556 research outputs found
Electron correlations and bond-length fluctuations in copper oxides: from Zhang--Rice singlets to correlation bags
We perform first principles, multiconfiguration calculations on clusters
including several CuO octahedra and study the ground-state electron
distribution and electron--lattice couplings when holes are added to the
undoped configuration. We find that the so-called Zhang--Rice state
on a single CuO plaquette is nearly degenerate with a state whose leading
configuration is of the form Cu -- O -- Cu . A strong coupling
between the electronic and nuclear motion gives rise to large inter-site charge
transfer effects for half-breathing displacements of the oxygen ions. Under the
assumption of charge segregation into alternating hole-free and hole-rich
stripes of Goodenough \cite{jbg_02,jbg_03}, our results seem to support the
vibronic mechanism and the traveling charge-density wave model from
Refs.\cite{jbg_02,jbg_03} for the superconductivity in copper oxides.Comment: submitted to Phys. Rev.
Renormalization of the quasiparticle hopping integrals by spin interactions in layered copper oxides
Holes doped within the square CuO2 network specific to the cuprate
superconducting materials have oxygen 2p character. We investigate the basic
properties of such oxygen holes by wavefunction-based quantum chemical
calculations on large embedded clusters. We find that a 2p hole induces
ferromagnetic correlations among the nearest-neighbor Cu 3d spins. When moving
through the antiferromagnetic background the hole must bring along this spin
polarization cloud at nearby Cu sites, which gives rise to a substantial
reduction of the effective hopping parameters. Such interactions can explain
the relatively low values inferred for the effective hoppings by fitting the
angle-resolved photoemission data. The effect of the background
antiferromagnetic couplings of renormalizing the effective nearest-neighbor
hopping is also confirmed by density-matrix renormalization-group model
Hamiltonian calculations for chains and ladders of CuO4 plaquettes
Dipole formation at metal/PTCDA interfaces: Role of the Charge Neutrality Level
The formation of a metal/PTCDA (3, 4, 9, 10-perylenetetracarboxylic
dianhydride) interface barrier is analyzed using weak-chemisorption theory. The
electronic structure of the uncoupled PTCDA molecule and of the metal surface
is calculated. Then, the induced density of interface states is obtained as a
function of these two electronic structures and the interaction between both
systems. This induced density of states is found to be large enough (even if
the metal/PTCDA interaction is weak) for the definition of a Charge Neutrality
Level for PTCDA, located 2.45 eV above the highest occupied molecular orbital.
We conclude that the metal/PTCDA interface molecular level alignment is due to
the electrostatic dipole created by the charge transfer between the two solids.Comment: 6 page
Point defects on graphene on metals
Understanding the coupling of graphene with its local environment is critical
to be able to integrate it in tomorrow's electronic devices. Here we show how
the presence of a metallic substrate affects the properties of an atomically
tailored graphene layer. We have deliberately introduced single carbon
vacancies on a graphene monolayer grown on a Pt(111) surface and investigated
its impact in the electronic, structural and magnetic properties of the
graphene layer. Our low temperature scanning tunneling microscopy studies,
complemented by density functional theory, show the existence of a broad
electronic resonance above the Fermi energy associated with the vacancies.
Vacancy sites become reactive leading to an increase of the coupling between
the graphene layer and the metal substrate at these points; this gives rise to
a rapid decay of the localized state and the quenching of the magnetic moment
associated with carbon vacancies in free-standing graphene layers
Enmiendas ideológicas al Buscón
En este trabajo se repasan algunos de los problemas generales que plantea la lectura e interpretación del Buscón. Aunque no existen datos inequÃvocos sobre la fecha de redacción de esta obra, Jauralde defiende una redacción única en torno a 1604 y afirma que no existe «ni un solo testimonio ni directo ni indirecto de que Quevedo retocara mÃnimamente el Buscón». En su opinión, la originalidad de esta pieza quevediana frente al Lazarillo y el Guzmán consiste «en una actitud más ideológica que formal». Después de repasar el proceso de transmisión del texto, analiza otros aspectos como su adscripción al género picaresco, su estructura, el ritmo narrativo y las diversas interpretaciones crÃticas que la obra ha suscitado.
This article tries to examine some problems arisen by the reading and interpretation of Quevedo's El Buscón. Although we lack indisputable information about the date in which this work was written, Jauralde believes that the work was written around 1604 and assures that there is not a direct testimony nor indirect proving that Quevedo revised El Buscón later. In his opinion, «the originality of this work compared with El Lazarillo or El Guzmán de Alfarache rests on the ideologic attitude more than on a formal one». After summarizing the process of transmission of the text, he analyses other aspects such as its being in the picaresque genre, its structure, the narrative rithm and the different critic inerpretations that El Buscón has received
Metallic properties of magnesium point contacts
We present an experimental and theoretical study of the conductance and
stability of Mg atomic-sized contacts. Using Mechanically Controllable Break
Junctions (MCBJ), we have observed that the room temperature conductance
histograms exhibit a series of peaks, which suggests the existence of a shell
effect. Its periodicity, however, cannot be simply explained in terms of either
an atomic or electronic shell effect. We have also found that at room
temperature, contacts of the diameter of a single atom are absent. A possible
interpretation could be the occurrence of a metal-to-insulator transition as
the contact radius is reduced, in analogy with what it is known in the context
of Mg clusters. However, our first principle calculations show that while an
infinite linear chain can be insulating, Mg wires with larger atomic
coordinations, as in realistic atomic contacts, are alwaysmetallic. Finally, at
liquid helium temperature our measurements show that the conductance histogram
is dominated by a pronounced peak at the quantum of conductance. This is in
good agreement with our calculations based on a tight-binding model that
indicate that the conductance of a Mg one-atom contact is dominated by a single
fully open conduction channel.Comment: 14 pages, 5 figure
The ecology of sexual dimorphism in size and shape of the freshwater blenny Salaria fluviatilis.
Sexual selection is considered the major cause of sexual dimorphism, but recent observations suggest that natural selection may play a more important role in the evolution of sex differentiation than previously recognized. Therefore, studying the trade-offs between natural selection and sexual selection is crucial to a better understanding of the ecology underlying the evolution of sexual dimorphism. The freshwater blenny Salaria fluviatilis, a fish inhabiting lakes and rivers around the Mediterranean Sea, displays strong sexual dimorphism in size, shape, and behavior (i.e., larger body and head size for males and higher swimming requirements for females during the reproductive period). We tested for differences in sexual dimorphism in size and shape between the populations from lake and river habitats with the goal of identifying the trade-offs between natural and sexual selection that underlie variations in sexual dimorphism in this species. Our results show i) differences in sexual size dimorphism (SSizeD) in accordance to Rensch's rule (i.e., larger individuals in rivers associated with higher SSizeD), and ii) a decrease in shape differentiation between males and females in lake populations. Together, this suggests that the different environmental conditions between lake and river habitats (e.g., resource limitations, predation pressure, water velocity) affect the relative importance of sexual selection in the display of sexual dimorphism within the species. This study highlights the importance of considering the environmental conditions to which populations are exposed to better understand the ecology underlying the evolution of sexual dimorphism
Electronic excited states of conjugated cyclic ketones and thioketones : A theoretical study
Absorption spectra of a series of cyclic conjugated ketones and thioketones have been computed at the multiconfigurational second-order multistate perturbation level of theory, the CASSCF/MS-CASPT2 method. Excitation energies, transition dipole moments, oscillator strengths, and static dipole moments are reported and discussed for excited states with energies lower than ≈ 7–8 eV. The main bands of the spectra have been assigned and characterized in most cases for the first time. The spectroscopy of the different systems is compared in detail. Thioketones in particular have low-energy and intense ππ∗ transitions which suggest corresponding enhanced nonlinear molecular optical properties. Additionally, some of the methods used to estimate these properties from spectroscopic data have been considered in order to analyze the main contributions to the nonlinear optical [email protected] ; [email protected]
Variable-Frequency Grid-Sequence Detector Based on a Quasi-Ideal Low-Pass Filter Stage and a Phase-Locked Loop
This paper proposes a filtered-sequence phase-locked loop (FSPLL) structure for detection of the positive sequence in
three-phase systems. The structure includes the use of the Park
transformation and moving average filters (MAF). Performance
of the MAF is mathematically analyzed and represented in Bode
diagrams. The analysis allows a proper selection of the window
width of the optimal filter for its application in the dq transformed
variables. The proposed detector structure allows fast detection
of the grid voltage positive sequence (within one grid voltage cycle).
The MAF eliminates completely any oscillation multiple of the
frequency for which it is designed; thus, this algorithm is not affected
by the presence of imbalances or harmonics in the electrical
grid. Furthermore, the PLL includes a simple-frequency detector
that makes frequency adaptive the frequency depending blocks.
This guarantees the proper operation of the FSPLL under large
frequency changes. The performance of the entire PLL-based detector
is verified through simulation and experiment. It shows veryPeer ReviewedPostprint (published version
Grid-Following Voltage Source Converters: Basic Schemes and Current Control Techniques to Operate with Unbalanced Voltage Conditions
The growing relevance of voltage source converters (VSCs), and the deep impact they have on the development and maintenance of the electrical grid, increase the necessity of further research on how to deal with nonideal grid conditions from the VSCs control. This paper is aimed to summarize the basic techniques and schemes that might be required for a grid-connected VSC to work under these conditions: grid synchronization schemes, sequence decomposition, current reference generation, and current controllers. At the same time, some alternative schemes that improves the basic ones are cited. Modelling and the two typical current controllers design and tuning under stationary and synchronous reference frames are also exhibited. Given the importance of the current control stage in the VSC behaviour, five control schemes, designed to track negative sequence currents, are shown and tested in simulation and experiments. According to the experiments, it is shown that the standard proportional-resonant controller achieves the best performance in negative sequence tracking due to the robustness of its non-ideal version, the improved implementation thanks to the delta operator, and the non-dependence on grid-synchronization schemes. Alternatively, one approach based on dual synchronous reference frame is also highlighted for easiness of implementation and good performance
- …