Probing of local electron states in Pb$_{1-x}$Sn$_x$Te(In) narrow-gap semiconductors by laser terahertz radiation

Local electron states in indium-doped lead telluride-based solid solutions exhibit a number of features which separate them from the diversity of impurity states in semiconductors. These features are most pronounced in terahertz photoconductivity. The results of the corresponding experiments performed during the last years and supported by the Russian Foundation for Basic Research are reviewed.