1,445 research outputs found
Materials Design using Correlated Oxides: Optical Properties of Vanadium Dioxide
Materials with strong electronic Coulomb interactions play an increasing role
in modern materials applications. "Thermochromic" systems, which exhibit
thermally induced changes in their optical response, provide a particularly
interesting case. The optical switching associated with the metal-insulator
transition of vanadium dioxide (VO2), for example, has been proposed for use in
"intelligent" windows, which selectively filter radiative heat in hot weather
conditions. In this work, we develop the theoretical tools for describing such
a behavior. Using a novel scheme for the calculation of the optical
conductivity of correlated materials, we obtain quantitative agreement with
experiments for both phases of VO2. On the example of an optimized
energy-saving window setup, we further demonstrate that theoretical materials
design has now come into reach, even for the particularly challenging class of
correlated electron systems.Comment: 4+x pages, 2 figure
Saturating Constructions for Normed Spaces II
We prove several results of the following type: given finite dimensional
normed space V possessing certain geometric property there exists another space
X having the same property and such that (1) log (dim X) = O(log (dim V)) and
(2) every subspace of X, whose dimension is not "too small," contains a further
well-complemented subspace nearly isometric to V. This sheds new light on the
structure of large subspaces or quotients of normed spaces (resp., large
sections or linear images of convex bodies) and provides definitive solutions
to several problems stated in the 1980s by V. Milman. The proofs are
probabilistic and depend on careful analysis of images of convex sets under
Gaussian linear maps.Comment: 35 p., LATEX; the paper is a follow up on math.FA/040723
The quantum Heisenberg antiferromagnet on the Sierpinski Gasket: An exact diagonalization study
We present an exact diagonalization study of the quantum Heisenberg
antiferromagnet on the fractal Sierpinski gasket for spin quantum numbers
s=1/2,s=1 and s=3/2. Since the fractal dimension of the Sierpinski gasket is
between one and two we compare the results with corresponding data of one- and
two-dimensional systems. By analyzing the ground-state energy, the low-lying
spectrum, the spin-spin correlation and the low-temperature thermodynamics we
find arguments, that the Heisenberg antiferromagnet on the Sierpinski gasket is
probably disordered not only in the extreme quantum case s=1/2 but also for s=1
and s=3/2. Moreover, in contrast to the one-dimensional chain we do not find a
distinct behavior between the half-integer and integer-spin Heisenberg models
on the Sierpinski gasket. We conclude that magnetic disorder may appear due to
the interplay of frustration and strong quantum fluctuations in this spin
system with spatial dimension between one and two.Comment: 12 pages (LaTeX), 7 figures, 3 tables, to appear in Physica
Investigation of quasi-periodic varaiations in hard X-rays of solar flares
The aim of the present paper is to use quasi-periodic oscillations in hard
X-rays (HXRs) of solar flares as a diagnostic tool for investigation of
impulsive electron acceleration. We have selected a number of flares which
showed quasi-periodic oscillations in hard X-rays and their loop-top sources
could be easily recognized in HXR images. We have considered MHD standing waves
to explain the observed HXR oscillations. We interpret these HXR oscillations
as being due to oscillations of magnetic traps within cusp-like magnetic
structures. This is confirmed by a good correlation between periods of the
oscillations and the sizes of the loop-top sources. We argue that a model of
oscillating magnetic traps is adequate to explain the observations. During the
compressions of a trap particles are accelerated, but during its expansions
plasma, coming from chromospheric evaporation, fills the trap, which explains
the large number of electrons being accelerated during a sequence of strong
impulses. The advantage of our model of oscillating magnetic traps is that it
can explain both the impulses of electron acceleration and quasi-periodicity of
their distribution in time.Comment: 21 pages, 11 figures, 3 tables, submitted to Solar Physic
Constraints on the total coupling strength to bosons in iron based superconductors
At present, there is still no consistent interpretation of the normal and
superconducting properties of Fe-based superconductors (FeSCs). The strength of
the el-el interaction and the role of correlation effects are under debate.
Here, we examine several common materials and illustrate various problems and
concepts that are generic for all FeSCs. Based on empirical observations and
qualitative insight from density functional theory, we show that the
superconducting and low-energy thermodynamic properties of the FeSCs can be
described semi-quantitively within multiband Eliashberg theory. We account for
an important high-energy mass renormalization phenomenologically,and in
agreement with constraints provided by thermodynamic, optical, and
angle-resolved photoemission data. When seen in this way, all FeSCs with
40~K studied so far are found to belong to an {\it
intermediate} coupling regime. This finding is in contrast to the strong
coupling scenarios proposed in the early period of the FeSC history.We also
discuss several related issues, including the role of band shifts as measured
by the positions of van Hove singularities, and the nature of a recently
suggested quantum critical point in the strongly hole-doped systems
AFeAs (A = K, Rb, Cs). Using high-precision full relativistic GGA-band
structure calculations, we arrive at a somewhat milder mass renormalization in
comparison with previous studies. From the calculated mass anisotropies of all
Fermi surface sheets, only the -pocket near the corner of the BZ
is compatible with the experimentally observed anisotropy of the upper critical
field. pointing to its dominant role in the superconductivity of these three
compounds.Comment: 19 pages, 9 figure
Probabilistic Approach to Structural Change Prediction in Evolving Social Networks
We propose a predictive model of structural
changes in elementary subgraphs of social network based on
Mixture of Markov Chains. The model is trained and verified
on a dataset from a large corporate social network analyzed
in short, one day-long time windows, and reveals distinctive
patterns of evolution of connections on the level of local
network topology. We argue that the network investigated in
such short timescales is highly dynamic and therefore immune
to classic methods of link prediction and structural analysis,
and show that in the case of complex networks, the dynamic
subgraph mining may lead to better prediction accuracy. The
experiments were carried out on the logs from the Wroclaw
University of Technology mail server
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