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NMR and spin/charge fluctuations in intermediate-valent SmB6

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Information on spin and charge fluctuations in the non-metallic intermediate-valent compound SmB6 has been obtained via 10B and 11B NMR between 2 and 300 K. Both the isotropic shift and the quadrupole coupling constant are temperature independent over this range, as expected if the fractional valence mixing is also temperature independent. Below 4.2 K the boron spin-lattice relaxation rate T 1-1 varies linearly with temperature, and yields a spin fluctuation rate τs-f1∼ 1013 sec-1, which corresponds to a spin fluctuation temperature Tsf∼ 50 K if characteristic of the stoichiometric compound. An increase of T1-1 above 20 K indicates the onset of Sm-spin relaxation by thermal excitations. The results are also consistent with relaxation by impurity-band states associated with vacancies, but the Korringa constant obtained on this assumption is unrealistically small. Comparison between data obtained from 10B and 11B resonances yields no indication of a contribution from electric field gradient fluctuations to the relaxation; an upper bound on the charge fluctuation time τcf≲3τsf is obtained. It is shown that correlations between Sm spin and charge fluctuations, if present, do not affect the nuclear relaxation in the extreme narrowing limit.

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