3,958 research outputs found
Polyradical character and spin frustration in fullerene molecules: An ab initio non-collinear Hartree--Fock study
Most {\em ab initio} calculations on fullerene molecules have been carried
out based on the paradigm of the H\"uckel model. This is consistent with the
restricted nature of the independent-particle model underlying such
calculations, even in single-reference-based correlated approaches. On the
other hand, previous works on some of these molecules using model Hamiltonians
have clearly indicated the importance of short-range inter-atomic spin-spin
correlations. In this work, we consider {\em ab initio} non-collinear
Hartree--Fock (HF) solutions for representative fullerene systems: the bowl,
cage, ring, and pentagon isomers of C, and the larger C,
C, C, C, and C fullerene cages. In all cases but
the ring we find that the HF minimum corresponds to a truly non-collinear
solution with a torsional spin density wave. Optimized geometries at the
generalized HF (GHF) level lead to fully symmetric structures, even in those
cases where Jahn-Teller distortions have been previously considered. The nature
of the GHF solutions is consistent with the -electron space becoming
polyradical in nature: each -orbital remains effectively singly occupied.
The spin frustration, induced by the pentagon rings in an otherwise
anti-ferromagnetic background, is minimized at the HF level by aligning the
spins in non-collinear arrangements. The long-range magnetic ordering observed
is reminiscent of the character of broken symmetry HF solutions in polyacene
systems.Comment: 16 figure
Multi-reference symmetry-projected variational approximation for the ground state of the doped one-dimensional Hubbard model
A multi-reference configuration mixing scheme is used to describe the ground
state, characterized by well defined spin and space group symmetry quantum
numbers as well as doping fractions , of one dimensional
Hubbard lattices with nearest-neighbor hopping and periodic boundary
conditions. Within this scheme, each ground state is expanded in a given number
of nonorthogonal and variationally determined symmetry-projected
configurations. The results obtained for the ground state and correlation
energies of half-filled and doped lattices with 30, 34 and 50 sites, compare
well with the exact Lieb-Wu solutions as well as with the ones obtained with
other state-of-the-art approximations. The structure of the intrinsic
symmetry-broken determinants resulting from the variational procedure is
interpreted in terms of solitons whose translational and breathing motions can
be regarded as basic units of quantum fluctuations. It is also shown that in
the case of doped 1D lattices, a part of such fluctuations can also be
interpreted in terms of polarons. In addition to momentum distributions, both
spin-spin and density-density correlation functions are studied as functions of
doping. The spectral functions and density of states, computed with an ansatz
whose quality can be well-controlled by the number of symmetry-projected
configurations used to approximate the electron systems, display
features beyond a simple quasiparticle distribution, as well as spin-charge
separation trends.Comment: 16 pages, 11 figure
Effects of Reserve Requirements in an Inflation Targeting Regime: The Case of Colombia
The Colombian economy and financial system have coped reasonably well with the effects of the global financial crisis. Hence, "unconventional" policy measures have not been at the center of the policy decisions and discussions. Nominal short term interest rates have remained the main monetary policy tool and "Quantitative easing" measures have not been central in the policy response. The one "unconventional" monetary instrument used by the Central Bank in Colombia has been changes in reserve requirements (RR) on financial system deposits. Interestingly, they were adopted before the global financial crisis, as a reaction to domestic credit conditions. The effects of RR on interest rate and interest rate pass-through in an inflation targeting regime are not as straightforward as those under a monetary targeting regime. Conceptually, those effects depend on the degree of substitution between deposits and central bank credit as sources of funds for banks and on the extent to which RR changes affect the risks facing banks. The empirical results for Colombia suggest that RR are important long run determinants of business loan interest rates and have been effective in strengthening the pass-through from policy to deposit and lending interest rates.Reserve Requirements, Inflation Targeting, Interest rate pass-through. Classification JEL: E51, E52, E58, G21.
Excited electronic states from a variational approach based on symmetry-projected Hartree--Fock configurations
Recent work from our research group has demonstrated that symmetry-projected
Hartree--Fock (HF) methods provide a compact representation of molecular ground
state wavefunctions based on a superposition of non-orthogonal Slater
determinants. The symmetry-projected ansatz can account for static correlations
in a computationally efficient way. Here we present a variational extension of
this methodology applicable to excited states of the same symmetry as the
ground state. Benchmark calculations on the C dimer with a modest basis
set, which allows comparison with full configuration interaction results,
indicate that this extension provides a high quality description of the
low-lying spectrum for the entire dissociation profile. We apply the same
methodology to obtain the full low-lying vertical excitation spectrum of
formaldehyde, in good agreement with available theoretical and experimental
data, as well as to a challenging model insertion pathway for BeH.
The variational excited state methodology developed in this work has two
remarkable traits: it is fully black-box and will be applicable to fairly large
systems thanks to its mean-field computational cost
Interplay Between Yu-Shiba-Rusinov States and Multiple Andreev Reflections
Motivated by recent scanning tunneling microscopy experiments on single
magnetic impurities on superconducting surfaces, we present here a
comprehensive theoretical study of the interplay between Yu-Shiba-Rusinov bound
states and (multiple) Andreev reflections. Our theory is based on a combination
of an Anderson model with broken spin degeneracy and nonequilibrium Green's
function techniques that allows us to describe the electronic transport through
a magnetic impurity coupled to superconducting leads for arbitrary junction
transparency. Using this combination we are able to elucidate the different
tunneling processes that give a significant contribution to the subgap
transport. In particular, we predict the occurrence of a large variety of
Andreev reflections mediated by Yu-Shiba-Rusinov bound states that clearly
differ from the standard Andreev processes in non-magnetic systems. Moreover,
we provide concrete guidelines on how to experimentally identify the subgap
features originating from these tunneling events. Overall, our work provides
new insight into the role of the spin degree of freedom in Andreev transport
physics.Comment: 15 pages, 10 figure
Multi-component symmetry-projected approach for molecular ground state correlations
The symmetry-projected Hartree--Fock ansatz for the electronic structure
problem can efficiently account for static correlation in molecules, yet it is
often unable to describe dynamic correlation in a balanced manner. Here, we
consider a multi-component, systematically-improvable approach, that accounts
for all ground state correlations. Our approach is based on linear combinations
of symmetry-projected configurations built out of a set of non-orthogonal,
variationally optimized determinants. The resulting wavefunction preserves the
symmetries of the original Hamiltonian even though it is written as a
superposition of deformed (broken-symmetry) determinants. We show how short
expansions of this kind can provide a very accurate description of the
electronic structure of simple chemical systems such as the nitrogen and the
water molecules, along the entire dissociation profile. In addition, we apply
this multi-component symmetry-projected approach to provide an accurate
interconversion profile among the peroxo and bis(-oxo) forms of
[CuO], comparable to other state-of-the-art quantum chemical
methods
Nuevos datos sobre "Myxomycetes" presentes en la provincia de Granada (España)
En el presente trabajo se aportan nuevos datos sobre el conocimiento de los Myxomycetes de Granada (sur de España), citĂĄndose catorce nuevas especies, entre las que ne destacan: Comatricha alta Preusz, Diderma trevelyanii (Grev.) Fr., Diderma umbilicatum Pers., Didymium bahiense Gottsberger, Didymium difforme (Pers.) S.F. Gray, Didymium trachysporum G. Lister, Lamproderma scintillans (Berk. & Br.) Morgan, Physarum contextum (Pers.) Pers., Physarum vernum Somm. y se amplĂa la corologia de varias especies ya citadas en anteriores trabajos. Se comenta la distribuciĂłn en la Peninsula IbĂ©rica, confeccionĂĄndose mapas de distribuciĂłn de dichos tazones basados en cuadrĂcula UTM de 50 Km.This article shows a study on the Myxomycetes on the province of Granada (Spain). We have found fourteen new taxa for this province. Some of these are: Comatricha alta Preusz, Diderma trevelyanii(Gres.) Fr., Diderma umbilicatum Pers., Didymium bahiense Gottsberger, Didymium difforme (Pers.) S.F. Gray, Didymium trachysporum G. Lister, Lamproderma scintillans (Berk. & Br.) Morgan, Physarum contextum (Pers.) Pers., Physarum vernum Somm. Distribution maps are given. We also included new dates about the chorology another species
Propiedades de transmisión de electrones de Dirac a través de superredes Cantor en grafenoTransmission properties of Dirac electrons through Cantor monolayer graphene superlattices
In this work we use the transfer matrix method to studythe tunneling of Dirac electrons through aperiodic monolayer graphene superlattices. We consider a graphene sheet deposited on top of slabs of Silicon-Oxide (SiO2) and Silicon-Carbide (SiC) substrates, in which we applied the Cantorâs series. We calculatethe transmittance for different fundamental parameters such as: starting width, incident energy, incident angle and generation number of the Cantorâs series. In this case, the transmittance as function of energy presents self-similar features as a function of the generation number. We also compute the angular distribution of the transmittance for fixed energies finding a self-similar patterns between generations. Finally, we calculate the scaling factor for some transmittance spectra, which effectively show scalability.En este trabajo usamos el mĂ©todo de la matriz de transferencia para estudiar el tunelamiento de los electrones de Dirac a travĂ©s de superredes aperiodicas en grafeno. Consideramosuna hoja de grafeno depositada encima de bloques de sustratos de Ăxido de Silicio (SiO2) y Carburo de Silicio (SiC), en los cuales aplicamos la serie de Cantor. Calculamos la transmitancia para diferentes parĂĄmetros fundamentales tales como: ancho de partida, energĂa de incidencia, ĂĄngulo de incidencia y nĂșmero de generaciĂłn de la serie de Cantor. En este caso, la transmitancia como funciĂłn de la energĂa presenta rasgos autosimilares al variar el nĂșmero de generaciĂłn. TambiĂ©n computamos la distribuciĂłn angular de la transmitancia para energĂas fijas econtrando un patrĂłn autosimilar entre generaciones. Por Ășltimo, calculamos los factores de escala para algunos espectros de la transmitancia, los cuales efectivamente muestran escalabilida
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