Many electron effects in semiconductor quantum dots

Semiconductor quantum dots (QDs) exhibit shell structures, very similar to atoms. Termed as ’artificial atoms’ by some, they are much larger (1 100 nm) than real atoms. One can study a variety of manyelectron effects in them, which are otherwise difficult to observe in a real atom. We have treated these effects within the local density approximation (LDA) and the Harbola-Sahni (HS) scheme. HS is free of the self-interaction error of the LDA. Our calculations have been performed in a three-dimensional quantum dot. We have carried out a study of the size and shape dependence of the level spacing. Scaling laws for the Hubbard ‘U’ are established.     

Effect of Co on the magnetism and phase stability of lithiated manganese oxides

We present first-principles calculations of the relative energies of various phases of lithiated manganese oxides with and without Co. We use the ultrasoft pseudopotential method as implemented in the Vienna ab initio simulation package (VASP). The calculations employ the local spin density approximation (LSDA) as well as the generalized gradient approximation (GGA). We consider monoclinic and rhombohedral structures in paramagnetic, ferromagnetic and antiferromagnetic (AF3) spin configurations. Spin-polarization significantly lowers the total energy in all cases. The effect of Co on the stability of these phases is discussed.     

Electronic structure, charge density, and chemical bonding properties of C11H8N2O o-methoxydicyanovinylbenzene (DIVA) single crystal

A comprehensive theoretical density functional theory (DFT) study of the electronic crystal structure, bonding properties, electron charge density of C₁₁H₈N₂O o-methoxydicyanovinylbenzene (DIVA) single crystals were performed. The exchange and correlation potential was described within a framework of the local density approximation (LDA) by Ceperley-Alder and gradient approximation (GGA) based on exchange–correlation energy optimization to calculate the total energy. In addition, we have used Engel–Vosko generalized gradient approximation (EV-GGA) and the modified Becke–Johnson potential (mBJ) for the electronic crystal structure, bonding properties, electron charge density calculations. There is systematically increasing in the energy gap from 2.25 eV (LDA), 2.34 eV (GGA), 2.50 eV (EV-GGA), 2.96 eV (mBJ). Our calculations show that this crystal possess direct energy gap. Furthermore, the electronic charge density space distribution contours in the (1 1 0) crystallographic plane clarifies the nature of chemical bonding.     

Formation of single-walled carbon nanotube via the interaction of graphene nanoribbons: ab initio density functional calculations

The interaction of bare graphene nanoribbons (GNRs) was investigated by ab initio density functional theory calculations with both the local density approximation (LDA) and the generalized gradient approximation (GGA). Remarkably, two bare 8-GNRs with zigzag-shaped edges are predicted to form an (8, 8) armchair single-wall carbon nanotube (SWCNT) without any obvious activation barrier. The formation of a (10, 0) zigzag SWCNT from two bare 10-GNRs with armchair-shaped edges has activation barriers of 0.23 and 0.61 eV for using the LDA and the revised PBE exchange correlation functional, respectively. Our results suggest a possible route to control the growth of specific types SWCNT via the interaction of GNRs.     

Variations in first principles calculated defect energies in GaAs and their effect on practical predictions

There is an abundant literature on calculations of formation and ionization energies of point defects in GaAs. Since most of these energies, especially the formation energies, are difficult to measure, the calculations are primary means of obtaining their values. However, based on the assumptions of the calculations, the reported values differ greatly among the various calculations. In this paper we discuss the sources of errors and their impact on practical predictions valuable in GaAs device fabrication. In particular, we have compared a large set of computed energies and selected the most appropriate values. Then, in the context of GaAs material quality, we investigated the impact of errors in calculation of formation energies on the performance of the GaAs substrate for device fabrication. We find that in spite of the errors inherent in ab initio calculations, it is possible to correctly predict the behaviour of GaAs substrate.     

Electronic structure and optical properties of thorium monopnictides

We have calculated the electronic density of states (DOS) and dielectric function for the ThX (X = P, As and Sb) using the linear muffin tin orbital method within atomic sphere approximation (LMTO-ASA) including the combined correction terms. The calculated electronic DOS of ThSb has been compared with the available experimental data and we find a good agreement. The calculated optical conductivity for ThP and ThAs is increasing monotonically, while for ThSb a sharp peak has been found at 6–5 eV. Unfortunately there are no experimental data to compare with calculated optical properties, we hope our calculations will motivate some experimentalists.     

Structures of Mn clusters

The geometries of several Mn clusters in the size range Mn₁₃-Mn{on23} are studied via the generalized gradient approximation to density functional theory. For the 13-and 19-atom clusters, the icosahedral structures are found to be most stable, while for the 15-atom cluster, the bcc structure is more favoured. The clusters show ferrimagnetic spin configurations.     

Obtaining Kohn-Sham potential without taking the functional derivative

Over the past decade and a half, many new accurate density functionals, based on the generalized gradient approximation, have been proposed, and they give energies close to chemical accuracy. However, accuracy of the energy functional does not guarantee that its functional derivative, which gives the corresponding potential, is also accurate all over space. For example, although the Becke88 exchange-energy functional gives very good exchange energies, its functional derivative goes as —1/2 m comparison to the correct —1 for r ⇇ where ris the distance of the electron from a finite system. On the other hand, accuracy of the potential is of prime importance if one is interested in properties other than the total energy; properties such as optical response depend crucially on the potential in the outer regions of a system. In this paper we present a different approach, based on the ideas of Harbola and Sahni, to obtain the potential directly from the energy density of a given approximation, without taking recourse to the functional derivative route. This leads to a potential that is as accurate as the functional itself. We demonstrate the accuracy of our approach by presenting some results obtained from the Becke88 functional.     

First-principles study of the electronic,optical and bonding properties in dolomite

The structural, electronic, optical properties and chemical bonding of dolomite CaMg(CO₃)₂ (rhombohedral calcite-type structure) are investigated using plane wave pseudopotential density-functional theory (DFT) method taking the local density approximation (LDA) and the generalized gradient approximation (GGA) as the exchange–correlation energy functional. The structural properties are consistent with the early experimental and theoretical results. The indirect electronic band gap is estimated to be ～5.0 eV, which is less than the optical band gap measured from the fundamental absorption edge of ～6.0 eV. The optical band gap is also consistent with the experimental band gap of similar calcite-type structure. A noticeable difference for the LDA and GGA derived transition peaks and a significant optical anisotropy are observed in the optical spectra. The analysis of electronic density of states, Mulliken charge and bonding population shows the coexistence of covalent and ionic bonding in the dolomite structure and the results are consistent with previous theoretical calculations.     

A first principles study of small Cun clusters based on local-density and generalized-gradient approximations to density functional theory

Neutral and anionic Cu_n clusters (Cu₂, Cu₃, Cu₆ and Cu₇⁻) are studied within density functional theory via (a) the local-density approximation (LDA) and (b) the generalized-gradient approximation (GGA) of Perdew and Wang (GGA-PW) for exchange and correlation. GGA reduces by ˜20% the binding energies, while the bond lengths are increased by ˜3–4%. The different levels of GGA approximation, involving optimization of the electronic density and/or of the geometry, are shown in detail. In the case of Cu₆ the GGA configurational ground state is a planar structure of D_3h symmetry. This result differs from the one obtained by LDA, where the three different isomers (one two-dimensional and two three-dimensional) were found to lie within 0.04 eV.     

Further study on structural and electronic properties of silicon phosphide compounds with 3:4 stoichiometry

Si₃N₄ has been extensively studied, due to its potential applications in electronic devices. Substitution of N by P results in Si₃P₄ which is a relatively unknown material. In this study, we carried out further investigation on the structural and electronic properties of Si₃P₄ using first principles total energy method based on the density functional theory and the generalized gradient approximation. Our calculations show that pseudocubic-Si₃P₄ is energetically favored. However, the present study based on the generalized gradient approximation predicts that the phase is more stable than the γ phase which was predicted to be more energetically stable in an earlier study based on the local density approximation. Other properties such as bulk modulus, band structure, of Si₃P₄ calculated using GGA are consistent with the LDA results.     

Ab initio molecular dynamics simulation of pressure-induced phase transformation in BeO

Ab initio molecular dynamics (MD) method has been used to study high pressure-induced phase transformation in BeO based on the local density approximation (LDA) and the generalized gradient approximation (GGA). Both methods show that the wurtzite (WZ) and zinc blende (ZB) BeO transforms to the rocksalt (RS) structure smoothly at high pressure. The transition pressures obtained from the LDA method are about 40 GPa larger than the GGA result for both WZ → RS and ZB → RS phase transformations, and the phase transformation mechanisms revealed by the LDA and GGA methods are different. For WZ → RS phase transformations both mechanisms obtained from the LDA and GGA methods are not comparable to the previous ab initio MD simulations of WZ BeO at 700 GPa based on the GGA method. It is suggested that the phase transformation mechanisms of BeO revealed by the ab initio MD simulations are affected remarkably by the exchange–correlation functional employed and the way of applying pressure.     

Neutral and anionic CuO2: an ab inito study

By using first-principles calculations within density functional theory via the local density approximation (LDA) and the generalized-gradient approximation (GGA) of Perdew and Wang for exchange and correlation, we calculate the equilibrium structures of CuO₂ and CuO₂⁻ clusters. In the case of CuO₂, three isomers (OCuO linear and two CuO₂ complexes, side-on and bent) lie within 0.5 eV, while the negatively charged cluster is most stable as a linear molecule. Our assignment of measured photo-electron spectra features on the basis of the electronic density of states (EDOS) suggests that the bent structure is the most stable among the two forms of CuO₂⁻ complexes.     

Double stripe reconstruction of the Pt(111) surface

We have studied the reconstruction of the Pt(111) surface theoretically, using a 2D generalization of the Frenkel-Kontorova model. The parameters in the model are obtained by performingab initio density functional theory calculations. The Pt(111) surface does not reconstruct under normal conditions but experiments have shown that there are two ways to induce the reconstruction: by increasing the temperature, or by depositing adatoms on the surface. The basic motif of this reconstruction is a ‘double stripers with an increased surface density and alternatinghcp andfcc domains, arranged to form a honeycomb pattern with a very large repeat distance of 100–300 Å. In this paper, we have studied the ‘double stripe’ reconstruction of the Pt(111) surface. In agreement with experiment, we find that it is favourable for the surface to reconstruct in the presence of adatoms, but not otherwise.     

β-CaSiO3晶体的电子结构及光学性质

基于第一性原理的平面波赝势方法(PWP)的局域密度近似(LDA)/广义梯度近似(GGA)计算了β-Ca-SiO3的几何结构、能带结构、态密度和光学性质。其晶胞参数优化结果与实验相比,LDA/GGA的相对误差为-3.62%/1.91%。对优化后的β-CaSiO3晶体进行能带结构分析表明,β-CaSiO3晶体为间接带隙结构,禁带宽度Eg(LDA)=5.53eV,Eg(GGA)=5.18eV。对态密度图及Mulliken电荷分布的分析表明,Ca的d轨道有电子分布,即Ca的s、p、d轨道均参与了成键。β-CaSiO3晶体中Ca与SiO3基团之间形成的化学键主要是离子键,而Si与O之间的化学键是共价键。     

The Effects of 5f Localization on Magnetic Properties of UAl3

The structural, electronic, and magnetic properties of UAl₃ have been calculated using density functional theory by the Wien2k package within LDA, GGA, LDA?+?U, and GGA?+?U approaches. The total energy calculations indicate that at zero pressure the ferromagnetic phase is the most stable phase. The energy band calculation and the density of state curves indicate that the localization of 5f electron and spin orbit coupling have a considerable effect on electronic properties of the UAl₃ compound. The calculations of the electric field gradient, magnetic moment, and optical properties of this compound have been performed under the presence and absence of spin orbit coupling. The contribution of different orbitals to the EFG shows that the strongest anisotropy in the charge distribution is due to the electrons in p orbitals. The electric field gradient and magnetic moment as a function of pressure have been investigated in the presence and absence of spin orbit coupling.     

第一原理计算Al_2Mg中间相的电子结构

对镁合金材料研发应用现状与发展趋势进行了论述,展望了镁合金研发应用的未来发展趋势。采用基于密度泛函理论的第一性原理平面波赝势法,通过选用广义梯度近似(GGA)和局域密度近似(LDA)分别计算了Al2Mg晶体的几何与电子结构,分析了其电子态密度以及电荷密度分布,研究了Al2Mg晶体的电子结构和成键特性。     

A density functional study of structural and elastic properties of LaN under high pressure

Structural and elastic properties of LaN at normal and high pressures are investigated using ab initio calculations based on full-potential linearized augmented plane wave (FP-LAPW) within both local density approximation (LDA) and generalized gradient approximation (GGA). Our results concerning equilibrium lattice parameter and bulk modulus agree well with the available experimental and previous theoretical findings. The transition pressure from NaCl (B1) to CsCl (B2) phase is found to be 31.05 GPa from LDA, and 42.2 GPa from GGA. To the best of our knowledge, the elastic properties for LaN in the B1 structure in the presence of pressure have never been reported so far. The linear pressure coefficients of elastic constants and their related bulk modulus are determined from the pressure dependence of these parameters. Furthermore, the mechanical stability criteria for LaN in B1 phase are found to be fulfilled at normal conditions.     

On the electronic structure and equation of state in high pressure studies of solids

We discuss the high pressure behaviour of zinc as an interesting example of controversy, and of extensive interplay between theory and experiment. We present its room temperature electronic structure calculations to study the temperature effect on the occurrence of its controversial axial ratio (c/a) anomaly under pressure, and the related electronic topological transition (ETT). We have employed a dense 63 X 63 X 29 k-point sampling of the Brillouin zone and find that the small (c/a) anomaly near 10 GPa pressure persists at room temperature. A weak signature of the anomaly can be seen in the pressure-volume curve, which gets enhanced in the universal equation of state, along with that of K-point ETTs. We attribute the change of slope in the universal equation of state near 10 GPa pressure, mainly to hybridization effects. The temperature effect in fact enhances the possibility of L-point ETT. We find that the L-point ETT is very sensitive to exchange correlation terms, and hence we suggest that further refinements in the theoretical techniques are needed to resolve the controversies on the ETT in Zn.