663 research outputs found
Ab initio study of the CE magnetic phase in half-doped manganites: Purely magnetic versus double exchange description
The leading electronic interactions governing the local physics of the CE
phase of half-doped manganites are extracted from correlated ab initio
calculations performed on an embedded cluster. The electronic structure of the
low-energy states is dominated by double exchange configurations and
O-2 to Mn-3d charge transfer configurations. The model spectra of
both a purely magnetic non-symmetric Heisenberg Hamiltonian involving a
magnetic oxygen and two non-symmetric double exchange models are compared to
the \textit{ab initio} one. While a satisfactory agreement between the
Heisenberg spectrum and the calculated one is obtained, the best description is
provided by a double exchange model involving excited non-Hund atomic states.
This refined model not only perfectly reproduces the spectrum of the embedded
cluster in the crystal geometry, but also gives a full description of the local
double-well potential energy curve of the ground state (resulting from the
interaction of the charge localized electronic configurations) and the local
potential energy curves of all excited states ruled by the double exchange
mechanism
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
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
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
Feasibility Study on Laser Cutting of Phenolic Resin Boards
AbstractLaser cutting is the most widely implemented application of lasers in industry. The many advantages of this process stimulate users in industry to cut many different materials, such as wood and wood composites âparticleboard, plywood, etc.â, which are being cut with excellent results and productivity. Phenolic resins boards are a new substitute of wood in highly aggressive environments. In the present work we study the feasibility of CO2 lasers to cut phenolic resin boards and assess the potential health hazards of the vapours and residues produced, since its thermal degradation may produce toxic organic vapors
Barrier formation at metal/organic interfaces: dipole formation and the Charge Neutrality Level
The barrier formation for metal/organic semiconductor interfaces is analyzed
within the Induced Density of Interface States (IDIS) model. Using weak
chemisorption theory, we calculate the induced density of states in the organic
energy gap and show that it is high enough to control the barrier formation. We
calculate the Charge Neutrality Levels of several organic molecules (PTCDA,
PTCBI and CBP) and the interface Fermi level for their contact with a Au(111)
surface. We find an excellent agreement with the experimental evidence and
conclude that the barrier formation is due to the charge transfer between the
metal and the states induced in the organic energy gap.Comment: 7 pages, Proceedings of ICFSI-9, Madrid, Spain (September 2003),
special issue of Applied Surface Science (in press
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
Comparison of bipolar sub-modules for the alternate arm converter
© 2016 IEEE.Research on dc-fault tolerant multilevel converters has gained noticeable attention over recent years. The alternate arm converter (AAC) is one of such emerging multilevel converter topologies, and a hybrid topology of the two-level converter and the modular multilevel converter (MMC). Bipolar sub-modules (SMs) that can produce both positive and negative voltages are the building blocks of the AAC. This paper analyses the operation of an AAC with the full-bridge SM (FB-SM) and the cross-connected SM (CC-SM). The conduction and switching losses of the two SM configurations are evaluated and compared, in order to identify the suitability of CC-SM for AACs and its performance compared to the FB-SM. The CC-SM with identical semiconductor devices has reduced losses compared to the CC-SM with higher rated devices in the cross-connected path. It is concluded that the CC-SM does not offer advantages in the losses, construction, and application to the AAC, compared to FB-SM
High-accuracy large-scale DFT calculations using localized orbitals in complex electronic systems: The case of graphene-metal interfaces
Over many years, computational simulations based on Density Functional Theory
(DFT) have been used extensively to study many different materials at the
atomic scale. However, its application is restricted by system size, leaving a
number of interesting systems without a high-accuracy quantum description. In
this work, we calculate the electronic and structural properties of a
graphene-metal system significantly larger than in previous plane-wave
calculations with the same accuracy. For this task we use a localised basis set
with the \textsc{Conquest} code, both in their primitive, pseudo-atomic orbital
form, and using a recent multi-site approach. This multi-site scheme allows us
to maintain accuracy while saving computational time and memory requirements,
even in our exemplar complex system of graphene grown on Rh(111) with and
without intercalated atomic oxygen. This system offers a rich scenario that
will serve as a benchmark, demonstrating that highly accurate simulations in
cells with over 3000 atoms are feasible with modest computational resources.Comment: 11 pages, 3 figures, submitted to J. Chem. Theor. Compu
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