7,630 research outputs found
One-dimensional orbital fluctuations and the exotic magnetic properties of YVO
Starting from the Mott insulator picture for cubic vanadates, we derive and
investigate the model of superexchange interactions between V ions, with
nearly degenerate orbitals occupied by two electrons each. The
superexchange interactions are strongly frustrated and demonstrate a strong
interrelation between possible types of magnetic and orbital order. We
elucidate the prominent role played by fluctuations of and orbitals
which generate ferromagnetic superexchange interactions even in the absence of
Hund's exchange. In this limit we find orbital valence bond state which is
replaced either by -type antiferromagnetic order with weak -type orbital
order at increasing Hund's exchange, or instead by -type antiferromagnetic
order when the lattice distortions stabilize -type orbital order. Both
phases are observed in YVO and we argue that a dimerized -type
antiferromagnetic phase with stronger and weaker FM bonds alternating along the
c axis may be stabilized by large spin-orbital entropy at finite temperature.
This suggests a scenario which explains the origin of the exotic -AF order
observed in YVO in the regime of intermediate temperatures and allows one
to specify the necessary ingredients of a more complete future theory.Comment: 23 pages, 15 figure
Orbital liquid in ferromagnetic manganites: The orbital Hubbard model for electrons
We have analyzed the symmetry properties and the ground state of an orbital
Hubbard model with two orbital flavors, describing a partly filled
spin-polarized band on a cubic lattice, as in ferromagnetic manganites.
We demonstrate that the off-diagonal hopping responsible for transitions
between and orbitals, and the absence of SU(2) invariance
in orbital space, have important implications. One finds that superexchange
contributes in all orbital ordered states, the Nagaoka theorem does not apply,
and the kinetic energy is much enhanced as compared with the spin case.
Therefore, orbital ordered states are harder to stabilize in the Hartree-Fock
approximation (HFA), and the onset of a uniform ferro-orbital polarization and
antiferro-orbital instability are similar to each other, unlike in spin case.
Next we formulate a cubic (gauge) invariant slave boson approach using the
orbitals with complex coefficients. In the mean-field approximation it leads to
the renormalization of the kinetic energy, and provides a reliable estimate for
the ground state energy of the disordered state. Using this approach one finds
that the HFA fails qualitatively in the regime of large Coulomb repulsion
-- the orbital order is unstable, and instead a strongly
correlated orbital liquid with disordered orbitals is realized at any electron
filling.Comment: 25 pages, 9 figure
The importance of transport model uncertainties for the estimation of CO2 sources and sinks using satellite measurements
This study presents a synthetic model intercomparison to investigate the importance of transport model errors for estimating the sources and sinks of CO2 using satellite measurements. The experiments were designed for testing the potential performance of the proposed CO2 lidar A-SCOPE, but also apply to other space borne missions that monitor total column CO2. The participating transport models IFS, LMDZ, TM3, and TM5 were run in forward and inverse mode using common a priori CO2 fluxes and initial concentrations. Forward simulations of column averaged CO2 (xCO2) mixing ratios vary between the models by s=0.5 ppm over the continents and s=0.27 ppm over the oceans. Despite the fact that the models agree on average on the sub-ppm level, these modest differences nevertheless lead to significant discrepancies in the inverted fluxes of 0.1 PgC/yr per 106 km2 over land and 0.03 PgC/yr per 106 km2 over the ocean. These transport model induced flux uncertainties exceed the target requirement that was formulated for the A-SCOPE mission of 0.02 PgC/yr per 106 km2, and could also limit the overall performance of other CO2 missions such as GOSAT. A variable, but overall encouraging agreement is found in comparison with FTS measurements at Park Falls, Darwin, Spitsbergen, and Bremen, although systematic differences are found exceeding the 0.5 ppm level. Because of this, our estimate of the impact of transport model uncerainty is likely to be conservative. It is concluded that to make use of the remote sensing technique for quantifying the sources and sinks of CO2 not only requires highly accurate satellite instruments, but also puts stringent requirements on the performance of atmospheric transport models. Improving the accuracy of these models should receive high priority, which calls for a closer collaboration between experts in atmospheric dynamics and tracer transpor
Environmental factors influence both abundance and genetic diversity in a widespread bird species.
Genetic diversity is one of the key evolutionary variables that correlate with population size, being of critical importance for population viability and the persistence of species. Genetic diversity can also have important ecological consequences within populations, and in turn, ecological factors may drive patterns of genetic diversity. However, the relationship between the genetic diversity of a population and how this interacts with ecological processes has so far only been investigated in a few studies. Here, we investigate the link between ecological factors, local population size, and allelic diversity, using a field study of a common bird species, the house sparrow (Passer domesticus). We studied sparrows outside the breeding season in a confined small valley dominated by dispersed farms and small-scale agriculture in southern France. Population surveys at 36 locations revealed that sparrows were more abundant in locations with high food availability. We then captured and genotyped 891 house sparrows at 10 microsatellite loci from a subset of these locations (N = 12). Population genetic analyses revealed weak genetic structure, where each locality represented a distinct substructure within the study area. We found that food availability was the main factor among others tested to influence the genetic structure between locations. These results suggest that ecological factors can have strong impacts on both population size per se and intrapopulation genetic variation even at a small scale. On a more general level, our data indicate that a patchy environment and low dispersal rate can result in fine-scale patterns of genetic diversity. Given the importance of genetic diversity for population viability, combining ecological and genetic data can help to identify factors limiting population size and determine the conservation potential of populations
Renormalizable parameters of the sine-Gordon model
The well-known phase structure of the two-dimensional sine-Gordon model is
reconstructed by means of its renormalization group flow, the study of the
sensitivity of the dynamics on microscopic parameters. Such an analysis
resolves the apparent contradiction between the phase structure and the
triviality of the effective potential in either phases, provides a case where
usual classification of operators based on the linearization of the scaling
relation around a fixed point is not available and shows that the Maxwell-cut
generates an unusually strong universality at long distances. Possible
analogies with four-dimensional Yang-Mills theories are mentioned, too.Comment: 12 pages, 7 figures. Revised form, to appear in Phys. Lett.
Breaking a one-dimensional chain: fracture in 1 + 1 dimensions
The breaking rate of an atomic chain stretched at zero temperature by a
constant force can be calculated in a quasiclassical approximation by finding
the localized solutions ("bounces") of the equations of classical dynamics in
imaginary time. We show that this theory is related to the critical cracks of
stressed solids, because the world lines of the atoms in the chain form a
two-dimensional crystal, and the bounce is a crack configuration in (unstable)
mechanical equilibrium. Thus the tunneling time, Action, and breaking rate in
the limit of small forces are determined by the classical results of Griffith.
For the limit of large forces we give an exact bounce solution that describes
the quantum fracture and classical crack close to the limit of mechanical
stability. This limit can be viewed as a critical phenomenon for which we
establish a Levanyuk-Ginzburg criterion of weakness of fluctuations, and
propose a scaling argument for the critical regime. The post-tunneling dynamics
is understood by the analytic continuation of the bounce solutions to real
time.Comment: 15 pages, 5 figure
Symmetry adapted finite-cluster solver for quantum Heisenberg model in two-dimensions: a real-space renormalization approach
We present a quantum cluster solver for spin- Heisenberg model on a
two-dimensional lattice. The formalism is based on the real-space
renormalization procedure and uses the lattice point group-theoretical analysis
and nonabelian SU(2) spin symmetry technique. The exact diagonalization
procedure is used twice at each renormalization group step. The method is
applied to the spin-half antiferromagnet on a square lattice and a calculation
of local observables is demonstrated. A symmetry based truncation procedure is
suggested and verified numerically.Comment: willm appear in J. Phys.
Adsorption of 2,2 '-dithiodipyridine as a tool for the assembly of silver nanoparticles
Silver nanostructured thin films stabilized by 2,2’-dithiodipyridine (2dtpy) were prepared. The Ag nanoparticles
were obtained by treating the complex [Ag(2dtpy)]NO3 with NaBH4 in a methanol–toluene mixture. The films
were transferred to borosilicate glass slips by a dip-coating method and were found to consist of Ag
nanoparticles possibly linked via 2dtpy molecules. Surface-enhanced Raman scattering (SERS) studies have
offered the possibility of investigating the adsorption modes of 2dtpy at the Ag nanoparticle surfaces in the
fil
Immersed boundary-finite element model of fluid-structure interaction in the aortic root
It has long been recognized that aortic root elasticity helps to ensure
efficient aortic valve closure, but our understanding of the functional
importance of the elasticity and geometry of the aortic root continues to
evolve as increasingly detailed in vivo imaging data become available. Herein,
we describe fluid-structure interaction models of the aortic root, including
the aortic valve leaflets, the sinuses of Valsalva, the aortic annulus, and the
sinotubular junction, that employ a version of Peskin's immersed boundary (IB)
method with a finite element (FE) description of the structural elasticity. We
develop both an idealized model of the root with three-fold symmetry of the
aortic sinuses and valve leaflets, and a more realistic model that accounts for
the differences in the sizes of the left, right, and noncoronary sinuses and
corresponding valve cusps. As in earlier work, we use fiber-based models of the
valve leaflets, but this study extends earlier IB models of the aortic root by
employing incompressible hyperelastic models of the mechanics of the sinuses
and ascending aorta using a constitutive law fit to experimental data from
human aortic root tissue. In vivo pressure loading is accounted for by a
backwards displacement method that determines the unloaded configurations of
the root models. Our models yield realistic cardiac output at physiological
pressures, with low transvalvular pressure differences during forward flow,
minimal regurgitation during valve closure, and realistic pressure loads when
the valve is closed during diastole. Further, results from high-resolution
computations demonstrate that IB models of the aortic valve are able to produce
essentially grid-converged dynamics at practical grid spacings for the
high-Reynolds number flows of the aortic root
Kirchhoff's Rule for Quantum Wires
In this article we formulate and discuss one particle quantum scattering
theory on an arbitrary finite graph with open ends and where we define the
Hamiltonian to be (minus) the Laplace operator with general boundary conditions
at the vertices. This results in a scattering theory with channels. The
corresponding on-shell S-matrix formed by the reflection and transmission
amplitudes for incoming plane waves of energy is explicitly given in
terms of the boundary conditions and the lengths of the internal lines. It is
shown to be unitary, which may be viewed as the quantum version of Kirchhoff's
law. We exhibit covariance and symmetry properties. It is symmetric if the
boundary conditions are real. Also there is a duality transformation on the set
of boundary conditions and the lengths of the internal lines such that the low
energy behaviour of one theory gives the high energy behaviour of the
transformed theory. Finally we provide a composition rule by which the on-shell
S-matrix of a graph is factorizable in terms of the S-matrices of its
subgraphs. All proofs only use known facts from the theory of self-adjoint
extensions, standard linear algebra, complex function theory and elementary
arguments from the theory of Hermitean symplectic forms.Comment: 40 page
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