Neutron scattering measurements have been performed with time-of-flight spectrometers on an YbB 12 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 SmB 6 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.

There are two structures in the photoemission spectra of CaVO3 z and SrVO3 z in the d-band region.One peak is within -1 eV of the Fermi level and has a sharp Fermi cutoff; The other is a broad peak centered at -1.6 eV. From our photoemission spectra of CaVO& z and SrVO3 z both with oxygen vacancies (5= -0. 1) and without vacancies (5=0), it is shown that the relative intensity of the peak near the Fermi level for CaVO& z drastically decreases with decreasing oxygen vacancy concentration (6). In contrast, no such drastic change in intensity is observed for SrVO3 z. This difference in the effect of oxygen vacancies between SrVO3 q and CaVO& z reflects the fact that a metal-insulator phase transition induced by bulk oxygen vacancies takes place in CaVO& t; but not in SrVO3

Peculiarities in the lattice dynamics of the Kondo insulator Y bB(12) have been studied by inelastic neutron scattering. Selected phonon modes were traced above and below the temperature region (T ~ 50 K) where the gap opens in the electron density of states. The intensities of some low-energy modes exhibit an anomalous temperature dependence for q vectors close to the Brillouin zone boundary, suggesting a renormalization of the phonon eigenvectors. This effect is thought to arise from a coupling with magnetic excitations of the same symmetry, which exist at nearby energies. It is argued that this magnetovibrational coupling may in turn play a role in the steep temperature crossover existing in Y bB(12) between the low-temperature (Kondo insulator) and high-temperature (incoherent spin-fluctuation) regimes, which is rapidly suppressed by lighter Zr substitution.

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