M. Cardona

Lattice properties of PbX (X=S, Se, Te): Experimental studies and ab initio calculations including spin-orbit effects

During the past five years the low-temperature heat capacity of simple semiconductors and insulators has received renewed attention. Of particular interest is its dependence on isotopic masses and the effect of spin-orbit coupling in ab initio calculations. Here we concentrate on the lead chalcogenides PbS, PbSe, and PbTe. These materials, with rock salt structure, have different natural isotopes for both cations and anions; a fact that allows a systematic experimental and theoretical study of isotopic effects, e.g., on the specific heat. Also, the large spin-orbit splitting of the 6p electrons of Pb and the 5p of Te, using a computer code which includes spin-orbit interaction, allows an investigation of the effect of this interaction on the phonon-dispersion relations and the temperature dependence of the specific heat, and on the lattice parameter. It is shown that agreement between measurements and calculations significantly improves when spin-orbit interaction is included.

Temperature-dependent Raman scattering of natural and isotopically substituted PbS

Lead sulfide is an important semiconductor that has found technological applications for over a century. Raman spectroscopy, a standard tool for the investigation and characterization of semiconductors, has limited application to this material because of the forbidden nature of its first order scattering and its opacity to visible lasers. Nevertheless, useful vibrational spectra from two-phonon processes are obtained with red lasers, probably because of a resonance in the concomitant electronic transitions. Herewith, we report temperature dependent Raman spectra covering the 10–300 K range, for two samples with different sulfur isotopic compositions. The results are analyzed by comparison with ab initio cal- culations of the lattice dynamics of PbS and the corresponding densities of one and two-phonon states. Emphasis is placed on the analysis of the two phonon band centered at ∼430 cm–1. The combined use of ab initio calculations and an extended temperature range (compared to previous reports) allows an unequivocal identification of some of the main features in the second-order Raman spectra. Our study also provides a much firmer background for the understanding of anomalous temperature dependences in the anharmonic shifts of second-order Raman spectra in PbS. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)