309 research outputs found
Symmetry-induced interference effects in metalloporphyrin wires
Organo-metallic molecular structures where a single metallic atom is embedded
in the organic backbone are ideal systems to study the effect of strong
correlations on their electronic structure. In this work we calculate the
electronic and transport properties of a series of metalloporphyrin molecules
sandwiched by gold electrodes using a combination of density functional theory
and scattering theory. The impact of strong correlations at the central
metallic atom is gauged by comparing our results obtained using conventional
DFT and DFT+U approaches. The zero bias transport properties may or may not
show spin-filtering behavior, depending on the nature of the d state closest to
the Fermi energy. The type of d state depends on the metallic atom and gives
rise to interference effects that produce different Fano features. The
inclusion of the U term opens a gap between the d states and changes
qualitatively the conductance and spin-filtering behavior in some of the
molecules. We explain the origin of the quantum interference effects found as
due to the symmetry-dependent coupling between the d states and other molecular
orbitals and propose the use of these systems as nanoscale chemical sensors. We
also demonstrate that an adequate treatment of strong correlations is really
necessary to correctly describe the transport properties of metalloporphyrins
and similar molecular magnets
Charge Kondo anomalies in PbTe doped with Tl impurities
We investigate the properties of PbTe doped with a small concentration of
Tl impurities acting as acceptors and described by Anderson impurities with
negative onsite correlation energy. We use the numerical renormalization group
method to show that the resulting charge Kondo effect naturally accounts for
the unusual low temperature and doping dependence of normal state properties,
including the self-compensation effect in the carrier density and the
non-magnetic Kondo anomaly in the resistivity. These are found to be in good
qualitative agreement with experiment. Our results for the Tl s-electron
spectral function provide a new interpretation of point contact data.Comment: 5 pages, 3 figures; published versio
Accurate screened exchange band structures for transition metal monoxides MnO, FeO, CoO and NiO
We report calculations of the band structures and density of states of the
four transition metal monoxides MnO, FeO, CoO and NiO using the hybrid density
functional sX-LDA. Late transition metal oxides are prototypical examples of
strongly correlated materials, which pose challenges for electronic structure
methods. We compare our results with available experimental data and show that
our calculations yield accurate predictions for the fundamental band gaps and
valence bands of FeO, CoO and NiO. For MnO, the band gaps are underestimated,
suggesting additional many-body effects that are not captured by our screened
hybrid functional approach.Comment: 9 pages, 3 figures, 3 table
Self-stabilizing algorithms for Connected Vertex Cover and Clique decomposition problems
In many wireless networks, there is no fixed physical backbone nor
centralized network management. The nodes of such a network have to
self-organize in order to maintain a virtual backbone used to route messages.
Moreover, any node of the network can be a priori at the origin of a malicious
attack. Thus, in one hand the backbone must be fault-tolerant and in other hand
it can be useful to monitor all network communications to identify an attack as
soon as possible. We are interested in the minimum \emph{Connected Vertex
Cover} problem, a generalization of the classical minimum Vertex Cover problem,
which allows to obtain a connected backbone. Recently, Delbot et
al.~\cite{DelbotLP13} proposed a new centralized algorithm with a constant
approximation ratio of for this problem. In this paper, we propose a
distributed and self-stabilizing version of their algorithm with the same
approximation guarantee. To the best knowledge of the authors, it is the first
distributed and fault-tolerant algorithm for this problem. The approach
followed to solve the considered problem is based on the construction of a
connected minimal clique partition. Therefore, we also design the first
distributed self-stabilizing algorithm for this problem, which is of
independent interest
Lattice Dynamics and Specific Heat of - GeTe: a theoretical and experimental study
We extend recent \textit{ab initio} calculations of the electronic band
structure and the phonon dispersion relations of rhombohedral GeTe to
calculations of the density of phonon states and the temperature dependent
specific heat. The results are compared with measurements of the specific heat.
It is discovered that the specific heat depends on hole concentration, not only
in the very low temperature region (Sommerfeld term) but also at the maximum of
(around 16 K). To explain this phenomenon, we have performed
\textit{ab initio} lattice dynamical calculations for GeTe rendered metallic
through the presence of a heavy hole concentration ( 2
10 cm). They account for the increase observed in the maximum of
.Comment: 8 pages, 7 figures, ref. 19 correcte
The Protein Ontology: a structured representation of protein forms and complexes
The Protein Ontology (PRO) provides a formal, logically-based classification of specific protein classes including structured representations of protein isoforms, variants and modified forms. Initially focused on proteins found in human, mouse and Escherichia coli, PRO now includes representations of protein complexes. The PRO Consortium works in concert with the developers of other biomedical ontologies and protein knowledge bases to provide the ability to formally organize and integrate representations of precise protein forms so as to enhance accessibility to results of protein research. PRO (http://pir.georgetown.edu/pro) is part of the Open Biomedical Ontology Foundry
Enhanced superconducting pairing interaction in indium-doped tin telluride
The ferroelectric degenerate semiconductor SnTe exhibits
superconductivity with critical temperatures, , of up to 0.3 K for hole
densities of order 10 cm. When doped on the tin site with greater
than indium atoms, however, superconductivity is observed up
to 2 K, though the carrier density does not change significantly. We present
specific heat data showing that a stronger pairing interaction is present for
than for . By examining the effect of In dopant atoms on
both and the temperature of the ferroelectric structural phase
transition, , we show that phonon modes related to this transition are
not responsible for this enhancement, and discuss a plausible candidate
based on the unique properties of the indium impurities.Comment: 7 page
ТЕРМОЭЛЕКТРИЧЕСКИЕ СВОЙСТВА МАТЕРИАЛА НА ОСНОВЕ (Bi,Sb)2Te3, ПОЛУЧЕННОГО МЕТОДОМ ИСКРОВОГО ПЛАЗМЕННОГО СПЕКАНИЯ
Thermoelectric properties of (Bi,Sb)2Te3 nanostructured bulk material as a function of composition and spark plasma sintering temperature were investigated. The Bi0,4Sb1,6Te3 alloys at sintering temperature 450—500 °C with ZT=1,25—1,28 figure of merit was fabricated. Thermoelectric properties ad a function ofsintering temperature above 400 °C correlate with fine structure change. It was found that point structure defects introduce essential contribution to the formation of nanostructured material thermoelectric properties.Исследована зависимость термоэлектрических свойств наноструктурированного объемногоматериала (Bi,Sb)2Te3 от состава и температуры SPS−спекания ТSPS. Обнаружено, что твердыйраствор Bi0,4Sb1,6Te3, спеченный при температуре 450—500 °С, имеет термоэлектрическуюэффективность ZT = 1,25÷1,28. Зависимость термоэлектрических свойств от температурыспекания ТSPS выше 400 °С коррелирует с изменением тонкой структуры материала, котороеопределяется перераспределением донорных точечныхдефектов вакансионного типа впроцессе повторной рекристаллизации. Установлено, что точечные структурные дефекты вносятсущественный вклад в формирование термоэлектрическихсвойств наноструктурированногоматериала
CТРУКТУРА ОБЪЕМНОГО ТЕРМОЭЛЕКТРИЧЕСКОГО МАТЕРИАЛА НА ОСНОВЕ (Bi,Sb)2Te3, ПОЛУЧЕННОГО МЕТОДОМ ИСКРОВОГО ПЛАЗМЕННОГО СПЕКАНИЯ
Bulk nanostructured BixSb1−xTe3 based material was fabricated with spark plasma sintering (SPS) method. The starting powders with a particle size of 10—12 nm were prepared by ball milling. The regularity of fine structure changes as a function of sintering temperature in the 250—550 °C range was investigated by X−raydiffractometry, scanning and transmission electron microscopy (TEM, HRTEM). For the first time coherent dispersion areas (CDA) size was found not to increase monotonously with sintering temperature, and at temperatures of above 400 °C the CDA size decreased as a result of repeated recrystallization. Apparently,the structural changes are associated with a redistribution of intrinsic vacancy type structural defects during sintering.Исследованы образцы объемного наноструктурированного материала на основе твердого раствора BixSb1−xTe3, полученного методом искрового плазменного спекания. Исходные порошки с размером частиц 10—12 нм изготовлены помолом в шаровой мельнице. С помощью методов рентгеновской дифрактометрии, растровой и просвечивающей (в том числе высокого разрешения) электронной микроскопии изучены закономерности изменения тонкой структуры в зависимости от температуры спекания в интервале 250—550 °С. Впервые установлено, что размеры областей когерентного рассеяния не возрастают монотонно с ростом температуры спекания, а при температуре выше 400 °С происходит их измельчение в результате процесса повторной рекристаллизации. Наиболее вероятно, что изменения структуры связаны с перераспределением собственных структурных дефектов вакансионного типа в процессе спекания образцов
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