172 research outputs found
Analysis of f-p model for octupole ordering in NpO2
In order to examine the origin of octupole ordering in NpO2, we propose a
microscopic model constituted of neptunium 5f and oxygen 2p orbitals. To study
multipole ordering, we derive effective multipole interactions from the f-p
model by using the fourth-order perturbation theory in terms of p-f hopping
integrals. Analyzing the effective model numerically, we find a tendency toward
\Gamma_{5u} antiferro-octupole ordering.Comment: 4 pages, 3 figure
Resonant x-ray scattering spectra from multipole orderings: Np M_{4,5} edges in NpO2
We study resonant x-ray scattering (RXS) at Np M_{4,5} edges in the
triple-\textbf{k} multipole ordering phase in NpO_{2}, on the basis of a
localized electron model. We derive an expression for RXS amplitudes to
characterize the spectra under the assumption that a rotational invariance is
preserved in the intermediate state of scattering process. This assumption is
justified by the fact that energies of the crystal electric field and the
intersite interaction is smaller than the energy of multiplet structures. This
expression is found useful to calculate energy profiles with taking account of
the intra-Coulomb and spin-orbit interactions. Assuming the \Gamma_{8}-quartet
ground state, we construct the triple-\textbf{k} ground state, and analyze the
RXS spectra. The energy profiles are calculated in good agreement with the
experiment, providing a sound basis to previous phenomenological analyses.Comment: 10 pages, 7 figure
Microscopic theory of multipole ordering in NpO2
In order to examine the mysterious ordered phase of NpO2 from a microscopic
viewpoint, we investigate an f-electron model on an fcc lattice constructed
based on a j-j coupling scheme. First, an effective model with multipole
interactions is derived in the strong-coupling limit. Numerical analysis of the
model clearly indicates that the interactions for \Gamma_{4u} and \Gamma_{5u}
moments are relevant to the ground state. Then, by applying mean-field theory
to the simplified model including only such interactions, we conclude that
longitudinal triple-q \Gamma_{5u} octupole order is realized in NpO2 through
the combined effects of multipole interactions and anisotropy of the
\Gamma_{5u} moment.Comment: 5 pages, 2 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
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