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

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

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

Outflow and dense gas emission from massive Infrared Dark Clouds

Infrared Dark Clouds are expected to harbor sources in different, very young evolutionary stages. To better characterize these differences, we observed a sample of 43 massive Infrared Dark Clouds, originally selected as candidate high-mass starless cores, with the IRAM 30m telescope covering spectral line tracers of low-density gas, high-density gas, molecular outflows/jets and temperature effects. The SiO(2-1) observations reveal detections toward 18 sources. Assuming that SiO is exclusively produced by sputtering from dust grains, this implies that at least in 40% of this sample star formation is on-going. A broad range of SiO line-widths is observed (between 2.2 and 65km/s), and we discuss potential origins for this velocity spread. While the low-density tracers 12CO(2-1) and 13CO(1-0) are detected in several velocity components, the high-density tracer H13CO+(1--0) generally shows only a single velocity component and is hence well suited for kinematic distance estimates of IRDCs. Furthermore, the H13CO+ line-width is on average 1.5 times larger than that of previously observed NH3(1,1). This is indicative of more motion at the denser core centers, either due to turbulence or beginning star formation activity. In addition, we detect CH3CN toward only six sources whereas CH3OH is observed toward approximately 40% of the sample. Estimates of the CH3CN and CH3OH abundances are low with average values of 1.2x10^{-10} and 4.3x10^{-10}, respectively. These results are consistent with chemical models at the earliest evolutionary stages of high-mass star formation. Furthermore, the CH3OH abundances compare well to recently reported values for low-mass starless cores.Comment: 22 pages (ApJ referee style), 7 figures, accepted for Ap

Transformer Uncertainty Estimation with Hierarchical Stochastic Attention

Transformers are state-of-the-art in a wide range of NLP tasks and have also been applied to many real-world products. Understanding the reliability and certainty of transformer model predictions is crucial for building trustable machine learning applications, e.g., medical diagnosis. Although many recent transformer extensions have been proposed, the study of the uncertainty estimation of transformer models is under-explored. In this work, we propose a novel way to enable transformers to have the capability of uncertainty estimation and, meanwhile, retain the original predictive performance. This is achieved by learning a hierarchical stochastic self-attention that attends to values and a set of learnable centroids, respectively. Then new attention heads are formed with a mixture of sampled centroids using the Gumbel-Softmax trick. We theoretically show that the self-attention approximation by sampling from a Gumbel distribution is upper bounded. We empirically evaluate our model on two text classification tasks with both in-domain (ID) and out-of-domain (OOD) datasets. The experimental results demonstrate that our approach: (1) achieves the best predictive performance and uncertainty trade-off among compared methods; (2) exhibits very competitive (in most cases, improved) predictive performance on ID datasets; (3) is on par with Monte Carlo dropout and ensemble methods in uncertainty estimation on OOD datasets.Comment: AAAI 202