Far-infrared study of impurity local modes in Co-doped PbTe

Far-infrared reflection spectra in wide temperature range were used to investigate the vibrational properties of the PbTe single crystals doped with Co. In the analysis of the experimental results we used dielectric function that took into account the existence of plasmon-ionised impurity-phonon interaction. The results obtained from the experimental spectra as the best fit, are in very good agreement with the theoretical prediction. Together with the strong coupling we obtained three local modes of Co at about 125, 190 and 275 cm-1, which corresponded to the impurity atoms in different valence states. The positions of these modes depend on the impurity center charge, and their intensities depend on the temperature and Co concentration.