E. V. Nefeodova

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.

Spin dynamics of the intermediate-valence compound EuCu2Si2

The dynamic magnetic response of the intermediate-valence compound EuCu2Si2 has been studied using inelastic neutron scattering. At low temperatures, strong renormalization of the 7F0 → 7F1 spin-orbit transition energy is detected; it is likely to be related to partial delocalization of the f electrons of Eu. An increase in the temperature increases the valence instability of europium and results in further changes in the magnetic excitation spectrum parameters and the appearance of an intense quasi-elastic component.

Lattice dynamics and magneto-elastic coupling in Kondo-insulator YbB12

Lattice dynamics and magneto-elastic coupling effects have been studied in the Kondo insulator YbB12 by means of inelastic neutron scattering. The analysis of the phonon density of states, dispersion, and symmetry properties is presented in connection with a possible magneto-elastic coupling. Manifestation of such effects was found for the phonons corresponding to the vibrations of Yb atoms.

Intermultiplet transitions and crystal field in mixed valence Sm3Te4

Abstract Time-of-flight inelastic neutron-scattering measurements have been carried out on polycrystalline samples of 154 Sm 3 Te 4 and La 3 Te 4 with incoming neutron energies of 67 and 300 meV. The results are, in general, consistent with the assumption that both Sm valence states, Sm 2+ and Sm 3+ , contribute to the magnetic scattering function as well-defined local-moment states subjected to the crystal field from the surroundings.

Spin–orbit transitions in mixed-valence samarium compounds

Abstract Inelastic neutron scattering spectroscopy of spin–orbit transitions has been carried out on mixed-valence Sm compounds with different degrees of hybridization. Contrasted behaviours are observed in Sm3Te4 (narrow, crystal-field-split, inter-multiplet Sm2+ and Sm3+ transitions) and in Sm0.8Y0.2S or Sm0.5La0.5B6 (broadened inter-multiplet transitions, plus an extra peak at lower energy possibly due to an exciton-like state as found previously in SmB6).

Dynamics of boron nanoclusters in RB12 (R = Yb, Lu) systems

The phonon dispersion in RB12 (R = Yb, Lu) systems is studied in detail using inelastic neutron scattering over a wide range of energies (up to 55 meV). It is demonstrated that the main features of the lattice dynamics in the RB12 crystals can be described in terms of a simple model of force constants with a strong hierarchy of interatomic interactions in the systems: B-B ≫ B-R ≫ R-R. An interesting feature of the low-energy portion of the phonon spectrum is the weakly dispersive optical mode. According to the model calculations, this mode is associated with the relative displacements and “breathing” vibrations of the boron nanoclusters B12.

Neutron scattering study of the magnetic excitation spectra in mixed-valence 154Sm3Te4

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.

Neutron studies of crystal-field effects in PrB6

The roles of various physical mechanisms in the properties of the ground state of the Pr ion in PrB6 are studied by measuring magnetic excitation spectra in the paramagnetic and magnetically ordered phases of polycrystalline PrB6. The ground state of the Pr ion in the paramagnetic phase is experimentally found to be triplet Γ5. During the transition into the magnetically ordered state, this triplet splits into three singlet levels. The results obtained do not exclude the appearance of an additional contribution to the splitting induced by a decrease in the local symmetry because of structural distortions.

Vibrational spectra of the YbB12 Kondo insulator

The density of phonon states for the YbB12 Kondo insulator is calculated from the inelastic neutron scattering spectra of this compound. It is established that thermal vibrations of rare-earth atoms predominantly occur in the low-energy range. These atoms are most weakly bound in the crystal structure of the YbB12 Kondo insulator. The high-energy part of the vibrational spectrum is determined by thermal vibrations of the boron atoms forming a rigidly connected structure of the compound. It is revealed that the temperature dependence of the intensity of the phonon peak attributed to thermal vibrations of the ytterbium atoms exhibits an anomalous behavior. This circumstance suggests that the magnetoelastic coupling occurring in the structure of the YbB12 Kondo insulator is relatively strong and can contribute to the magnetic excitation spectrum of this compound.

Dynamical magnetic correlations in the YbB12 kondo insulator: Neutron investigations with a polarization analysis

The results of investigations into the spin dynamics in the YbB12 Kondo insulator with the use of inelastic neutron scattering, including experiments with a polarization analysis, are discussed. It is shown that, at low temperatures, the dynamic magnetic response in the structure of the YbB12 Kondo insulator is characterized by three dispersive excitations with a nontrivial q-dependence of the intensity. An increase in the temperature results in a crossover to the single-site spin fluctuation regime accompanied by suppression of collective excitations.