583 research outputs found

    Secondary Roads Are Important Too

    Get PDF

    Lattice Properties of PbX (X = S, Se, Te): Experimental Studies and ab initio Calculations Including Spin-Orbit Effects

    Full text link
    During the past five years the low temperature heat capacity of simple semiconductors and insulators has received renewed attention. Of particular interest has been 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 allows, using a computer code which includes spin-orbit interaction, 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.Comment: 25 pages, 12 Figures, 1 table, submitted to PR

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

    Full text link
    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.Comment: 10 pages, submitted to PR

    Treatment of Localized Morphea with Zyderm Collagen Implant

    Full text link
    Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/74946/1/j.1524-4725.1984.tb01267.x.pd

    Treatment Site Reactions to Zyderm Collagen Implantation

    Full text link
    Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98779/1/j.1524-4725.1983.tb00819.x.pd

    Electronic and phononic properties of the chalcopyrite CuGaS2

    Full text link
    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.Comment: 9 pages, 8 Figures, submitted to Phys. Rev

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

    Full text link
    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.Comment: 13 pages, 15 figure

    Heat Capacity of PbS: Isotope Effects

    Full text link
    In recent years, the availability of highly pure stable isotopes has made possible the investigation of the dependence of the physical properties of crystals, in particular semiconductors, on their isotopic composition. Following the investigation of the specific heat (CpC_p, CvC_v) of monatomic crystals such as diamond, silicon, and germanium, similar investigations have been undertaken for the tetrahedral diatomic systems ZnO and GaN (wurtzite structure), for which the effect of the mass of the cation differs from that of the anion. In this article we present measurements for a semiconductor with rock salt structure, namely lead sulfide. Because of the large difference in the atomic mass of both constituents (MPbM_{\rm Pb}= 207.21 and (MSM_{\rm S}=32.06 a.m.u., for the natural isotopic abundance) the effects of varying the cation and that of the anion mass are very different for this canonical semiconductor. We compare the measured temperature dependence of Cp≈CvC_p \approx C_v, and the corresponding derivatives with respect to (MPbM_{\rm Pb} and MSM_{\rm S}), with \textit{\textit{ab initio}} calculations based on the lattice dynamics obtained from the local density approximation (LDA) electronic band structure. Quantitative deviations between theory and experiment are attributed to the absence of spin-orbit interaction in the ABINIT program used for the electronic band structure calculations.Comment: 17 pages including 10 Fig

    Raman and Infrared-Active Phonons in Hexagonal HoMnO3_3 Single Crystals: Magnetic Ordering Effects

    Full text link
    Polarized Raman scattering and infrared reflection spectra of hexagonal HoMnO3_3 single crystals in the temperature range 10-300 K are reported. Group-theoretical analysis is performed and scattering selection rules for the second order scattering processes are presented. Based on the results of lattice dynamics calculations, performed within the shell model, the observed lines in the spectra are assigned to definite lattice vibrations. The magnetic ordering of Mn ions, which occurs below TN_N=76 K, is shown to effect both Raman- and infrared-active phonons, which modulate Mn-O-Mn bonds and, consequently, Mn exchange interaction.Comment: 8 pages, 6 figure
    • …
    corecore