Electronic transport and thermal and optical properties ofCa1−xLaxB6

We present a study of transport, thermal, and optical properties of stoichiometric ${\mathrm{CaB}}_6$ and La-doped ${\mathrm{CaB}}_6.$ For stoichiometric ${\mathrm{CaB}}_6$ a strong increase of the resistivity with decreasing temperature and anomalies in the low-temperature behavior of the resistivity and the specific heat have been observed. The application of an external magnetic field at low temperatures is shown to lead to an anomalous magnetoresistance in the case of stoichiometric ${\mathrm{CaB}}_6.$ The optical conductivity exhibits a strong doping dependence. Rather unexpectedly, small changes in the chemical composition lead to significant changes in the electronic interband transitions in the visible-UV spectral range. The relevance of our results with respect to the recently suggested excitonic scenario for explaining the physical properties of alkaline-earth hexaborides is discussed.