981 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
Polish energy sector in order to reaching the standings of european union climate and energy policy
Signatures of electronic correlations in iron silicide
The intermetallic FeSi exhibits an unusual temperature dependence in its
electronic and magnetic degrees of freedom, epitomized by the crossover from a
low temperature non-magnetic semiconductor to a high temperature paramagnetic
metal with a Curie-Weiss like susceptibility. Many proposals for this
unconventional behavior have been advanced, yet a consensus remains elusive.
Using realistic many-body calculations, we here reproduce the signatures of the
metal-insulator crossover in various observables: the spectral function, the
optical conductivity, the spin susceptibility, and the Seebeck coefficient.
Validated by quantitative agreement with experiment, we then address the
underlying microscopic picture. We propose a new scenario in which FeSi is a
band-insulator at low temperatures and is metalized with increasing temperature
through correlation induced incoherence. We explain that the emergent
incoherence is linked to the unlocking of iron fluctuating moments which are
almost temperature independent at short time scales. Finally, we make explicit
suggestions for improving the thermoelectric performance of FeSi based systems.Comment: 4+ pages, and supplementary materia
The impact of COVID-19 on the banking sector. Are we heading for the next banking crisis?
The aim of this research is to study the effect of the COVID-19 pandemic on the banking sector and to assess if COVID-19 was a trigger for the banking crisis. In order to analyse the extent of the impact of the COVID-19 pandemic the beta of the banking sector was calculated and analysed for selected countries. In addition, using the panel data technique over the period from the 30th of December 2019 until the 24th of September 2021, we examined variables contributing to the banking crisis development. The results suggest that the pandemic and government bond yield spread contributed to higher volatility and risk in the banking sector but did not lead to the banking crisis
On almost randomizing channels with a short Kraus decomposition
For large d, we study quantum channels on C^d obtained by selecting randomly
N independent Kraus operators according to a probability measure mu on the
unitary group U(d). When mu is the Haar measure, we show that for
N>d/epsilon^2. For d=2^k (k qubits), this includes Kraus operators
obtained by tensoring k random Pauli matrices. The proof uses recent results on
empirical processes in Banach spaces.Comment: We added some background on geometry of Banach space
Learning Arbitrary Statistical Mixtures of Discrete Distributions
We study the problem of learning from unlabeled samples very general
statistical mixture models on large finite sets. Specifically, the model to be
learned, , is a probability distribution over probability
distributions , where each such is a probability distribution over . When we sample from , we do not observe
directly, but only indirectly and in very noisy fashion, by sampling from
repeatedly, independently times from the distribution . The problem is
to infer to high accuracy in transportation (earthmover) distance.
We give the first efficient algorithms for learning this mixture model
without making any restricting assumptions on the structure of the distribution
. We bound the quality of the solution as a function of the size of
the samples and the number of samples used. Our model and results have
applications to a variety of unsupervised learning scenarios, including
learning topic models and collaborative filtering.Comment: 23 pages. Preliminary version in the Proceeding of the 47th ACM
Symposium on the Theory of Computing (STOC15
Linear independence of localized magnon states
At the magnetic saturation field, certain frustrated lattices have a class of
states known as "localized multi-magnon states" as exact ground states. The
number of these states scales exponentially with the number of spins and
hence they have a finite entropy also in the thermodynamic limit
provided they are sufficiently linearly independent. In this article we present
rigorous results concerning the linear dependence or independence of localized
magnon states and investigate special examples. For large classes of spin
lattices including what we called the orthogonal type and the isolated type as
well as the kagom\'{e}, the checkerboard and the star lattice we have proven
linear independence of all localized multi-magnon states. On the other hand the
pyrochlore lattice provides an example of a spin lattice having localized
multi-magnon states with considerable linear dependence.Comment: 23 pages, 6 figure
Mass enhancements and band shifts in strongly hole overdoped Fe-based pnictide superconductors: KFeAs and CsFeAs
The interplay of high and low-energy mass renormalizations with band-shifts
reflected by the positions of van Hove singularities (VHS) in the normal state
spectra of the highest hole-overdoped and strongly correlated AFeAs
(A122) with A = K, Cs is discussed phenomenologically based on ARPES data and
GGA band-structure calculations with full spin-orbit coupling. The big increase
of the Sommerfeld coefficient from K122 to Cs122 is ascribed to an
enhanced coupling to low-energy bosons in the vicinity of a quantum critical
point to an unknown, yet incommensurate phase different from the commensurate
Mott one. We find no sizeable increase in correlations for Cs122 in contrast to
F. Eilers et al., PRL v. 116, 237003 (2016) [3]. The empirical (ARPES) VHS
positions as compared with GGA-predictions point even to slightly weaker
correlations in Cs122 in accord with low- magnetic susceptibility
data and a decreasing Wilson ratio .Comment: 8 pages, 7 figures, updated references, and a Note for arXiv-reader
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