Local density of states of dc-biased superlattices with time-dependent imperfection

The single-particle Green's function for a dc-biased superlattices with single impurity potential varying harmonically with time has been obtained in the framework of U(t,t') method and Floquet--Green's function. The calculation of the local density of states shows that new states will emerge between the resonant Wannier--Stark states as a result of the intervention of time-dependent impurity potential, and the increase in electric field strength of impurity will result in the growing of the number of new states between the gaps of neighbouring Stark ladders.     

Effect of carbon network defects on the electronic structure of semiconductor single-wall carbon nanotubes

For a single-wall (14, 0) carbon nanotube, the total density of electronic states of the ideal structure and of some possible defect structures is calculated in the framework of the band theory approach using Gaussian-type orbitals and the approximation of the generalized density gradient. It is shown that allowance for defects of the atomic structure of a nanotube makes it possible to adequately describe the existing experimental data on nanotube electronic structure. In the framework of the same approach, the total density of electronic states is calculated for an intermolecular contact of (5, 5) and (10, 0) single-wall carbon nanotubes formed due to the creation of a 5–7 defect. It is shown that the electronic states related to the contact region and the 5–7 defect lie in vicinity of the Fermi level.     

Density Operator in Terms of Coherent States Representation with the Applications in the Quantum Information

In the quantum information theory operates with qubits and N-qubits that can be express through coherent states. Density operator admits a representation in terms of coherent states formalism. Consequently, in this paper the notions of qubit and density operators are described in the framework of coherent states. We have expressed a qubit as a coherent state, and thus a sequence of qubits becomes the tensor product of the coherent states. For the ensembles of qubits, it could be used the density operator, in order to describe the informational content of the ensemble. The coherent states representation may play an important role in the quantum information theory and the use of this formalism is not only theoretical, but also, due to its applications, of some practical relevance.     

Thermodynamic Properties of Non-equilibrium States in Quantum Field Theory

Within the framework of relativistic quantum field theory, a novel method is established which allows for distinguishing non-equilibrium states admitting locally a thermodynamic interpretation. The basic idea is to compare these states with global equilibrium states (KMS states) by means of local thermal observables. With the help of such observables, the states can be ordered into classes of increasing local thermal stability. Moreover, it is possible to identify states exhibiting certain specific thermal properties of interest, such as a definite local temperature or entropy density. The method is illustrated in a simple model describing the spatio-temporal evolution of a “big heat bang.”     

铝中单空位正电子态的完全自洽计算

从第一性原理和两分量密度泛函理论出发,应用离散变分法和嵌入集团模型,计算了铝中单空位的正电子态。分析了外部晶格势场、集团大小和芯冻结近似对固体中正电子分布、正电子湮没特性的影响。同时讨论了完全自洽方法和简单方法对固体中正电子态计算的影响。 关键词：     

Groupoids,von Neumann Algebras and the Integrated Density of States

We study spectral properties of random operators in the general setting of groupoids and von Neumann algebras. In particular, we establish an explicit formula for the canonical trace of the von Neumann algebra of random operators and define an abstract density of states. While the treatment applies to a general framework we lay special emphasis on three particular examples: random Schrödinger operators on manifolds, quantum percolation and quasi–crystal Hamiltonians. For these examples we show that the distribution function of the abstract density of states coincides with the integrated density of states defined via an exhaustion procedure.     

Density of states of two-dimensional aluminum nanoclusters in the Hubbard model

The density of states of a two-dimensional square nanosystem composed of N × N aluminum atoms (N = 3?30) is calculated in the framework of the Hubbard model. It is demonstrated that, at a small parameter N, the density of states depends substantially on the number of atoms and on the position of a particular atom in the lattice. As the parameter N increases, the density of states for the vertex and edge atoms tends to the value of the density of states for the bulk atoms. The temperature of the system is implicitly included by specifying the energy of hopping in the initial Hamiltonian.     

LiNH2 的晶格动力学、介电性质和热力学性质第一性原理研究

采用基于密度泛函理论的平面波赝势方法,在局域密度近似下采用线性响应的密度泛函微扰理论计算了LiNH₂的晶格动力学、介电性质和热力学性质,得到了布里渊区高对称方向上的声子色散曲线和相应的声子态密度,分析了 LiNH₂的红外和拉曼活性声子频率,同时给出它的介电张量和玻恩有效电荷张量. 研究表明,LiNH₂存在小的各向异性,计算所得结果与实验值和其他理论值符合较好.最后,利用得到的声子态密度进一步预测了LiNH₂的热力学性质 关键词： 密度泛函理论 晶格动力学 热力学性质 第一性原理计算     

Einfluß der Schalenstruktur und der Paarung auf die Dichte der Energiezustände von Atomkernen

The density of nuclear single particle states is calculated within the framework of the spherical oscillator model including spinorbit and pairing interactions. The use of the oscillator energies leads to a strong influence of shell-effects on the level density which are slightly reduced, if pairing interactions are taken into account.     

Band structure of tetragonal BaTiO <Emphasis Type="Bold">3</Emphasis>

The electronic structure, total density of states DOS and electronic density in ferroelectric tetragonal crystal BaTiO₃ are studied using WIEN2k package. This employs the full potential-linearized augmented plan wave FP-LAPW method in the framework of the density functional theory DFT with the generalized gradient approximation (GGA). The results show an indirect band gap of 2.30 eV at the Γ point in the Brillouin zone. The calculated band structure and density of states of BaTiO₃ agree with the previous experimental and theoretical results, as do the charge distribution and the prediction of the nature of the chemical bonding. Received 11 December 2002 / Received in final form 3 February 2003 Published online 1st April 2003 RID="a" ID="a"e-mail: salehihamid@yahoo.com     

Ab initio study of palladium and silicon carbide

Ab initio methods have been used to investigate the properties of Pd as impurity in bulk SiC at five charged states within the framework of density functional theory using the local spin density approximation. It was found that Pd interstitials and substitutionals have similar energy to their intrinsic counterparts. In addition, Pd substitutes for a vacancy, di-vacancy, and tri-vacancy with similar energies. Pd diffuses through SiC via an interstitial mechanism employing the tetrahedral sites and Pd can substitute for Si and C at positive charged states. Removing electrons (p-type doping) from SiC lowers the formation and migration energies of Pd defects in SiC for most configurations.     

锂离子电池负极材料CuSn的Li嵌入性质的研究

使用基于混合基表示的第一原理赝势法，研究了锂离子电池非碳类负极材料CuSn的Li嵌入时的形成能以及相应的电子结构.还给出了Li嵌入时的体积变化，能带结构、电子态密度以及电荷密度分布等性质, 并讨论了CuSn作为负极材料的特点.计算发现，Cu-Sn化合物在闪锌矿结构时，Li嵌入主体材料时的嵌入形成能大致在3.5eV附近. 关键词： 锂离子电池 负极材料 CuSn 电子结构     

Spin Waves in a Surface-Diluted Quant um Spin system

A theoretical investigation of the effects of surface dilution on spin waves in a semi-infinite transverse Ising model (TIM) is presented. Within the framework of the double-time Green's function theory, the loctl density of spin-wave states is obtained by the application of the cluster-Bethe-lattice (CBL) method. Numerical calculations of paramagnetic phases reveal that surface dilution will lead to appearance of the localized spin-wave states whose frequencies are bulk independent while the corresponding density of states is a function of the exchange interactions. Increase of the bulk exchange interactions between the surface and the bulk spins tends to diminish the localized spin-wave states. Dependence of the local density of states on the concentration p of the magnetic atoms is similar with that in a three-dimensional infinite TIM which has been discussed in our previous paper.     

Cluster model calculation and photoemission studies of molecularly adsorbed NO on Ni

Self-consistent scattered wave cluster model calculations for the NONi system are presented with NO bonded N-down in a fourfold coordination site. The results suggest that the charge is transferred from metal into the single occupied 2π¹ molecular orbital of NO which is thus significantly perturbed. The interaction involves the metal s, p, and d-derived states. We calculate total and local “density of states” defined within the framework of the cluster model and compare the results with Ultraviolet Photoemission experiments recorded at 80 K. At 300 K partial dissociation occurs as judged from UPS and XPS. Increased photoemission observed at about 2 eV below the Fermi level in the molecularly adsorbed state is attributed to the alteration of density of states due to the interaction of the 2π¹ molecular orbital of NO with the substrate. A detailed analysis of the nature of the relevant molecular orbitals is also given.     

First principles calculations of structural,electronic and optical properties of various phases of CaS

First principles calculations have been performed within the framework of density functional theory to investigate the structural, electronic and optical properties of all four possible B1, B2, B3 and B4 phases of CaS. Apart from the standard local density approximation (LDA) and GGA (PBE), a more accurate nonempirical density functional generalized gradient approximation (GGA), as proposed by Wu and Cohen [Phys. Rev. B 73, 235116 (2006)] for the exchange-correlation energy, E_XC, has been attempted in these calculations. Calculated electronic structure and the density of states are analyzed in terms of the contribution of Ca d states and S s and p states in determining the nature of the fundamental band gap in various phases. Reflectivity, R (ω), the real and imaginary part of the dielectric functions, ε(ω), have been calculated for all the phases and the results have been discussed and compared with the existing experimental data.     

Conductance of disordered graphene sheets: A real space approach

In this work the conducting properties of graphene lattice (buckled as well as planar) having different concentrations of defects are studied with the help of real space block recursion method introduced by Haydock et al. Since the defects are completely random, reciprocal space based methods which need artificial periodicity are not applicable here. Different resonant states appear because of the presence of topological and local defects which are calculated within the framework of Green function. Except random voids, in all other density of states (DOS) spectra there are signatures of Breit–Wigner and Fano resonance at occupied and unoccupied regime respectively. Although Fano resonance states are not prominent for graphene with random voids, however Stone–Wales (SW) type defect can naturally introduce their resonance states. The appearance of localized states depends strongly on the concentration of defects.     

Bose glass and superfluid phase transitions of exciton-polaritons in GaN microcavities

Microcavity exciton-polaritons within GaN-based structures are the object of the present work. The impact of the structural imperfections on the properties of the two-dimensional polariton gas is investigated through the calculation of its phase diagram. We demonstrate that the presence of disorder first induces a quasi-phase transition of the polariton system towards a Bose-glass phase before it becomes superfluid as its density increases. Calculations of the density of states as well as the condensate wavefunction and the related spectrum of elementary excitations in the framework of the Gross-Pitaevskii theory provide further insight into the properties of exciton-polaritons in GaN-based microcavities.     

Metastable molecular fluid hydrogen at high pressures

Warm dense hydrogen is studied in the region of fluid–fluid phase transition within the framework of the density functional theory. We report a procedure of obtaining metastable states and calculate the equation of state. Metastable states are diagnosed by pair correlation functions and values of conductivity. We obtain a strong overlapping through the density of metastable and equilibrium branches of pressure isotherms. This indicates the plasma nature of the phase transition.