N. Bernhoeft

Triple-q Octupolar Ordering in NpO2.

We report the results of resonant x-ray scattering experiments performed at the Np M(4,5) edges in NpO2. 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 polarization and azimuthal dependence of the intensity of the resonant peaks are well reproduced assuming anisotropic tensor susceptibility scattering from a triple-q(-->) longitudinal antiferroquadrupolar structure. Electric-quadrupole order in NpO2 could be driven by the ordering at T0 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.

Multipolar ordering in NpO2 below 25 K

The phase transition at T0 = 25 K in NpO2 and the single-ion nature of the Np 5f electrons is examined in the light of the results of resonant x-ray scattering experiments at the M4 Np edge. These experiments exclude usual magnetic dipole ordering at T0, and provide direct evidence of long-range order of the electric quadrupole moment with ?5 symmetry. The phase transition is purely electronic and does not involve either internal or external crystallographic distortions, so the symmetry of the system remains cubic. The primary order parameter (OP) is associated with ?4t magnetic octupoles, ordering in a triple-q longitudinal structure defined by the three wavevectors of the 001 star. Magnetic octupolar order breaks invariance under time reversal and induces the order of electric quadrupoles as the secondary OP. The resulting ground state is a singlet with zero dipole magnetic moment.

Polarization analysis in the 3-25 keV range at the ESRF magnetic scattering beamline.

The ESRF magnetic scattering beamline has been optimized for easy tunability of the polarization and energy in the 3-40 keV range. The linear horizontal polarization from the undulator reaches 99.9%, with a flux of approximately 10(12) photons s(-1) at the sample. The diffractometer can operate in horizontal and vertical geometries, with an energy or polarization analyser. The capabilities of this beamline in terms of flux, energy tunability and polarization, permitted polarization analysis of resonant magnetic scattering from antiferromagnetic UPd(2)Si(2) at both the L(2)- and M(4)-edges of uranium, to separate the contributions of the 5f and 6d electrons to the magnetism.

Resonant x-ray scattering study of Np Rh Ga 5 and Np Co Ga 5

We report a resonant x-ray scattering (RXS) study of antiferromagnetic neptunium compounds NpCoGa_5 and NpRhGa_5 at the Np M_4 and Ga K-edges. Large resonant signals of magnetic dipole character are observed below the Neel temperatures at both edges. The signals at the Np edges confirm the behaviour determined previously from neutron diffraction, i.e. the moments along [001] in NpCoGa_{5} and in NpRhGa_5 a reorientation of the moments from the c-axis direction to the ab plane. In the latter material, on application of magnetic field of 9 Tesla along the [010] direction we observe a change in the population of different [110]-type domains. We observe also a magnetic dipole signal at the Ga K-edge, similarly to the reported UGa_3 case, that can be interpreted within a semi-localized model as an orbital polarization of the Ga 4p states induced via strong hybridization with the Np 5f valence band. Quantitative analysis of the signal shows that the Ga dipole on the two different Ga sites follows closely the Np magnetic moment reorientation in NpRhGa_5. The ratios of the signals on the two inequivalent Ga sites are not the same for the different compounds.

Resonant x-ray scattering fromUAs0.8Se0.2:Multi-kconfigurations

Using resonant x-ray scattering to perform diffraction experiments at the U M

Magnetization dynamics in the normal and superconducting phases of UPd2Al3: II. Inferences on the nodal gap symmetry

_{4}

Resonant x-ray scattering study of NpRhGa5 and NpCoGa5

edge novel reflections of the generic form

Use of polarization analysis in the understanding of resonant magnetic x-ray scattering at the L2,3 edges of rare earths

have been observed in UAs

Antiferromagnetic layer formation in UAs

_{0.8}