Magnetic spectral response and lattice properties in mixed-valence Sm1-x Yx S solid solutions studied with x-ray diffraction, x-ray absorption spectroscopy, and inelastic neutron scattering

摘要

Mixed-valence phenomena occurring in the "black" (B) and "gold" (G) phases ofSm1-x Yx S have been studied by x-ray diffraction, x-ray absorptionspectroscopy, and inelastic neutron scattering. Lattice-constant andphonon-dispersion results confirm that the valence instability occurs alreadyinside the B phase. On the other hand, pronounced temperature anomalies in thethermal expansion \alpha(T), as well as in the Sm mean-square displacementsdenote the onset of the B-G transition for the compositions x = 0.33 and 0.45.It is argued that these anomalies primarily denote an effect of electron-phononcoupling. The magnetic spectral response, measured on both powder and singlecrystals, is dominated by the Sm2+ spin-orbit component close to 36 meV. Astrongly overdamped Sm3+ contribution appears only for x >= 0.33 nearroom-temperature. The quasielastic signal is strongly suppressed below 70 K,reflecting the formation of the singlet mixed-valence ground state. Quiteremarkably, the signal around 36 meV is found, from the single-crystal spectra,to arise from two distinct, dispersive, interacting branches. The lower peak,confirmed to exist from x = 0.17 to x = 0.33 at least, is tentatively ascribedto an excitation specific to the mixed-valence regime, reminiscent of the"exciton" peak reported previously for SmB6 .