Mixed-valence states in narrow-gap IV-VI semiconductors with rare-earth ions

The mechanism of valence-state formation in the IV-VI alloys doped with rar e-earth impurities is analyzed with respect to certain properties of narrow -gap semiconductors. The mean-field approximation in the slave-boson repres entation is used to account for the strong electron correlation at the impu rity. The energy, width, and filling factor of the impurity level are calcu lated as a function of both the band gap and position of the chemical poten tial. We also calculate the temperature dependence of the magnetic suscepti bility of an impurity. It is shown that mixing to coupled conduction- and v alence-band states makes the properties of rare-earth dopants sensitive to the band gap and Fermi level in the narrow-gap semiconductor host. [S0163-1 829(99)08723-8].