LOW-ENERGY OPTICAL-EXCITATION IN RARE-EARTH HEXABORIDES

Reflectivity spectra of trivalent rare-earth hexaborides (RB6; R = La, Ce, Pr, Nd, Gd, Th, Dy, Ho, and Y) and mixed-valent SmB6 have been me asured in the photon energy region from 1 meV to 40 eV at 300 and 9 K. We discuss the absorption structure due to conduction electrons. Even in LaB6 and YB6, which have no 4f electrons, the optical conductivity spectra were not fitted by a simple Drude model and a frequency-depen dent relaxation time was observed. This was attributed to the electron -phonon and electron-electron scattering. In RB6 with 4f electrons, a common absorption structure was observed at the energy position of abo ut 0.6 eV. The intensity was found to be almost proportional to the 4f spin angular momentum. From the f-sum rule, the absorption was found to be due to the conduction electrons. The origin was assigned to be d ue to the transition or to the relevant exciton absorption from the sa ddle points of SIGMA1 at the neck point to the saddle points of GAMMA1 2 and GAMMA25 assisted by the scattering of the intra-atomic 5d-4f Cou lomb exchange interaction, in particular enhanced in the heavy RB6 com pounds by the lattice instability. In SmB6, the absorption that was se en in trivalent RB6'S was also observed in addition to absorptions fro m 4f states of Sm2+ to 5d states and from 5d to 4f in Sm3+. This is th ought to mean that the Fermi level of SmB6 is located at the same ener gy position in the band structure as that of the trivalent RB6.