Free carriers, coupled modes, and the generalized dielectric function in PbTe

The reflectivity of n-PbTe has been measured at 300^∘{}K in the range 16-235 cm^-1 for samples with carrier concentrations between 2 ×{} 10¹⁷ and 2 ×{} 10¹⁹ cm^-3. Both the conventional two-oscillator dielectric function and the generalized function εK proposed by Kukharskii were used to fit the data with equally valid results. Unlike previous low-temperature measurements there was no serious difference between the dc and optical values for the free-carrier scattering time τ except at the highest concentration where τdcτₒₚₜ=3. This discrepancy may be related to screening effects. Kukharskii's extended Lyddane-Sachs-Teller relation was found to hold despite the relatively heavy damping. εK was used to analyze the damped plasmon-phonon modes and indicated the presence of an overdamped mode at very low frequencies. The lattice damping constant was found to be 21±{}5 cm^-1, a value comparable to that measured in PbSe but much larger than is typical for other semiconductors.