G. Siegle

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.

Lattice dynamics and specific heat of alpha-GeTe: Theoretical and experimental study

We extend recent ab initio calculations of the electronic band structure and the phonon-dispersion relations of rhombohedral GeTe to calculations of the density of phonon states and the temperature-dependent specific heat. The results are compared to measurements of the specific heat. It is discovered that the specific heat depends on hole concentration, not only in the very low temperature region (Sommerfeld term) but also at the maximum of C-p/T-3 (around 16 K). To explain this phenomenon, we have performed ab initio lattice-dynamical calculations for GeTe rendered metallic through the presence of a heavy-hole concentration (p similar to 2x10(21) cm(-3)). They account for the increase observed in the maximum of C-p/T-3.

Electronic and phononic properties of the chalcopyrite CuGaS2

The availability of ab initio electronic calculations and the concomitant techniques for deriving the corresponding lattice dynamics have been profusely used for calculating thermodynamic and vibrational properties of semiconductors, as well as their dependence on isotopic masses. The latter have been compared with experimental data for elemental and binary semiconductors with different isotopic compositions. Here we present theoretical and experimental data for several vibronic and thermodynamic properties of CuGa2, a canonical ternary semiconductor of the chalcopyrite family. Among these properties are the lattice parameters, the phonon dispersion relations and densities of states (projected on the Cu, Ga, and S constituents), the specific heat and the volume thermal expansion coefficient. The calculations were performed with the ABINIT and VASP codes within the LDA approximation for exchange and correlation and the results are compared with data obtained on samples with the natural isotope composition for Cu, Ga and S, as well as for isotope enriched samples.

Vibrational and Thermal Properties of ZnX (X=Se, Te): Density Functional Theory (LDA and GGA) versus Experiment

We calculated the phonon dispersion relations of ZnX (X=Se, Te) employing ab initio techniques. These relations have been used to evaluate the temperature dependence of the respective specific heats of crystals with varied isotopic compositions. These results have been compared with mea- surements performed on crystals down to 2 K. The calculated and measured data are generally in excellent agreement with each other. Trends in the phonon dispersion relations and the correspond- ing densities of states for the zinc chalcogenide series of zincblende-type materials are discussed.

Electronic and phononic properties of cinnabar: Ab initio calculations and some experimental results

We report ab initio calculations of the electronic band structure, the corresponding optical spectra, and the phonon dispersion relations of trigonal alpha-HgS (cinnabar). The calculated dielectric functions are compared with unpublished optical measurements by Zallen and coworkers. The phonon dispersion relations are used to calculate the temperature and isotopic mass dependence of the specific heat which has been compared with experimental data obtained on samples with the natural isotope abundances of the elements Hg and S (natural minerals and vapor phase grown samples) and on samples prepared from isotope enriched elements by vapor phase transport. Comparison of the calculated vibrational frequencies with Raman and ir data is also presented. Contrary to the case of cubic beta-HgS (metacinnabar), the spin-orbit splitting of the top valence bands at the Gamma-point of the Brillouin zone (Delta_0) is positive, because of a smaller admixture of 5d core electrons of Hg. Calculations of the lattice parameters, and the pressure dependence of Delta_0 and the corresponding direct gap E_0~2eV are also presented. The lowest absorption edge is confirmed to be indirect.