K. S. Nemkovski

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.

Spin dynamics in Yb- and Sm-based systems with the nonmagnetic ground state

The nature and formation conditions of the ground state and the dynamic magnetic response (the magnetic excitation spectrum) of the YbB12 and Sm(Y)S systems with the nonmagnetic ground state have been studied using inelastic neutron scattering.

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.

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.

Magnetic dynamics in an electron-doped YbB12 condo insulator

Neutron spectroscopy results are presented for electron-doped YbB12-based systems. Electrondoping effects were obtained through the partial substitution of Yb for four-valence Zr, and through the introduction of a small amount of carbon in the boron sublattice of a YbB12 condo insulator. The different character of the original YbB12 gap-type magnetic-excitation-spectrum changes was observed.

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.

Evolution of the magnetic excitation spectra of the YbB12 kondo insulator under variations in temperature

The results obtained from experiments on inelastic neutron scattering in the YbB12 Kondo insulator are discussed. The structure of the magnetic excitation spectra and their evolution under variations in temperature are interpreted in the framework of a simple phenomenological model with due regard for the crystal field effects renormalized as a result of the influence of the intermediate valence state.