Superconductivity in the YIr2Si2 and LaIr2Si2 Polymorphs

We report on existence of superconductivity in YIr2Si2 and LaIr2Si2 compounds in relation to crystal structure. The two compounds crystallize in two structural polymorphs, both tetragonal. The high temperature polymorph (HTP) adopts the CaBe2Ge2-structure type (space group P4/nmm) while the low temperature polymorph (LTP) is of the ThCr2Si2 type (I4/mmm). By studying polycrystals prepared by arc melting we have observed that the rapidly cooled samples retain the HTP even at room temperature (RT) and below. Annealing such samples at 900C followed by slow cooling to RT provides the LTP. Both, the HTP and LTP were subsequently studied with respect to magnetism and superconductivity by electrical resistivity, magnetization, AC susceptibility and specific heat measurements. The HTP and LTP of both compounds respectively, behave as Pauli paramagnets. Superconductivity has been found exclusively in the HTP of both compounds below Tsc (= 2.52 K in YIr2Si2 and 1.24 K in LaIr2Si2). The relations of magnetism and superconductivity with the electronic and crystal structure are discussed with comparing experimental data with the results of first principles electronic structure calculations

Multi-k⃗\vec{k} Configurations

Using resonant x-ray scattering to perform diffraction experiments at the U M$_{4}$ edge novel reflections of the generic form have been observed in UAs$_{0.8}$Se$_{0.2}$ where $\vec{k} = $, with $k = {1/2}$ reciprocal lattice units, is the wave vector of the primary (magnetic) order parameter. The reflections, with $10^{-4}$ of the magnetic intensities, cannot be explained on the basis of the primary order parameter within standard scattering theory. A full experimental characterisation of these reflections is presented including their energy, azimuthal and temperature dependencies. On this basis we establish that the reflections most likely arise from the electric dipole operator involving transitions between the core 3d and partially filled $5f$ states. The temperature dependence couples the peak to the triple-$\vec{k}$ region of the phase diagram: Below $\sim 50$ K, where previous studies have suggested a transition to a double-$\vec{k}$ state, the intensity of the $$ is dramatically reduced. Whilst we are unable to give a definite explanation of how these novel reflections appear, this paper concludes with a discussion of possible ideas for these reflections in terms of the coherent superposition of the 3 primary (magnetic) order parameters

Performance of molybdenum with UF4 at high temperatures as a wall material for space reactors

Thin 99.99 pet purity molybdenum (Mo) foils for use in gas core reactors have been tested in both liquid and gaseous uranium tetrafluoride (UF4) at temperature ranges of 1500 to 2000 K under argon pressures approaching 600 torr for exposure times of 15 to 75 minutes. Weight change measurements have been performed before and after the experiments. Surface and cross-sectional analyses of the samples have been done using scanning electron microscopy (SEM), electron microprobe (EMP), and energy dispersive spectroscopy (EDS). The Mo container, which was exposed for nearly 9 hours to both liquid and gas phase UF4, was also examined. No reaction or diffusion of uranium or fluorine has been found on the cross sections of the samples under the experimental conditions. These results indicate that molybdenum can be a candidate wall material in contact with UF4 as used as a circulating fuel at ultrahigh temperatures

Electronic properties of mixed uranium-thorium monosulphides (U xTh1-x)S

We have measured bulk magnetization, neutron magnetic scattering and specific heat between 1.5 and 300 K of mixed uranium-thorium monosulphides (U xTh1-x)S with large uranium concentrations (x ≽ 0.20). Long-range ferromagnetic order is observed at low temperature for 0.43 < x ≤ 1, but no long-range magnetic order occurs for x < 0.43. Magnetization and Curie temperature decrease linearly with uranium content. The electronic specific heat is maximum at the critical concentration xc = 0.43. Our experiments failed to reveal Schottky anomalies in specific heat or crystal-field levels by inelastic neutron scattering. Our results favour a band description rather than a localised description for uranium 5f electrons.Nous avons mesuré l'aimantation, la diffusion magnétique des neutrons et la chaleur spécifique entre 1,5 et 300 K de monosulfures mixtes d'uranium et de thorium (UxTh1-x)S contenant une proportion importante d'uranium (x ≽ 0,20). L'ordre ferromagnétique à grande distance est observé à basse température pour 0,43 < x ≤ 1, mais pour x < 0,43, il n'y a pas d'ordre magnétique à grande distance. L'aimantation spontanée et la température de Curie décroissent linéairement avec la concentration en uranium. La chaleur spécifique électronique est maximale à la concentration critique xc = 0,43. Nos mesures de chaleur spécifique n'ont pas montré d'anomalies de Schottky et nous n'avons pas observé de niveaux de champ cristallin en diffusion inélastique de neutrons. L'ensemble de nos résultats suggère une description de bande plutôt qu'une description localisée pour les électrons 5f de l'uranium

NEUTRON DIFFRACTION AND MAGNETIZATION STUDY OF THE MAGNETIC PHASE DIAGRAM OF UAs0.75Se0.25 SINGLE CRYSTAL

The magnetic phase diagram (magnetic field versus temperature) of UAs0.75Se0.25 has been determined by neutron diffraction and magnetization of single crystals in magnetic fields up to 67.2 and 190 kOe, respectively, applied along the cubic [001] axis