Crystal structure and physical properties of AmPd5Al2

We report on the crystal structure, magnetic susceptibility, specific heat, electrical and thermoelectrical properties of AmPd5Al2, the americium counterpart of the unconventional superconductor NpPd5Al2. AmPd5Al2 crystallizes in the ZrNi2Al5-type of structure with lattice parameters: a = 4.1298 A and c = 14.7925 A. Magnetic measurements of AmPd5Al2 indicate a paramagnetic behavior with no hint of magnetic ordering nor superconductivity down to 2 K. This aspect is directly related to its 5f6 electronic configuration with J = 0. The specific heat measurements confirm the non magnetic ground state of this compound. The low temperature electronic specific heat \gamma_el = 20 mJ mol-1K-2 is clearly enhanced as compared to americium metal. All transport measurements obtained point to a metallic behavior in AmPd5Al2.Comment: 7 pages, 6 figures; Accepted to be published in Physical Review B (January 24, 2011

Investigation of electrorefining of metallic alloy fuel onto solid Al cathodes

This work concerned the electrorefining of UZr and UPuZr alloys on a solid aluminium cathode, in the LiCl-KCl eutectic melt containing U3+, Pu3+, Np3+, Zr2+ or Zr 4+, Am3+, Nd3+, Y3+, Ce3+ and Gd3+ chlorides. During constant current electrolyses, the use of a cathodic cut-off potential (-1.25 V vs. Ag/AgCl) allowed to selectively deposit actinides (mainly U), while lanthanides remainedin the salt. The aim was to determine the maximal load achievable on a single aluminium electrode. The total exchange charge was 4300 C, which represents the deposition of 3.72 g of actinides in 4.17 g Al, yielding a composition of 44.6 wt% An in Al. It was shown that the melting of the cathode contributed to increase the total amount of actinides deposited on the aluminium

Magnetic and electrical properties of dhcp NpPd3 and U(1-x)Np(x)Pd3

We have made an extensive study of the magnetic and electrical properties of double-hexagonal closepacked NpPd3 and a range of U(1-x)Np(x)Pd3 compounds with x=0.01, 0.02, 0.05, and 0.50 using magnetization, magnetic susceptibility, electrical resistivity, and heat capacity measurements on polycrystalline samples, performed in the temperature range 2-300 K and in magnetic fields up to 9 T. Two transitions are observed in NpPd3 at T=10 and 30 K. Dilute Np samples (x<0.05) exhibit quadrupolar transitions, with the transition temperatures reduced from those of pure UPd3.Comment: 10 pages, 18 figure

Magnetic Excitations in NpCoGa5

We report the results of inelastic neutron scattering experiments on NpCoGa$_{5}$, an isostructural analogue of the PuCoGa$_{5}$ superconductor. Two energy scales characterize the magnetic response in the antiferromagnetic phase. One is related to a non-dispersive excitation between two crystal field levels. The other at lower energies corresponds to dispersive fluctuations emanating from the magnetic zone center. The fluctuations persist in the paramagnetic phase also, although weaker in intensity. This supports the possibility that magnetic fluctuations are present in PuCoGa$_{5}$, where unconventional d-wave superconductivity is achieved in the absence of magnetic order.Comment: 4 pages, 5 figure

Triple-q octupolar ordering in NpO_2

We report the results of resonant X-ray scattering experiments performed at the Np M_4,5 edges in NpO_2. Below T_0 = 25 K, the development of long-range order of Np electric quadrupoles is revealed by the growth of superlattice Bragg peaks. The electronic transition is not accompanied by any measurable crystallographic distortion, either internal or external, so the symmetry of the system remains cubic. The polarization and azimuthal dependence of the intensity of the resonant peaks is well reproduced assuming Templeton scattering from a triple-q longitudinal antiferroquadrupolar structure. Electric quadrupole order in NpO_2 could be driven by the ordering at T_0 of magnetic octupoles of Gamma_5 symmetry, splitting the Np ground state quartet and leading to a singlet ground state with zero dipole magnetic moment.Comment: 4 Pages, 3 Figures, submitted to Phys. Rev. Lett. v2: resubmitted after referee report

Possible mechanism of superconductivity in PuCoGa5 probed by self-irradiation damage

Measurements of the electrical resistivity of a polycrystalline PuCoGa5 sample reveal significant modifications of the superconducting properties as a function of time, due to the increase of defects and impurities resulting from self-irradiation damage. More than four years of aging were necessary to detect a deviation from linearity in the time dependence of the critical temperature. The observed behavior is understood in the framework of the Eliashberg theory, confirming the ¿dirty¿ d-wave character which was already suggested by nuclear magnetic resonance. We show that experimental data accumulated so far can be well reproduced by assuming a phononic mechanism for superconductivity, with reasonable values of the electron-phonon coupling and Coulomb pseudopotential. Further experiments are then required to assess the role of spin fluctuations in stabilizing the superconducting state in this compound.JRC.E.6-Actinides researc