Quaternary borocarbides: New class of intermetallic superconductors

Our recent discovery of superconductivity (SC) in the four-element multiphase Y-Ni-B-C system at an elevated temperature (TC approximately 12 K) has opened up great possibilities of identifying new superconducting materials and generating new physics. Superconductivity with Tc (greater than 20 K) higher than that known so far in bulk intermetallics has been observed in multiphase Y-Pd-B-C and Th-Pd-B-C systems and a family of single phase materials RENi2B2C (RE= Y, rare earth) have been found. Our investigations show YNi2B2C to be a strong coupling hard type-II SC. HC2(T) exhibits an unconventional temperature dependence. Specific heat and magnetization studies reveal coexistence of SC and magnetism in RNi2B2C (R = Ho, Er, Tm) with magnetic ordering temperatures (Tc approximately 8 K, 10.5 K, 11 K and Tm approximately 5 K, approximately 7K, approximately 4 K respectively) that are remarkably higher than those in known magnetic superconductors . Mu-SR studies suggest the possibility of Ni atoms carrying a moment in TmNi2B2C. Resistivity results suggests a double re-entrant transition (SC-normal-SC) in HoNi2B2C. RENi2B2C (RE = Ce, Nd, Gd) do not show SC down to 4.2 K. The Nd- and Gd-compounds order magnetically at approximately 4.5 K and approximately 19.5 K, respectively. Two SC transitions are observed in Y-Pd-B-C (Tc approximately 22 K, approximately 10 K) and in Th-Pd-B-C (Tc approximately 20 K, approximately 14 K) systems, which indicate that there are at least two structures which support SC in these borocarbides. In our multiphase ThNi2B2C we observe SC at approximately 6 K. No SC was seen in multiphase UNi2B2C, UPd2B2C, UOs2Ge2C and UPd5B3C(0.35) down to 4.2 K. Tc in YNi2B2C is depressed by substitutions (Gd, Th and U at Y-sites and Fe, Co at Ni-sites)

Absolute dimensions of eclipsing binaries. XXVI, Setting a new standard : masses, radii, and abundances for the F-type systems AD Bootis, VZ Hydrae, and WZ Ophiuchi

Context. Accurate mass, radius, and abundance determinations from binaries provide important information on stellar evolution, fundamental to central fields in modern astrophysics and cosmology. Aims. We aim to determine absolute dimensions and abundances for the three F-type main-sequence detached eclipsing binaries ADBoo, VZHya, and WZOph and to perform a detailed comparison with results from recent stellar evolutionary models. Methods. uvby light curves and uvbyβ standard photometry were obtained with the Strömgren Automatic Telescope at ESO, La Silla, radial velocity observations at CfA facilities, and supplementary high-resolution spectra with ESO’s FEROS spectrograph. State-ofthe-art methods were applied for the analyses: the EBOP andWilson-Devinney binary models, two-dimensional cross-correlation and disentangling, and the VWA abundance analysis tool. Results. Masses and radii that are precise to 0.5–0.7% and 0.4–0.9%, respectively, have been established for the components, which span the ranges of 1.1 to 1.4 M and 1.1 to 1.6 R. The [Fe/H] abundances are from –0.27 to +0.10, with uncertainties between 0.07 and 0.15 dex. We find indications of a slight α-element overabundance of [α/Fe] ∼ +0.1 for WZOph. The secondary component of ADBoo and both components of WZOph appear to be slightly active. Yale-Yonsai and Victoria-Regina evolutionary models fit the components of ADBoo and VZHya almost equally well, assuming coeval formation, at ages of about 1.75/1.50 Gyr (ADBoo) and 1.25/1.00 Gyr (VZHya). BaSTI models, however, predict somewhat different ages for the primary and secondary components. For WZOph, the models from all three grids are significantly hotter than observed. A low He content, decreased envelope convection coupled with surface activity, and/or higher interstellar absorption would remove the discrepancy, but its cause has not been definitively identified. Conclusions. We have demonstrated the power of testing and comparing recent stellar evolutionary models using eclipsing binaries, provided their abundances are known. The strongest limitations and challenges are set by Teff and interstellar absorption determinations, and by their effects on and correlation with abundance results

Local structure study about Co in YBa2_2(Cu1−x_{1-x}Cox_x)3_3O7−δ_{7-\delta} thin films using polarized XAFS

We have studied the local structure around Co in YBa$_2$(Cu$_{1-x}$Co$_x$)$_3$O$_{7-\delta}$ thin films with three different concentrations: x=0.07, 0.10, 0.17, and in a PrBa$_2$(Cu$_{1-x}$Co$_x$)$_3$O$_{7-\delta}$ thin film of concentration x=0.05 using the X-ray Absorption Fine Structure (XAFS) technique. Data were collected at the Co $K$-edge with polarizations both parallel and perpendicular to the film surface. We find that the oxygen neighbors are well ordered and shortened in comparison with YBCO Cu-O values to 1.80 \AA{} and 1.87 \AA{} in the $c$-axis and $ab$-plane, respectively. A comparison of further neighbors in the thin film and powder data show that these peaks in the film are suppressed in amplitude relative to the powder samples, which suggests there is more disorder and/or distortions of the Co environment present in the thin films.Comment: 14 pages; To be submitted to Phys. Rev.

CNx-modified Fe3O4 as Pt nanoparticle support for the oxygen reduction reaction

A novel electrocatalyst support material, nitrogendoped carbon (CNx)-modified Fe3O4 (Fe3O4-CNx), was synthesized through carbonizing a polypyrrole-Fe3O4 hybridized precursor. Subsequently, Fe3O4-CNx-supported Pt (Pt/Fe3O4-CNx) nanocomposites were prepared by reducing Pt precursor in ethylene glycol solution and evaluated for the oxygen reduction reaction (ORR). The Pt/Fe3O4-CNx catalysts were characterized by X-ray diffraction, Raman spectra, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The electrocatalytic activity and stability of the as-prepared electrocatalysts toward ORR were studied by cyclic voltammetry and steady-state polarization measurements. The results showed that Pt/ Fe3O4-CNx catalysts exhibited superior catalytic performance for ORR to the conventional Pt/C and Pt/C-CNx catalysts.Web of Scienc

Antiferromagnetic ordering in CePdSn

A new equiatomic cerium containing ternary compound CePdSn has been synthesized and is found to crystallize in the orthorhombic TiNiSi type structure. Magnetic susceptibility and electrical resistivity measurements reveal that this compound is ordered antiferromagnetically with a Neel temperature (T<SUB>N</SUB>) of 7.5K. In the paramagnetic state, the susceptibility follows Curie-Weiss behavior between 50-300K with an effective moment close to that of free Ce<SUP>3+</SUP> ion and a large negative paramagnetic Curie temperature. Deviation from Curie-Weiss behavior of the susceptibility is observed at low temperatures. The T<SUB>N</SUB> of CePdSn is anomalously high compared to that expected, on the basis of de Gennes rule, from the T<SUB>N</SUB>=14.6K of GdPdSn

Evidence for the revival of superconductivity in Nd<SUB>1.82</SUB>Ce<SUB>0.18</SUB>CuO<SUB>4-&#948;</SUB> by doping with divalent alkaline-earth elements

The revival of superconductivity in the electron-excess T'-type compound Nd1.82Ce0.18CuO4-&#948; by hole doping has been observed. The normal-state resistivity (&#961;) changes from semiconducting (d&#961;/dT &#60; 0) to metallic (d&#961;/dT &#62; 0) as the hole doping increases up to a certain level