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Register a new userVibrational and Thermal Properties of ZnX (X=Se, Te): Density Functional Theory (LDA and GGA) versus Experiment
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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.
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Al4SiC4 is a wide band gap semiconductor with numerous potential technological applications. We report here the first thorough experimental Raman and Infrared (IR) investigation of vibrational properties of Al4SiC4 single crystals grown by high temperature solution growth method. The experimental results are compared with the full theoretical analysis of vibrational properties based on Density Functional Theory calculations that are revisited here. We have obtained a good agreement between the experimental and calculated Raman phonon modes and this allowed the symmetry assignment of all the measured Raman modes. We have revisited the DFT calculation of the IR active phonon modes and our results for LO-TO splitting indicate a substantial decrease of the variation of omega(LO-TO) compared with the previous reported calculation. Moreover, most of the IR modes have been symmetry assigned from the comparison of the experimental IR spectra with the corresponding Raman spectra and the Al4SiC4 calculated phonon modes.
The electronic structure and properties of PuO$_{2}$ and Pu$_{2}$O$_{3}$ have been studied from first principles by the all-electron projector-augmented-wave (PAW) method. The local density approximation (LDA)+$U$ and the generalized gradient approximation (GGA)+$U$ formalism have been used to account for the strong on-site Coulomb repulsion among the localized Pu $5f$ electrons. We discuss how the properties of PuO$_{2}$ and Pu$_{2}$O$_{3}$ are affected by the choice of $U$ as well as the choice of exchange-correlation potential. Also, oxidation reaction of Pu$_{2}$O$_{3}$, leading to formation of PuO$_{2}$, and its dependence on $U$ and exchange-correlation potential have been studied. Our results show that by choosing an appropriate $U$ it is promising to correctly and consistently describe structural, electronic, and thermodynamic properties of PuO$_{2}$ and Pu$_{2}$O$_{3}$, which enables it possible the modeling of redox process involving Pu-based materials.
We discuss the structural and electronic properties of tetragonal CuO grown on SrTiO3(100) by means of hybrid density functional theory. Our analysis explains the anomalously large Cu-O vertical distance observed in the experiments (~2.7 A) in terms of a peculiar frustration between two competing local Cu-O environments characterized by different in-plane and out-of-plane bond lengths and Cu electronic populations. The proper inclusion of substrate effects is crucial to understand the tetragonal expansion and to reproduce correctly the measured valence band spectrum for a CuO thickness of 3-3.5 unit cells, in agreement with the experimentally estimated thickness.
We study the electronic properties of the $textrm{Fe}textrm{Se}_{1-x}textrm{Te}_x$ system ($x=0$, 0.25, 0.5, 0.75, and 1) from the perspective of X-ray spectroscopy and density functional theory (DFT). The analysis performed on the density of states reveals marked differences in the distribution of the $5p$ states of Te for $x>0$. We think that this finding can be associated with the fact that superconductivity is suppressed in FeTe. Moreover, using resonant inelastic X-ray scattering, we estimate the spin state of our system which can be correlated to the magnetic order. We find that the spin state of the $textrm{Fe}textrm{Se}_{1-x}textrm{Te}_x$ system fluctuates, as a function of $x$, between $S=0$ and $S=2$ with Fe in FeSe in the highest spin state. Finally, our DFT calculations nicely reproduce the X-ray emission spectra performed at the Fe $L$-edge (which probe the occupied states) and suggest that the $textrm{Fe}textrm{Se}_{1-x}textrm{Te}_x$ system can be considered at most as a moderately correlated system.
We will discuss the key concepts in density functional theory (DFT), how it can be used to model experimental data, and consider how the synergy between DFT and experiment can give significant insights. The discussion will centre on the scanning tunnelling microscope (STM) and surface problems, tracking the authors personal interest, though the general principles are widely applicable.
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