Superconducting properties of the pyrochlore oxide Cd2Re2O7

We report the superconducting properties of the pyrochlore oxide Cd2Re2O7. The bulk superconducting transition temperature Tc is about 1.0 K, and the upper critical field Hc2 determined by the measurement of specific heat under magnetic fields is 0.29 T. The superconducting coherence length is estimated to be 34 nm. Specific heat data measured on single crystals suggest that the superconducting gap of Cd2Re2O7 is nodeless.Comment: 6 pages, 6 figures, 1 table, to be published in J. Chem. Phys. Solid

The rp Process Ashes from Stable Nuclear Burning on an Accreting Neutron Star

We calculate the nucleosynthesis during stable nuclear burning on an accreting neutron star. This is appropriate for weakly magnetic neutron stars accreting at near-Eddington rates in low mass X-ray binaries, and for most accreting X-ray pulsars. We show that the nuclear burning proceeds via the rapid proton capture process (rp process), and makes nuclei far beyond the iron group. The final mixture of nuclei consists of elements with a range of masses between approximately A=60 and A=100. The average nuclear mass of the ashes is set by the extent of helium burning via (alpha,p) reactions, and depends on the local accretion rate. Our results imply that the crust of these accreting neutron stars is made from a complex mixture of heavy nuclei, with important implications for its thermal, electrical and structural properties. A crustal lattice as impure as our results suggest will have a conductivity set mostly by impurity scattering, allowing more rapid Ohmic diffusion of magnetic fields than previously estimated.Comment: To appear in the Astrophysical Journal (33 pages, LaTeX, including 11 postscript figures

Structural Phase Transition in the Superconducting Pyrochlore Oxide Cd2Re2O7

We report a structural phase transition found at Ts = 200 K in a pyrochlore oxide Cd2Re2O7 which shows superconductivity at Tc = 1.0 K. X-ray diffractionexperiments indicate that the phase transition is of the second order, from a high-temperature phase with the ideal cubic pyrochlore structure (space group Fd-3m) to a low-temperature phase with another cubic structure (space group F-43m). It is accompanied by a dramatic change in the resistivity and magnetic susceptibility and thus must induce a significant change in the electronic structure of Cd2Re2O7.Comment: 4 pages, 4figures, proceeding for ISSP

Low Temperature Symmetry of Pyrochlore Oxide Cd2Re2O7

We report the X-ray study for the pyrochlore oxide Cd2Re2O7. Two symmetry-lowering structural transitions were observed at Ts1=200K and Ts2=120K. The former is of the second order from the ideal cubic pyrochlore structure with space group Fd-3m to a tetragonally distorted structure with I-4m2, while the latter is of the first order likely to another tetragonal space group I4122. We discuss the feature of the lattice deformation.Comment: 4 pages, 4 figure

The second phase transition in the pyrochlore oxide Cd2Re2O7

Evidence for another phase transition at 120 K in the metallic pyrochlore oxide Cd2Re2O7, following the structural transition at 200 K and followed by the superconducting transition at 1.0 K, is given through resistivity, magnetoresistance, specific heat, and X-ray diffraction measurements. The results indicate unique successive structural and electronic transitions occurring in the pyrochlore compound, revealing an interesting interplay between the crystal and electronic structures on the itinerant electron system in the pyrochlore lattice

High Pressure Effects on Superconductivity in the beta-pyrochlore Oxides AOs2O6 (A=K, Rb, Cs)

Recently new pyrochlore oxides superconductors AOs2O6 (A=K,Rb,Cs) were found and we measured the pressure dependence of magnetization up to 1.2 GPa in order to deduce the pressure effect of Tc in the three beta-pyrochlore oxides. It is found that the initial pressure dependence of Tc is positive for all the compounds. Only KOs2O6 exhibits a saturation in Tc at 0.56 GPa and the downturn at higher pressureComment: 7 pages, 2 figure

Hall effect in superconducting Fe(Se0.5Te0.5) thin films

The Hall effect is investigated for eight superconducting Fe(Se_0.5_Te_0.5_) thin films grown on MgO and LaSrAlO_4_ substrates with different transition temperatures (T_c_). The normal Hall coefficients (R_H_) have positive values with magnitude of 1 - 1.5 x 10^-3^ cm^3^/C at room temperature for the all samples. With decreasing temperature, we find two characteristic types of behavior in R_H_(T) depending on T_c_. For thin films with lower T_c_ (typically T_c_ < 5 K), R_H_ start decreasing approximately below T = 250 K toward a negative side, some of which shows sign reversal at T = 50 - 60 K, but turns positive toward T = 0 K. On the other hand for the films with higher T_c_ (typically T_c_ > 9 K), R_ H_ leaves almost unchanged down to T = 100 K, and then starts decreasing toward a negative side. Around the temperatures when R_H_ changes its sign from positive to negative, obvious nonlinearity is observed in the field-dependence of Hall resistance as to keep the low-field R_H_ positive while the high-field R_H_ negative. Thus the electronic state just above T_c_ is characterized by n_e_ (electron density) > n_h_ (hole density) with keeping \mu_e_ < \mu_h_. These results suggest the dominance of electron density to the hole density is an essential factor for the occurence of superconductivity in Fe-chalcogenide superconductors.Comment: 11 pages, 4 figures, revised version for Physical Review B. accepted for publication in Physical Review

Parker-Jeans Instability of Gaseous Disks Including the Effect of Cosmic Rays

We use linear analysis to examine the effect of cosmic rays (CRs) on the Parker-Jeans instability of magnetized self-gravitating gaseous disks. We adopt a slab equilibrium model in which the gravity (including self-gravity) is perpendicular to the mid-plane, the magnetic field lies along the slab. CR is described as a fluid and only along magnetic field lines diffusion is considered. The linearised equations are solved numerically. The system is susceptible to Parker-Jeans instability. In general the system is less unstable when the CR diffusion coefficient is smaller (i.e., the coupling between the CRs and plasma is stronger). The system is also less unstable if CR pressure is larger. This is a reminiscence of the fact that Jeans instability and Parker instability are less unstable when the gas pressure is larger (or temperature is higher). Moreover, for large CR diffusion coefficient (or small CR pressure), perturbations parallel to the magnetic field are more unstable than those perpendicular to it. The other governing factor on the growth rate of the perturbations in different directions is the thickness of the disk or the strength of the external pressure on the disk. In fact, this is the determining factor in some parameter regimes.Comment: 19pages, 14figures submitted to Ap

New beta-Pyrochlore Oxide Superconductor CsOs2O6

The discovery of a new beta-pyrochlore oxide superconductor CsOs2O6 with Tc = 3.3 K is reported. It is the third superconductor in the family of beta-pyrochlore oxides, following KOs2O6 with Tc = 9.6 K and RbOs2O6 with Tc = 6.3 K. The Tc of this series decreases with increasing the ionic radius of alkaline metal ions, imposing negative chemical pressure upon the Os pyrochlore lattice.Comment: submitted to J. Phys. Soc Jp

Rotational Evolution During Type I X-Ray Bursts

The rotation rates of six weakly-magnetic neutron stars accreting in low-mass X-ray binaries have most likely been measured by Type I X-ray burst observations with RXTE. The nearly coherent oscillations detected during the few seconds of thermonuclear burning are most simply understood as rotational modulation of brightness asymmetries on the neutron star surface. We show that, as suggested by Strohmayer and colleagues, the frequency changes of 1-2 Hz observed during bursts are consistent with angular momentum conservation as the burning shell hydrostatically expands and contracts. We calculate how vertical heat propagation through the radiative outer layers of the atmosphere and convection affect the coherence of the oscillation. We show that the evolution of the rotational profile depends strongly on whether the burning layers are composed of pure helium or mixed hydrogen/helium. Our results help explain the absence (presence) of oscillations from hydrogen-burning (helium-rich) bursts that was found by Muno and collaborators. We investigate angular momentum transport within the burning layers and the recoupling of the burning layers with the star. We show that the Kelvin-Helmholtz instability is quenched by the strong stratification, and that mixing between the burning fuel and underlying ashes by the baroclinic instability does not occur. However, the baroclinic instability may have time to operate within the differentially rotating burning layer, potentially bringing it into rigid rotation.Comment: To appear in The Astrophysical Journal; minor corrections made to tables and figure