No abstract available

[en] Short note

[en] 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. -- Graphical abstract: The neutron data indicate that the spin gap is not suppressed by the dilution process even for large Lu concentrations. However, the breakdown 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

[en] Polycrystalline YbB_x (x = 5.7-6.3) were prepared and their thermoelectric properties were measured to discuss the possibility of p-type thermoelectric material. P-type Seebeck coefficient was observed at around room temperature in the specimens with nominal composition of x=6.1 and 6.3, though they turned into n-type at high temperature due to thermal excitation of n-type carrier. Seebeck coefficient and electrical conductivity of the p-type YbB₆ were as high as those of n-type alkaline-earth hexaborides, suggesting the possibility of p-type counterpart to the n-type ones.

[en] Neutron scattering measurements have been performed with time-of-flight spectrometers on an YbB₁₂ polycrystal. The spectra show clearly the presence of magnetic excitations and a gap at around 14 meV. At low temperature, two broad signals (FWHM ∼ 7 meV), one asymmetric, the other one of Lorentzian shape, are well defined. After subtracting the phonon contribution, the former signal is further decomposed into a sharp 15.5 meV peak and a broader 20 meV peak. The 15.5 meV peak behaves similarly to that of the exciton at 14.5 meV for SmB₆ as regards both the Q- and T-dependencies. The 20 meV peak was identified as manifesting a transition from a localized Kondo singlet with a gap to a magnetic state. With decreasing temperature, the gap disappears and the scattering becomes quasi-elastic, like for the metallic Kondo system. The second signal, centred at 37 meV, was assigned previously to a crystal-field excitation. However, the observed behaviour is anomalous, indicating a shift of the intensity from the lower peaks to the higher peak, suggesting a change in the T_K-value due to lattice distortion. (author)

[en] Thermoelectric power of the Kondo insulators is analyzed theoretically. It is pointed out that Kelvin's relation found in 1851 must be modified for insulators at low temperatures. A realistic tight-binding model is constructed for the description of the conduction bands of the most typical Kondo insulator YbB₁₂. It will be a good starting point for the development of a realistic theory of the Kondo insulators with strong correlation

[en] Monophase YbB₁₂ samples have been used to determine the magnetic properties, the lattice parameter and the displacement of the K-series X-ray lines of YbB₁₂ relatively to three-valent reference compound YbF₉. Ytterbium in the dodecaboride is shown to have a three-valent state in its hexaboride ytterbium is two-valent

No abstract available

[en] B₁₂ icosahedra or related structure elements determine the different modifications of elementary boron and numerous boron-rich compounds from α-rhombohedral boron with 12 to YB₆₆ type with about 1584 atoms per unit cell. Typical are well-defined high density intrinsic defects: Jahn-Teller distorted icosahedra, vacancies, incomplete occupancies, statistical occupancies and antisite defects. The correlation between intrinsic point defects and electron deficiencies solves the discrepancy between theoretically predicted metal and experimentally proved semiconducting character. The electron deficiencies generate split-off valence states, which are decisive for the electronic transport, a superposition of band-type and hopping-type conduction. Their share depends on actual conditions like temperature or pre-excitation. The theoretical model of bipolaron hopping is incompatible with numerous experiments. Technical application of the typically p-type icosahedral boron-rich solids requires suitable n-type counterparts; doping and other possibilities are discussed.

No abstract available