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We have tested the conduction band electrons of lanthanum hexaboride that is among the most effective electron-beam sources with one of the highest brightness of thermionic emission. We performed infrared spectroscopic, DC (direct current) resistivity and Hall-effect studies of LaB6 single crystals with various 10B and 11B isotope contents. We find that only a small amount of conduction electrons behave as Drude-type charge carriers while about 70% of the electrons are involved in collective oscillations of electron density coupled to vibrations of both the Jahn-Teller unstable rigid boron cage and rattling modes of La-ions loosely bound to the lattice. We suggest that exactly these non-equilibrium conduction electrons determine the extraordinary low work function of thermoemission in LaB6.

By measuring room temperature infrared (40–35000 cm–1) reflectivity of metallic LuB12 single crystals with different isotopic compositions (natB, 10B, 11B), we find that to model the spectrum we had to introduce, additionally to Drude free-carrier component, a broad excitation with unusually large dielectric contribution (Δe = 8000 ± 4000), which is characterized by a non-Lorentzian lineshape. It is suggested that the origin of the excitation is connected with cooperative dynamics of Jahn–Teller active B12 molecules producing quasilocal vibrations (rattling modes) of caged lutetium ions. The coupling of the Lu3+ rattling motions with the charge carriers of conduction band is proposed to be the reason of strongly damped character of the excitation.

The infrared reflection and transmission spectra of Bi2Te2Se single crystals grown by the modified Bridgeman method have been studied in the spectral range of 30–10000 cm−1 at temperatures of 5–300 K. The bandgap and its temperature dependence, optical function spectra, and concentration of free charge carriers in Bi2Te2Se have been determined.

The results of infrared reflectivity measurements for the iron-based high-temperature superconductor Ba(Fe0.9Co0.1)2As2 are reported. The reflectivity is found to be close to unity at frequencies w lower than 2Delta/h (2Delta is the superconducting gap and h is Planck's constant). This is evidence for the s+/- or s+/+ symmetry of the superconducting order parameter in the studied compound. The infrared reflectivity spectra of Ba(Fe0.9Co0.1)2As2 manifest opening of several superconducting gaps at temperatures lower than critical Tc., Comment: 4 pages, 2 figures