Neutron studies of crystal-field effects in PrB<Subscript>6</Subscript>

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 YbB<Subscript>12</Subscript> 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.     

Spin-gap magnetic response in (Yb, Lu)B12

To clarify the role of Yb-Yb correlations in the formation of the gap-like excitation spectrum of YbB₁₂, the spin dynamics of strongly diluted Yb_0.25Lu_0.75B₁₂ have been studied by inelastic neutron scattering in a wide temperature range. The data indicate that the spin gap is not suppressed by the dilution process even for large concentrations of Lu. However, the breaking-down of the Yb-sublattice periodicity leads to a strong smearing of the low-energy features and to a moderate suppression of the high-energy peak in the magnetic spectral response of YbB₁₂. The interrelation of the spin and charge gaps is discussed.     

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.

     

Spin-glass, antiferromagnetism and kondo behavior in Ce2Au1−x Co x Si3 alloys

Recently, the solid solution Ce₂Au_{1− x Co} xSi₃ has been shown to exhibit many magnetic anomalies associated with the competition between magnetic ordering and the Kondo effect. Here we report high pressure electrical resistivity of Ce₂AuSi₃, ac susceptibility (X) and magnetoresistance of various alloys of this solid solution in order to gain better knowledge of the magnetism of these alloys. High pressure resistivity behavior is consistent with the proposal that Ce₂AuSi₃ lies at the left-hand side of the maximum in Doniach’s magnetic phase diagram. The ac X data reveal that there are in fact two magnetic transitions, one at 2 K and the other at 3 K for this compound, both of which are spin-glass-like. However, as the Co concentration is increased, antiferromagnetism is stabilized for intermediate compositions before attaining non-magnetism for the Co end member.     

The growth of a single crystal of Sr3CuIrO6 and its magnetic behavior compared to polycrystals

We have grown single crystals of the psuedo-one-dimensional compound Sr₃CuIrO₆, a K₄CdCl₆-derived monoclinic structure with Cu-Ir chains along the [101] direction. We present the ac and dc magnetization behavior of the single crystals in comparison with that of the polycrystalline form reported earlier. There is a distinct evidence for at least two magnetic transitions, at 5 K (T ₁) and 19 K (T ₂), with different relative magnitudes in the single and polycrystals. The low temperature magnetic relaxation behavior of both the forms is found to be widely different, exhibiting unexpected time dependence.     

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.     

Lattice dynamics in the Kondo insulator YbB12

The phonon dispersion in the Kondo-insulator YbB₁₂ and its structure analogue LuB₁₂ has been studied in a wide energy range (up to 55 meV) by means of inelastic neutron scattering. The specific shape of phonon dispersion curves for low-frequency lattice vibrations could be described on the basis of a strong hierarchy suggested for the interactions between boron and rare-earth (RE) atoms: B-B?B-RE?RE-RE.