Showing papers on "Absorption (logic) published in 2016"

Observation of Anomalous Internal Pair Creation in Be 8 : A Possible Indication of a Light, Neutral Boson

Abstract: Electron-positron angular correlations were measured for the isovector magnetic dipole 17.6 MeV (J^{π}=1^{+}, T=1) state→ground state (J^{π}=0^{+}, T=0) and the isoscalar magnetic dipole 18.15 MeV (J^{π}=1^{+}, T=0) state→ground state transitions in ^{8}Be. Significant enhancement relative to the internal pair creation was observed at large angles in the angular correlation for the isoscalar transition with a confidence level of >5σ. This observation could possibly be due to nuclear reaction interference effects or might indicate that, in an intermediate step, a neutral isoscalar particle with a mass of 16.70±0.35(stat)±0.5(syst) MeV/c^{2} and J^{π}=1^{+} was created.

MINUTE-TIMESCALE >100 MeV γ-RAY VARIABILITY during the GIANT OUTBURST of QUASAR 3C 279 OBSERVED by FERMI-LAT in 2015 June

Abstract: On 2015 June 16, Fermi-LAT observed a giant outburst from the flat spectrum radio quasar 3C 279 with a peak $>100$ MeV flux of $\sim3.6\times10^{-5}\;{\rm photons}\;{\rm cm}^{-2}\;{\rm s}^{-1}$ averaged over orbital period intervals. It is the historically highest $\gamma$-ray flux observed from the source including past EGRET observations, with the $\gamma$-ray isotropic luminosity reaching $\sim10^{49}\;{\rm erg}\;{\rm s}^{-1}$. During the outburst, the Fermi spacecraft, which has an orbital period of 95.4 min, was operated in a special pointing mode to optimize the exposure for 3C 279. For the first time, significant flux variability at sub-orbital timescales was found in blazar observations by Fermi-LAT. The source flux variability was resolved down to 2-min binned timescales, with flux doubling times less than 5 min. The observed minute-scale variability suggests a very compact emission region at hundreds of Schwarzschild radii from the central engine in conical jet models. A minimum bulk jet Lorentz factor ($\Gamma$) of 35 is necessary to avoid both internal $\gamma$-ray absorption and super-Eddington jet power. In the standard external-radiation-Comptonization scenario, $\Gamma$ should be at least 50 to avoid overproducing the synchrotron-self-Compton component. However, this predicts extremely low magnetization ($\sim5\times10^{-4}$). Equipartition requires $\Gamma$ as high as 120, unless the emitting region is a small fraction of the dissipation region. Alternatively, we consider $\gamma$ rays originating as synchrotron radiation of $\gamma_{\rm e}\sim1.6\times10^6$ electrons, in magnetic field $B\sim1.3$ kG, accelerated by strong electric fields $E\sim B$ in the process of magnetoluminescence. At such short distance scales, one cannot immediately exclude production of $\gamma$ rays in hadronic processes.

Compton-thick Accretion in the local Universe

Abstract: Heavily obscured accretion is believed to represent an important stage in the growth of supermassive black holes, and to play an important role in shaping the observed spectrum of the Cosmic X-ray Background (CXB). Hard X-ray (E$>$10 keV) selected samples are less affected by absorption than samples selected at lower energies, and are therefore one of the best ways to detect and identify Compton-thick (CT, $\log N_{\rm\,H}\geq 24$) Active Galactic Nuclei (AGN). In this letter we present the first results of the largest broad-band (0.3-150 keV) X-ray spectral study of hard X-ray selected AGN to date, focusing on the properties of heavily obscured sources. Our sample includes the 834 AGN (728 non-blazar, average redshift $z\simeq 0.055$) reported in the 70-months catalog of the all-sky hard X-ray Swift/BAT survey. We find 55 CT AGN, which represent $7.6^{+1.1}_{-2.1}\%$ of our non-blazar sample. Of these, 26 are reported as candidate CT AGN for the first time. We correct for selection bias and derive the intrinsic column density distribution of AGN in the local Universe in two different luminosity ranges. We find a significant decrease in the fraction of obscured Compton-thin AGN for increasing luminosity, from $46\pm3\%$ (for $\log L_{\rm\,14-195} = 40-43.7$) to $39\pm3\%$ (for $\log L_{\rm\,14-195} = 43.7-46$). A similar trend is also found for CT AGN. The intrinsic fraction of CT AGN with $\log N_{\rm\,H}=24-25$ normalised to unity in the $\log N_{\rm H} = 20-25$ range is $27\pm4\%$, and is consistent with the observed value obtained for AGN located within 20 Mpc.

Are iris bombs connected to ellerman bombs

Abstract: Recent observations by the Interface Region Imaging Spectrograph (IRIS) have revealed pockets of hot gas ($\sim$2--8$\times$10$^{4}$ K) potentially resulting from magnetic reconnection in the partially ionized lower solar atmosphere (IRIS bombs; IBs). Using joint observations between IRIS and the Chinese New Vacuum Solar Telescope, we have identified ten IBs. We find that three are unambiguously and three others are possibly connected to Ellerman bombs (EBs), which show intense brightening of the extended H$_{\alpha}$ wings without leaving an obvious signature in the H$_{\alpha}$ core. These bombs generally reveal the following distinct properties: (1) The O~{\sc{iv}}~1401.156A and 1399.774A lines are absent or very weak; (2) The Mn~{\sc{i}}~2795.640A line manifests as an absorption feature superimposed on the greatly enhanced Mg~{\sc{ii}}~k line wing; (3) The Mg~{\sc{ii}}~k and h lines show intense brightening in the wings and no dramatic enhancement in the cores; (4) Chromospheric absorption lines such as Ni~{\sc{ii}}~1393.330A and 1335.203A are very strong; (5) The 1700A images obtained with the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory reveal intense and compact brightenings. These properties support the formation of these bombs in the photosphere, demonstrating that EBs can be heated much more efficiently than previously thought. We also demonstrate that the Mg~{\sc{ii}}~k and h lines can be used to investigate EBs similarly to H$_{\alpha}$, which opens a promising new window for EB studies. The remaining four IBs obviously have no connection to EBs and they do not have the properties mentioned above, suggesting a higher formation layer possibly in the chromosphere.

Pure negatively charged state of the NV center in n -type diamond

Abstract: Optical illumination on negatively charged nitrogen-vacancy $(\mathrm{N}{\mathrm{V}}^{\ensuremath{-}})$ centers inevitably causes stochastic charge-state transitions between the $\mathrm{N}{\mathrm{V}}^{\ensuremath{-}}$ and the neutral charge state of the NV center. It limits the steady-state population of $\mathrm{N}{\mathrm{V}}^{\ensuremath{-}}$ to 5% at minimum (\ensuremath{\sim}610 nm) and 80% (\ensuremath{\sim}532 nm) at maximum in intrinsic diamond depending on the wavelength. Here, we show Fermi-level control by phosphorus doping generates 99.4 \ifmmode\pm\else\textpm\fi{} 0.1% $\mathrm{N}{\mathrm{V}}^{\ensuremath{-}}$ under 1-\ensuremath{\mu}W and 593-nm excitation which is close to maximum absorption of $\mathrm{N}{\mathrm{V}}^{\ensuremath{-}}$. The pure $\mathrm{N}{\mathrm{V}}^{\ensuremath{-}}$ shows a fivefold increase in luminescence and a fourfold enhancement of an optically detected magnetic resonance under 593-nm excitation compared with those in intrinsic diamond.

GaN: From three- to two-dimensional single-layer crystal and its multilayer van der Waals solids

Abstract: Three-dimensional (3D) GaN is a III-V compound semiconductor with potential optoelectronic applications. In this paper, starting from 3D GaN in wurtzite and zinc-blende structures, we investigated the mechanical, electronic, and optical properties of the 2D single-layer honeycomb structure of GaN ($g\ensuremath{-}\mathrm{GaN}$) and its bilayer, trilayer, and multilayer van der Waals solids using density-functional theory. Based on high-temperature ab initio molecular-dynamics calculations, we first showed that $g\ensuremath{-}\mathrm{GaN}$ can remain stable at high temperature. Then we performed a comparative study to reveal how the physical properties vary with dimensionality. While 3D GaN is a direct-band-gap semiconductor, $g\ensuremath{-}\mathrm{GaN}$ in two dimensions has a relatively wider indirect band gap. Moreover, 2D $g\ensuremath{-}\mathrm{GaN}$ displays a higher Poisson ratio and slightly less charge transfer from cation to anion. In two dimensions, the optical-absorption spectra of 3D crystalline phases are modified dramatically, and their absorption onset energy is blueshifted. We also showed that the physical properties predicted for freestanding $g\ensuremath{-}\mathrm{GaN}$ are preserved when $g\ensuremath{-}\mathrm{GaN}$ is grown on metallic as well as semiconducting substrates. In particular, 3D layered blue phosphorus, being nearly lattice-matched to $g\ensuremath{-}\mathrm{GaN}$, is found to be an excellent substrate for growing $g\ensuremath{-}\mathrm{GaN}$. Bilayer, trilayer, and van der Waals crystals can be constructed by a special stacking sequence of $g\ensuremath{-}\mathrm{GaN}$, and they can display electronic and optical properties that can be controlled by the number of $g\ensuremath{-}\mathrm{GaN}$ layers. In particular, their fundamental band gap decreases and changes from indirect to direct with an increasing number of $g\ensuremath{-}\mathrm{GaN}$ layers.

Are we witnessing the epoch of reionisation at $z=7.1$ from the spectrum of J1120+0641?

Abstract: We quantify the presence of Ly\alpha\ damping wing absorption from a partially-neutral intergalactic medium (IGM) in the spectrum of the $z=7.08$ QSO, ULASJ1120+0641. Using a Bayesian framework, we simultaneously account for uncertainties in: (i) the intrinsic QSO emission spectrum; and (ii) the distribution of cosmic HI patches during the epoch of reionisation (EoR). For (i) we use a new intrinsic Ly\alpha\ emission line reconstruction method (Greig et al.), sampling a covariance matrix of emission line properties built from a large database of moderate-$z$ QSOs. For (ii), we use the Evolution of 21-cm Structure (EOS; Mesinger et al.) simulations, which span a range of physically-motivated EoR models. We find strong evidence for the presence of damping wing absorption redward of Ly\alpha\ (where there is no contamination from the Ly\alpha\ forest). Our analysis implies that the EoR is not yet complete by $z=7.1$, with the volume-weighted IGM neutral fraction constrained to $\bar{x}_{\rm H\,{\scriptsize I}} = 0.40\substack{+0.21 -0.19}$ at $1\sigma$ ($\bar{x}_{\rm H\,{\scriptsize I}} = 0.40\substack{+0.41 -0.32}$ at $2\sigma$). This result is insensitive to the EoR morphology. Our detection of significant neutral HI in the IGM at $z=7.1$ is consistent with the latest Planck 2016 measurements of the CMB Thompson scattering optical depth (Planck Collaboration XLVII).

Influence of halide composition on the structural, electronic, and optical properties of mixed CH 3 NH 3 Pb ( I 1 − x Br x ) 3 perovskites calculated using the virtual crystal approximation method

Abstract: Extensive studies have demonstrated the promising capability of the organic-inorganic hybrid halide perovskite ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}$ in solar cells with a high power conversion efficiency exceeding 20%. However, the intrinsic as well as extrinsic instabilities of this material remain the major challenge to the commercialization of perovskite-based solar cells. Mixing halides is expected to resolve this problem. Here, we investigate the effect of chemical substitution in the position of the halogen atom on the structural, electronic, and optical properties of mixed halide perovskites ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}\mathrm{Pb}{({\mathrm{I}}_{1\ensuremath{-}x}{\mathrm{Br}}_{x})}_{3}$ with a pseudocubic phase using the virtual crystal approximation method within density functional theory. With an increase of Br content $x$ from 0.0 to 1.0, the lattice constant decreases in proportion to $x$ with the function of $a(x)=6.420\ensuremath{-}0.333x$ (\AA{}), while the band gap and the exciton binding energy increase with the quadratic function of ${E}_{g}(x)=1.542+0.374x+0.185{x}^{2}$ (eV) and the linear function of ${E}_{b}(x)=0.045+0.057x$ (eV), respectively. The photoabsorption coefficients are also calculated, showing a blueshift of the absorption onsets for higher Br contents. We calculate the phase decomposition energy of these materials and analyze the electronic charge density difference to estimate the material stability. Based on the calculated results, we suggest that the best match between efficiency and stability can be achieved at $x\ensuremath{\approx}0.2$ in ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}\mathrm{Pb}{({\mathrm{I}}_{1\ensuremath{-}x}{\mathrm{Br}}_{x})}_{3}$ perovskites.

Theory of single photon detection by 'dirty' current-carrying superconducting strip based on the kinetic equation approach

Abstract: Using kinetic equation approach we study dynamics of electrons and phonons in current-carrying superconducting nanostrips after absorption of single photon of near-infrared or optical range. We find that the larger the ratio $C_e/C_{ph}|_{T_c}$ ($T_c$ is a critical temperature of superconductor, $C_e$ and $C_{ph}$ are specific heat capacities of electrons and phonons, respectively) the larger part of photon's energy goes to electrons, they become stronger heated and, hence, could thermalize faster during initial stage of hot spot formation. Thermalization time $\tau_{th}$ could be less than one picoseconds for superconductors with $C_e/C_{ph}|_{T_c}\gg 1$ and small diffusion coefficient $D\simeq 0.5 cm^2/s$ when thermalization occurs mainly due to electron-phonon and phonon-electron scattering in relatively small volume $\sim \xi^2d$ ($\xi$ is a superconducting coherence length, $d<\xi$ is a thickness of the strip). At larger times due to diffusion of hot electrons effective temperature inside the hot spot decreases, the size of hot spot increases, superconducting state becomes unstable and normal domain spreads in the strip at current larger than so-called detection current. We find dependence of detection current on the photon's energy, place of its absorption in the strip, width of the strip, magnetic field and compare it with existing experiments. Our results demonstrate that materials with $C_e/C_{ph}|_{T_c} \ll 1$ are bad candidates for single photon detectors due to small transfer of photon's energy to electronic system and large $\tau_{th}$. We also predict that even several microns wide dirty superconducting bridge is able to detect single near-infrared or optical photon if its critical current exceeds 70 $\%$ of depairing current and $C_e/C_{ph}|_{T_c} \gtrsim 1$.

Probing ultrafast \pi\pi*/n\pi* internal conversion in organic chromophores via K-edge resonant absorption

Abstract: Organic chromophores with heteroatoms possess an important excited state relaxation channel from an optically allowed {\pi}{\pi}* to a dark n{\pi}*state. We exploit the element and site specificity of soft x-ray absorption spectroscopy to selectively follow the electronic change during the {\pi}{\pi}*/n{\pi}* internal conversion. As a hole forms in the n orbital during {\pi}{\pi}*/n{\pi}* internal conversion, the near edge x-ray absorption fine structure (NEXAFS) spectrum at the heteroatom K-edge exhibits an additional resonance. We demonstrate the concept with the nucleobase thymine, a prototypical heteroatomic chromophore. With the help of time resolved NEXAFS spectroscopy at the oxygen K-edge, we unambiguously show that {\pi}{\pi}*/n{\pi}* internal conversion takes place within (60 \pm 30) fs. High-level coupled cluster calculations on the isolated molecules used in the experiment confirm the superb electronic structure sensitivity of this new method for excited state investigations.

Van Hove singularities and spectral smearing in high-temperature superconducting H 3 S

Abstract: The superconducting phase of hydrogen sulfide at ${T}_{c}=200$ K observed by Drozdov and collaborators at pressures around 200 GPa is simple bcc $Im\overline{3}m {\mathrm{H}}_{3}\mathrm{S}$ from a combination of theoretical and experimental confirmation. The various ``extremes'' that are involved---high pressure implying extreme reduction of volume, extremely high H phonon energy scale around 1400 K, extremely high temperature for a superconductor---necessitates a close look at new issues raised by these characteristics in relation to high ${T}_{c}$ itself. First principles methods are applied to analyze the ${\mathrm{H}}_{3}\mathrm{S}$ electronic structure, beginning with the effect of sulfur and then focusing on the origin and implications of the two van Hove singularities (vHs) providing an impressive peak in the density of states near the Fermi energy. Implications arising from strong coupling Migdal-Eliashberg theory are studied. It becomes evident that electron spectral density smearing due to virtual phonon emission and absorption must be accounted for in a correct understanding of this unusual material and to obtain accurate theoretical predictions. Means for increasing ${T}_{c}$ in ${\mathrm{H}}_{3}\mathrm{S}$-like materials are noted.

The existence of designs via iterative absorption

Abstract: In a recent breakthrough, Keevash proved the Existence conjecture for combinatorial designs, which has its roots in the 19th century. We give a new proof, based on the method of iterative absorption. Our main result concerns $K^{(r)}_{q}$-decompositions of hypergraphs whose clique distribution fulfils certain uniformity criteria. These criteria offer considerable flexibility. This enables us to strengthen the results of Keevash as well as to derive a number of new results, for example a resilience version and minimum degree version.

Many-electron effects on the dielectric function of cubic In 2 O 3 : Effective electron mass, band nonparabolicity, band gap renormalization, and Burstein-Moss shift

Abstract: We systematically investigate the influence of free-electron concentrations from $1.5\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ up to $1.6\ifmmode\times\else\texttimes\fi{}{10}^{21}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ on the optical properties of single-crystalline ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ in the cubic bixbyite structure. Dielectric functions of bulk crystals and epitaxial films on various substrates are determined by spectroscopic ellipsometry from the mid-infrared $(37\phantom{\rule{4pt}{0ex}}\mathrm{meV}\ensuremath{\approx}300\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1})$ into the ultraviolet $(6.5\phantom{\rule{4pt}{0ex}}\mathrm{eV})$ spectral region. Eight transverse optical phonon modes are resolvable for low carrier-density material. The analysis of the plasma frequencies yields effective electron masses which increase from a zero-density mass of ${m}^{*}=0.18{m}_{0}$ to $0.4{m}_{0}$ at $n={10}^{21}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. This mirrors the nonparabolicity of the conduction band being described by an analytical expression. The onset of absorption due to dipole-allowed interband transitions is found at $3.8\phantom{\rule{4pt}{0ex}}\mathrm{eV}$ for $n\ensuremath{\le}{10}^{19}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. It undergoes a blue-shift (effective Burstein-Moss shift) for higher electron densities as a result of the dominating phase-space filling compared to band gap renormalization. A comprehensive model describing the absorption onset is developed, taking nonparabolicity into account, yielding an accurate description and explanation of the observations. The agreement of modeled and measured absorption onset independently supports the effective electron masses derived from infrared data. The high-frequency dielectric constant of undoped ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ is found to be ${\ensuremath{\varepsilon}}_{\ensuremath{\infty}}=(4.08\ifmmode\pm\else\textpm\fi{}0.02)$.

The evolution of neutral gas in damped Lyman α systems from the XQ-100 survey

Abstract: We present a sample of 38 intervening Damped Lyman $\alpha$ (DLA) systems identified towards 100 $z>3.5$ quasars, observed during the XQ-100 survey. The XQ-100 DLA sample is combined with major DLA surveys in the literature. The final combined sample consists of 742 DLAs over a redshift range approximately $1.6 < z_{\rm abs} < 5.0$. We develop a novel technique for computing $\Omega_{\rm HI}^{\rm DLA}$ as a continuous function of redshift, and we thoroughly assess and quantify the sources of error therein, including fitting errors and incomplete sampling of the high column density end of the column density distribution function. There is a statistically significant redshift evolution in $\Omega_{\rm HI}^{\rm DLA}$ ($\geq 3 \sigma$) from $z \sim 2$ to $z \sim$ 5. In order to make a complete assessment of the redshift evolution of $\Omega_{\rm HI}$, we combine our high redshift DLA sample with absorption surveys at intermediate redshift and 21cm emission line surveys of the local universe. Although $\Omega_{\rm HI}^{\rm DLA}$, and hence its redshift evolution, remains uncertain in the intermediate redshift regime ($0.1 < z_{\rm abs} < 1.6$), we find that the combination of high redshift data with 21cm surveys of the local universe all yield a statistically significant evolution in $\Omega_{\rm HI}$ from $z \sim 0$ to $z \sim 5$ ($\geq 3 \sigma$). Despite its statistical significance, the magnitude of the evolution is small: a linear regression fit between $\Omega_{\rm HI}$ and $z$ yields a typical slope of $\sim$0.17$\times 10^{-3}$, corresponding to a factor of $\sim$ 4 decrease in $\Omega_{\rm HI}$ between $z=5$ and $z=0$.

Defect-induced Burstein-Moss shift in reduced V 2 O 5 nanostructures

Abstract: The main effects of oxygen vacancy defects on the electronic and optical properties of ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ nanowires were studied through in situ Raman, photoluminescence, absorption, and photoemission spectroscopy. Both thermal reduction and electrochemical reduction via lithium insertion leads to the creation of oxygen vacancy defects in the crystal that gives rise to new electronic midgap defect states at energy 0.75 eV below the conduction band edge. The defect formation results in delocalization and injection of excess electrons into the conduction band, as opposed to localized electron injection as previously suggested. Contrary to what is seen in most oxides, the presence of vacancy defects leads to band filling and an increase in the optical band gap of ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ from 1.95 to 2.45 eV, which is attributed to the Burstein-Moss effect. Other observed changes in the optical properties are correlated to the changes in the electronic structure of the oxide as a result of defect formation. Further, in situ Raman measurements during the electrochemical reduction at room temperature show that the oxygen atom that is most readily reduced is the threefold coordinated oxygen (O3).

Investigation of Electrical Characteristics in Al/CdS-PVA/p-Si (MPS) Structures Using Impedance Spectroscopy Method

Abstract: The cadmium sulfide (CdS) nanopowders have been prepared by ball-milling method, and CdS-polyvinyl alcohol (PVA) nanocompound in the form of film has been deposited on a p-Si wafer as an interfacial layer by spin-coating method. The impedance characteristics of the fabricated Al/CdS-PVA/p-Si (metal-polymer-semiconductor)-type structures were studied in the frequency and voltage range of 5 kHz–5 MHz and ±1 V, respectively, by considering interface states ( $D_{\mathrm {it}})$ , series resistance ( $R_{s})$ , and interfacial layer effects at 300 K. While the voltage and frequency dependence profiles of $D_{\mathrm {it}}$ were evaluated from the low-high frequency capacitance ( $C_{\mathrm {LF}}$ – $C_{\mathrm {HF}}$ ) and Hill-Coleman methods, $R_{s}$ profiles were evaluated from the Nicollian and Brews method. Doping concentration atoms ( $N_{A})$ and barrier height [ $\Phi _{B}$ (capacitance–voltage ( $C$ – $V$ ))] values were also obtained from the reverse bias $C^{-2}$ versus V plots for each frequency. While $D_{\mathrm {it}}$ and $R_{s}$ values decrease with increasing frequency almost exponentially, $\Phi _{B}$ ( $C$ – $V$ ) increases linearly. Therefore, both the measured capacitance ( $C_{m})$ and conductance ( $G_{m}/\omega $ ) values were corrected to eliminate the $R_{s}$ effect. The experimental results show that $R_{s}$ value is more effective on the impedance measurements at high frequencies in the accumulation region, but $D_{\mathrm {it}}$ is effective at low frequencies in the depletion region.

Absorption of very high energy gamma rays in the Milky Way

Abstract: Galactic gamma ray astronomy at very high energy (${E}_{\ensuremath{\gamma}}\ensuremath{\gtrsim}30\text{ }\text{ }\mathrm{TeV}$) is a vital tool in the study of the nonthermal universe. The interpretation of the observations in this energy region requires the precise modeling of the attenuation of photons due to pair production interactions ($\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}$) where the targets are the radiation fields present in interstellar space. For gamma rays with energy ${E}_{\ensuremath{\gamma}}\ensuremath{\gtrsim}300\text{ }\text{ }\mathrm{TeV}$ the attenuation is mostly due to the photons of the cosmic microwave background radiation. At lower energy the most important targets are infrared photons with wavelengths in the range $\ensuremath{\lambda}\ensuremath{\simeq}50--500\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ emitted by dust. The evaluation of the attenuation requires a good knowledge of the density, and energy and angular distributions of the target photons for all positions in the Galaxy. In this work we discuss a simple model for the infrared radiation that depends on only few parameters associated to the space and temperature distributions of the emitting dust. The model allows to compute with good accuracy the effects of absorption for any space and energy distribution of the diffuse Galactic gamma ray emission. The absorption probability due to the Galactic infrared radiation is maximum for ${E}_{\ensuremath{\gamma}}\ensuremath{\simeq}150\text{ }\text{ }\mathrm{TeV}$, and can be as large as ${P}_{\text{abs}}\ensuremath{\simeq}0.45$ for distant sources on lines of sight that pass close to the Galactic center. The systematic uncertainties on the absorption probability are estimated as $\mathrm{\ensuremath{\Delta}}{P}_{\text{abs}}\ensuremath{\lesssim}0.08$.

The spectral slope and escape fraction of bright quasars at z ∼ 3.8: the contribution to the cosmic UV background

Abstract: We use a sample of 1669 QSOs ($r<20.15$, $3.6

Steady flow of Burgers’ nanofluid over a stretching surface with heat generation/absorption

Abstract: Analysis has been conducted to analyze the steady free convection boundary layer flow of a Burgers’ nanofluid near a linear stretched sheet. The effects of the heat generation/absorption and nanoparticles on the flow are considered. Similarity transformations are presented to convert the coupled nonlinear partial differential equations into coupled nonlinear ordinary differential equations. The reduced coupled transformed boundary layer equations of Burgers’ nanofluid model are then solved analytically using the homotopy analysis method. The variations of the dimensionless velocity, temperature, and nanoparticle with various physical parameters, namely the Deborah numbers $$ \beta_{1} $$ , $$ \beta_{2}, $$ and $$ \beta_{3} $$ , the Prandtl number $$ \Pr $$ , the Brownian motion parameter $$ N_{\text{b}} $$ , the thermophoresis parameter $$ N_{\text{t}}, $$ and the Lewis number $$ {\text{Le}} $$ are graphed and discussed.

Investigation of the environment around close-in transiting exoplanets using cloudy

Abstract: It has been suggested that hot stellar wind gas in a bow shock around an exoplanet is sufficiently opaque to absorb stellar photons and give rise to an observable transit depth at optical and UV wavelengths. In the first part of this paper, we use the CLOUDY plasma simulation code to model the absorption from X-ray to radio wavelengths by 1-D slabs of gas in coronal equilibrium with varying densities ($10^{4}-10^{8} \, {\rm cm^{-3}}$) and temperatures ($2000-10^{6} \ {\rm K}$) illuminated by a solar spectrum. For slabs at coronal temperatures ($10^{6} \ {\rm K}$) and densities even orders of magnitude larger than expected for the compressed stellar wind ($10^{4}-10^{5} \, {\rm cm^{-3}}$), we find optical depths orders of magnitude too small ($> 3\times10^{-7}$) to explain the $\sim3\%$ UV transit depths seen with Hubble. Using this result and our modeling of slabs with lower temperatures ($2000-10^4 {\rm K}$), the conclusion is that the UV transits of WASP-12b and HD 189733b are likely due to atoms originating in the planet, as the stellar wind is too highly ionized. A corollary of this result is that transport of neutral atoms from the denser planetary atmosphere outward must be a primary consideration when constructing physical models. In the second part of this paper, additional calculations using CLOUDY are carried out to model a slab of planetary gas in radiative and thermal equilibrium with the stellar radiation field. Promising sources of opacity from the X-ray to radio wavelengths are discussed, some of which are not yet observed.

First-principles study of Ce 3 + -doped lanthanum silicate nitride phosphors: Neutral excitation, Stokes shift, and luminescent center identification

Abstract: We study from first principles two lanthanum silicate nitride compounds, ${\mathrm{LaSi}}_{3}{\mathrm{N}}_{5}$ and ${\mathrm{La}}_{3}{\mathrm{Si}}_{6}{\mathrm{N}}_{11}$, pristine as well as doped with ${\mathrm{Ce}}^{3+}$ ion, in view of explaining their different emission color, and characterizing the luminescent center. The electronic structures of the two undoped hosts are similar, and do not give a hint to quantitatively describe such difference. The $4f\ensuremath{\rightarrow}5d$ neutral excitation of the ${\mathrm{Ce}}^{3+}$ ions is simulated through a constrained density functional theory method coupled with a $\mathrm{\ensuremath{\Delta}}\mathrm{SCF}$ analysis of total energies, yielding absorption energies. Afterwards, atomic positions in the excited state are relaxed, yielding the emission energies and Stokes shifts. Based on these results, the luminescent centers in ${\mathrm{LaSi}}_{3}{\mathrm{N}}_{5}$:Ce and ${\mathrm{La}}_{3}{\mathrm{Si}}_{6}{\mathrm{N}}_{11}$:Ce are identified. The agreement with the experimental data for the computed quantities is quite reasonable and explains the different color of the emitted light. Also, the Stokes shifts are obtained within 20% difference relative to experimental data.

Aeronomical constraints to the minimum mass and maximum radius of hot low-mass planets

Abstract: Stimulated by the discovery of a number of close-in low-density planets, we generalise the Jeans escape parameter taking hydrodynamic and Roche lobe effects into account. We furthermore define $\Lambda$ as the value of the Jeans escape parameter calculated at the observed planetary radius and mass for the planet's equilibrium temperature and considering atomic hydrogen, independently of the atmospheric temperature profile. We consider 5 and 10 $M_{\oplus}$ planets with an equilibrium temperature of 500 and 1000 K, orbiting early G-, K-, and M-type stars. Assuming a clear atmosphere and by comparing escape rates obtained from the energy-limited formula, which only accounts for the heating induced by the absorption of the high-energy stellar radiation, and from a hydrodynamic atmosphere code, which also accounts for the bolometric heating, we find that planets whose $\Lambda$ is smaller than 15-35 lie in the "boil-off" regime, where the escape is driven by the atmospheric thermal energy and low planetary gravity. We find that the atmosphere of hot (i.e. $T_{\rm eq}\gtrapprox$ 1000 K) low-mass ($M_{\rm pl}\lessapprox$ 5 $M_{\oplus}$) planets with $\Lambda$ < 15-35 shrinks to smaller radii so that their $\Lambda$ evolves to values higher than 15-35, hence out of the boil-off regime, in less than $\approx$500 Myr. Because of their small Roche lobe radius, we find the same result also for hot (i.e. $T_{\rm eq}\gtrapprox$ 1000 K) higher mass ($M_{\rm pl}\lessapprox$ 10 $M_{\oplus}$) planets with $\Lambda$ < 15-35, when they orbit M-dwarfs. For old, hydrogen-dominated planets in this range of parameters, $\Lambda$ should therefore be $\geq$15-35, which provides a strong constraint on the planetary minimum mass and maximum radius and can be used to predict the presence of aerosols and/or constrain planetary masses, for example.

Strong interlayer coupling mediated giant two-photon absorption in MoS e 2 /graphene oxide heterostructure: Quenching of exciton bands

Abstract: A complex few-layer $\mathrm{MoS}{\mathrm{e}}_{2}$/graphene oxide (GO) heterostructure with strong interlayer coupling was prepared by a facile hydrothermal method. In this strongly coupled heterostructure, we demonstrate a giant enhancement of two-photon absorption that is in stark contrast to the reverse saturable absorption of a weakly coupled $\mathrm{MoS}{\mathrm{e}}_{2}$/GO heterostructure and saturable absorption of isolated $\mathrm{MoS}{\mathrm{e}}_{2}$. Spectroscopic evidence of our study indicates that the optical signatures of isolated $\mathrm{MoS}{\mathrm{e}}_{2}$ and GO domains are significantly modified in the heterostructure, displaying a direct coupling of both domains. Furthermore, our first-principles calculations indicate that strong interlayer coupling between the layers dramatically suppresses the $\mathrm{MoS}{\mathrm{e}}_{2}$ excitonic bands. We envision that our findings provide a powerful tool to explore different optical functionalities as a function of interlayer coupling, which may be essential for the development of device technologies.

The primordial deuterium abundance: subDLA system at zabs = 2.437 towards the QSO J 1444+2919

Abstract: We report a new detection of neutral deuterium in the sub Damped Lyman Alpha system with low metallicity [O/H]\,=\,$-2.042 \pm 0.005$ at $z_{\rm abs}=2.437$ towards QSO~J\,1444$+$2919. The hydrogen column density in this system is log$N$(H\,{\sc i})~$=19.983\pm0.010$ and the measured value of deuterium abundance is log(D/H)~$=-4.706\pm0.007_{\rm stat}\pm0.067_{\rm syst}$. This system meets the set of strict selection criteria stated recently by Cooke et al. and, therefore, widens the {\it Precision Sample} of D/H. However, possible underestimation of systematic errors can bring bias into the mean D/H value (especially if use a weighted mean). Hence, it might be reasonable to relax these selection criteria and, thus, increase the number of acceptable absorption systems with measured D/H values. In addition, an unweighted mean value might be more appropriate to describe the primordial deuterium abundance. The unweighted mean value of the whole D/H data sample available to date (15 measurements) gives a conservative value of the primordial deuterium abundance (D/H)$_{\rm p}=(2.55\pm 0.19)\times10^{-5}$ which is in good agreement with the prediction of analysis of the cosmic microwave background radiation for the standard Big Bang nucleosynthesis. By means of the derived (D/H)$_{\rm p}$ value the baryon density of the Universe $\Omega_{\rm b}h^2=0.0222\pm0.0013$ and the baryon-to-photon ratio $\eta_{10} = 6.09\pm 0.36$ have been deduced. These values have confident intervals which are less stringent than that obtained for the {\it Precision Sample} and, thus, leave a broader window for new physics. The latter is particularly important in the light of the lithium problem.

Optical absorption and conductivity in quasi-two-dimensional crystals from first principles: Application to graphene

Abstract: This paper gives a theoretical formulation of the electromagnetic response of the quasi-two-dimensional crystals suitable for investigation of optical activity and polariton modes. The response to external electromagnetic field is described by current-current response tensor ${\mathrm{\ensuremath{\Pi}}}_{\ensuremath{\mu}\ensuremath{ u}}$ calculated by solving the Dyson equation in the random phase approximation, where current-current interaction is mediated by the photon propagator ${D}_{\ensuremath{\mu}\ensuremath{ u}}$. The irreducible current-current response tensor ${\mathrm{\ensuremath{\Pi}}}_{\ensuremath{\mu}\ensuremath{ u}}^{0}$ is calculated from the ab initio Kohn-Sham orbitals. The accuracy of ${\mathrm{\ensuremath{\Pi}}}_{\ensuremath{\mu}\ensuremath{ u}}^{0}$ is tested in the long-wavelength limit where it gives correct Drude dielectric function and conductivity. The theory is applied to the calculation of optical absorption and conductivity in pristine and doped single-layer graphene and successfully compared with previous calculations and measurements.

Large Size Czochralski Growth and Scintillation Properties of $\text{Mg}^{2+}$ Co-doped $\text{Ce}:\text{Gd}_{3}\text{Ga}_{3}\text{Al}_{2}\text{O}_{12}$

Abstract: The 3 inch size Mg co-doped ${\rm Ce}:{\rm Gd}_{3} {\rm Al}_{2}{\rm Ga}_{3} {\rm O}_{12}$ single crystals were prepared by the Czochralski (Cz) method. Absorption and luminescence spectra were measured together with several other scintillation characteristics, namely the scintillation decay and light yield to reveal the effect of Mg co-doping. The timing resolution measurement for a pair of $3 \times 3 \times 3\;\hbox{mm}^3$ size GAGG:Ce.Mg scintillator crystals was performed using Si-PMs.

BANYAN. VIII. New Low-Mass Stars and Brown Dwarfs with Candidate Circumstellar Disks

Abstract: We present the results of a search for new circumstellar disks around low-mass stars and brown dwarfs with spectral types >K5 that are confirmed or candidate members of nearby young moving groups. Our search input sample was drawn from the BANYAN surveys of Malo et al. and Gagne et al. Two-Micron All-Sky Survey and Wide-field Infrared Survey Explorer data were used to detect near- to mid-infrared excesses that would reveal the presence of circumstellar disks. A total of 13 targets with convincing excesses were identified: four are new and nine were already known in the literature. The new candidates are 2MASS J05010082$-$4337102 (M4.5), J08561384$-$1342242 (M8$\,\gamma$), J12474428$-$3816464 (M9$\,\gamma$) and J02265658$-$5327032 (L0$\,\delta$), and are candidate members of the TW Hya ($\sim10\pm 3\,$Myr), Columba ($\sim 42^{+6}_{-4}\,$Myr) and Tucana-Horologium ($\sim 45\pm 4\,$Myr) associations, with masses of $120$ and $13-18\,M_{\mathrm{Jup}}$. The M8$-$L0 objects in Columba and Tucana-Horologium are potentially among the first substellar disk systems aged $\sim 40\,$Myr. Estimates of the new candidates' mean disk temperatures and fractional luminosities are in the ranges $\sim 135-520\,$K and $0.021-0.15$, respectively. New optical spectroscopy of J0501$-$4337 reveals strong H$\alpha$ emission, possibly indicating ongoing accretion, provides a detection of lithium absorption and a radial velocity measurement that is consistent with a membership to Columba. We also present a near-infrared spectrum of J0226$-$5327 that reveals Paschen $\beta$ emission and shows signs of low surface gravity, consistent with accretion from a disk and a young age.

Static magnetic proximity effect in Pt / Ni 1 − x Fe x bilayers investigated by x-ray resonant magnetic reflectivity

Abstract: We present x-ray resonant magnetic reflectivity (XRMR) as a very sensitive tool to detect proximity induced interface spin polarization in Pt/FM heterostructures. Different XRMR experiments are carried out and the results are evaluated for their dependence on the magneto-optical depth profile, the photon energy, the optical parameters, and the ferromagnetic material. We demonstrate that a detailed analysis of the reflected x-ray intensity gives insight into the spatial distribution of the spin polarization of a nonmagnetic metal across the interface to a ferromagnetic layer. The evaluation of the experimental results with simulations based on optical data from ab initio calculations provides the induced magnetic moment per Pt atom in the spin-polarized volume adjacent to the ferromagnet. For a series with different ferromagnetic materials consisting of Pt/Fe, $\text{Pt}/{\mathrm{Ni}}_{33}{\mathrm{Fe}}_{67}, \text{Pt}/{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ (permalloy), and Pt/Ni bilayers we find the largest spin polarization in Pt/Fe and a much smaller magnetic proximity effect in Pt/Ni. Additional XRMR experiments with varying photon energy are in good agreement with the theoretical predictions for the energy dependence of the magneto-optical parameters and allow identifying the optical dispersion $\ensuremath{\delta}$ and absorption $\ensuremath{\beta}$ across the $\mathrm{Pt}\phantom{\rule{0.28em}{0ex}}{\mathrm{L}}_{3}$-absorption edge.

Charge state dynamics of the nitrogen vacancy center in diamond under 1064-nm laser excitation

Abstract: The photophysics and charge state dynamics of the nitrogen vacancy (NV) center in diamond has been extensively investigated, but is still not fully understood. In contrast to previous work, we find that ${\mathrm{NV}}^{0}$ converts to ${\mathrm{NV}}^{\ensuremath{-}}$ under excitation with low power near-infrared (1064-nm) light, resulting in increased photoluminescence from the ${\mathrm{NV}}^{\ensuremath{-}}$ state. We used a combination of spectral and time-resolved photoluminescence experiments and rate-equation modeling to conclude that ${\mathrm{NV}}^{0}$ converts to ${\mathrm{NV}}^{\ensuremath{-}}$ via absorption of 1064-nm photons from the valence band of diamond. We report fast quenching and recovery of the photoluminescence from both charge states of the NV center under low power 1064-nm laser excitation, which has not been previously observed. We also find, using optically detected magnetic resonance experiments, that the charge transfer process mediated by the 1064-nm laser is spin dependent.

Precise limits on cosmological variability of the fine-structure constant with zinc and chromium quasar absorption lines

Abstract: The strongest transitions of Zn and CrII are the most sensitive to relative variations in the fine-structure constant ($\Delta\alpha/\alpha$) among the transitions commonly observed in quasar absorption spectra. They also lie within just 40A of each other (rest frame), so they are resistant to the main systematic error affecting most previous measurements of $\Delta\alpha/\alpha$: long-range distortions of the wavelength calibration. While Zn and CrII absorption is normally very weak in quasar spectra, we obtained high signal-to-noise, high-resolution echelle spectra from the Keck and Very Large Telescopes of 9 rare systems where it is strong enough to constrain $\Delta\alpha/\alpha$ from these species alone. These provide 12 independent measurements (3 quasars were observed with both telescopes) at redshifts 1.0--2.4, 11 of which pass stringent reliability criteria. These 11 are all consistent with $\Delta\alpha/\alpha=0$ within their individual uncertainties of 3.5--13 parts per million (ppm), with a weighted mean $\Delta\alpha/\alpha = 1.2\pm1.7_{\rm stat}\pm0.9_{\rm sys}$ ppm (1$\sigma$ statistical and systematic uncertainties), indicating no significant cosmological variations in $\alpha$. This is the first statistical sample of absorbers that is resistant to long-range calibration distortions (at the $<$1 ppm level), with a precision comparable to previous large samples of $\sim$150 (distortion-affected) absorbers. Our systematic error budget is instead dominated by much shorter-range distortions repeated across echelle orders of individual spectra.