Neutron scattering studies of mixed-valence semiconductors

Neutron scattering experiments on the mixed-valence (MV) compounds SmB6 and TmSe are reported. The inelastic magnetic response of SmB6 at T=2 K, measured on a double-isotope single crystal, displays a strongly damped peak at 35 meV corresponding to the intermultiplet transition of Sm2+. At lower energies (hω∼14 meV), a narrow magnetic excitation is observed, with remarkable scattering-vector and temperature dependences of its intensity. This novel feature is discussed in terms of recent theoretical works describing the formation of an anisotropic local bound state in semiconducting MV materials. If the average samarium valence is decreased by substituting La for Sm, a peak is found to appear at higher energies. The elastic magnetic form factor of SmB6 was determined using polarised neutrons and no significant difference is observed in its Q-dependence with respect to that of pure divalent samarium. This surprising behaviour is consistent with previous measurements on the gold (high-pressure) phase of SmS.The above results are compared with those already reported for other MV materials. In particular existing information for TmSe is supplemented by recent inelastic scattering measurements carried out on a large single crystal. (orig.)