1,047 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
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
Electronic correlations in FeGa3 and the effect of hole doping on its magnetic properties
We investigate signatures of electronic correlations in the narrow-gap semiconductor FeGa 3 by means of electrical resistivity and thermodynamic measurements performed on single crystals of FeGa 3 , Fe 1−x Mn x Ga 3 , and FeGa 3−y Zn y , complemented by a study of the 4d analog material RuGa 3 . We find that the inclusion of sizable amounts of Mn and Zn dopants into FeGa 3 does not induce an insulator-to-metal transition. Our study indicates that both substitution of Zn onto the Ga site and replacement of Fe by Mn introduces states into the semiconducting gap that remain localized even at highest doping levels. Most importantly, using neutron powder diffraction measurements, we establish that FeGa 3 orders magnetically above room temperature in a complex structure, which is almost unaffected by the doping with Mn and Zn. Using realistic many-body calculations within the framework of dynamical mean field theory (DMFT), we argue that while the iron atoms in FeGa 3 are dominantly in an S=1 state, there are strong charge and spin fluctuations on short-time scales, which are independent of temperature. Further, the low magnitude of local contributions to the spin susceptibility advocates an itinerant mechanism for the spin response in FeGa 3 . Our joint experimental and theoretical investigations classify FeGa 3 as a correlated band insulator with only small dynamical correlation effects, in which nonlocal exchange interactions are responsible for the spin gap of 0.4 eV and the antiferromagnetic order. We show that hole doping of FeGa 3 leads, within DMFT, to a notable strengthening of many-body renormalizations
Realistic many-body models for Manganese Monoxide under pressure
In materials like transition metals oxides where electronic Coulomb
correlations impede a description in terms of standard band-theories, the
application of genuine many-body techniques is inevitable. Interfacing the
realism of density-functional based methods with the virtues of Hubbard-like
Hamiltonians, requires the joint ab initio construction of transfer integrals
and interaction matrix elements (like the Hubbard U) in a localized basis set.
In this work, we employ the scheme of maximally localized Wannier functions and
the constrained random phase approximation to create effective low-energy
models for Manganese monoxide, and track their evolution under external
pressure. We find that in the low pressure antiferromagnetic phase, the
compression results in an increase of the bare Coulomb interaction for specific
orbitals. As we rationalized in recent model considerations [Phys. Rev. B 79,
235133 (2009)], this seemingly counter-intuitive behavior is a consequence of
the delocalization of the respective Wannier functions. The change of screening
processes does not alter this tendency, and thus, the screened on-site
component of the interaction - the Hubbard U of the effective low-energy system
- increases with pressure as well. The orbital anisotropy of the effects
originates from the orientation of the orbitals vis-a-vis the deformation of
the unit-cell. Within the high pressure paramagnetic phase, on the other hand,
we find the significant increase of the Hubbard U is insensitive to the orbital
orientation and almost exclusively owing to a substantial weakening of
screening channels upon compression.Comment: 13 pages, 6 figure
Effective band-structure in the insulating phase versus strong dynamical correlations in metallic VO2
Using a general analytical continuation scheme for cluster dynamical mean
field calculations, we analyze real-frequency self-energies, momentum-resolved
spectral functions, and one-particle excitations of the metallic and insulating
phases of VO2. While for the former dynamical correlations and lifetime effects
prevent a description in terms of quasi-particles, the excitations of the
latter allow for an effective band-structure. We construct an
orbital-dependent, but static one-particle potential that reproduces the full
many-body spectrum. Yet, the ground state is well beyond a static one-particle
description. The emerging picture gives a non-trivial answer to the decade-old
question of the nature of the insulator, which we characterize as a ``many-body
Peierls'' state.Comment: 5 pages, 4 color figure
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
On the porosity of barrier layers
Barrier layers are defined as the layer between the pycnocline and the thermocline when the latter are different as a result of salinity stratification. We present a revisited 2-degree resolution global climatology of monthly mean oceanic Barrier Layer (BL) thickness first proposed by de Boyer Montégut et al. (2007). In addition to using an extended data set, we present a modified computation method that addresses the observed porosity of BLs. We name porosity the fact that barrier layers distribution can, in some areas, be very uneven regarding the space and time scales that are considered. This implies an intermittent alteration of air-sea exchanges by the BL. Therefore, it may have important consequences for the climatic impact of BLs. Differences between the two computation methods are small for robust BLs that are formed by large-scale processes. However, the former approach can significantly underestimate the thickness of short and/or localized barrier layers. This is especially the case for barrier layers formed by mesoscale mechanisms (under the intertropical convergence zone for example and along western boundary currents) and equatorward of the sea surface salinity subtropical maxima. Complete characterisation of regional BL dynamics therefore requires a description of the robustness of BL distribution to assess the overall impact of BLs on the process of heat exchange between the ocean interior and the atmosphere
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
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