Single domain YBCO/Ag bulk superconductors fabricated by seeded infiltration and growth

We have applied the seeded infiltration and growth (IG) technique to the processing of samples containing Ag in an attempt to fabricate Ag-doped Y-Ba-Cu-O (YBCO) bulk superconductors with enhanced mechanical properties. The IG technique has been used successfully to grow bulk Ag-doped YBCO superconductors of up to 25 mm in diameter in the form of single grains. The distribution of Ag in the parent Y-123 matrix fabricated by the IG technique is observed to be at least as uniform as that in samples grown by conventional top seeded melt growth (TSMG). Fine Y-211 particles were observed to be embedded within the Y-123 matrix for the IG processed samples, leading to a high critical current density, Jc, of over 70 kA/cm2 at 77.3 K in self-field. The distribution of Y-211 in the IG sample microstructure, however, is inhomogeneous, which leads to a variation in the spatial distribution of Jc throughout the bulk matrix. A maximum-trapped field of around 0.43 T at 1.2 mm above the sample surface (i.e. including 0.7 mm for the sensor mould thickness) is observed at liquid nitrogen temperature, despite the relatively small grain size of the sample (20 mm diameter × 7 mm thickness)

T/B scaling without quasiparticle mass divergence: YbCo2Ge4

YbCo$_2$Ge$_4$ is a clean paramagnetic Kondo lattice which displays non-Fermi liquid behavior. We report a detailed investigation of the specific heat, magnetic Gr\"uneisen parameter ($\Gamma_{\rm mag}$) and temperature derivative of the magnetization ($M$) on a high-quality single crystal at temperatures down to $0.1$~K and magnetic fields up to 7~T. $\Gamma_{\rm mag}$ and $dM/dT$ display a divergence upon cooling and obey $T/B$ scaling. Similar behavior has previously been found in several other Yb-based Kondo lattices and related to a zero-field quantum critical point without fine tuning of pressure or composition. However, in the approach of $B\rightarrow 0$ the electronic heat capacity coefficient of YbCo$_2$Ge$_4$ saturates at low $T$, excluding ferromagnetic quantum criticality. This indicates that $T/B$ scaling is insufficient to prove a zero-field quantum critical point.Comment: 6 pages, 6 figures (including supplemental material

Frustration-induced eta inversion in the S=1/2 bond-alternating spin chain

We study the frustration-induced enhancement of the incommensurate correlation for a bond-alternating quantum spin chain in a magnetic field, which is associated with a quasi-one-dimensional organic compound F5PNN. We investigate the temperature dependence of the staggered susceptibilities by using the density matrix renormalization group, and then find that the incommensurate correlation becomes dominant in a certain range of the magnetic field. We also discuss the mechanism of this enhancement on the basis of the mapping to the effective S=1/2 XXZ chain and a possibility of the field-induced incommensurate long range order.Comment: 4 pages, 5 figures, replaced with revised version accepted to PR

Has Blending Compromised Cepheid-Based Determinations of the Extragalactic Distance Scale?

We examine the suggestion that half of the HST Key Project- and Sandage/Saha-observed galaxies have had their distances systematically underestimated, by 0.1-0.3 mag in the distance modulus, due to the underappreciated influence of stellar profile blending on the WFC chips. The signature of such an effect would be a systematic trend in (i) the Type Ia supernovae corrected peak luminosity and (ii) the Tully-Fisher residuals, with increasing calibrator distance, and (iii) a differential offset between PC and WFC distance moduli, within the same galaxy. The absence of a trend would be expected if blending were negligible (as has been inherently assumed in the analyses of the aforementioned teams). We adopt a functional form for the predicted influence of blending that is consistent with the models of Mochejska et al. and Stanek & Udalski, and demonstrate that the expected correlation with distance predicted by these studies is not supported by the data. We conclude that the Cepheid-based extragalactic distance scale has not been severely compromised by the neglect of blending.Comment: 14 pages, 2 figures, 1 table, LaTeX, accepted for publication in Astrophysical Journal Letters, also available at http://casa.colorado.edu/~bgibson/publications.htm

Neutron Skin Thickness of 90Zr Determined By Charge Exchange Reactions

Charge exchange spin-dipole (SD) excitations of 90Zr are studied by the 90Zr(p,n) and 90Zr(n,p) reactions at 300 MeV. A multipole decomposition technique is employed to obtain the SD strength distributions in the cross section spectra. For the first time, a model-independent SD sum rule value is obtained: 148+/-12 fm^2. The neutron skin thickness of 90Zr is determined to be 0.07+/-0.04 fm from the SD sum rule value.Comment: 4 pages, 2 figures, submitted to Phys. Rev.

Unified picture of Q-balls and boson stars via catastrophe theory

We make an analysis of Q-balls and boson stars using catastrophe theory, as an extension of the previous work on Q-balls in flat spacetime. We adopt the potential $V_3(\phi)={m^2\over2}\phi^2-\mu\phi^3+\lambda\phi^4$ for Q-balls and that with $\mu =0$ for boson stars. For solutions with $|g^{rr}-1|\sim 1$ at its peak, stability of Q-balls has been lost regardless of the potential parameters. As a result, phase relations, such as a Q-ball charge versus a total Hamiltonian energy, approach those of boson stars, which tell us an unified picture of Q-balls and boson stars.Comment: 10 pages, 13 figure