phase difference correlation length inside layers on B_\perp and estimate the average length of pieces of vortex lines confined inside columnar defects as a function of the filling factor f=B_\perp / B_\phi. The maximum length, about 15 interlayer distances, is reached near f=0.35.Comment: 4 pages, 4 figure

Reentrant softening as precursor to reentrant melting of the vortex-lattice in YBCO single crystal

A vibrating sample technique was used to study the elastic behavior of the magnetic vortex system in YBa{sub 2}Cu{sub 3}O{sub 7} single crystal. The setup consists of a system of two weakly coupled mechanical oscillators (transducer, sample), the frequency and Q of which depends sensitively on the frequencies of the two subsystems as well as the coupling between both. By sweeping a magnetic field at temperatures below the superconducting transition temperature {Tc} the authors observe pronounced attenuation peaks of temperature-dependent characteristic field strengths H{sub 1} and H{sub 2}. These fields mark temperature-dependent points of constant elasticity of the vortex-ensemble. Since softening precedes the melting of the vortex-lattice by approaching H{sub o1} as well as H{sub o2}, the observed angular dependence of H{sub 1} and H{sub 2} is interpreted as due to reentrant softening as precursor to reentrant melting

Dynamic signatures of driven vortex motion.

We probe the dynamic nature of driven vortex motion in superconductors with a new type of transport experiment. An inhomogeneous Lorentz driving force is applied to the sample, inducing vortex velocity gradients that distinguish the hydrodynamic motion of the vortex liquid from the elastic and-plastic motion of the vortex solid. We observe elastic depinning of the vortex lattice at the critical current, and shear induced plastic slip of the lattice at high Lorentz force gradients

Induced point and correlated disorder pinning in untwinned YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} crystals

The magnetic phase diagram of untwinned single crystals of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} is studied via transport measurements. The authors show that the first order vortex melting line which terminates at a critical point in high magnetic fields is strongly dependent on point disorder induced by proton irradiation. In addition, they report on the effect of heavy ion irradiation induced columnar defects on vortex pinning

Pasture Access Effects on the Welfare of Dairy Cows Housed in Free-Stall Barns

Despite considerable research regarding the benefits of natural living conditions on several aspects of the health and well-being of dairy cows, the effects of pasture access on their overall welfare are less studied. In this comparative study, the Welfare Quality® protocol was applied in 22 zero-grazing and 17 grazing access farms with an ulterior statistical exploration of the differences found. Moreover, correlations were calculated between pasture access and animal-based welfare measures. Aside from the multiple benefits of pasturing identified within the welfare measures, criteria, and principles, in the overall classification, the farms with permanent confinement ranked lower than the grazing farms. Although both systems used free-stall barns, allowing the cows’ movement, the grazing animals showed improved overall welfare. Yet, the origin-related adaptation of the animals could play a role. The authors recommend research-based tailoring whenever these conditions are intended to be transposed in technology, especially in intensive systems