583 research outputs found
Optical properties of small polarons from dynamical mean-field theory
The optical properties of polarons are studied in the framework of the
Holstein model by applying the dynamical mean-field theory. This approach
allows to enlighten important quantitative and qualitative deviations from the
limiting treatments of small polaron theory, that should be considered when
interpreting experimental data. In the antiadiabatic regime, accounting on the
same footing for a finite phonon frequency and a finite electron bandwidth
allows to address the evolution of the optical absorption away from the
well-understood molecular limit. It is shown that the width of the multiphonon
peaks in the optical spectra depends on the temperature and on the frequency in
a way that contradicts the commonly accepted results, most notably in the
strong coupling case. In the adiabatic regime, on the other hand, the present
method allows to identify a wide range of parameters of experimental interest,
where the electron bandwidth is comparable or larger than the broadening of the
Franck-Condon line, leading to a strong modification of both the position and
the shape of the polaronic absorption. An analytical expression is derived in
the limit of vanishing broadening, which improves over the existing formulas
and whose validity extends to any finite-dimensional lattice. In the same
adiabatic regime, at intermediate values of the interaction strength, the
optical absorption exhibits a characteristic reentrant behavior, with the
emergence of sharp features upon increasing the temperature -- polaron
interband transitions -- which are peculiar of the polaron crossover, and for
which analytical expressions are provided.Comment: 16 pages, 6 figure
Properties of quasi-periodic pulsations in solar flares from a single active region
We investigate the properties of a set of solar flares originating from a
single active region (AR) that exhibit QPPs, and look for signs of the QPP
periods relating to AR properties. The AR studied, best known as NOAA 12192,
was unusually long-lived and produced 181 flares. Data from the GOES, EVE,
Fermi, Vernov and NoRH observatories were used to determine if QPPs were
present in the flares. For the soft X-ray GOES and EVE data, the time
derivative of the signal was used. Power spectra of the time series data
(without any form of detrending) were inspected, and flares with a peak above
the 95% confidence level in the spectrum were labelled as having candidate
QPPs. The confidence levels were determined taking account of uncertainties and
the possible presence of red noise. AR properties were determined using HMI
line of sight magnetograms. A total of 37 flares (20% of the sample) show good
evidence of having QPPs, and some of the pulsations can be seen in data from
multiple instruments and in different wavebands. The QPP periods show a weak
correlation with the flare amplitude and duration, but this may be due to an
observational bias. A stronger correlation was found between the QPP period and
duration of the QPP signal, which can be partially but not entirely explained
by observational constraints. No correlations were found with the AR area,
bipole separation, or average magnetic field strength. The fact that a
substantial fraction of the flare sample showed evidence of QPPs using a strict
detection method with minimal processing of the data demonstrates that these
QPPs are a real phenomenon, which cannot be explained by the presence of red
noise or the superposition of multiple unrelated flares. The lack of
correlation between the QPP periods and AR properties implies that the
small-scale structure of the AR is important, and/or that different QPP
mechanisms act in different cases.Comment: 23 pages, 57 figures. Accepted for publication by Astronomy &
Astrophysic
Diffusion and Transport Coefficients in Synthetic Opals
Opals are structures composed of the closed packing of spheres in the size
range of nano-to-micro meter. They are sintered to create small necks at the
points of contact. We have solved the diffusion problem in such structures. The
relation between the diffusion coefficient and the termal and electrical
conductivity makes possible to estimate the transport coefficients of opal
structures. We estimate this changes as function of the neck size and the
mean-free path of the carriers. The theory presented is also applicable to the
diffusion problem in other periodic structures.Comment: Submitted to PR
Effect of pressure on the polarized infrared optical response of quasi-one-dimensional LaTiO
The pressure-induced changes in the optical properties of the
quasi-one-dimensional conductor LaTiO were studied by
polarization-dependent mid-infrared micro-spectroscopy at room temperature. For
the polarization of the incident radiation parallel to the conducting
direction, the optical conductivity spectrum shows a pronounced mid-infrared
absorption band, exhibiting a shift to lower frequencies and an increase in
oscillator strength with increasing pressure. On the basis of its pressure
dependence, interpretations of the band in terms of electronic transitions and
polaronic excitations are discussed. Discontinuous changes in the optical
response near 15 GPa are in agreement with a recently reported pressure-induced
structural phase transition and indicate the onset of a dimensional crossover
in this highly anisotropic system.Comment: 7 pages, 7 figure
Relief and geology of the north polar region of the planet Venus
Description of topographic features is given for the North polar region of the planet Venus. Principal geomorphic types of terrain are characterized as well as their geologic relations. Relative ages of geologic units in Venus North polar region are discussed
Metallic Xenon, Molecular Condensates, and Superconductivity
A possibility of explaining the light absorption observed to occur under
pressure-induced xenon metallization as due to the transition to the
superconducting state is analyzed. The mechanism of the van der Waals bonding
is discussed.Comment: LaTeX 2.09 (RevTeX), 4 pages, 4 PostScript figures included in tex
Feasibility of a Small, Rapid Optical-to-IR Response, Next Generation Gamma Ray Burst Mission
We present motivations for and study feasibility of a small, rapid optical to
IR response gamma ray burst (GRB) space observatory. By analyzing existing GRB
data, we give realistic detection rates for X-ray and optical/IR instruments of
modest size under actual flight conditions. Given new capabilities of fast
optical/IR response (about 1 s to target) and simultaneous multi-band imaging,
such an observatory can have a reasonable event rate, likely leading to new
science. Requiring a Swift-like orbit, duty cycle, and observing constraints, a
Swift-BAT scaled down to 190 square cm of detector area would still detect and
locate about 27 GRB per yr. for a trigger threshold of 6.5 sigma. About 23
percent of X-ray located GRB would be detected optically for a 10 cm diameter
instrument (about 6 per yr. for the 6.5 sigma X-ray trigger).Comment: Elaborated text version of a poster presented at 2012 Malaga/Marbella
symposiu
Rationality of the moduli spaces of plane curves of sufficiently large degree
We prove that the moduli space of plane curves of degree d is rational for
all sufficiently large d.Comment: 18 pages; 1 figure; Macaulay2 scripts used can be found at
http://www.uni-math.gwdg.de/bothmer/rationality/ or at the end of the latex
source fil
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