Microscopic electronic wave function and interactions between quasiparticles in empirical tight-binding theory

International audienceA procedure to obtain single-electron wave functions within the tight-binding formalism is proposed. It is based on linear combinations of Slater-type orbitals whose screening coefficients are extracted from the optical matrix elements of the tight-binding Hamiltonian. Bloch functions obtained for zinc-blende semiconductors in the extended-basis spds∗ tight-binding model demonstrate very good agreement with first-principles wave functions. We apply this method to the calculation of the electron-hole exchange interaction, and obtain the dispersion of excitonic fine structure in bulk GaAs. Beyond semiconductor nanostructures, this work is a fundamental step toward modeling many-body effects from post-processing single-particle wave functions within the tight-binding theory

Ab initio calculations of polarization, piezoelectric constants, and elastic constants of InAs and InP in the wurtzite phase

International audienceWe report first-principle density functional calculations of the spontaneous polarization, piezoelectric stress constants, and elastic constants for the III–V wurtzite structure semiconductors InAs and InP. Using the density functional theory implemented in the VASP code, we obtain polarization values–0.011 and–0.013 C/m2, and piezoelectric constants e33 (e31) equal to 0.091 (–0.026) and 0.012 (–0.081) C/m2 for structurally relaxed InP and InAs respectively. These values are consistently smaller than those of nitrides. Therefore, we predict a smaller built-in electric field in such structures

Molecular Analysis of the Mechanical Behavior of Plasticized Amorphous Polymer

Plasticization effects on the mechanical behavior were investigated on two families of materials based on poly(methyl methacrylate) (PMMA) and poly(vinyl chloride) (PVC), respectively. For this purpose, PMMA was blended with poly(vinylidene fluoride) (PVDF) by co-precipitation from solution, all over the PVDF range 0-40 wt% where the samples remain amorphous. Di-octylphtalate (DOP) was mechanically dispersed in PVC over the DOP range 0-20 wt%. The relaxation behavior of the samples was studied by differential scanning calorimetry at heating rate of 10 °C$\cdot$min-1 and by dynamic mechanical analysis at the frequency 1 Hz over the temperature range $-100~^{\circ}$C/150 °C. Stress strain curves were recorded during compression testing at a deformation rate of 2.10-3 s-1. Data analysis was carried out on the molecular scale; it permitted to highlight the influence of the β elaxation motions on the plastic behavior. Consideration of the non elastic part of the energy to yield was clearly related to the contribution of $\alpha$ and $\beta$ motions

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Amorphous and Semicrystalline Blends of Poly(vinylidene fluoride) and Poly(methyl methacrylate): Characterization and Modeling of the Mechanical Behavior After extensive studies starting in the 1970s in relation to miscibility and piezoelectric properties, the blends of poly(vinylidene fluoride) (PVDF) and poly(methyl methacrylate) (PMMA) have been revisited with the aim of assessing their mechanical behavior. Depending on the amount of PVDF, either amorphous or semicrystalline blends are produced. Typically, the blends remain amorphous when their PVDF content does not exceed 40 wt. %. Blend composition influence on the values of the glass transition temperature, T g , and on its mechanical expression, T a , is extensively discussed. Then, emphasis is put on the stress-strain behavior in tension and compression over the low deformation range covering the elastic, anelastic, and viscoplastic response. The reported data depend, as expected, on temperature and strain rate and also, markedly, on blend composition and degree of crystallinity. Molecular arguments, based on the contribution of the glass transition motions are proposed to account for the observed behavior. Thanks to the understanding of phenomena at the molecular level, accurate models can be selected in the view of mechanical modeling