P. A. Alekseev

Magnetic excitation spectrum of mixed-valence SmB6 studied by neutron scattering on a single crystal

The paramagnetic response of mixed-valence samarium hexaboride has been studied by inelastic neutron scattering using a low-absorption double-isotope single crystal of 154Sm11 B6. Measurements were performed for energy transfers 0<or=h(cross) omega <or=50 meV at temperatures ranging from 2 to 100 K. Two main contributions were observed in the spectra: a broad intermultiplet transition of single-ion type at h(cross) omega approximately=36 meV, and a narrow low-energy excitation centred at h(cross) omega approximately=14 meV, which is strongly anisotropic and temperature dependent and exhibits a weak but visible dispersion. The latter excitation appears to be an inherent feature of the mixed-valence state due to f-electron hybridization effects, which denotes the formation of a local bound state around the Sm sites.

Lattice Dynamics of Intermediate Valence Semiconductor SmB6.

All acoustic and the lowest optic branches in three main symmetry directions have been determined by inelastic neutron scattering at room temperature using a double isotope monocrystal 154Sm11B6. In comparison with the integer valence system LaB6 the phonons are softer, and the LA branches exhibit remarkable anomalies. These features are interpreted on the basis of a specific coupling of the lattice vibrations with the dipole (f-d) and monopole (f-f) excitations of the Sm shells, the latter being characteristic for the mixed valence semiconductor. A weak broad signal has been observed in the energy gap between acoustic and optic modes, which can be ascribed to an additional mode appearing due to the nonadiabatic interaction of phonons with valence fluctuations.

Neutron Scattering Study of the Intermediate-Valent Ground State in SmB6

Inelastic-neutron-scattering spectra were measured on the polycrystalline double isotopic sample 154Sm 11B6 in a wide range of the energy transfer. The intermultiplet transitions corresponding to f6 and f5 configurations of the intermediate-valent state of the Sm ion, as well as a low-energy excitation with unusual momentum transfer and temperature dependences have been observed in the magnetic part of the scattering function.

Evidence for Short-Range Antiferromagnetic Fluctuations in Kondo-Insulating YbB12

The spin dynamics of mixed-valence YbB12 has been studied by inelastic neutron scattering on a highquality single crystal. In the Kondo-insulating regime realized at low temperature, the spectra exhibit a spin-gap structure with two sharp, dispersive, in-gap excitations at � h! � 14:5 and � 20 meV. The lower mode is shown to be associated with short-range correlations near the antiferromagnetic wave vector q0 �� 1 ; 1 ; 1 � . Its properties are in overall agreement with those expected for a ‘‘spin-exciton’’ branch in an indirect hybridization gap semiconductor.

Magnetic excitations in SmB6 single crystals

Abstract Two, essentially dispersionless, magnetic excitations, at energies ℏ ω 1 ≃ 36 and ℏ ω 2 ≃ 14 meV, have been investig ated by inelastic neutron scattering measurements on a single crystal of mixed-valent 154 Sm 11 B 6 . The latter one decreases markedly upon heating, or with increasing | Q | . It is ascribed to the spin-flip of 5d electrons which form a bound state with the 4f electrons of a given site by tunnelling over the octahedron defined by its six nearest Sm neighbours.

Gap formation in CeNiSn at low temperatures

Abstract Transport, thermoelectric, calorimetric properties and inelastic neutron spectra of the CeNiSn and solid solutions Ce1-xLaxNiSn (0

Low-temperature effects in magnetic spectral response of CeAl3-based systems

Magnetic excitation spectra of Ce ions in CeAl3 and in the substituted systems Ce0.9Y0.1Al3 and Ce0.9Gd0.1Al3 were studied by inelastic neutron scattering in the temperature range 0.03 K < T < 100 K. Below ≈ 60 K peak positions are strongly temperature dependent. The effect is enhanced due to chemical pressure and looks like a pure single ion feature given by the interaction of the Ce ion with the conduction electrons. Additional investigations of the spectra of the paramagnetic impurity ion Pr3+ in CeAl3 and in the reference system LaAl3 showed an anomalous attenuation of the relaxation broadening for the CEF excitations in the CeAl3 matrix. All observed features are interpreted as a consequence of the Kondo ground state formation, strongly influenced by the interaction between the crystalline electric field and the local moment of the Ce Kondo ion.

Neutron scattering studies of mixed-valence semiconductors

Abstract 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 ( h ω ≈ 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.

Temperature effects in phonon dispersion of SmB6 intermediate valence semiconductor

Abstract Temperature dependence of previously discovered anomalies in SmB 6 acoustic and low-lying phonon branches was investigated on three-axis and time-of-flight spectrometers. The largest changes in phonon frequencies with respect to the room temperature data were found for longitudinal branches while all transverse phonons hardened slightly. The unusual nature of the previously observed dispersionless mode in the energy gap between acoustic and optic phonons was confirmed. Some new peculiarities of the SmB 6 excitation spectra were obtained at low temperature.

Magnetic excitations in EuCu2(SixGe1−x)2: from mixed valence towards magnetism

We report the inelastic neutron scattering study of spin dynamics in EuCu(2)(Si(x)Ge(1-x))(2) (x = 1, 0.9, 0.75, 0.6), performed in a wide temperature range. At x = 1 the magnetic excitation spectrum was found to be represented by the double-peak structure well below the energy range of the Eu(3+) spin-orbit (SO) excitation (7)F(0)→(7)F(1), so that at least the high-energy spectral component can be assigned to the renormalized SO transition. Change of the Eu valence towards 2 + with increased temperature and/or Ge concentration results in further renormalization (lowering the energy) and gradual suppression of both inelastic peaks in the spectrum, along with developing sizeable quasielastic signal. The origin of the spectral structure and its evolution is discussed in terms of excitonic model for the mixed valence state.