Maximally-localized Wannier Functions in Antiferromagnetic MnO within the FLAPW Formalism

We have calculated the maximally-localized Wannier functions of MnO in its antiferromagnetic (AFM) rhombohedral unit cell, which contains two formula units. Electron Bloch functions are obtained with the linearized augmented plane-wave method within both the LSD and the LSD+U schemes. The thirteen uppermost occupied spin-up bands correspond in a pure ionic scheme to the five Mn 3d orbitals at the Mn_1 (spin-up) site, and the four O 2s/2p orbitals at each of the O_1 and O_2 sites. Maximal localization identifies uniquely four Wannier functions for each O, which are trigonally-distorted sp^3-like orbitals. They display a weak covalent bonding between O 2s/2p states and minority-spin d states of Mn_2, which is absent in a fully ionic picture. This bonding is the fingerprint of the interaction responsible for the AFM ordering, and its strength depends on the one-electron scheme being used. The five Mn Wannier functions are centered on the Mn_1 site, and are atomic orbitals modified by the crystal field. They are not uniquely defined by the criterion of maximal localization and we choose them as the linear combinations which diagonalize the r^2 operator, so that they display the D_3d symmetry of the Mn_1 site.Comment: 11 pages, 6 PostScript figures. Uses Revtex4. Hi-res figures available from the author

The issues of weed infestation with environmentally hazardous plants and methods of their control

The authors analyze expansion of segetal and ruderal vegetation on agricultural lands in Leningrad and Tomsk oblasts, typical for the European and Asian parts of Russia. The spreading conditions, composition of species, biological features and ecological requirements of the most aggressive species are identified. Some effective ways of weed control are suggested

Dynamical mean-field theory using Wannier functions: A flexible route to electronic structure calculations of strongly correlated materials

International audienceA versatile method for combining density functional theory in the local density approximation with dynamical mean-field theory (DMFT) is presented. Starting from a general basis-independent formulation, we use Wannier functions as an interface between the two theories. These functions are used for the physical purpose of identifying the correlated orbitals in a specific material, and also for the more technical purpose of interfacing DMFT with different kinds of band-structure methods (with three different techniques being used in the present work). We explore and compare two distinct Wannier schemes, namely the maximally localized Wannier function and the Nth order muffin-tin-orbital methods. Two correlated materials with different degrees of structural and electronic complexity, SrV O3 and BaV S3, are investigated as case studies. SrV O3 belongs to the canonical class of correlated transition-metal oxides, and is chosen here as a test case in view of its simple structure and physical properties. In contrast, the sulfide BaV S3 is known for its rich and complex physics, associated with strong correlation effects and low-dimensional characteristics. Insights into the physics associated with the metal-insulator transition of this compound are provided, particularly regarding correlation-induced modifications of its Fermi surface. Additionally, the necessary formalism for implementing self-consistency over the electronic charge density in a Wannier basis is discussed. © 2006 The American Physical Society

Spontaneous polarization and piezoelectric constants of III-V nitrides

The spontaneous polarization, dynamical Born charges, and piezoelectric constants of the III-V nitrides AlN, GaN, and InN are studied ab initio using the Berry phase approach to polarization in solids. The piezoelectric constants are found to be up 10 times larger than in conventional III-V's and II-VI's, and comparable to those of ZnO. Further properties at variance with those of conventional III-V compounds are the sign of the piezoelectric constants (positive as in II-VI's) and the very large spontaneous polarization.Comment: RevTeX 4 pages, improved upon revie

Transition from tunneling to direct contact in tungsten nanojunctions

We apply the mechanically controllable break junctions technique to investigate the transition from tunneling to direct contact in tungsten. This transition is quite different from that of other metals and is determined by the local electronic properties of the tungsten surface and the relief of the electrodes at the point of their closest proximity. The conductance traces show a rich variety of patterns from the avalanche-like jump to a mesoscopic contact to the completely smooth transition between direct contact and tunneling. Due to the occasional absence of an adhesive jump the conductance of the contact can be continuously monitored at ultra-small electrode separations. The conductance histograms of tungsten are either featureless or show two distinct peaks related to the sequential opening of spatially separated groups of conductance channels. The role of surface states of tungsten and their contribution to the junction conductance at sub-Angstrom electrode separations are discussed.Comment: 6 pages, 6 figure

Electronic and structural properties of superconducting MgB2_2, CaSi2_2 and related compounds

We report a detailed study of the electronic and structural properties of the 39K superconductor \mgbtwo and of several related systems of the same family, namely \mgalbtwo, \bebtwo, \casitwo and \cabesi. Our calculations, which include zone-center phonon frequencies and transport properties, are performed within the local density approximation to the density functional theory, using the full-potential linearized augmented plane wave (FLAPW) and the norm-conserving pseudopotential methods. Our results indicate essentially three-dimensional properties for these compounds; however, strongly two-dimensional $\sigma$-bonding bands contribute significantly at the Fermi level. Similarities and differences between \mgbtwo and \bebtwo (whose superconducting properties have not been yet investigated) are analyzed in detail. Our calculations for \mgalbtwo show that metal substitution cannot be fully described in a rigid band model. \casitwo is studied as a function of pressure, and Be substitution in the Si planes leads to a stable compound similar in many aspects to diborides.Comment: Revised version, Phys.Rev.B in pres

First-principles study of (BiScO3){1-x}-(PbTiO3){x} piezoelectric alloys

We report a first-principles study of a class of (BiScO3)_{1-x}-(PbTiO3)_x (BS-PT) alloys recently proposed by Eitel et al. as promising materials for piezoelectric actuator applications. We show that (i) BS-PT displays very large structural distortions and polarizations at the morphotropic phase boundary (MPB) (we obtain a c/a of ~1.05-1.08 and P_tet of ~1.1 C/m^2); (ii) the ferroelectric and piezoelectric properties of BS-PT are dominated by the onset of hybridization between Bi/Pb-6p and O-2p orbitals, a mechanism that is enhanced upon substitution of Pb by Bi; and (iii) the piezoelectric responses of BS-PT and Pb(Zr_{1-x}Ti_x)O3 (PZT) at the MPB are comparable, at least as far as the computed values of the piezoelectric coefficient d_15 are concerned. While our results are generally consistent with experiment, they also suggest that certain intrinsic properties of BS-PT may be even better than has been indicated by experiments to date. We also discuss results for PZT that demonstrate the prominent role played by Pb displacements in its piezoelectric properties.Comment: 6 pages, with 3 postscript figures embedded. Uses REVTEX and epsf macros. Also available at http://www.physics.rutgers.edu/~dhv/preprints/ji_bi/index.htm

"CSTC T-PPR": ARCHITECTURAL AND HISTORICAL DEVELOPMENT OF ODESSA IN ХІХ ... THE BEGINNING OF THE XX-th CENTURIES

It is offered to create in the city of Odessa "the Corporate scientific and technical complex town-planning power reconstruction "CSTC T-PPR", as innovative organizational structure which uses in practice the saved up scientific and technical potential for reconstruction of buildings of historical building of Odessa under standards power efficiency. It is executed researches of historically-style development which is characterized by the big scope of civil work and presence of various styles. Superstructures were carried out in a city practically during all history its development. The mansard floor is an effective variant a superstructure buildings of background building. Пропонується створити у місті Одесі "Корпоративний науково-технічний комплекс містобудівної енергореконструкції "КНТК МЕРек", як інноваційну організаційну структуру, яка використовує на практиці накопичений науково-технічний потенціал для реконструкції будівель історичної забудови Одеси за стандартами енергоефективності. Виконано дослідження історично-стильового розвитку Одеси в ХІХ … на початку ХХ століть, який характеризується великим розмахом будівельних робіт та наявністю різноманітних стилів (класицизм, бароко, романтизм, конструктивізм). Враховуючи, що розташування нового будинку в щільному ряду історичної забудови заборонено, то виходом з такої ситуації є надбудова будівлі новим об'ємом. Надбудови виконувалися в місті практично в ході всієї історії його розвитку. Ефективним варіантом надбудови будівель фонової забудови є мансардний поверх.