Showing papers on "Absorption (logic) published in 2014"

Lyman-$\alpha$ Transit Spectroscopy and the Neutral Hydrogen Tail of the Hot Neptune GJ436b

Abstract: To date, more than 750 planets have been discovered orbiting stars other than the Sun. Two sub-classes of these exoplanets, "hot Jupiters" and their less massive counterparts "hot Neptunes," provide a unique opportunity to study the extended atmospheres of planets outside of our solar system. We describe here the first far-ultraviolet transit study of a hot Neptune, specifically GJ436b, for which we use HST/STIS Lyman-$\alpha$ spectra to measure stellar flux as a function of time, observing variations due to absorption from the planetary atmosphere during transit. This analysis permits us to derive information about atmospheric extent, mass-loss rate from the planet, and interactions between the star and planet. We observe an evolution of the Lyman-$\alpha$ lightcurve with a transit depth of GJ436b from $8.8\pm4.5\%$ near mid-transit, to $22.9\pm3.9\%$ $\sim2$ hours after the nominal geometric egress of the planet. Using data from the time-tag mode and considering astrophysical noise from stellar variability, we calculate a post-egress occultation of $23.7\pm4.5\%$, demonstrating that the signature is statistically significant and of greater amplitude than can be attributed to stellar fluctuations alone. The extended egress absorption indicates the probable existence of a comet-like tail trailing the exoplanet. We calculate a mass-loss rate for GJ436b in the range of $3.7\times10^6 -1.1\times10^{9}$ g s$^{-1}$, corresponding to an atmospheric lifetime of $4\times10^{11}-2\times10^{14}$ years.

Chiral anomaly and optical absorption in Weyl semimetals

Abstract: Weyl semimetals are a three-dimensional topological phase of matter with isolated band touchings in the Brillouin zone. These points have an associated chirality, and many of the proposals to detect the Weyl semimetal state rely on the chiral anomaly. A consequence of the chiral anomaly is that under the application of an $\mathbf{E}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{B}$ field, charge is transferred between points of opposite chirality. In this paper we propose an optical absorption experiment that provides evidence for the chiral anomaly. We use the Kubo formula, and find that an applied $\mathbf{E}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{B}$ induces the formation of steplike features at finite frequency in the interband optical conductivity. We study the effect of scattering and finite temperatures on this feature and find that it should be observable at low temperatures in pure samples. Finally we discuss how the application of an $\mathbf{E}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{B}$ field can be used to map out the frequency dependence of the scattering rate.

The Physical conditions, metallicity and metal abundance ratios in a highly magnified galaxy at z = 3.6252

Abstract: We present optical and near-IR imaging and spectroscopy of SGAS J105039.6$+$001730, a strongly lensed galaxy at z $=$ 3.6252 magnified by $>$30$\times$, and derive its physical properties. We measure a stellar mass of log(M$_{*}$/M$_{\odot}$) $=$ 9.5 $\pm$ 0.35, star formation rates from [O II]$\lambda$$\lambda$3727 and H-$\beta$ of 55 $\pm$ 20 and 84 $\pm$ 17 M$_{\odot}$ yr$^{-1}$, respectively, an electron density of n$_{e} \leq$ 10$^{3}$ cm$^{-2}$, an electron temperature of T$_{e} \leq$ 14000 K, and a metallicity of 12+log(O/H) $=$ 8.3 $\pm$ 0.1. The strong C III]$\lambda$$\lambda$1907,1909 emission and abundance ratios of C, N, O and Si are consistent with well-studied starbursts at z $\sim$ 0 with similar metallicities. Strong P Cygni lines and He II$\lambda$1640 emission indicate a significant population of Wolf-Rayet stars, but synthetic spectra of individual populations of young, hot stars do not reproduce the observed integrated P Cygni absorption features. The rest-frame UV spectral features are indicative of a young starburst with high ionization, implying either 1) an ionization parameter significantly higher than suggest by rest-frame optical nebular lines, or 2) differences in one or both of the initial mass function and the properties of ionizing spectra of massive stars. We argue that the observed features are likely the result of a superposition of star forming regions with different physical properties. These results demonstrate the complexity of star formation on scales smaller than individual galaxies, and highlight the importance of systematic effects that result from smearing together the signatures of individual star forming regions within galaxies.

Wear, Plasticity, and Rehybridization in Tetrahedral Amorphous Carbon

Abstract: Wear in self-mated tetrahedral amorphous carbon (ta-C) films is studied by molecular dynamics and near-edge X-ray absorption fine structure spectroscopy. Both theory and experiment demonstrate the formation of a soft amorphous carbon (a-C) layer with increased sp2 content, which grows faster than an a-C tribolayer found on self-mated diamond sliding under similar conditions. The faster $$\hbox{sp}^{3} \rightarrow\,\hbox{ sp}^{2}$$ transition in ta-C is explained by easy breaking of prestressed bonds in a finite, nanoscale ta-C region, whereas diamond amorphization occurs at an atomically sharp interface. A detailed analysis of the underlying rehybridization mechanism reveals that the $$\hbox{sp}^{3}\, \rightarrow\hbox{ sp}^{2}$$ transition is triggered by plasticity in the adjacent a-C. Rehybridization therefore occurs in a region that has not yet experienced plastic yield. The resulting soft a-C tribolayer is interpreted as a precursor to the experimentally observed wear.

Lyman-alpha Forest Tomography from Background Galaxies: The First Megaparsec-Resolution Large-Scale Structure Map at z>2

Abstract: We present the first observations of foreground Lyman-$\alpha$ forest absorption from high-redshift galaxies, targeting 24 star-forming galaxies (SFGs) with $z\sim 2.3-2.8$ within a $5' \times 15'$ region of the COSMOS field. The transverse sightline separation is $\sim 2\,h^{-1}\mathrm{Mpc}$ comoving, allowing us to create a tomographic reconstruction of the 3D Ly$\alpha$ forest absorption field over the redshift range $2.20\leq z\leq 2.45$. The resulting map covers $6\,h^{-1}\mathrm{Mpc} \times 14\,h^{-1}\mathrm{Mpc}$ in the transverse plane and $230\,h^{-1}\mathrm{Mpc}$ along the line-of-sight with a spatial resolution of $\approx 3.5\,h^{-1}\mathrm{Mpc}$, and is the first high-fidelity map of large-scale structure on $\sim\mathrm{Mpc}$ scales at $z>2$. Our map reveals significant structures with $\gtrsim 10\,h^{-1}\mathrm{Mpc}$ extent, including several spanning the entire transverse breadth, providing qualitative evidence for the filamentary structures predicted to exist in the high-redshift cosmic web. Simulated reconstructions with the same sightline sampling, spectral resolution, and signal-to-noise ratio recover the salient structures present in the underlying 3D absorption fields. Using data from other surveys, we identified 18 galaxies with known redshifts coeval with our map volume enabling a direct comparison to our tomographic map. This shows that galaxies preferentially occupy high-density regions, in qualitative agreement with the same comparison applied to simulations. Our results establishes the feasibility of the CLAMATO survey, which aims to obtain Ly$\alpha$ forest spectra for $\sim 1000$ SFGs over $\sim 1 \,\mathrm{deg}^2$ of the COSMOS field, in order to map out IGM large-scale structure at $\langle z \rangle \sim 2.3$ over a large volume $(100\,h^{-1}\mathrm{Mpc})^3$.

Search for MeV dark photons in a light-shining-through-walls experiment at CERN

Abstract: In addition to gravity, there might be another very weak interaction between the ordinary and dark matter transmitted by ${U}^{\ensuremath{'}}(1)$ gauge bosons ${A}^{\ensuremath{'}}$ (dark photons) mixing with our photons. If such ${A}^{\ensuremath{'}}$'s exist, they could be searched for in a light-shining-through-a-wall experiment with a high-energy electron beam. The electron energy absorption in a calorimeter (CAL1) is accompanied by the emission of bremsstrahlung ${A}^{\ensuremath{'}}$'s in the reaction $eZ\ensuremath{\rightarrow}eZ{A}^{\ensuremath{'}}$ of electrons scattering on nuclei due to the $\ensuremath{\gamma}\ensuremath{-}{A}^{\ensuremath{'}}$ mixing. A part of the primary beam energy is deposited in the CAL1, while the rest of the energy is transmitted by the ${A}^{\ensuremath{'}}$ through the ``CAL1 wall'' and deposited in another downstream calorimeter CAL2 by the ${e}^{+}{e}^{\ensuremath{-}}$ pair from the ${A}^{\ensuremath{'}}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}$ decay in flight. Thus, the ${A}^{\ensuremath{'}}$'s could be observed by looking for an excess of events with the two-shower signature generated by a single high-energy electron in the CAL1 and CAL2. A proposal to perform such an experiment to probe the still unexplored area of the mixing strength ${10}^{\ensuremath{-}5}\ensuremath{\lesssim}\ensuremath{\epsilon}\ensuremath{\lesssim}{10}^{\ensuremath{-}3}$ and masses ${M}_{{A}^{\ensuremath{'}}}\ensuremath{\lesssim}100\text{ }\text{ }\mathrm{MeV}$ by using 10--300 GeV electron beams from the CERN SPS is presented. The experiment can provide complementary coverage of the parameter space, which is intended to be probed by other searches. It has also a capability for a sensitive search for ${A}^{\ensuremath{'}}$'s decaying invisibly to dark-sector particles, such as dark matter, which could cover a significant part of the still allowed parameter space.

Forging the link between nuclear reactions and nuclear structure.

Abstract: A comprehensive description of all single-particle properties associated with the nucleus $^{40}\mathrm{Ca}$ is generated by employing a nonlocal dispersive optical potential capable of simultaneously reproducing all relevant data above and below the Fermi energy. The introduction of nonlocality in the absorptive potentials yields equivalent elastic differential cross sections as compared to local versions but changes the absorption profile as a function of angular momentum suggesting important consequences for the analysis of nuclear reactions. Below the Fermi energy, nonlocality is essential to allow for an accurate representation of particle number and the nuclear charge density. Spectral properties implied by ($e$, ${e}^{\ensuremath{'}}p$) and ($p$, $2p$) reactions are correctly incorporated, including the energy distribution of about 10% high-momentum nucleons, as experimentally determined by data from Jefferson Lab. These high-momentum nucleons provide a substantial contribution to the energy of the ground state, indicating a residual attractive contribution from higher-body interactions for $^{40}\mathrm{Ca}$ of about $0.64\text{ }\text{ }\mathrm{MeV}/\mathrm{A}$.

E(B-V), N(H I) and N(H_2)

Abstract: We consider the structure of the N(H I) - E(B-V) relationship when H I is measured in the 21 cm radio line and \EBV\ is defined by far-IR dust-derived measures. We derive reddening-dependent corrections to N(H I) based on interferometric absorption measurements over the past 30 years that follow a single power-law relationship $\int \tau(H I) dv = 14.07 ~\kms\ $\EBV$^{1.074}$ at 0.02 $\la$ \EBV\ $\la$ 3 mag. Corrections to 21cm line-derived H I column densities are too small to have had any effect on the ratio N(H I)/\EBV\ $= 8.3 \times 10^{21}\pcc$ mag$^{-1}$ we derived at 0.015 $\la$ \EBV\ $\la$ 0.075 mag and \absb\ $\ge$ 20\degr; they are also too small to explain the break in the slope of the N(H I) - \EBV\ relation at \EBV\ $\ga$ 0.1 mag that we demonstrated around the Galaxy at \absb $\ge 20$\degr. The latter must therefore be attributed to the onset of \HH-formation and we show that models of \HH\ formation in a low density diffuse molecular gas can readily explain the inflected N(H I)- \EBV\ relationship. Below \absb\ = 20\degr\ N(H I)/\EBV\ measured at 0.015 $\la$ \EBV\ $\la$ 0.075 mag increases steadily down to \absb\ = 8\degr\ where sightlines with small \EBV\ no longer occur. By contrast, the ratio N(H I)/\EBV\ measured over all \EBV\ declines to N(H I)/\EBV\ $= 5-6 \times 10^{21}\pcc$ mag$^{-1}$ at \absb\ $\la 30$\degr, perhaps providing an explanation of the difference between our results and the gas/reddening ratios measured previously using stellar spectra.

Orbital splitting and optical conductivity of the insulating state of NbO 2

Abstract: Optical properties from 0.2 to 6.5 eV of epitaxial ${\mathrm{NbO}}_{2}$ $(4{d}^{1} \text{system})$ films in their insulating states have been investigated by spectroscopic ellipsometry. The optical spectra are compared to that of epitaxial ${\mathrm{VO}}_{2}$ $(3{d}^{1} \text{system})$ in its insulating phase. Both compounds are insulators at room temperature and undergo temperature-induced metal-insulator transitions. We find a ${d}_{||}\ensuremath{-}{e}_{\mathrm{g}}^{\ensuremath{\pi}}$ orbital splitting energy of $\ensuremath{\sim}1.6$ eV in ${\mathrm{NbO}}_{2}$ compared to $\ensuremath{\sim}1.3$ eV in ${\mathrm{VO}}_{2}$; orbital splitting is a key ingredient that stabilizes the cation-dimerized insulating states of both materials. The edge of the $\text{O}2p$-like valence band is estimated to be 3.2 eV below the Fermi level through x-ray photoelectron spectroscopy measurements, suggesting that electrons from $\text{O}2p$ states do not contribute to absorption below this energy. This allows us to also assign an observed optical peak at $\ensuremath{\sim}3.0$ eV to the $\mathrm{Nb}\phantom{\rule{0.28em}{0ex}}4{d}_{||}\ensuremath{\rightarrow}4{d}_{||}^{*}$ transition.

Tracing the Cosmic Metal Evolution in the Low-redshift Intergalactic Medium

Abstract: Using the Cosmic Origins Spectrograph aboard the Hubble Space Telescope, we measured the abundances of six ions (C III, C IV, Si III, Si IV, N V, and O VI) in the low-redshift (z ≤ 0.4) intergalactic medium (IGM). Both C IV and Si IV have increased in abundance by a factor of ∼10 from z ≈ 5.5 to the present. We derive ion mass densities, ρ{sub ion} ≡ Ω{sub ion}ρ{sub cr}, with Ω{sub ion} expressed relative to the closure density. Our models of mass-abundance ratios, (Si III/Si IV) =0.67{sub −0.19}{sup +0.35}, (C III/C IV) =0.70{sub −0.20}{sup +0.43}, and (Ω{sub C} {sub III}+Ω{sub C} {sub IV})/(Ω{sub Si} {sub III}+Ω{sub Si} {sub IV})=4.9{sub −1.1}{sup +2.2}, are consistent with the photoionization parameter log U = –1.5 ± 0.4, hydrogen photoionization rate Γ{sub H} = (8 ± 2) × 10{sup –14} s{sup –1} at z < 0.4, and specific intensity I {sub 0} = (3 ± 1) × 10{sup –23} erg cm{sup –2} s{sup –1} Hz{sup –1} sr{sup –1} at the Lyman limit. Consistent ionization corrections for C and Si are scaled to an ionizing photon flux Φ{sub 0} = 10{sup 4} cm{sup –2} s{sup –1}, baryon overdensity Δ {sub b} ≈ 200more » ± 50, and ''alpha-enhancement'' (Si/C enhanced to three times its solar ratio). We compare these metal abundances to the expected IGM enrichment and abundances in higher photoionized states of carbon (C V) and silicon (Si V, Si VI, and Si VII). Our ionization modeling infers IGM metal densities of (5.4 ± 0.5) × 10{sup 5} M {sub ☉} Mpc{sup –3} in the photoionized Lyα forest traced by the C and Si ions and (9.1 ± 0.6) × 10{sup 5} M {sub ☉} Mpc{sup –3} in hotter gas traced by O VI. Combining both phases, the heavy elements in the IGM have mass density ρ {sub Z} = (1.5 ± 0.8) × 10{sup 6} M {sub ☉} Mpc{sup –3} or Ω {sub Z} ≈ 10{sup –5}. This represents 10% ± 5% of the metals produced by (6 ± 2) × 10{sup 8} M {sub ☉} Mpc{sup –3} of integrated star formation with yield y{sub m} = 0.025 ± 0.010. The missing metals at low redshift may reside within galaxies and in undetected ionized gas in galactic halos and circumgalactic medium.« less

CO/H2 Abundance Ratio 10?4 in a Protoplanetary Disk

Abstract: The relative abundances of atomic and molecular species in planet-forming disks around young stars provide important constraints on photochemical disk models and provide a baseline for calculating disk masses from measurements of trace species. A knowledge of absolute abundances, those relative to molecular hydrogen (H{sub 2}), are challenging because of the weak rovibrational transition ladder of H{sub 2} and the inability to spatially resolve different emission components within the circumstellar environment. To address both of these issues, we present new contemporaneous measurements of CO and H{sub 2} absorption through the 'warm molecular layer' of the protoplanetary disk around the Classical T Tauri Star RW Aurigae A. We use a newly commissioned observing mode of the Hubble Space Telescope Cosmic Origins Spectrograph to detect warm H{sub 2} absorption in this region for the first time. An analysis of the emission and absorption spectrum of RW Aur shows components from the accretion region near the stellar photosphere, the molecular disk, and several outflow components. The warm H{sub 2} and CO absorption lines are consistent with a disk origin. We model the 1092-1117 A spectrum of RW Aur to derive log{sub 10} N(H{sub 2}) = 19.90{sub −0.22}{sup +0.33} cm{sup –2} at T {submore » rot}(H{sub 2}) = 440 ± 39 K. The CO A - X bands observed from 1410 to 1520 A are best fit by log{sub 10} N(CO) = 16.1 {sub −0.5}{sup +0.3} cm{sup –2} at T {sub rot}(CO) = 200{sub −125}{sup +650} K. Combining direct measurements of the H I, H{sub 2}, and CO column densities, we find a molecular fraction in the warm disk surface of f {sub H2} ≥ 0.47 and derive a molecular abundance ratio of CO/H{sub 2} = 1.6{sub −1.3}{sup +4.7} × 10{sup –4}, both consistent with canonical interstellar dense cloud values.« less

Relaxation of excited spin, orbital, and valley qubit states in ideal silicon quantum dots

Abstract: We review and expand on previous work that treats relaxation physics of low-lying excited states in ideal, single-electron, silicon quantum dots in the context of quantum computing. These states are of three types: orbital, valley, and spin. The relaxation times depend sensitively on system parameters such as the dot size and the external magnetic field. Generally, however, orbital relaxation times are short in strained silicon (${10}^{\ensuremath{-}7}$ to ${10}^{\ensuremath{-}12}$ s), spin-relaxation times are long (${10}^{\ensuremath{-}6}$ to $\ensuremath{\gg}$1 s), while valley relaxation times are expected to lie in between. The focus is on relaxation due to emission or absorption of phonons, but for spin relaxation we also consider competing mechanisms such as charge noise. Where appropriate, comparison is made to reference systems such as quantum dots in III-V materials and silicon donor states. The phonon-bottleneck effect is shown to be rather small in the regime of interest. We compare the theoretical predictions to some recent spin relaxation experiments and comment on the possible effects of nonideal dots.

Quantum control of molecular gas hydrodynamics.

Abstract: We demonstrate that strong impulsive gas heating or heating suppression at standard temperature and pressure can occur from coherent rotational excitation or deexcitation of molecular gases using a sequence of nonionizing laser pulses. For the case of excitation, subsequent collisional decoherence of the ensemble leads to gas heating significantly exceeding that from plasma absorption under the same laser focusing conditions. In both cases, the macroscopic hydrodynamics of the gas can be finely controlled with $\ensuremath{\sim}40\text{ }\text{ }\mathrm{fs}$ temporal sensitivity.

Origins of electronic band gap reduction in Cr/N codoped TiO2.

Abstract: Recent studies indicated that noncompensated cation-anion codoping of wide-band-gap oxide semiconductors such as anatase ${\mathrm{TiO}}_{2}$ significantly reduces the optical band gap and thus strongly enhances the absorption of visible light [W. Zhu et al., Phys. Rev. Lett. 103, 226401 (2009)]. We used soft x-ray spectroscopy to fully determine the location and nature of the impurity levels responsible for the extraordinarily large ($\ensuremath{\sim}1\text{ }\mathrm{eV}$) band gap reduction of noncompensated codoped rutile ${\mathrm{TiO}}_{2}$. It is shown that $\mathrm{Cr}/\mathrm{N}$ codoping strongly enhances the substitutional N content, compared to single element doping. The band gap reduction is due to the formation of Cr $3{d}^{3}$ levels in the lower half of the gap while the conduction band minimum is comprised of localized Cr $3d$ and delocalized N $2p$ states. Band gap reduction and carrier delocalization are critical elements for efficient light-to-current conversion in oxide semiconductors. These findings thus raise the prospect of using codoped oxide semiconductors with specifically engineered electronic properties in a variety of photovoltaic and photocatalytic applications.

Charge-transfer excited states in aqueous DNA: Insights from many-body Green's function theory.

Abstract: Charge-transfer (CT) excited states play an important role in the excited-state dynamics of DNA in aqueous solution. However, there is still much controversy on their energies. By ab initio many-body Green's function theory, together with classical molecular dynamics simulations, we confirm the existence of CT states at the lower energy side of the optical absorption maximum in aqueous DNA as observed in experiments. We find that the hydration shell can exert strong effects ($\ensuremath{\sim}1\text{ }\text{ }\mathrm{eV}$) on both the electronic structure and CT states of DNA molecules through dipole electric fields. In this case, the solvent cannot be simply regarded as a macroscopic screening medium as usual. The influence of base stacking and base pairing on the CT states is also discussed.

Tunable optical absorption and interactions in graphene via oxygen plasma

Abstract: We report significant changes of optical conductivity (${\ensuremath{\sigma}}_{1}$) in single-layer graphene induced by mild oxygen plasma exposure and explore the interplay between carrier doping, disorder, and many-body interactions from their signatures in the absorption spectrum. The first distinctive effect is the reduction of the excitonic binding energy that can be extracted from the renormalized saddle point resonance at 4.64 eV. Secondly, ${\ensuremath{\sigma}}_{1}$ is nearly completely suppressed (${\ensuremath{\sigma}}_{1}\ensuremath{\ll}{\ensuremath{\sigma}}_{0}$) below an exposure-dependent threshold in the near-infrared range. The clear steplike suppression follows the Pauli blocking behavior expected for doped monolayer graphene. The nearly zero residual conductivity below \ensuremath{\omega} \ensuremath{\sim} 2${E}_{F}$ can be interpreted as arising from the weakening of the electronic self-energy. Our data shows that mild oxygen exposure can be used to controllably dope graphene without introducing the strong physical and chemical changes that are common in other approaches to oxidized graphene, allowing a controllable manipulation of the optical properties of graphene.

One-dimensional stochastic differential equations with generalized drift ∗

Abstract: We consider one-dimensional stochastic differential equations with generalized drift which involve the local time $L^X$ of the solution process $ X_t = X_0 + \int_0^t b(X_s) \, {\rm d} B_s + \int_{\bf R} L^X(t,y)$ where $b$ is a measurable real function, $B$ is a Wiener process, and $ u$ denotes a set function which is defined on the bounded Borel sets of the real line ${\bf R}$ such that it is a finite signed measure on ${\cal B}([-N,N])$ for every $N \in {\bf N}$. This kind of equation is, in dependence of using the right, the left, or the symmetric local time, usually studied under the atom condition $ u(\{x\}) -1/2$, and $| u(\{x\})| < 1$, respectively. This condition allows us to reduce an equation with generalized drift to an equation without drift and to derive conditions on existence and uniqueness of solutions from corresponding results for equations without drift. The main aim of the present paper is to treat the cases $ u(\{x\}) \ge 1/2$, $ u(\{x\}) \le -1/2$, and $|\...

Probing the mass and anisotropy of the Milky Way gaseous halo: Sight-lines toward Mrk 421 and PKS 2155-304

Abstract: We recently found that the halo of the Milky Way contains a large reservoir of warm-hot gas that accounts for large fraction of the missing baryons from the Galaxy. The average physical properties of this circumgalactic medium (CGM) are determined by combining average absorption and emission measurements along several extragalactic sightlines. However, there is a wide distribution of both, the halo emission measure and the O vii column density, suggesting that the Galactic warm-hot gaseous halo is anisotropic. We present Suzaku observations of fields close to two sightlines along which we have precise O vii absorption measurements with Chandra. The column densities along these two sightlines are similar within errors, but we find that the emission measures are different: 0.0025±0.0006 cm−6 pc near the Mrk 421 direction and 0.0042±0.0008 cm−6 pc close to the PKS 2155-304 sightline. Therefore the densities and pathlengths in the two directions must be different, providing a suggestive evidence that the warm-hot gas in the CGM of the Milky Way is not distributed uniformly. However, the formal errors on derived parameters are too large to make such a claim. In the Mrk 421 direction we derive the density of $1.6^{+2.6}_{-0.8} \times 10^{-4}~\mbox{cm}^{-3}$ and pathlength of $334^{+685}_{-274}~\mbox{kpc}$ . In the PKS 2155-304 direction we measure the gas density of $3.6^{+4.5}_{-1.8} \times10^{-4}~\mbox{cm}^{-3}$ and path-length of $109^{+200}_{-82}~\mbox{kpc}$ . Thus the density and pathlength along these sightlines are consistent with each other within errors. The average density and pathlength of the two sightlines are similar to the global averages, so the halo mass is still huge, over 10 billion solar masses. With more such studies, we will be able to better characterize the CGM anisotropy and measure its mass more accurately. We can then compare the observational results with theoretical models and investigate if/how the CGM structure is related to the larger scale environment of the Milky Way. We also show that the Galactic disk makes insignificant contribution to the observed O vii absorption; a similar conclusion was also reached by Henley and Shelton (2013) about the emission measure. We further argue that any density inhomogeneity in the warm-hot gas, be it from clumping, from the disk, or from a non-constant density gradient, would strengthen our result in that the Galactic halo path-length and the mass would become larger than what we estimate here. As such, our results are conservative and robust.

Local structure in perovskite ( Ba , Sr ) TiO 3 : Reverse Monte Carlo refinements from multiple measurement techniques

Abstract: Local and average structures of ceramic ${\mathrm{Ba}}_{1\ensuremath{-}x}$Sr${}_{x}$${\text{TiO}}_{3}$ solid solutions have been reconciled using Reverse Monte Carlo (RMC) refinements of atomic positions that employ simultaneous fitting of neutron total scattering data, x-ray absorption fine structure, and patterns of diffuse scattering in electron diffraction. These refinements enable explicit reconstruction of three-dimensional atomic configurations without any effective parameters. The results reveal cube- and parallelepiped-shaped probability density distributions for Ti atoms in the cubic and tetragonal phases of BaTiO${}_{3}$, respectively. These distributions are consistent with the split Ti sites (eight for the cubic and four for the tetragonal polymorphs) separated by \ensuremath{\approx}0.2 \AA{}. The characters of Ti distributions are retained in the solid solutions, but the magnitude of Ti off-centering decreases as $x$ increases. The Ti displacements remain correlated along the octahedral chains, at least up to $x$ = 0.5, as manifested in the sheets of diffuse scattering in electron diffraction patterns; the correlation parameters and lengths have been quantified using RMC analyses. The ion-size difference between Ba and Sr is accommodated through the approximately isotropic relaxation of the oxygen atoms, which shift toward Sr; a similar relaxation is observed for the Ti atoms. Local Ti off-centering diminishes as the number of Sr atoms in the coordination groups [TiO${}_{x}$Ba${}_{8\ensuremath{-}n}$Sr${}_{n}$] increases.

The Disk Wind in the Rapidly Spinning Stellar-Mass Black Hole 4U 1630-472 Observed with NuSTAR

Abstract: We present an analysis of a short NuSTAR observation of the stellar-mass black hole and low-mass X-ray binary 4U 1630$-$472. Reflection from the inner accretion disk is clearly detected for the first time in this source, owing to the sensitivity of NuSTAR. With fits to the reflection spectrum, we find evidence for a rapidly spinning black hole, $a_{*}=0.985^{+0.005}_{-0.014}$ ($1\sigma$ statistical errors). However, archival data show that the source has relatively low radio luminosity. Recently claimed relationships between jet power and black hole spin would predict either a lower spin or a higher peak radio luminosity. We also report the clear detection of an absorption feature at $7.03\pm0.03$ keV, likely signaling a disk wind. If this line arises in dense, moderately ionized gas ($\log\xi=3.6^{+0.2}_{-0.3}$) and is dominated by He-like Fe XXV, the wind has a velocity of $v/c=0.043^{+0.002}_{-0.007}$ (12900$^{+600}_{-2100}$ km s$^{-1}$). If the line is instead associated with a more highly ionized gas ($\log\xi=6.1^{+0.7}_{-0.6}$), and is dominated by Fe XXVI, evidence of a blue-shift is only marginal, after taking systematic errors into account. Our analysis suggests the ionized wind may be launched within 200$-$1100 Rg, and may be magnetically driven.

VERITAS Observations of the BL Lac Object PG 1553+113

Abstract: We present results from VERITAS observations of the BL Lac object PG 1553+113 spanning the years 2010, 2011, and 2012. The time-averaged spectrum, measured between 160 and 560\,GeV, is well described by a power law with a spectral index of $4.33 \pm 0.09$. The time-averaged integral flux above $200\,$GeV measured for this period was $(1.69 \pm 0.06) \times 10^{-11} \, \mathrm{ph} \, \mathrm{cm}^{-2} \, \mathrm{s}^{-1}$, corresponding to 6.9\% of the Crab Nebula flux. We also present the combined $\gamma$-ray spectrum from the Fermi Large Area Telescope and VERITAS covering an energy range from 100~MeV to 560~GeV. The data are well fit by a power law with an exponential cutoff at $\rm {101.9 \pm 3.2 \, \mathrm{GeV}} $. The origin of the cutoff could be intrinsic to PG~1553+113 or be due to the $\gamma$-ray opacity of our universe through pair production off the extragalactic background light (EBL). Given lower limits to the redshift of $\rm z egthinspace > egthinspace 0.395$ based on optical/UV observations of PG~1553+113, the cutoff would be dominated by EBL absorption. Conversely, the small statistical uncertainties of the VERITAS energy spectrum have allowed us to provide a robust upper limit on the redshift of PG 1553+113 of $z egthinspace \leq egthinspace 0.62$. A strongly-elevated mean flux of $(2.50 \pm 0.14) \times 10^{-11} \, \mathrm{ph} \, \mathrm{cm}^{-2} \, \mathrm{s}^{-1}$ (10.3\% of the Crab Nebula flux) was observed during 2012, with the daily flux reaching as high as $(4.44 \pm 0.71) \times 10^{-11} \, \mathrm{ph} \, \mathrm{cm}^{-2} \, \mathrm{s}^{-1}$ (18.3\% of the Crab Nebula flux) on MJD 56048. The light curve measured during the 2012 observing season is marginally inconsistent with a steady flux, giving a $\chi^2$ probability for a steady flux of 0.03\%.

Experimental realization of a semiconducting full-Heusler compound: Fe 2 TiSi

Abstract: Single-phase films of the full-Heusler compound ${\mathrm{Fe}}_{2}\mathrm{TiSi}$ have been prepared by magnetron sputtering. The compound is found to be a semiconductor with a gap of 0.4 eV. The electrical resistivity has a logarithmic temperature dependence up to room temperature due to Kondo scattering of a dilute free electron gas off superparamagnetic impurities. The origin of the electron gas is extrinsic due to residual off-stoichiometry. Density functional theory calculations of the electronic structure are in excellent agreement with electron energy loss, optical, and x-ray absorption experiments. ${\mathrm{Fe}}_{2}\mathrm{TiSi}$ may find applications as a thermoelectric material.

Phonon-assisted optical excitation in the narrow bandgap Mott insulator Sr3Ir2O7

Abstract: We examined the temperature ($T$) evolution of the optical conductivity spectra of ${Sr}_{3}$${Ir}_{2}$${O}_{7}$ over a wide range of 10--400 K. The system was barely insulating, exhibiting a small indirect bandgap of $\ensuremath{\le}$0.1 eV. The low-energy features of the optical d-d excitation ($\ensuremath{\hbar}\ensuremath{\omega}$ 0.3 eV) evolved drastically, whereas such evolution was not observed for the O $K$-edge x-ray-absorption spectra. This suggests that the $T$ evolution in optical spectra is not caused by a change in the bare (undressed) electronic structure, but instead presumably originates from an abundance of phonon-assisted indirect excitations. Our results showed that the low-energy excitations were dominated by phonon-absorption processes which involved, in particular, the optical phonons. This implies that phonon-assisted processes significantly facilitate the charge dynamics in barely insulating ${Sr}_{3}$${Ir}_{2}$${O}_{7}$.

Shaping the distribution of vertical velocities of antihydrogen in GBAR

Abstract: GBAR is a project aiming at measuring the free-fall acceleration of gravity for antimatter, namely antihydrogen atoms ( $$\overline{\mathrm {H}}$$ ). The precision of this timing experiment depends crucially on the dispersion of initial vertical velocities of the atoms as well as on the reliable control of their distribution. We propose to use a new method for shaping the distribution of the vertical velocities of $$\overline{\mathrm {H}}$$ , which improves these factors simultaneously. The method is based on quantum reflection of elastically and specularly bouncing $$\overline{\mathrm {H}}$$ with small initial vertical velocity on a bottom mirror disk, and absorption of atoms with large initial vertical velocities on a top rough disk. We estimate statistical and systematic uncertainties, and we show that the accuracy for measuring the free fall acceleration $$\overline{g}$$ of $$\overline{\mathrm {H}}$$ could be pushed below $$10^{-3}$$ under realistic experimental conditions.

CH in absorption in IRAS 16293−2422

Abstract: While recent studies of the solar-mass protostar IRAS16293-2422 have focused on its inner arcsecond, the wealth of Herschel/HIFI data has shown that the structure of the outer envelope and of the transition region to the more diffuse ISM is not clearly constrained. We use rotational ground-state transitions of CH (methylidyne), as a tracer of the lower-density envelope. Assuming LTE, we perform a $\chi^2$ minimization of the high spectral resolution HIFI observations of the CH transitions at ~532 and ~536 GHz in order to derive column densities in the envelope and in the foreground cloud. We obtain column densities of (7.7$\pm$0.2)$\times10^{13}$ cm$^{-2}$ and (1.5$\pm$0.3)$\times10^{13}$ cm$^{-2}$, respectively. The chemical modeling predicts column densities of (0.5-2)$\times10^{13}$ cm$^{-2}$ in the envelope (depending on the cosmic-ray ionization rate), and 5$\times10^{11}$ to 2.5$\times10^{14}$ cm$^{-2}$ in the foreground cloud (depending on time). Both observed abundances are reproduced by the model at a satisfactory level. The constraints set by these observations on the physical conditions in the foreground cloud are however weak. Furthermore, the CH abundance in the envelope is strongly affected by the rate coefficient of the reaction H+CH$\rightarrow$C+H$_2$ ; further investigation of its value at low temperature would be necessary to facilitate the comparison between the model and the observations.

Quantitative Analysis of the Optical Losses in CZTS Thin-Film Semiconductors

Abstract: For the first time, a quantitative approach was carried out to model the optical losses in several $\hbox{Cu}_{2}\hbox{ZnSnS}_{4}$ (CZTS) based thin-film heterojunction solar cells having different window and buffer layers. The modeling indicates that the heterojunction of glass/FTO/ $\hbox{TiO}_{2}$/$\hbox{In}_{2}\hbox{S}_{3}$ /CZTS has a lower reflection and higher transmission coefficient than glass/FTO/ $\hbox{In}_{2}\hbox{S}_{3}$/CZTS and conventional glass/FTO/CdS/CZTS structures. These optical losses at the interfaces (reflection) and inside the thickness (absorption) of stacked layers were calculated based on the known optical constants such as refractive index and extinction coefficient. While $\hbox{TiO}_{2}$ windowed device resulted in a promising high short-circuit current and energy conversion efficiency, the $\hbox{In}_{2}\hbox{S}_{3}$ windowed and conventional CdS windowed devices showed to be less efficient. We compared our theoretical analysis with the practical reports given for such structures in the literature. Further studies are required to reduce the reflection and absorption losses of CZTS structures in practice. So far, the overall performance parameters (e.g., $J_{{\rm sc}}$ and ${\eta}$) are much less than our theoretical estimations. The size of the ${\rm TiO}_{2}$ nanoparticles on the layer is about 100 nm.

Collisionless absorption, hot electron generation, and energy scaling in intense laser-target interaction

Abstract: Among the various attempts to understand collisionless absorption of intense ultrashort laser pulses a variety of models has been invented to describe the laser beam target interaction. In terms of basic physics collisionless absorption is understood now as the interplay of the oscillating laser field with the space charge field produced in the plasma. A first approach to this idea is realized in Brunel's model the essence of which consists in the formation of an oscillating charge cloud in the vacuum in front of the target. The investigation of statistical ensembles of orbits shows that the absorption process is localized at the ion-vacuum interface and in the skin layer: Single electrons enter into resonance with the laser field thereby undergoing a phase shift which causes orbit crossing and braking of Brunel's laminar flow. This anharmonic resonance acts like an attractor for the electrons and leads to the formation of a Maxwellian tail in the electron energy spectrum. Most remarkable results of our investigations are the Brunel-like hot electron distribution at the relativistic threshold; the minimum of absorption at $I\lambda^2 \cong (0.3-1.2)\times 10^{21}$ W/cm$^2\mu$m$^2$, in the plasma target with the electron density of $n_e \lambda^2\sim 10^{23}$cm$^{-3}\mu$m$^2;$ the drastic reduction of the number of hot electrons in this domain and their reappearance in the highly relativistic domain; strong coupling of the fast electron jets with the return current through Cherenkov emission of plasmons. The hot electron energy scaling shows a strong dependence on intensity in the moderately relativistic domain $I\lambda^2 \cong (10^{18} - 10^{20})$ W/cm$^2\mu$m$^2$, a scaling in vague accordance with current published estimates in the range $I\lambda^2 \cong (0.14-3.5)\times 10^{21}$ W/cm$^2\mu$m$^2$, and a distinct power increase beyond $I=3.5\times 10^{21}$ W/cm$^2\mu$m$^2$.

Structual study of Galactic hot gas toward Markarian 421 from X-ray absorption and emission lines

Abstract: We present a structural study of the hot ISM in the Galactic halo along the sight line toward the bright active galactic nucleus Mkn 421. The OVII and OVIII absorption lines were measured with the Low Energy Transmission Grating Spectrograph aboard Chandra toward Mkn 421, and the OVII and OVIII emission lines were observed in the adjacent fields of the sight line with the X-ray Imaging Spectrometer aboard Suzaku. We jointly analyzed the absorption and the emission spectra assuming exponential distributions of the gas temperature and density from the Galactic plane, and constrained the temperature and density at the plane to be $(3.2^{+0.6}_{-0.7})\times 10^6\,\mathrm{K}$ and $(1.2^{+0.5}_{-0.4})\times 10^{-3}\,\mathrm{cm^{-3}}$, with the scale heights of $1.6^{+1.7}_{-0.7}\,\mathrm{kpc}$ and $>2.8\,\mathrm{kpc}$ respectively. The results are consistent with those obtained in the LMC X-3 direction and the PKS 2155-304 direction, describing a thick disk-like hot gas with its height of a few kpc from the Galactic plane.

New accurate measurement of 36ArH+ and 38ArH+ ro-vibrational transitions by high resolution IR absorption spectroscopy

Abstract: The protonated Argon ion, $^{36}$ArH$^{+}$, has been identified recently in the Crab Nebula (Barlow et al. 2013) from Herschel spectra. Given the atmospheric opacity at the frequency of its $J$=1-0 and $J$=2-1 rotational transitions (617.5 and 1234.6 GHz, respectively), and the current lack of appropriate space observatories after the recent end of the Herschel mission, future studies on this molecule will rely on mid-infrared observations. We report on accurate wavenumber measurements of $^{36}$ArH$^{+}$ and $^{38}$ArH$^{+}$ rotation-vibration transitions in the $v$=1-0 band in the range 4.1-3.7 $\mu$m (2450-2715 cm$^{-1}$). The wavenumbers of the $R$(0) transitions of the $v$=1-0 band are 2612.50135$\pm$0.00033 and 2610.70177$\pm$0.00042 cm$^{-1}$ ($\pm3\sigma$) for $^{36}$ArH$^{+}$ and $^{38}$ArH$^{+}$, respectively. The calculated opacity for a gas thermalized at a temperature of 100 K and a linewidth of 1 km.s$^{-1}$ of the $R$(0) line is $1.6\times10^{-15}\times N$($^{36}$ArH$^+$). For column densities of $^{36}$ArH$^+$ larger than $1\times 10^{13}$ cm$^{-2}$, significant absorption by the $R$(0) line can be expected against bright mid-IR sources.