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Persistent URL http://purl.org/net/epubs/work/29772
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Record Id 29772
Title Neptunium octupole and hexadecapole motif in NpO2 directly from electric-dipole (E1) enhanced x-ray Bragg diffraction
Contributors
Abstract The phase transition in NpO2 at To (approximately = to) 25.5K is accompanied by the onset of superlattice reflections in the x-ray Bragg diffraction pattern, with intensity enhanced by an electric-dipole (E1) event. Additional experiments using other techniques indicate no ordering at To of Np magnetic moments. Absence of long-range magnetic order below To fits with the outcome of a polarization analysis of superlattice intensities at 12K; signals are observed in both the unrotated (sigma'sigma) and rotated (pi'sigma) channels of scattering while magnetic (dipole) moments would contribute only in the rotated channel. We demonstrate that these empirical findings, together with a narrow energy profile of the Bragg intensity at the Np M4 edge, are consistent with magnetic and charge contributions to the E1 Bragg amplitude described by Np 5f multipoles of ranks 3 (octupole) and 4 (hexadecapole). Key to our understanding of the x-ray diffraction data gathered in the vicinity of the Np M4 edge is recognition of an exchange field creating 3d(subscript 3/2) core-level structure. The particular importance of the exchange field at the Np M4 edge is emphatically revealed in calculations of the corresponding XAS spectrum with and without the core-valence interaction. From the experimental information about NpO2 we can infer a ground-state wave function for the Np 5f(superscript 3) valence shell and estimate saturation values for the octupole and hexadecapole. We are led to null values for Np multipoles of ranks 2 (quadrupole) and 5 (triakontadipole).
Organisation CCLRC
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Related Research Object(s): 1740
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Language English (EN)
Type Details URI(s) Local file(s) Year
Report RAL Technical Reports RAL-TR-2003-002. 2003. raltr-2003002.pdf 2003