6,344 research outputs found
Near degeneracy and pseudo Jahn-Teller effects in mixed-valence ladders: the phase transition in NaVO
We analyze the electronic structure of a mixed-valence ladder system. We find
that structural anisotropy and complex electron correlations lead to on-rung
charge localization and insulating character. Charge fluctuations within the
rung of the ladder interact strongly to the lattice degrees of freedom, which
gives rise to large pseudo Jahn--Teller effects. The phase transition in
NaVO should be driven by this kind of mechanism.Comment: 5 pages, 2 figures, 1 table, submitted to PR
The Emergence of El-Ni\~{n}o as an Autonomous Component in the Climate Network
We construct and analyze a climate network which represents the
interdependent structure of the climate in different geographical zones and
find that the network responds in a unique way to El-Ni\~{n}o events. Analyzing
the dynamics of the climate network shows that when El-Ni\~{n}o events begin,
the El-Ni\~{n}o basin partially loses its influence on its surroundings. After
typically three months, this influence is restored while the basin loses almost
all dependence on its surroundings and becomes \textit{autonomous}. The
formation of an autonomous basin is the missing link to understand the
seemingly contradicting phenomena of the afore--noticed weakening of the
interdependencies in the climate network during El-Ni\~{n}o and the known
impact of the anomalies inside the El-Ni\~{n}o basin on the global climate
system.Comment: 5 pages,10 figure
Impurity-doping induced ferroelectricity in frustrated antiferromagnet CuFeO2
Dielectric responses have been investigated on the triangular-lattice
antiferromagnet CuFeO2 and its site-diluted analogs CuFe1-xAlxO2 (x=0.01 and
0.02) with and without application of magnetic field. We have found a
ferroelectric behavior at zero magnetic field for x=0.02. At any doping level,
the onset field of the ferroelectricity always coincides with that of the
noncollinear magnetic structure while the transition field dramatically
decreases to zero field with Al doping. The results imply the further
possibility of producing the ferroelectricity by modifying the frustrated spin
structure in terms of site-doping and external magnetic field.Comment: 4 pages, 4 figure
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.
Diurnal experiment data report, 19-20 March 1974
Temperature and wind data are presented from 70 small meteorological sounding rockets launched from eight selected launch sites in the Western Hemisphere. Table 1 gives a complete listing of the launch sites involved and the altitude of temperature and wind observations successfully completed
A Generalized Preferential Attachment Model for Business Firms Growth Rates: II. Mathematical Treatment
We present a preferential attachment growth model to obtain the distribution
of number of units in the classes which may represent business firms
or other socio-economic entities. We found that is described in its
central part by a power law with an exponent which depends on
the probability of entry of new classes, . In a particular problem of city
population this distribution is equivalent to the well known Zipf law. In the
absence of the new classes entry, the distribution is exponential. Using
analytical form of and assuming proportional growth for units, we derive
, the distribution of business firm growth rates. The model predicts that
has a Laplacian cusp in the central part and asymptotic power-law tails
with an exponent . We test the analytical expressions derived using
heuristic arguments by simulations. The model might also explain the
size-variance relationship of the firm growth rates.Comment: 19 pages 6 figures Applications of Physics in Financial Analysis,
APFA
The Growth of Business Firms: Theoretical Framework and Empirical Evidence
We introduce a model of proportional growth to explain the distribution of
business firm growth rates. The model predicts that the distribution is
exponential in the central part and depicts an asymptotic power-law behavior in
the tails with an exponent 3. Because of data limitations, previous studies in
this field have been focusing exclusively on the Laplace shape of the body of
the distribution. In this article, we test the model at different levels of
aggregation in the economy, from products to firms to countries, and we find
that the model's predictions agree with empirical growth distributions and
size-variance relationships.Comment: 22 pages, 5 Postscript figures, uses revtex4. to be published in
Proc. Natl. Acad. Sci. (2005
- …