No Arabic abstract
The phonon and electronic properties, the Eliashberg function and the temperature dependence of resistance of electride Ca2N are investigated by the DFT-LDA plane-wave method. The phonon dispersion, the partial phonon density of states and the atomic eigenvectors of zero-center phonons are studied. The electronic band dispersion and partial density of states conclude that Ca2N is a metal and the Ca 3p, 4s and N 2p orbitals are hybridized. For the analysis of an electron - phonon interaction (EPI) and its contribution to resistance the Eliashberg function was calculated and a temperature dependence of resistance caused EPI was found. The present results are in good agreement with experiment data.
One novel family of two-dimensional IV-V compounds have been proposed, whose dynamical stabilities and electronic properties have been systematically investigated using the density functional theory. Extending from our previous work, two phases of carbon phosphorus bilayers alpha- and beta-C$_{2}$P$_{2}$ have been proposed. Both of them are dynamically stable and thermally stable at 300K. They possess intrinsic HSE gaps of 2.70 eV and 2.67 eV, respectively. Similar alpha- and beta-C$_{2}$Y$_{2}$ (Y= As, Sb, and Bi) can be obtained if the phosphorus atoms in the alpha- and beta-C$_{2}$P$_{2}$ replaced by other pnictogens, respectively. If the C atoms in the alpha- and beta-C$_{2}$Y$_{2}$ (Y= P, As, Sb, and Bi) are further replaced by other IV elements X (X=Si, Ge, Sn, and Pb), respectively, more derivatives of alpha- and beta-X$_{2}$Y$_{2}$ (Y=N, P, As, Sb, and Bi) also can be obtained. It was found that the majority of them are dynamically stable. The proposed compounds range from metal to insulators depending on their constitutions. All insulated compounds can undergo a transition from insulator to metal induced by biaxial strain. Some of them can undergo a transition from indirect band gap to direct band gap. These new compounds can become candidates as photovoltaic device, thermoelectric material field as well as lamellated superconductors.
A systemically theoretical study has been presented to explored the crystal structures and electronic characteristics of polycyclic aromatic hydrocarbons (PAHs), such as solid phenanthrene, picene, 1,2;8,9-dibenzopentacene, and 7-phenacenes, since these PAHs exhibited the superconductivity when potassium doping into. For tripotassium-doped phenanthrene and picene, we demonstrate the K atomic positions to fit the experimental lattice parameters, and analyze the distinction between the stablest configuration and the fitted experimental one. Based on the first-principles calculations, for the first time, we predict the possible crystal configurations of pristine and tripotassium-doped 1,2;8,9-dibenzopentacene and 7-phenacenes, respectively. For these four PAHs, the electronic structures after doping are investigated in details. The results show that the electronic characters near the Fermi level are high sensitive to structure. Because of the change of the benzene rings arrangement, the 1,2;8,9-dibenzopentacene exhibits visibly different band structures from other three PAHs. In these metallic PAHs, two bands cross the Fermi level which results in the complicated multiband feature of Fermi surfaces. Fascinatingly, we find that the electronic states of potassium contribute to the Fermi surfaces especially for K-3$d$ electrons, which improves a way to understand this superconductivity. As a result, we suggest that the rigid-band picture is invalidated due to the hybridization between K atoms and PAH molecules as well as the rearrangement and distortion of PAH molecules.
First-principle FLAPW-GGA band structure calculations are employed to obtain the structural, electronic properties and chemical bonding picture for two related layered phases, namely, quaternary oxyarsenides LaZnAsO and YZnAsO. These compounds are found to be direct-transition type semiconductors with the GGA gaps of about 0.65-1.30 eV. The peculiarities of chemical bonding in these phases are investigated and discussed in comparison with quaternary oxyarsenide LaFeAsO - a basic phase for the newly discovered 26-52K superconductors.
The PEO3:LiCF3SO3 polymer electrolyte has attracted significant research due to its enhanced stability at the lithium/polymer interface of high conductivity polymer batteries. Experimental studies have shown that, depending on the preparation conditions, both the PEO3:LiCF3SO3 crystalline complex and the PEO3:LiCF3SO3 amorphous phase can be formed. However, previous theoretical investigations focused on the short chain amorphous PEO3:LiCF3SO3 system. We report ab initio density-functional-theory calculations of crystalline PEO3:LiCF3SO3. The calculated results about the bonding configuration, electronic structures, and conductivity properties are in good agreement with the experimental measurements.
Density Functional Theory calculations traditionally suffer from an inherent cubic scaling with respect to the size of the system, making big calculations extremely expensive. This cubic scaling can be avoided by the use of so-called linear scaling algorithms, which have been developed during the last few decades. In this way it becomes possible to perform ab-initio calculations for several tens of thousands of atoms or even more within a reasonable time frame. However, even though the use of linear scaling algorithms is physically well justified, their implementation often introduces some small errors. Consequently most implementations offering such a linear complexity either yield only a limited accuracy or, if one wants to go beyond this restriction, require a tedious fine tuning of many parameters. In our linear scaling approach within the BigDFT package, we were able to overcome this restriction. Using an ansatz based on localized support functions expressed in an underlying Daubechies wavelet basis -- which offers ideal properties for accurate linear scaling calculations -- we obtain an amazingly high accuracy and a universal applicability while still keeping the possibility of simulating large systems with only a moderate demand of computing resources.