22,027 research outputs found
On the Penrose Inequality for general horizons
For asymptotically flat initial data of Einstein's equations satisfying an
energy condition, we show that the Penrose inequality holds between the ADM
mass and the area of an outermost apparent horizon, if the data are restricted
suitably. We prove this by generalizing Geroch's proof of monotonicity of the
Hawking mass under a smooth inverse mean curvature flow, for data with
non-negative Ricci scalar. Unlike Geroch we need not confine ourselves to
minimal surfaces as horizons. Modulo smoothness issues we also show that our
restrictions on the data can locally be fulfilled by a suitable choice of the
initial surface in a given spacetime.Comment: 4 pages, revtex, no figures. Some comments added. No essential
changes. To be published in Phys. Rev. Let
Radiative Transfer on Perturbations in Protoplanetary Disks
We present a method for calculating the radiative tranfer on a protoplanetary
disk perturbed by a protoplanet. We apply this method to determine the effect
on the temperature structure within the photosphere of a passive circumstellar
disk in the vicinity of a small protoplanet of up to 20 Earth masses. The
gravitational potential of a protoplanet induces a compression of the disk
material near it, resulting in a decrement in the density at the disk's
surface. Thus, an isodensity contour at the height of the photosphere takes on
the shape of a well. When such a well is illuminated by stellar irradiation at
grazing incidence, it results in cooling in a shadowed region and heating in an
exposed region. For typical stellar and disk parameters relevant to the epoch
of planet formation, we find that the temperature variation due to a
protoplanet at 1 AU separation from its parent star is about 4% (5 K) for a
planet of 1 Earth mass, about 14% (19 K) for planet of 10 Earth masses, and
about 18% (25 K) for planet of 20 Earth masses, We conclude that even such
relatively small protoplanets can induce temperature variations in a passive
disk. Therefore, many of the processes involved in planet formation should not
be modeled with a locally isothermal equation of state.Comment: 23 pages, 8 figures (including 3 color figs). Submitted to Ap
Compression and diffusion: a joint approach to detect complexity
The adoption of the Kolmogorov-Sinai (KS) entropy is becoming a popular
research tool among physicists, especially when applied to a dynamical system
fitting the conditions of validity of the Pesin theorem. The study of time
series that are a manifestation of system dynamics whose rules are either
unknown or too complex for a mathematical treatment, is still a challenge since
the KS entropy is not computable, in general, in that case. Here we present a
plan of action based on the joint action of two procedures, both related to the
KS entropy, but compatible with computer implementation through fast and
efficient programs. The former procedure, called Compression Algorithm
Sensitive To Regularity (CASToRe), establishes the amount of order by the
numerical evaluation of algorithmic compressibility. The latter, called Complex
Analysis of Sequences via Scaling AND Randomness Assessment (CASSANDRA),
establishes the complexity degree through the numerical evaluation of the
strength of an anomalous effect. This is the departure, of the diffusion
process generated by the observed fluctuations, from ordinary Brownian motion.
The CASSANDRA algorithm shares with CASToRe a connection with the Kolmogorov
complexity. This makes both algorithms especially suitable to study the
transition from dynamics to thermodynamics, and the case of non-stationary time
series as well. The benefit of the joint action of these two methods is proven
by the analysis of artificial sequences with the same main properties as the
real time series to which the joint use of these two methods will be applied in
future research work.Comment: 27 pages, 9 figure
Routes for efficiency enhancement in fluorescent TADF exciplex host OLEDs gained from an electroâoptical device model
Fluorescence-based organic light-emitting diodes (OLEDs) using thermally activated delayed fluorescence (TADF) have increasingly attracted attention in research and industry. One method to implement TADF is based on an emitter layer composed of an exciplex host and a fluorescent dopant. Even though the experimental realization of this concept has demonstrated promising external quantum efficiencies, the full potential of this approach has not yet been assessed. To this end, a comprehensive electro-optical device model accounting for the full exciton dynamics including triplet harvesting and exciton quenching is presented. The model parameters are fitted to multiple output characteristics of an OLED comprising a TADF exciplex host with a fluorescent emitter, showing an external quantum efficiency of >10%. With the model at hand, an emission zone analysis and a parameter study are performed, and possible routes for further efficiency enhancement are presented
Versatile spectral imaging with an algorithm-based spectrometer using highly tuneable quantum dot infrared photodetectors
We report on the implementation of an algorithm-based spectrometer capable of reconstructing the spectral shape of materials in the mid-wave infrared (MWIR) and long-wave infrared (LWIR) wavelengths using only experimental photocurrent measurements from quantum dot infrared photodetectors (QDIPs). The theory and implementation of the algorithm will be described, followed by an investigation into this algorithmic spectrometer's performance. Compared to the QDIPs utilized in an earlier implementation, the ones used here have highly varying spectral shapes and four spectral peaks across the MWIR and LWIR wavelengths. It has been found that the spectrometer is capable of reconstructing broad spectral features of a range of bandpass infrared filters between wavelengths of 4 and 12 mu m as well as identifying absorption features as narrow as 0.3 mu m in the IR spectrum of a polyethylene sheet
How many photons are needed to distinguish two transparencies?
We give a bound on the minimum number of photons that must be absorbed by any
quantum protocol to distinguish between two transparencies. We show how a
quantum Zeno method in which the angle of rotation is varied at each iteration
can attain this bound in certain situations.Comment: 5 pages, 4 figure
Nonflammable Lithium Metal Full Cells with Ultra-high Energy Density Based on Coordinated Carbonate Electrolytes
Coupling thin Li metal anodes with high-capacity/high-voltage cathodes such as LiNi0.8Co0.1Mn0.1O2 (NCM811) is a promising way to increase lithium battery energy density. Yet, the realization of high-performance full cells remains a formidable challenge. Here, we demonstrate a new class of highly coordinated, nonflammable carbonate electrolytes based on lithium bis(fluorosulfonyl)imide (UFSI) in propylene carbonate/fluoroethylene carbonate mixtures. Utilizing an optimal salt concentr ation (4 M LiFSI) of the electrolyte results in a unique coordination structure of Li+-FSI-solvent cluster, which is critical for enabling the formation of stable interfaces on both the thin Li metal anode and high-voltage NCM811 cathode. Under highly demanding cell configuration and operating conditions (Li metal anode = 35 mu m, areal capacity/charge voltage of NCM811 cathode = 4.8 mAh cm(-2)/4 .6 V, and anode excess capacity [relative to the cathode] = 0.83), the Li metal-based full cell provides exceptional electrochemical performance (energy densities = 679 Wh kg(cell)(-1)/1,024 Wh L-cell(-1)) coupled with nonflammability
Superconducting energy gap in MgCNi3 single crystals: Point-contact spectroscopy and specific-heat measurements
Specific heat has been measured down to 600 mK and up to 8 Tesla by the
highly sensitive AC microcalorimetry on the MgCNi3 single crystals with Tc ~ 7
K. Exponential decay of the electronic specific heat at low temperatures proved
that a superconducting energy gap is fully open on the whole Fermi surface, in
agreement with our previous magnetic penetration depth measurements on the same
crystals. The specific-heat data analysis shows consistently the strong
coupling strength 2D/kTc ~ 4. This scenario is supported by the direct gap
measurements via the point-contact spectroscopy. Moreover, the spectroscopy
measurements show a decrease in the critical temperature at the sample surface
accounting for the observed differences of the superfluid density deduced from
the measurements by different techniques
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