Invariant Form of Hyperfine Interaction with Multipolar Moments : Observation of Octupolar Moments in NpO_2 and Ce_<1-x>La_xB_6 by NMR(Condensed Matter: Electronic Structure, Electrical, Magnetic and Optical Properties)

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The invariant form of the hyperfine interaction between multipolar moments and the nuclear spin is derived, and applied to discuss possibilities to identify the antiferro-octupolar (AFO) moments by NMR experiments. The ordered phase of NpO_2 and the phase IV of Ce_<1-x>La_xB_6 are studied in detail. Recent ^<17>O NMR data for polycrystalline samples of NpO_2 are discussed theoretically from our formulation. The observed feature of the splitting of ^<17>O NMR spectrum into a sharp line and a broad line, their intensity ratio, and the magnetic field dependence of the shift and of the width can be consistently explained on the basis of the triple q AFO ordering model proposed by Paixao et al. [Phys. Rev. Lett. 89 (2002) 187202] Thus, the present theory shows that the ^<17>O NMR spectra give a strong support to the model. The four O sites in the fcc NpO_2 become inequivalent due to the secondary triple q ordering of AF-quadrupoles : one cubic and three non-cubic sites. It turns out that the hyperfine field due to the antiferro-dipole and AFO moments induced by the magnetic field, and the quadrupolar field at non-cubic sites are key ingredients to understand the observed spectra. The controversial problem of the nature of phase IV in Ce_<1-x>La_xB_6 is also studied. It is pointed out that there is a unique feature in the NMR spectra, if the Γ_5(T^β_x=T^β_y=T^β_z) AFO ordering is realized in Ce_<1-x>La_xB_6. Namely, the hyperfine splitting of a B atom pair on the (1/2, 1/2, ±u) sites crosses zero on the (110) plane when the magnetic field is rotated around the [001] axis.
社団法人日本物理学会の論文
2005-01-15

Invariant Form of Hyperfine Interaction with Multipolar Moments : Observation of Octupolar Moments in NpO_2 and Ce_<1-x>La_xB_6 by NMR(Condensed Matter: Electronic Structure, Electrical, Magnetic and Optical Properties)

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