Neutron scattering study of the magnetic excitation spectra in mixed-valence¹⁵⁴Sm₃Te₄

Abstract

The magnetic properties of the unconventional mixed-valence compound Sm3 Te4 have been studied by inelastic neutron scattering. Time-of-flight experiments with different incident energies (Ei = 300 meV, 67 meV and 3.2 meV) and energy resolutions have been performed on isotopic (low-absorption) 154 Sm3 Te4 and on the reference compound La3 Te4 . Spectral contributions from two coexisting valence states Sm2+ and Sm3+ were clearly identified. In contrast to previous results for SmB6 , the spectra can be analysed in terms of well defined 4f 5 and 4f 6 local moments subjected to the crystal field. However, the data reveal that di- and trivalent ions, which occupy identical crystallographic sites, feel quite different crystal-field potentials. Quasielastic scattering was found to exist down to the minimum temperature (1.5 K) of the measurements, with a half-width of about 0.45 meV. This implies that spin fluctuations, with a much higher fluctuation rate than the valence fluctuations reported in a recent NMR study, exist just above the spin-glass transition.

Magnetic spectral response and lattice properties in mixed-valenceSm1−xYxSsolid solutions studied with x-ray diffraction, x-ray absorption spectroscopy, and inelastic neutron scatte

“…From such a picture, the occurrence of two well defined peaks seems very unlikely. This argument is further supported by previous neutron measurements 35 on the inhomogeneous MV compound Sm 3 Te 4 , in which inter-atomic distances are close to those in SmS. In that compound, the ratio of Sm 2+ to Sm 3+ is exactly one half, and the probabilities of finding 2+ and 3+ ions on the first rare-earth coordination sphere of Sm are comparable.…”

Electronic Structure of Strongly Correlated Systems

“…Assuming ideal stoichiometry, simple charge balance for the nominal formula Sm 2+ (Sm 3+ ) 2 (X 2− ) 4 implies that no free carriers are available to populate the conduction band, which explains the lack of metallic conduction observed experimentally. Thermally activated electrical transport takes place, however, through the hopping of electrons among the rare-earth sites [443]. Thus, near room temperature, the Sm ions fluctuate rapidly between two configurations 4 f 5 and 4 f 6 , giving rise to the so-called "thermal valence fluctuations."…”