Investigation of the influence of nonstoichiometry and doping with carbon and nitrogen on the electronic spectrum of rutile by the coherent potential method

The electronic spectra and magnetic properties of rutile (TiO₂), nonstoichiometric rutile (TiO_{2 ? δ}), and stoichiometric and nonstoichiometric rutiles doped with carbon or nitrogen (TiO_{2 ? y ? δ}C _y and TiO_{2 ? y ? δ}N _y , y(δ) = 0, 0.03, 0.06) have been calculated using the coherent potential method. The used method of investigation has made it possible for the first time to calculate the disordered arrangement of impurity atoms and vacancies in the oxygen sublattice with their arbitrary concentration. The changes in the electronic spectrum and magnetic properties with variations in the concentration of dopants have been investigated. The possible photocatalytic activity of considered compositions has been analyzed.     

碳掺杂ZnO的电子结构和光学性质

采用基于密度泛函理论框架下的第一性原理计算研究碳掺杂ZnO的电子结构和光学性质.计算结果表明：C原子替代O原子和C原子替代Zn原子两种掺杂体系的电子结构存在明显差异,这主要是由于C原子的电子分布及对周围原子的影响不同;碳掺杂ZnO光学性质的变化集中在低能量区,而高能量区的光学性质没有明显变化.结合电子结构定性解释了光学性质的变化. 关键词： ZnO 碳掺杂 电子结构 光学性质     

Analyticity of the coherent potential approximation

A general proof is presented that the coherent potential approximation does not violate analyticity requirements.     

Electronic structure and optical properties of anatase doped with bismuth and carbon

The calculations of the electronic structure of pure anatase and the anatase doped with carbon and/or bismuth have been carried out using the ab initio tight-binding linear muffin-tin orbital (TB-LMTO) method in the local spin density approximation with the inclusion of single-site Coulomb correlations (LSDA + U). The dielectric function, absorption coefficient, and refractive index have been calculated in the random phase approximation. It has been found that, upon doping, narrow bands of carbon and bismuth impurity states are formed in the band gap. The calculations of the optical absorption coefficient have demonstrated that the C,Bi-doping can lead to the absorption in the visible region and an enhancement of the absorption in the near-ultraviolet region. Therefore, the C,Bi-doping can increase the photocatalytic activity on the surface of doped anatase.     

Electronic structure of boron nitride nanostructures doped with a carbon atom

We have investigated, using first-principles calculations, the role of a substitutional carbon atom on the electronic properties of boron nitride monolayers, nanotubes, and nanocones. It is shown that electron states in the energy-gap are independent of the curvature, being the same for the monolayer, for the cone and for the tube. It is also found, that the presence of carbon in the boron nitride compounds induces a spin polarization, with magnetic moment of 1.0 μ_B, which does not depend on the curvature.     

Electronic band structure and distribution of excited electrons in the conduction band of anatase doped with boron, nitrogen, and carbon

A method for calculating the distribution of excited electrons in the conduction band of semiconductors has been proposed. This method takes into account both the excitation of electrons by means of an external light source and the transitions to the bottom of the conduction band due to the electron-phonon interaction. The interaction of electrons with the light field has been calculated from first principles in the dipole approximation using the linear muffin-tin orbital method. The electron-phonon interaction has been calculated in terms of the density functional perturbation theory. The method has been applied to the calculation of the quasi-steady-state distribution function of excited electrons in anatase doped with boron, nitrogen, and carbon. The correlations of the distribution function with the photocatalytic activity of doped anatase have been discussed.     

Electronic density of states for the linear random binary alloy in the three-site coherent potential approximation

Morita and Chen's scheme of generalizing the coherent potential approximation in which the diagonal and off-diagonal randomness are included, is applied to the calculation of the electronic density of states for the linear random binary alloy. The result obtained in the three-site coherent potential approximation is reported.     

Electronic density of states for the linear random binary alloy in the two-site coherent potential approximation

Morita and Chen's scheme of generalizing the coherent potential approximation, in which the diagonal and off-diagonal randomness are included, is applied to the calculation of the electronic density of states for the linear random binary alloy. The result obtained in the two-site coherent potential approximation is reported.     

Electronic structure and the optical and photocatalytic properties of anatase doped with vanadium and carbon

The electronic structures of undoped anatase and anatase doped with carbon and vanadium have been calculated using the ab initio tight-binding linear muffin-tin orbital (TB-LMTO) method in the LSDA + U approximation. It has been shown that the doping of TiO₂ leads to the formation of narrow bands of the C and Vimpurity states in the band gap. The calculations of the imaginary part of the dielectric function have made it possible to estimate the intensity of the optical absorption. It has been established that the doping with vanadium and carbon leads to optical absorption in the visible range and to an increase in the absorption in the ultraviolet range up to 4 eV. This should result in an increase in the photocatalytic activity on the surface of the doped anatase. The experimental determination of the photocatalytic activity of whiskers of the anatase doped with carbon and vanadium in the reaction of hydroquinone oxidation has confirmed the increase in the activity of the doped materials under exposure to ultraviolet, visible, and blue light. The phenomenon of dark catalysis in the anatase doped with carbon and vanadium has been interpreted within the concept of low-energy electronic excitatio ns between the impurity levels of carbon.     

Electronic structure of carbon nanotubes modified by alkali metal atoms

The electronic structure and parameters of the energy band structure of (n, 0)-type nanotubes modified by alkali metal atoms (Li, Na) and intercalated by potassium atoms are studied. The quantum-chemical semiempirical MNDO method and a model of the covalent cyclic cluster built in via ionic bonding are used to model infinitely long nanotubes. The electronic density of states of modified nanotubes is found. It is shown that semiconductor-metal transitions can occur in semiconductor nanotubes and that semimetal nanotubes can undergo metal-metal transitions.     

C掺杂金红石相TiO2的电子结构和光学性质的第一性原理研究

运用第一性原理的局域密度近似+U(0 ≤U≤9 eV)方法研究了本征金红石相TiO₂在不同U值(对Ti-3d电子)下的禁带宽度、晶体结构以及不同比例C元素掺杂的金红石相TiO₂的电子结构和光学性质, 研究表明, TiO₂的禁带宽度和晶格常数随着U值的增加而增大. 综合考虑取U=3 eV并对其计算结果进行修正. 对于掺杂体系, 发现C 元素的掺杂在金红石相TiO₂中引入杂质能级, 杂质能级主要由O-2p轨道和C-2p轨道耦合形成, 杂质能级的引入可以增加TiO₂对可见光的响应, 从而使TiO₂的吸收范围增大. C原子掺杂最佳比例为8.3%, 此时光学吸收边的红移程度最明显, 可增大光吸收效率, 从而提高了TiO₂光催化效率.     

Kondo coupling and Curie temperature of in the coherent potential approximation

The ferromagnetism of Ga_1-xMn_xAs is studied in the coherent potential approximation (CPA). In this work, we used the exact Hilbert transformation of the face-centered cubic (fcc) density of states (DOS), which is different from the usual semi-circle DOS employed in previous works. Using Weiss molecular theory, we obtained a nonlinear relation of Curie temperature with respect to Kondo coupling. Our calculated T_C agrees very well with measured values.     

Energy structure of Al1−xGaxAs and Al1−xGaxSb in the coherent potential approximation

Using the pseudopotential method in the coherent potential approximation we calculate the energy structure of the solid solutions Al_1–xGa_xAs and Al_1–xGa_xSb. We analyze the effect of chemical disorder on the interband transitions and the electron mobility. We show that it is important to take into account the effects of nondiagonal disorder.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 15–22, July, 1986.     

Analytic extension of the coherent potential approximation to clusters

We put forward the homomorphic cluster coherent potential approximation (HCPA) for substitutionally disordered systems as one possible way of avoiding the breakdown of analyticity in average Green's functions even when effects of clusters are properly taken into account. We assert that a cluster CPA yields an analytic physical solution if a one-particle total Hamiltonian is partitioned into the sum of homomorphic single-cluster Hamiltonians and the CPA condition is applied to one of these homomorphic sub-Hamiltonians.     

Electronic theory of surface segregation in the coherent potential approximation: The surface segregation in CoNi, IrPt and RhPt

The surface segregation in CoNi, IrPt and RhPt is calculated within the coherent potential approximation. It is assumed that the metals form substitutional alloys and only diagonal disorder is taken into account. The condition of the local neutrality resulting in the presence of diagonal potentials in the Hamiltonian is also introduced. The alloys are selected in such a way that: (i) the non-diagonal disorder can be neglected with a relatively good approximation, (ii) the d-band filling is not too large to ensure the procedure of determining diagonal potentials from the local neutrality condition to work properly and (iii) the experimental data are available.     

Diagrammatic approach to the off-diagonal randomness in the coherent potential approximation

The off-diagonal randomness in the coherent potential approximation (CPA) is self-consistently treated by extending Nickel and Krumhansl's diagram-resummation technique CPA(2). New diagrams are introduced to represent the off-diagonal potentials and multiple occupancy correction is made in a completely self-consistent way.     

One-particle properties of compositionally modulated alloys in the coherent potential approximation

We propose a simple generalization of the original single-site coherent potential approximation (CPA) applicable to compositionally modulated alloys. Although the original single-site CPA gives an almost structureless density of states, we find that for strong modulation and relatively strong scattering our generalization of the CPA yields densities of states which exhibit considerable structure and which represent the exact densities of states remarkably well. In this sense our generalization is more accurate than the original CPA from which it was obtained.