Anisotropic optical conductivity of the putative Kondo insulator CeRu4Sn6

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 CeRu${}_4$Sn${}_6$ 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.