Electronic state of PuCoGa5 and NpCoGa5 as probed by polarized neutrons.

By using single crystals and polarized neutrons, we have measured the orbital and spin components of the microscopic magnetization in the paramagnetic state of NpCoGa(5) and PuCoGa(5). The microscopic magnetization of NpCoGa(5) agrees with that observed in bulk susceptibility measurements and the magnetic moment has spin and orbital contributions as expected for intermediate coupling. In contrast, for PuCoGa(5), which is a superconductor with a high transition temperature, the microscopic magnetization in the paramagnetic state is small, temperature-independent, and significantly below the value found with bulk techniques at low temperatures. The orbital moment dominates the magnetization.

High pressure study of Pu(0.92)Am(0.08) binary alloy.

The phase transitions (by means of x-ray diffraction) and electrical resistivity of a Pu(0.92)Am(0.08) binary alloy were determined under pressure (up to 2 GPa). The evolution of atomic volume with pressure gives detailed information concerning the degree of localization of 5f electronic states and their delocalization process. A quasi-linear V = f(P) dependence reflects subtle modifications of the electronic structure when P increases. The electrical resistivity measurements reveal the very high stability of the δ phase for pressures less than 0.7 GPa, since no martensitic-like transformation occurs at low temperature. Remarkable electronic behaviours have also been observed. Finally, resistivity curves have shown the temperature dependence of the phase transformations together with unexpected kinetic effects.

Photoelectron spectroscopy of NpPd(3) and PuPd(3).

We present the results of x-ray and ultraviolet photoelectron spectroscopy of NpPd(3) and PuPd(3). The spectra indicate that for both compounds, the 5f electrons are well localized on the actinide sites. Comparison with bulk measurements indicates that for NpPd(3) the electrons have a valence of Np(3+) and thus a ground state 5f(4) with a Hund's rules (5)I(4) configuration. Similarly for PuPd(3), we find a Pu(3+) valence, 5f(5) ground state and a Hund's rules (6)H(5/2) configuration.

Magnetic properties of NpNiGa(5).

We have investigated a polycrystalline sample of NpNiGa(5) by magnetization, specific heat and (237)Np Mössbauer spectroscopy. We confirm that a ferromagnetic order sets in at T(C)≈30 K and that NpNiGa(5) undergoes a second transition at T(N)≈18 K. Mössbauer data indicate that the transition at T(N) is accompanied by a steep increase of the Np magnetic moment, which saturates to a value of 0.94 µ(B), in good agreement with the recently published neutron diffraction results. The Sommerfeld specific heat coefficient γ = 114 mJ mol(-1) K(-2) is the highest among the NpTGa(5) compounds. It compares to the ones observed in the PuTGa(5) superconductors. The trend of the isomer shift along the NpTGa(5) (T = Fe, Co, Ni, Rh) series points to a tendency towards localization of the 5f electrons when increasing the number of d electrons of the transition metal T. Neither fully localized nor fully itinerant models are able to reproduce the whole experimental data. Realistic models should include the dual nature of the 5f electrons, i.e. consider the difference in the degree of itinerancy among 5f electrons in the NpTGa(5) compounds.

Determination of the antiferroquadrupolar order parameters in UPd(3).

By combining accurate heat capacity and x-ray resonant scattering results we have resolved the long standing question regarding the nature of the quadrupolar ordered phases in UPd(3). The order parameter of the highest temperature quadrupolar phase has been uniquely determined to be antiphase Q{zx} in contrast to the previous conjecture of Q{x{2}-y{2}}. The azimuthal dependence of the x-ray scattering intensity from the quadrupolar superlattice reflections indicates that the lower temperature phases are described by a superposition of order parameters. The heat capacity features associated with each of the phase transitions characterize their order, which imposes restrictions on the matrix elements of the quadrupolar operators.

Specific heat of delta-Pu stabilized by Am.

Detailed specific heat C(p) measurements of delta-Pu stabilized by Am (8%-20%) were performed in the temperature range 4.5-300 K. The coefficient of the electronic specific heat gamma, which reflects the quasiparticle density of states at the Fermi level E(F), is smaller than originally assumed and, depending on the estimate of phonon contributions, a value between 35 and 55 mJ/mol K2 can be deduced for Pu-8% Am. For higher Am concentrations, which expand the lattice, gamma decreases slightly with the Am content. An applied magnetic field of 9 T had no effect on C(p). The results strongly suggest that itinerant 5f states at E(F) are not appropriate for describing delta-Pu.

Structural tuning of unconventional superconductivity in PuMGa5 (M=Co,Rh).

The superconducting properties of the recently discovered PuMGa5 (M=Co,Rh) superconductors, including the power law behavior of the specific heat, the evolution of the superconducting transition T(c) temperature with pressure, and the linear relation between T(c) and ratio of tetragonal lattice parameters c/a, are compared to those of the heavy fermion CeMIn5 (M=Co,Rh,Ir) unconventional superconductors. The striking similarity of the properties between the two families of superconductors suggests a common physics and a common (magnetically mediated) mechanism of superconductivity.

Plutonium-based superconductivity with a transition temperature above 18 K.

Plutonium is a metal of both technological relevance and fundamental scientific interest. Nevertheless, the electronic structure of plutonium, which directly influences its metallurgical properties, is poorly understood. For example, plutonium's 5f electrons are poised on the border between localized and itinerant, and their theoretical treatment pushes the limits of current electronic structure calculations. Here we extend the range of complexity exhibited by plutonium with the discovery of superconductivity in PuCoGa5. We argue that the observed superconductivity results directly from plutonium's anomalous electronic properties and as such serves as a bridge between two classes of spin-fluctuation-mediated superconductors: the known heavy-fermion superconductors and the high-T(c) copper oxides. We suggest that the mechanism of superconductivity is unconventional; seen in that context, the fact that the transition temperature, T(c) approximately 18.5 K, is an order of magnitude greater than the maximum seen in the U- and Ce-based heavy-fermion systems may be natural. The large critical current displayed by PuCoGa5, which comes from radiation-induced self damage that creates pinning centres, would be of technological importance for applied superconductivity if the hazardous material plutonium were not a constituent.

5f-electron localization in PuSe and PuSb

Thin layers of PuSb and PuSe were studied by photoelectron spectroscopy. X-ray photoelectron spectroscopy and high-resolution valence-band ultraviolet photoelectron spectroscopy spectra show localization of the 5f states and a low density of states at E(F) in PuSb. In PuSe, which can be classified as a heavy fermion system with low carrier density, we observed three narrow peaks in the valence band, which can be related to the 5f emission. These three features are very sensitive to stoichiometry deviations and disappear for PuSe prepared at T = 77 K.