Collective infrared excitation in rare-earth GdxLa1-xB6 hexaborides

Using Fourier-transform infrared spectroscopy and optical ellipsometry, room temperature spectra of complex conductivity of single crystals of hexaborides GdxLa1-xB6, x(Gd) = 0, 0.01, 0.1, 0.78, 1, are determined in the frequency range 30-35000 cm(-1). In all compounds, in addition to the Drude free-carrier spectral component, broad excitations are discovered with unusually large dielectric contributions Delta epsilon = 7 000-15 000 and non-Lorentzian line shapes. It is suggested that the origin of the excitations is connected with the dynamic cooperative Jahn-Teller effect of B-6 clusters. Analysis of the spectra together with the results of dc and Hall resistivity measurements show that only 25-50% of the conduction band electrons are contributing to the free carrier ac conductivity, with the rest being involved in the formation of an overdamped excitation, thus providing a possible explanation, in terms of nonequilibrium (hot) electrons in these hexaborides, of both the remarkably low work function of thermoemission of Gd(x)La(1-x)B(6 )and the non-Fermi-liquid behavior in GdB6 crystals.