14,605 research outputs found
Schechter vs. Schechter: Sub-Arcsecond Gravitational Lensing and Inner Halo Profiles
Sub-arcsecond lensing statistics depend sensitively on the inner mass
profiles of low-mass objects and the faint-end slopes of the Schechter
luminosity function and the Press-Schechter mass function. By requiring the
luminosity and mass functions to give consistent predictions for the
distribution of image separation below 1'', we show that dark matter halos with
masses below 10^12 M_sun cannot have a single type of profile, be it the
singular isothermal sphere (SIS) or the shallower ``universal'' dark matter
profile. Instead, consistent results are achieved if we allow a fraction of the
halos at a given mass to be luminous with the SIS profile, and the rest be dark
with an inner logarithmic slope shallower than -1.5 to compensate for the
steeper faint-end slope of the mass function compared with the luminosity
function. We quantify how rapidly the SIS fraction must decrease with
decreasing halo mass, thereby providing a statistical measure for the
effectiveness of feedback processes on the baryon content in low-mass halos.Comment: 13 pages, 4 figures. CLASS lensing data added; minor revisions. ApJL
in pres
Preliminary design of the Visible Spectro-Polarimeter for the Advanced Technology Solar Telescope
The Visible Spectro-Polarimeter (ViSP) is one of the first light instruments
for the Advanced Technology Solar Telescope (ATST). It is an echelle
spectrograph designed to measure three different regions of the solar spectrum
in three separate focal planes simultaneously between 380 and 900 nm. It will
use the polarimetric capabilities of the ATST to measure the full Stokes
parameters across the line profiles. By measuring the polarization in
magnetically sensitive spectral lines the magnetic field vector as a function
of height in the solar atmosphere can be obtained, along with the associated
variation of the thermodynamic properties. The ViSP will have a spatial
resolution of 0.04 arcsec over a 2 arcmin field of view (at 600 nm). The
minimum spectral resolving power for all the focal planes is 180,000. The
spectrograph supports up to 4 diffraction gratings and is fully automated to
allow for rapid reconfiguration.Comment: 8 pages, 5 figures, proceedings of SPIE Astronomical Telescopes +
Instrumentation 2012 Conference 8446 (1-5 July 2012
Optimized thermoelectric properties of Mo_3Sb_(7-x)Te_x with significant phonon scattering by electrons
Heavily doped compounds Mo_3Sb_(7−x)Te_x (x = 0, 1.0, 1.4, 1.8) were synthesized by solid state reaction and sintered by spark plasma sintering. Both X-ray diffraction and electron probe microanalysis indicated the maximum solubility of Te was around x = 1.8. The trends in the electrical transport properties can generally be understood using a single parabolic band model, which predicts that the extremely high carrier concentration of Mo_3Sb_7 (~10^(22) cm^(−3)) can be reduced to a nearly optimized level (~2 × 10^(21) cm^(−3)) for thermoelectric figure of merit (zT) by Te-substitution with x = 1.8. The increased lattice thermal conductivity by Te-doping was found to be due to the decreased Umklapp and electron–phonon scattering, according to a Debye model fitting. The thermoelectric figure of merit (zT) monotonously increased with increasing temperature and reached its highest value of about 0.51 at 850 K for the sample with x = 1.8, making these materials competitive with the state-of-the-art thermoelectric SiGe alloys. Evidence of significant electron–phonon scattering is found in the thermal conductivity
Low effective mass leading to high thermoelectric performance
High Seebeck coefficient by creating large density-of-states effective mass through either electronic structure modification or manipulating nanostructures is commonly considered as a route to advanced thermoelectrics. However, large density-of-state due to flat bands leads to large transport effective mass, which results in a simultaneous decrease of mobility. In fact, the net effect of such a high effective mass is a lower thermoelectric figure of merit, zT, when the carriers are predominantly scattered by phonons according to the deformation potential theory of Bardeen–Shockley. We demonstrate that the beneficial effect of light effective mass contributes to high zT in n-type thermoelectric PbTe, where doping and temperature can be used to tune the effective mass. This clear demonstration of the deformation potential theory to thermoelectrics shows that the guiding principle for band structure engineering should be low effective mass along the transport direction
Combination of large nanostructures and complex band structure for high performance thermoelectric lead telluride
The complexity of the valence band structure in p-type PbTe has been shown to enable a significant enhancement of the average thermoelectric figure of merit (zT) when heavily doped with Na. It has also been shown that when PbTe is nanostructured with large nanometer sized Ag_2Te precipitates there is an enhancement of zT due to phonon scattering at the interfaces. The enhancement in zT resulting from these two mechanisms is of similar magnitude but, in principle, decoupled from one another. This work experimentally demonstrates a successful combination of the complexity in the valence band structure with the addition of nanostructuring to create a high performance thermoelectric material. These effects lead to a high zT over a wide temperature range with peak zT > 1.5 at T > 650 K in Na-doped PbTe/Ag_2Te. This high average zT produces 30% higher efficiency (300–750 K) than pure Na-doped PbTe because of the nanostructures, while the complex valence band structure leads to twice the efficiency as the related n-type La-doped PbTe/Ag_2Te without such band structure complexity
Weak electron–phonon coupling contributing to high thermoelectric performance in n-type PbSe
PbSe is a surprisingly good thermoelectric material due, in part, to its low thermal conductivity that had been overestimated in earlier measurements. The thermoelectric figure of merit, zT, can exceed 1 at high temperatures in both p-type and n-type PbSe, similar to that found in PbTe. While the p-type lead chalcogenides (PbSe and PbTe) benefit from the high valley degeneracy (12 or more at high temperature) of the valence band, the n-type versions are limited to a valley degeneracy of 4 in the conduction band. Yet the n-type lead chalcogenides achieve a zT nearly as high as the p-type lead chalcogenides. This effect can be attributed to the weaker electron–phonon coupling (lower deformation potential coefficient) in the conduction band as compared with that in the valence band, which leads to higher mobility of electrons compared to that of holes. This study of PbSe illustrates the importance of the deformation potential coefficient of the charge-carrying band as one of several key parameters to consider for band structure engineering and the search for high performance thermoelectric materials
Transport composite fuselage technology: Impact dynamics and acoustic transmission
A program was performed to develop and demonstrate the impact dynamics and acoustic transmission technology for a composite fuselage which meets the design requirements of a 1990 large transport aircraft without substantial weight and cost penalties. The program developed the analytical methodology for the prediction of acoustic transmission behavior of advanced composite stiffened shell structures. The methodology predicted that the interior noise level in a composite fuselage due to turbulent boundary layer will be less than in a comparable aluminum fuselage. The verification of these analyses will be performed by NASA Langley Research Center using a composite fuselage shell fabricated by filament winding. The program also developed analytical methodology for the prediction of the impact dynamics behavior of lower fuselage structure constructed with composite materials. Development tests were performed to demonstrate that the composite structure designed to the same operating load requirement can have at least the same energy absorption capability as aluminum structure
Effect of structural relaxation on the electronic structure of graphene on hexagonal boron nitride
We performed calculations of electronic, optical and transport properties of
graphene on hBN with realistic moir\'e patterns. The latter are produced by
structural relaxation using a fully atomistic model. This relaxation turns out
to be crucially important for electronic properties. We describe experimentally
observed features such as additional Dirac points and the "Hofstadter
butterfly" structure of energy levels in a magnetic field. We find that the
electronic structure is sensitive to many-body renormalization of the local
energy gap.Comment: 5 pages, 6 figures. Supplementary material is available at
http://www.theorphys.science.ru.nl/people/yuan/attachments/sm_hbn.pd
Dimension Independent Disentanglers from Unentanglement and Applications
Quantum entanglement is a key enabling ingredient in diverse applications.
However, the presence of unwanted adversarial entanglement also poses
challenges in many applications.
In this paper, we explore methods to "break" quantum entanglement.
Specifically, we construct a dimension-independent k-partite disentangler
(like) channel from bipartite unentangled input. We show: For every , there is an efficient channel such that for every bipartite separable
state , the output is
close to a k-partite separable state. Concretely, for some distribution
on states from , Moreover, . Without the
bipartite unentanglement assumption, the above bound is conjectured to be
impossible.
Leveraging our disentanglers, we show that unentangled quantum proofs of
almost general real amplitudes capture NEXP, greatly relaxing the nonnegative
amplitudes assumption in the recent work of QMA^+(2)=NEXP. Specifically, our
findings show that to capture NEXP, it suffices to have unentangled proofs of
the form where has non-negative amplitudes, only has negative amplitudes and with . Additionally, we present a protocol achieving an almost largest
possible gap before obtaining QMA^R(k)=NEXP$, namely, a 1/poly(n) additive
improvement to the gap results in this equality.Comment: 28 page
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