Anisotropic optical conductivity of the putative Kondo insulator CeRu ₄Sn₆

Kondo insulators and in particular their noncubic representatives have remained poorly understood. Here we report on the development of an anisotropic energy pseudogap in the tetragonal compound CeRu₄Sn₆ employing optical reflectivity measurements in broad frequency and temperature ranges. The low-energy optical conductivity shows semiconductor-like features within the a-a plane but a Drude-like response along the c axis, signaling weak metallicity. Local density approximation plus dynamical mean-field theory calculations reproduce these observations qualitatively and help to identify their origin. For large parts of the Brillouin zone, the strongly correlated band structure presents a narrow direct gap within the Kondo resonance. Only for the c direction, due to anisotropy in the hybridization, there is a pronounced band crossing at the Fermi level.