Anisotropic optical conductivity of the putative Kondo insulator   CeRu$_4$Sn$_6$
  Kondo insulators and in particular their non-cubic 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, and local density approximation plus dynamical mean field theory calculations. The calculations provide evidence for a Kondo insulator-like response within the $a-a$ plane and a more metallic response along the c axis and qualitatively reproduce the experimental observations, helping to identify their origin. 
V. Guritanu, P. Wissgott, T. Weig, H. Winkler, J. Sichelschmidt, M.   Scheffler, A. Prokofiev, S. Kimura, T. Iizuka, A. M. S...
cond-mat.str-el