Enhancement of Critical Current Density in low level Al-doped MgB2

Two sets of MgB2 samples doped with up to 5 at. % of Al were prepared in different laboratories using different procedures. Decreases in the a and c lattice parameters were observed with Al doping confirming Al substitution onto the Mg site. The critical temperature (Tc) remained largely unchanged with Al doping. For 1 - 2.5 at.% doping, at 20K the in-field critical current densities (Jc's) were enhanced, particularly at lower fields. At 5K, in-field Jc was markedly improved, e.g. at 5T Jc was enhanced by a factor of 20 for a doping level of 1 at.% Al. The improved Jcs correlate with increased sample resistivity indicative of an increase in the upper critical field, Hc2, through alloying.Comment: 17 pages, 4 figures, to be published in Superconductor Science and Technolog

Weak Lensing Effects on the Galaxy Three-Point Correlation Function

We study the corrections to the galaxy three-point correlation function (3PCF) induced by weak lensing magnification due to the matter distribution along the line of sight. We consistently derive all the correction terms arising up to second order in perturbation theory and provide analytic expressions as well as order of magnitude estimates for their relative importance. The magnification contributions depend on the geometry of the projected triangle on the sky plane, and scale with different powers of the number count slope and redshift of the galaxy sample considered. We evaluate all terms numerically and show that, depending on the triangle configuration as well as the galaxy sample considered, weak lensing can in general significantly contribute to and alter the three-point correlation function observed through galaxy and quasar catalogs.Comment: 24 pages, 11 figures; version accepted for publication in Phys. Rev. D; v2: typos corrected, figure caption clarifie

Evolution of the fishtail-effect in pure and Ag-doped MG-YBCO

We report on magnetic measurements carried out in a textured YBa$_2$Cu$_3$O$_{7-\delta}$ and YBa$_2$(Cu$_{1-x}$Ag$_x$)$_3$O$_{7-\delta}$ (at $x \approx$ 0.02) crystals. The so-called fishtail-effect (FE) or second magnetization peak has been observed in a wide temperature range 0.4~$<T/T_c<$~0.8 for $\textbf{H}\parallel c$. The origin of the FE arises for the competition between surface barrier and bulk pinning. This is confirmed in a non-monotonically behavior of the relaxation rate $R$. The value $H_{max}$ for Ag-doped crystals is larger than for the pure one due to the presence of additional pinning centers, above all on silver atoms.Comment: 6 pages, 6 figure

Preparation of atomically clean and flat Si(100) surfaces by low-energy ion sputtering and low-temperature annealing

Si(100) surfaces were prepared by wet-chemical etching followed by 0.3-1.5keV Ar ion sputtering, either at elevated or room temperature. After a brief anneal under ultrahigh vacuum conditions, the resulting surfaces were examined by scanning tunneling microscopy. We find that wet-chemical etching alone cannot produce a clean and flat Si(100) surface. However, subsequent 300eV Ar ion sputtering at room temperature followed by a 973K anneal yields atomically clean and flat Si(100) surfaces suitable for nanoscale device fabrication.Comment: 13 pages, 3 figures, to be published in Applied Surface Scienc

Edge and Bulk Transport in the Mixed State of a Type-II Superconductor

By comparing the voltage-current (V-I) curves obtained before and after cutting a sample of 2H-NbSe2, we separate the bulk and edge contributions to the transport current at various dissipation levels and derive their respective V- I curves and critical currents. We find that the edge contribution is thermally activated across a current dependent surface barrier. By contrast the bulk V-I curves are linear, as expected from the free flux flow model. The relative importance of bulk and edge contributions is found to depend on dissipation level and sample dimensions. We further show that the peak effect is a sharp bulk phenomenon and that it is broadened by the edge contribution

Disorder and thermally driven vortex-lattice melting in La{2-x}Sr{x}CuO{4} crystals

Magnetization measurements in La{2-x}Sr{x}CuO{4} crystals indicate vortex order-disorder transition manifested by a sharp kink in the second magnetization peak. The transition field exhibits unique temperature dependence, namely a strong decrease with temperature in the entire measured range. This behavior rules out the conventional interpretation of a disorder-driven transition into an entangled vortex solid phase. It is shown that the transition in La{2-x}Sr{x}CuO{4} is driven by both thermally- and disorder-induced fluctuations, resulting in a pinned liquid state. We conclude that vortex solid-liquid, solid-solid and solid to pinned-liquid transitions are different manifestations of the same thermodynamic order-disorder transition, distinguished by the relative contributions of thermal and disorder-induced fluctuations.Comment: To be published in phys. Rev. B Rapid Com

Asymmetric Field Profile in Bose Glass Phase of Irradiated YBa2Cu3O7-d: Loss of Interlayer Coherence around 1/3 of Matching Field

Magneto-optical imaging in YBa2Cu3O7-d with tilted columnar defects (CD's) shows an asymmetric critical-state field profile. The observed hysteretic shift of the profile ridge (trough) from the center of the sample is explained by in-plane magnetization originated from vortex alignment along CD's. The extracted ratio of the in-plane to out-of-plane magnetization component has a maximum at 1/5 of matching field ($B_\Phi$) and disappears above $B_\Phi/3$, suggesting a reduction of interlayer coherence well bellow $B_\Phi$ in the Bose glass phase. Implications are discussed in comparison with the vortex liquid recoupling observed in irradiated Bi2Sr2CaCu2O8+y.Comment: Revtex, 4 pages, 5 figures, also see a movie at (http://www.ap6.t.u-tokyo.ac.jp/kitaka/Research/d-line/index_e.htm). This manuscript will appear in Phys. Rev. Let

The dynamical viability of scalar-tensor gravity theories

We establish the dynamical attractor behavior in scalar-tensor theories of dark energy, providing a powerful framework to analyze classes of theories, predicting common evolutionary characteristics that can be compared against cosmological constraints. In the Jordan frame the theories are viewed as a coupling between a scalar field, \Phi, and the Ricci scalar, R, F(\Phi)R. The Jordan frame evolution is described in terms of dynamical variables m \equiv d\ln F/d\ln \Phi and r \equiv -\Phi F/f, where F(\Phi) = d f(\Phi)/d\Phi. The evolution can be alternatively viewed in the Einstein frame as a general coupling between scalar dark energy and matter, \beta. We present a complete, consistent picture of evolution in the Einstein and Jordan frames and consider the conditions on the form of the coupling F and \beta required to give the observed cold dark matter (CDM) dominated era that transitions into a late time accelerative phase, including transitory accelerative eras that have not previously been investigated. We find five classes of evolutionary behavior of which four are qualitatively similar to those for f(R) theories (which have \beta=1/2). The fifth class exists only for |\beta| < \sqrt{3}/4, i.e. not for f(R) theories. In models giving transitory late time acceleration, we find a viable accelerative region of the (r,m) plane accessible to scalar-tensor theories with any coupling, \beta (at least in the range |\beta| \leq 1/2, which we study in detail), and an additional region open only to theories with |\beta| < \sqrt{3}/4.Comment: 24 pages, 3 figure

Spherically symmetric solutions in f(R)-gravity via Noether Symmetry Approach

We search for spherically symmetric solutions of f(R) theories of gravity via the Noether Symmetry Approach. A general formalism in the metric framework is developed considering a point-like f(R)-Lagrangian where spherical symmetry is required. Examples of exact solutions are given.Comment: 17 pages, to appear in Class. Quant. Gra

Curvaton Dynamics and the Non-Linearity Parameters in Curvaton Model

We investigate the curvaton dynamics and the non-linearity parameters in curvaton model with potential slightly deviating from the quadratic form in detail. The non-linearity parameter $g_{NL}$ will show up due to the curvaton self-interaction. We also point out that the leading order of non-quadratic term in the curvaton potential can be negative, for example in the axion-type curvaton model. If a large positive $g_{NL}$ is detected, the axion-type curvaton model will be preferred.Comment: 14 pages, 4 figures; refs adde