51 research outputs found
Enhanced thermoelectric response of hole-doped LaNiO by ab initio calculations
Thermoelectric properties of the system LaNiO have been
studied ab initio. Large Seebeck coefficient values are predicted for the
parent compound, and to some extent remain in the hole-doped metallic phase,
accompanied of an increase in the conductivity. This system, due to its layered
structure would be a suitable candidate for an improvement of its
thermoelectric figure of merit by nanostructurization in thin films, that has
already been shown to increase the electrical conductivity (). Our
calculations show that in the region around LaNiO the system has a
large thermopower at high temperatures and also a substantially increased
. Films grown with this low-doping concentration will show an optimal
relationship between thermopower and . This result is obtained for
various exchange-correlation schemes (correlated, uncorrelated and
parameter-free) that we use to analyze the electronic structure of the
hole-doped compound.Comment: 10 pages, 6 figure
Electronic structure of VO: charge ordering, metal-insulator transition and magnetism
The low and high-temperature phases of VO have been studied by
\textit{ab initio} calculations. At high temperature, all V atoms are
electronically equivalent and the material is metallic. Charge and orbital
ordering, associated with the distortions in the V pseudo-rutile chains, occur
below the metal-insulator transition. Orbital ordering in the low-temperature
phase, different in V and V chains, allows to explain the
distortion pattern in the insulating phase of VO. The in-chain magnetic
couplings in the low-temperature phase turn out to be antiferromagnetic, but
very different in the various V and V bonds. The V dimers
formed below the transition temperature form spin singlets, but V ions,
despite dimerization, apparently participate in magnetic ordering.Comment: 10 pages, 6 figures, 2 table
Optimum sizing of launching nose of prestressed concrete bridges
El diseño del pico de lanzamiento de un puente lanzado determina su proceso constructivo y, por lo tanto, también sus dimensiones. La optimización del pico de lanzamiento puede plantearse como primer paso para la mejora del diseño de un puente lanzado. El procedimiento convencional de diseño del pico de lanzamiento se basa en emplear la técnica de prueba y error para reducir el momento flector sobre el apoyo frontal del tablero de hormigón pretensado durante el proceso de lanzamiento. Esta forma de trabajo en ningún caso garantiza alcanzar la mejor de entre todas las soluciones posibles, pues se basa en la experiencia e intuición del proyectista, además de limitarse necesariamente a escasos tanteos. Dado que los puentes lanzados constituyen una tipologÃa constructiva importante, deben incorporar todas las capacidades disponibles de innovación del diseño, entre las que se encuentra la optimización matemática. Este trabajo propone una formulación objetiva y rigurosa para optimizar el pico de lanzamiento de un puente lanzado adaptada a los condicionantes reales que tienen en la práctica los proyectistas de puentes. Comparando los resultados obtenidos mediante procedimientos convencionales y con técnicas de optimización se puede comprobar el grado de validez de algunas afirmaciones, que se dan por supuestas en los métodos clásicos de diseño de picos de lanzamiento, pero que en realidad no se sustentan en fundamento teórico alguno, demostrándose la utilidad de la optimización matemática para la mejora del diseño.The design of the launching nose of an incrementally launched bridge determines its constructive process and, therefore, also its dimensions. The optimization of the launching nose can raise as a first step to improve the design of a launched bridge. The conventional design process of a launching nose is based on trial and error method to reduce bending moment of prestressed concrete deck at the foremost support during launch. In this way, there is no guarantee that the obtained solution is the best among all the possible solutions since they all depend on the experience and intuition of a designer, and they are also restricted by a limited number of possible iterations. Given that launched bridges constitute an important constructive typology, all the available capacities of design innovation should be incorporated, among which it can be found numerical optimization. This research work proposes an objective and rigorous formulation to optimize a launching nose of launched bridge under real constraints that a bridge designer can encounter in practice. Comparing the results obtained by conventional process and that by optimization techniques, it can be verified that some of the assumptions, considered in classical design methods of a launching nose, are not based on any theoretical foundation. This fact demonstrates the utility of numerical optimization to improve a design.Peer Reviewe
Unexpected Magnetism of Small Silver Clusters
The ground-state electronic, structural, and magnetic properties of small
silver clusters, Ag (2n22), have been studied using a linear
combination of atomic Gaussian-type orbitals within the density functional
theory. The results show that the silver atoms, which are diamagnetic in bulk
environment, can be magnetic when they are grouped together in clusters. The
Ag cluster with icosahedral symmetry has the highest magnetic moment per
atom among the studied silver clusters. The cluster symmetry and the reduced
coordination number specific of small clusters reveal as a fundamental factor
for the onset of the magnetism.Comment: 4 pages, 4 figure
Reliability-based design optimization of composite stiffened panels in post-buckling regime
This paper focuses on Deterministic and Reliability Based Design Optimization (DO and RBDO) of composite stiffened panels considering post-buckling regime and progressive failure analysis. The ultimate load that a post-buckled panel can hold is to be maximised by changing the stacking sequence of both skin and stringers composite layups. The RBDO problem looks for a design that collapses beyond the shortening of failure obtained in the DO phase with a target reliability while considering uncertainty in the elastic properties of the composite material. The RBDO algorithm proposed is decoupled and hence separates the Reliability Analysis (RA) from the deterministic optimization. The main code to drive both the DO and RBDO approaches is written in MATLAB and employs Genetic Algorithms (GA) to solve the DO loops because discrete design variables and highly nonlinear response functions are expected. The code is linked with Abaqus to perform parallel explicit nonlinear finite element analyses in order to obtain the structural responses at each generation. The RA is solved through an inverse Most Probable failure Point (MPP) search algorithm that benefits from a Polynomial Chaos Expansion with Latin Hypercube Sampling (PCE-LHS) metamodel when the structural responses are required. The results led to small reductions in the maximum load that the panels can bear but otherwise assure that they will collapse beyond the shortening of failure imposed with a high reliability
Pressure-induced metal-insulator transition in MgV_2O_4
On the basis of experimental thermoelectric power results and ab initio
calculations, we propose that a metal-insulator transition takes place at high
pressure (approximately 6 GPa) in MgV_2O_4.Comment: 2 pages, 3 figures, accepted in Physica B (Strongly Correlated
Electron Systems '07
Magnetic polarons in doped 1D antiferromagnetic chain
The structure of magnetic polarons (ferrons) is studied for an 1D
antiferromagnetic chain doped by non-magnetic donor impurities. The conduction
electrons are assumed to be bound by the impurities. Such a chain can be
described as a set of ferrons at the antiferromagnetic background. We found
that two types of ferrons can exist in the system. The ground state of the
chain corresponds to the ferrons with the sizes of the order of the
localization length of the electron near the impurity. The ferrons of the
second type produce a more extended distortion of spins in the chain. They are
stable within a finite domain of the system parameters and can be treated as
excitations above the ground state. The ferrons in the excited states can
appear in pairs only. The energy of the excited states decreases with the
growth in density of impurities. This can be interpreted as a manifestation of
an attractive interaction between ferrons.Comment: 6 pages, 5 figures, RevTex4, submitted to PR
Enhanced dimerization of TiOCl under pressure: spin-Peierls - to - Peierls transition
We report high-pressure x-ray diffraction and magnetization measurements
combined with ab-initio calculations to demonstrate that the high-pressure
optical and transport transitions recently reported in TiOCl, correspond in
fact to an enhanced Ti3+-Ti3+ dimerization existing already at room
temperature. Our results confirm the formation of a metal-metal bond between
Ti3+ ions along the b-axis of TiOCl, accompanied by a strong reduction of the
electronic gap. The evolution of the dimerization with pressure suggests a
crossover from the spin-Peierls to a conventional Peierls situation at high
pressures.Comment: 9pages, 4 figure
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