Showing papers on "Absorption (logic) published in 2018"

The growth and entrainment of cold gas in a hot wind

Abstract: Both absorption and emission line studies show that cold gas around galaxies is commonly outflowing at speeds of several hundred km$\,\textrm{s}^{-1}$. This observational fact poses a severe challenge to our theoretical models of galaxy evolution since most feedback mechanisms (e.g., supernovae feedback) accelerate hot gas, and the timescale it takes to accelerate a blob of cold gas via a hot wind is much larger than the time it takes to destroy the blob. We revisit this long-standing problem using three-dimensional hydrodynamical simulations with radiative cooling. Our results confirm previous findings, that cooling is often not efficient enough to prevent the destruction of cold gas. However, we also identify regions of parameter space where the cooling efficiency of the mixed, `warm' gas is sufficiently large to contribute new comoving cold gas which can significantly exceed the original cold gas mass. This happens whenever, $t_{\mathrm{cool, mix}}/t_{\mathrm{cc}} < 1$, where $t_{\mathrm{cool,mix}}$ is the cooling time of the mixed warm gas and $t_{\mathrm{cc}}$ is the cloud-crushing time. This criterion is always satisfied for a large enough cloud. Cooling `focuses' stripped material onto the tail where mixing takes place and new cold gas forms. A sufficiently large simulation domain is crucial to capturing this behavior.

Shaken snow globes : kinematic tracers of the multiphase condensation cascade in massive galaxies, groups, and clusters.

Abstract: We propose a novel method to constrain turbulence and bulk motions in massive galaxies, galaxy groups, and clusters, exploring both simulations and observations. As emerged in the recent picture of top-down multiphase condensation, hot gaseous halos are tightly linked to all other phases in terms of cospatiality and thermodynamics. While hot halos (~107 K) are perturbed by subsonic turbulence, warm (~104 K) ionized and neutral filaments condense out of the turbulent eddies. The peaks condense into cold molecular clouds (<100 K) raining in the core via chaotic cold accretion (CCA). We show that all phases are tightly linked in terms of the ensemble (wide-aperture) velocity dispersion along the line of sight. The correlation arises in complementary long-term AGN feedback simulations and high-resolution CCA runs, and is corroborated by the combined Hitomi and new Integral Field Unit measurements in the Perseus cluster. The ensemble multiphase gas distributions (from the UV to the radio band) are characterized by substantial spectral line broadening (σ v,los ≈ 100–200 $\mathrm{km}\,{{\rm{s}}}^{-1}$) with a mild line shift. On the other hand, pencil-beam detections (as H i absorption against the AGN backlight) sample the small-scale clouds displaying smaller broadening and significant line shifts of up to several 100 $\mathrm{km}\,{{\rm{s}}}^{-1}$ (for those falling toward the AGN), with increased scatter due to the turbulence intermittency. We present new ensemble σ v,los of the warm Hα+[N ii] gas in 72 observed cluster/group cores: the constraints are consistent with the simulations and can be used as robust proxies for the turbulent velocities, in particular for the challenging hot plasma (otherwise requiring extremely long X-ray exposures). Finally, we show that the physically motivated criterion C ≡ t cool/t eddy ≈ 1 best traces the condensation extent region and the presence of multiphase gas in observed clusters and groups. The ensemble method can be applied to many available spectroscopic data sets and can substantially advance our understanding of multiphase halos in light of the next-generation multiwavelength missions.

21 cm limits on decaying dark matter and primordial black holes

Abstract: Recently the Experiment to Detect the Global Epoch of Reionization Signature reported the detection of a 21 cm absorption signal stronger than astrophysical expectations. In this paper we study the impact of radiation from dark matter (DM) decay and primordial black holes (PBHs) on the 21 cm radiation temperature in the reionization epoch, and impose a constraint on the decaying dark matter and PBH energy injection in the intergalactic medium, which can heat up neutral hydrogen gas and weaken the 21 cm absorption signal. We assume a strong coupling limit in the Lyman-$\ensuremath{\alpha}$ background and consider decay channels $\mathrm{DM}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}},\ensuremath{\gamma}\ensuremath{\gamma}$, ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$, ${\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$, $b\overline{b}$ and the $1{0}^{15--17}\text{ }\text{ }\mathrm{g}$ mass range for primordial black holes, and require that the heating of the neutral hydrogen does not negate the 21 cm absorption signal. For ${e}^{+}{e}^{\ensuremath{-}}$, $\ensuremath{\gamma}\ensuremath{\gamma}$ final states and PBH cases we find strong 21 cm bounds that can be more stringent than the current extragalactic diffuse photon bounds. For the $\mathrm{DM}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}$ channel, the lifetime bound is ${\ensuremath{\tau}}_{\mathrm{DM}}g{10}^{27}\text{ }\text{ }\mathrm{s}$ for sub-GeV dark matter. The bound is ${\ensuremath{\tau}}_{\mathrm{DM}}\ensuremath{\ge}{10}^{26}\text{ }\text{ }\mathrm{s}$ for the sub-GeV $\mathrm{DM}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$ channel and reaches $1{0}^{27}\text{ }\text{ }\mathrm{s}$ for MeV DM. For $b\overline{b}$ and ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ cases, the 21 cm constraint is better than all the existing constraints for ${m}_{\mathrm{DM}}l30\text{ }\text{ }\mathrm{GeV}$ where the bound on ${\ensuremath{\tau}}_{\mathrm{DM}}\ensuremath{\ge}{10}^{26}\text{ }\text{ }\mathrm{s}$. For both DM decay and primordial black hole cases, the 21 cm bounds significantly improve over the cosmic microwave background damping limits from Planck data.

Modeling the Radio Background from the First Black Holes at Cosmic Dawn: Implications for the 21 cm Absorption Amplitude

Abstract: We estimate the 21 cm Radio Background from accretion onto the first intermediate-mass Black Holes between $z\approx 30$ and $z\approx 16$. Combining potentially optimistic, but plausible, scenarios for black hole formation and growth with empirical correlations between luminosity and radio-emission observed in low-redshift active galactic nuclei, we find that a model of black holes forming in molecular cooling halos is able to produce a 21 cm background that exceeds the Cosmic Microwave Background (CMB) at $z \approx 17$ though models involving larger halo masses are not entirely excluded. Such a background could explain the surprisingly large amplitude of the 21 cm absorption feature recently reported by the EDGES collaboration. Such black holes would also produce significant X-ray emission and contribute to the $0.5-2$ keV soft X-ray background at the level of $\approx 10^{-13}-10^{-12}$ erg sec$^{-1}$ cm$^{-2}$ deg$^{-2}$, consistent with existing constraints. In order to avoid heating the IGM over the EDGES trough, these black holes would need to be obscured by Hydrogen column depths of $ N_\text{H} \sim 5 \times 10^{23} \text{cm}^{-2}$. Such black holes would avoid violating contraints on the CMB optical depth from Planck if their UV photon escape fractions were below $f_{\text{esc}} \lesssim 0.1$, which would be a natural result of $N_\text{H} \sim 5 \times 10^{23} \text{cm}^{-2}$ imposed by an unheated IGM.

Optical properties and gamma-shielding features of bismuth borate glasses

Abstract: In the present work, six bismuth borate glass samples with chemical formula (75−x)B2O3–xBi2O3–10CaO–10Na2O–5Al2O3, (x = 0.0–25.0 mol%) have been manufactured. UV–visible spectral distributions for the investigated system were measured in the range of 250–850 nm. The optical absorption data showed the existence of allowed indirect transitions. The optical energy gap values ( $$E_{{\text{g}}}^{{{\text{opt}}.}}$$ ) via Tauc’s model vary from 3.39 to 2.45 eV. The complex dielectric constant ( $$\hat {\varepsilon }$$ ) was evaluated. All the optical parameters are found to be sensitive to the concentration of Bi2O3. Also, gamma photons’ attenuation features such as the mass attenuation coefficient (µ/ρ), gamma photon transmission function (I/Io), effective atomic number (Zeff), and half value thickness (HVT) for the investigated glass samples were estimated using MCNPX code and XCOM program in the range of 0.356–1.33 MeV. Replacement of B2O3 by Bi2O3 enhances the ability of glass samples to attenuate gamma photons. The results revealed that the studied glasses can be considered as candidate for optical fiber, devices, and gamma shieling.

Constraining Primordial Black Holes with the EDGES 21-cm Absorption Signal : arXiv

Abstract: The EDGES experiment has recently measured an anomalous global 21-cm spectrum due to hydrogen absorptions at redshifts of about $z\ensuremath{\sim}17$. Model independently, the unusually low temperature of baryons probed by this observable sets strong constraints on any physical process that transfers energy into the baryonic environment at such redshifts. Here, we make use of the 21-cm spectrum to derive bounds on the energy injection due to a possible population of $\mathcal{O}(1--100){M}_{\ensuremath{\bigodot}}$ primordial black holes, which induce a wide spectrum of radiation during the accretion of the surrounding gas. After calculating the total radiative intensity of a primordial black hole population, we estimate the amount of heat and ionizations produced in the baryonic gas and compute the resulting thermal history of the Universe with a modified version of RECFAST code. Finally, by imposing that the temperature of the gas at $z\ensuremath{\sim}17$ does not exceed the indications of EDGES, we constrain the possible abundance of primordial black holes. Depending on uncertainties related to the accretion model, we find that $\mathcal{O}(10){M}_{\ensuremath{\bigodot}}$ primordial black holes can only contribute to a fraction ${f}_{\mathrm{PBH}}l(1--{10}^{\ensuremath{-}3})$ of the total dark matter abundance.

Charge-Spin Correlation in van der Waals Antiferromagnet NiPS 3

Abstract: Strong charge-spin coupling is found in a layered transition-metal trichalcogenide ${\mathrm{NiPS}}_{3}$, a van der Waals antiferromagnet, from studies of the electronic structure using several experimental and theoretical tools: spectroscopic ellipsometry, x-ray absorption, photoemission spectroscopy, and density functional calculations. ${\mathrm{NiPS}}_{3}$ displays an anomalous shift in the optical spectral weight at the magnetic ordering temperature, reflecting strong coupling between the electronic and magnetic structures. X-ray absorption, photoemission, and optical spectra support a self-doped ground state in ${\mathrm{NiPS}}_{3}$. Our work demonstrates that layered transition-metal trichalcogenide magnets are useful candidates for the study of correlated-electron physics in two-dimensional magnetic materials.

Two-dimensional n -InSe/ p -GeSe(SnS) van der Waals heterojunctions: High carrier mobility and broadband performance

Abstract: Recently, constructing van der Waals (vdW) heterojunctions by stacking different two-dimensional (2D) materials has been considered to be effective strategy to obtain the desired properties. Here, through first-principles calculations, we find theoretically that the 2D $n$-InSe/$p$-GeSe(SnS) vdW heterojunctions are the direct-band-gap semiconductor with typical type-II band alignment, facilitating the effective separation of photogenerated electron and hole pairs. Moreover, they possess the high optical absorption strength ($\ensuremath{\sim}{10}^{5}$), broad spectrum width, and excellent carrier mobility ($\ensuremath{\sim}{10}^{3}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$). Interestingly, under the influences of the interlayer coupling and external electric field, the characteristics of type-II band alignment is robust, while the band-gap values and band offset are tunable. These results indicate that 2D $n$-InSe/$p$-GeSe(SnS) heterojunctions possess excellent optoelectronic and transport properties, and thus can become good candidates for next-generation optoelectronic nanodevices.

Constraining noncold dark matter models with the global 21-cm signal

Abstract: Any particle dark matter (DM) scenario featuring a suppressed power spectrum of astrophysical relevance results in a delay of galaxy formation. As a consequence, such scenarios can be constrained using the global 21-cm absorption signal initiated by the UV radiation of the first stars. The Experiment to Detect the Global Epoch of Reionization Signature (EDGES) recently reported the first detection of such an absorption signal at redshift $\ensuremath{\sim}17$. While its amplitude might indicate the need for new physics, we solely focus on the timing of the signal to test noncold DM models. Assuming conservative limits for the stellar-to-baryon fraction (${f}_{*}l0.03$) and for the minimum cooling temperature (${T}_{\mathrm{vir}}g{10}^{3}$ Kelvin) motivated by radiation-hydrodynamic simulations, we are able to derive unprecedented constraints on a variety of noncold DM models. For example, the mass of thermal warm DM is limited to ${m}_{\mathrm{TH}}g6.1\text{ }\text{ }\mathrm{keV}$, while mixed DM scenarios (featuring a cold and a hot component) are constrained to a hot DM fraction below 17 percent. The ultralight axion DM model is limited to masses ${m}_{a}g8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}\text{ }\text{ }\mathrm{eV}$, a regime where its wave-like nature is pushed far below the kiloparsec scale. Finally, sterile neutrinos from resonant production can be fully disfavored as a dominant DM candidate. The results of this paper show that the 21-cm absorption signal is a powerful discriminant of noncold dark matter, allowing for significant improvements over to the strongest current limits. Confirming the result from EDGES is paramount in this context.

Interlayer and intralayer excitons in MoS 2 / WS 2 and MoSe 2 / WSe 2 heterobilayers

Abstract: Accurately described excitonic properties of transition metal dichalcogenide heterobilayers (HBLs) are crucial to comprehend the optical response and the charge carrier dynamics of them. Excitons in multilayer systems possess an inter- or intralayer character whose spectral positions depend on their binding energy and the band alignment of the constituent single layers. In this paper, we report the electronic structure and the absorption spectra of ${\mathrm{MoS}}_{2}/{\mathrm{WS}}_{2}$ and ${\mathrm{MoSe}}_{2}/{\mathrm{WSe}}_{2}$ HBLs from first-principles calculations. We explore the spectral positions, binding energies, and the origins of inter- and intralayer excitons and compare our results with experimental observations. The absorption spectra of the systems are obtained by solving the Bethe-Salpeter equation on top of a ${\mathrm{G}}_{0}{\mathrm{W}}_{0}$ calculation, which corrects the independent-particle eigenvalues obtained from density-functional theory. Our calculations reveal that the lowest energy exciton in both HBLs possess an interlayer character which is decisive regarding their possible device applications. Due to the spatially separated nature of the charge carriers, the binding energy of interlayer excitons might be expected to be considerably smaller than that of intralayer ones. However, according to our calculations, the binding energy of lowest energy interlayer excitons is only $\ensuremath{\sim}20%$ lower due to the weaker screening of the Coulomb interaction between layers of the HBLs. Therefore, it can be deduced that the spectral positions of the interlayer excitons with respect to intralayer ones are mostly determined by the band offset of the constituent single layers. By comparing oscillator strengths and thermal occupation factors, we show that in luminescence at low temperature, the interlayer exciton peak becomes dominant, while in absorption it is almost invisible.

Sub-Doppler Frequency Metrology in HD for Tests of Fundamental Physics.

Abstract: Weak transitions in the (2,0) overtone band of the hydrogen deuteride molecule at $\ensuremath{\lambda}=1.38\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ were measured in saturated absorption using the technique of noise-immune cavity-enhanced optical heterodyne molecular spectroscopy. Narrow Doppler-free lines were interrogated with a spectroscopy laser locked to a frequency comb laser referenced to an atomic clock to yield transition frequencies [$R(1)=217105181895(20)\text{ }\text{ }\mathrm{kHz}$; $R(2)=219042856621(28)\text{ }\text{ }\mathrm{kHz}$; $R(3)=220704304951(28)\text{ }\text{ }\mathrm{kHz}$] at three orders of magnitude improved accuracy. These benchmark values provide a test of QED in the smallest neutral molecule, and they open up an avenue to resolve the proton radius puzzle, as well as constrain putative fifth forces and extra dimensions.

Unveiling the mechanisms of the spin Hall effect in Ta

Abstract: Spin-to-charge current interconversions are widely exploited for the generation and detection of pure spin currents and are key ingredients for future spintronic devices including spin-orbit torques and spin-orbit logic circuits. In the case of the spin Hall effect, different mechanisms contribute to the phenomenon and determining the leading contribution is peremptory for achieving the largest conversion efficiencies. Here, we experimentally demonstrate the dominance of the intrinsic mechanism of the spin Hall effect in highly resistive Ta. We obtain an intrinsic spin Hall conductivity for \ensuremath{\beta}-Ta of $\ensuremath{-}820\ifmmode\pm\else\textpm\fi{}120(\ensuremath{\hbar}/e)\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ from spin absorption experiments in a large set of lateral spin valve devices. The predominance of the intrinsic mechanism in Ta allows us to linearly enhance the spin Hall angle by tuning the resistivity of Ta, reaching up to \ensuremath{-}35 \ifmmode\pm\else\textpm\fi{} 3%, the largest reported value for a pure metal.

Optical Absorption and Emission Mechanisms of Single Defects in Hexagonal Boron Nitride

Abstract: We investigate the polarization selection rules of sharp zero-phonon lines (ZPLs) from isolated defects in hexagonal boron nitride (HBN) and compare our findings with the predictions of a Huang-Rhys model involving two electronic states. Our survey, which spans the spectral range $\ensuremath{\sim}550--740\text{ }\text{ }\mathrm{nm}$, reveals that, in disagreement with a two-level model, the absorption and emission dipoles are often misaligned. We relate the dipole misalignment angle ($\mathrm{\ensuremath{\Delta}}\ensuremath{\theta}$) of a ZPL to its energy shift from the excitation energy ($\mathrm{\ensuremath{\Delta}}E$) and find that $\mathrm{\ensuremath{\Delta}}\ensuremath{\theta}\ensuremath{\approx}0\ifmmode^\circ\else\textdegree\fi{}$ when $\mathrm{\ensuremath{\Delta}}E$ corresponds to an allowed HBN phonon frequency and that $0\ifmmode^\circ\else\textdegree\fi{}\ensuremath{\le}\mathrm{\ensuremath{\Delta}}\ensuremath{\theta}\ensuremath{\le}90\ifmmode^\circ\else\textdegree\fi{}$ when $\mathrm{\ensuremath{\Delta}}E$ exceeds the maximum allowed HBN phonon frequency. Consequently, a two-level Huang-Rhys model succeeds at describing excitations mediated by the creation of one optical phonon but fails at describing excitations that require the creation of multiple phonons. We propose that direct excitations requiring the creation of multiple phonons are inefficient due to the low Huang-Rhys factors in HBN and that these ZPLs are instead excited indirectly via an intermediate electronic state. This hypothesis is corroborated by polarization measurements of an individual ZPL excited with two distinct wavelengths that indicate a single ZPL may be excited by multiple mechanisms. These findings provide new insight on the nature of the optical cycle of novel defect-based single-photon sources in HBN.

The Near-Infrared Transmission Spectra of TRAPPIST-1 Planets b, c, d, e, f, and g and Stellar Contamination in Multi-Epoch Transit Spectra

Abstract: The seven approximately Earth-sized transiting planets in the \object{TRAPPIST-1} system provide a unique opportunity to explore habitable zone and non-habitable zone small planets within the same system. Its habitable zone exoplanets -- due to their favorable transit depths -- are also worlds for which atmospheric transmission spectroscopy is within reach with the Hubble Space Telescope (HST) and with the James Webb Space Telescope (JWST). We present here an independent reduction and analysis of two \textit{HST} Wide Field Camera 3 (WFC3) near-infrared transit spectroscopy datasets for six planets (b through g). Utilizing our physically-motivated detector charge trap correction and a custom cosmic ray correction routine, we confirm the general shape of the transmission spectra presented by \textbf{\citet{deWit2016, deWit2018}}. Our data reduction approach leads to a 25\% increase in the usable data and reduces the risk of confusing astrophysical brightness variations (e.g., flares) with instrumental systematics. No prominent absorption features are detected in any individual planet's transmission spectra; by contrast, the combined spectrum of the planets shows a suggestive decrease around 1.4\,$\micron$ similar to an inverted water absorption feature. Including transit depths from \textit{K2}, the SPECULOOS-South Observatory, and \textit{Spitzer}, we find that the complete transmission spectrum is fully consistent with stellar contamination owing to the transit light source effect. These spectra demonstrate how stellar contamination can overwhelm planetary absorption features in low-resolution exoplanet transit spectra obtained by \textit{HST} and \textit{JWST} and also highlight the challenges in combining multi epoch observations for planets around rapidly rotating spotted stars.

Orbital Angular Momentum Transfer to Stably Trapped Elastic Particles in Acoustical Vortex Beams.

Abstract: The controlled rotation of solid particles trapped in a liquid by an ultrasonic vortex beam is observed. Single polystyrene beads, or clusters, can be trapped against gravity while simultaneously rotated. The induced rotation of a single particle is compared to a torque balance model accounting for the acoustic response of the particle. The measured torque ($\ensuremath{\sim}10\text{ }\text{ }\mathrm{pN}\text{ }\mathrm{m}$ for a driving acoustic power $\ensuremath{\sim}40\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$) suggests two dominating dissipation mechanisms of the acoustic orbital angular momentum responsible for the observed rotation. The first takes place in the bulk of the absorbing particle, while the second arises as dissipation in the viscous boundary layer in the surrounding fluid. Importantly, the dissipation processes affect both the dipolar and quadrupolar particle vibration modes suggesting that the restriction to the well-known Rayleigh scattering regime is invalid to model the total torque even for spheres much smaller than the sound wavelength. The findings show that a precise knowledge of the probe elastic absorption properties is crucial to perform rheological measurements with maneuverable trapped spheres in viscous liquids. Further results suggest that the external rotational steady flow must be included in the balance and can play an important role in other liquids.

Diffuse Galactic gamma ray flux at very high energy

Abstract: The observation of the diffuse Galactic gamma-ray flux is the most powerful tool to study cosmic rays in different regions of the Galaxy, because the energy and angular distributions of the photons encode information about the density and spectral shape of relativistic particles in the entire Milky Way. An open problem of fundamental importance is whether cosmic rays in distant regions of the Milky Way have the same spectral shape observed at the Earth or not. If the spectral shape of protons and nuclei is equal in all the Galaxy, the dominant, hadronic component of the diffuse gamma-ray flux must have an angular distribution that, after correcting for absorption effects, is energy independent. To study experimentally the validity of this factorization of the energy and angular dependence of the diffuse flux it is necessary to compare observations in a very broad energy range. The extension of the observations to energies ${E}_{\ensuremath{\gamma}}\ensuremath{\simeq}0.1--10\text{ }\text{ }\mathrm{PeV}$ is of great interest, because it allows the study of the cosmic ray spectra around the feature known as the ``knee.'' The absorption probability for photons in this energy range is not negligible, and distorts the energy and angular distributions of the diffuse flux, therefore a precise calculation of the absorption effects is necessary for the interpretation of the data. In this work we present predictions of the diffuse gamma-ray flux at very high energy, constructed under different hypothesis for the space dependence of the cosmic ray energy spectra, and discuss the potential of the observations for present and future detectors.

Implementation of the infinite-range exterior complex scaling to the time-dependent complete-active-space self-consistent-field method

Abstract: We present a numerical implementation of the infinite-range exterior complex scaling [Scrinzi, Phys. Rev. A 81, 053845 (2010)] as an efficient absorbing boundary to the time-dependent complete-active-space self-consistent field method [Sato, Ishikawa, B\ifmmode \check{r}\else \v{r}\fi{}ezinov\'a, Lackner, Nagele, and Burgd\"orfer, Phys. Rev. A 94, 023405 (2016)] for multielectron atoms subject to an intense laser pulse. We introduce Gauss-Laguerre-Radau quadrature points to construct discrete variable representation basis functions in the last radial finite element extending to infinity. This implementation is applied to strong-field ionization and high-harmonic generation in He, Be, and Ne atoms. It efficiently prevents unphysical reflection of photoelectron wave packets at the simulation boundary, enabling accurate simulations with substantially reduced computational cost, even under significant ($\ensuremath{\approx}50%$) double ionization. For the case of a simulation of high-harmonic generation from Ne, for example, 80% cost reduction is achieved, compared to a mask-function absorption boundary.

Hydrogen and Sodium Absorption in the Optical Transmission Spectrum of WASP-12b

Abstract: We have obtained $> 10$ hours of medium resolution ($R \sim 15000$) spectroscopic exposures on the transiting exoplanet host star WASP-12, including $\sim2$ hours while its planet, WASP-12b, is in transit, with the Hobby-Eberly Telescope (HET). The out-of-transit and in-transit spectra are coadded into master out-of-transit and in-transit spectra, from which we create a master transmission spectrum. Strong, statistically significant absorption features are seen in the transmission spectrum at H$\alpha$ and \ion{Na}{1} (the Na D doublet). There is the suggestion of pre- and post-transit absorption in both H$\alpha$ and \ion{Na}{1} when the transmission spectrum is examined as a function of phase. The timing of the pre-transit absorption is roughly consistent with previous results for metal absorption in WASP-12b, and the level of the \ion{Na}{1} absorption is consistent with a previous tentative detection. No absorption is seen in the control line of \ion{Ca}{1} at $\lambda$6122. We discuss in particular whether or not the WASP-12b H$\alpha$ absorption signal is of circumplanetary origin---an interpretation that is bolstered by the pre- and post-transit evidence---which would make it one of only a small number of detections of circumplanetary H$\alpha$ absorption in an exoplanet to date, the most well-studied being HD 189733b. We further discuss the notable differences between the HD 189733 and WASP-12 systems, and the implications for a physical understanding of the origin of the absorption.

Silicon-Based Optical Mirror Coatings for Ultrahigh Precision Metrology and Sensing.

Abstract: Thermal noise of highly reflective mirror coatings is a major limit to the sensitivity of many precision laser experiments with strict requirements such as low optical absorption. Here, we investigate amorphous silicon and silicon nitride as an alternative to the currently used combination of coating materials, silica, and tantala. We demonstrate an improvement by a factor of $\ensuremath{\approx}55$ with respect to the lowest so far reported optical absorption of amorphous silicon at near-infrared wavelengths. This reduction was achieved via a combination of heat treatment, final operation at low temperature, and a wavelength of $2\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ instead of the more commonly used 1550 nm. Our silicon-based coating offers a factor of 12 thermal noise reduction compared to the performance possible with silica and tantala at 20 K. In gravitational-wave detectors, a noise reduction by a factor of 12 corresponds to an increase in the average detection rate by three orders of magnitude ($\ensuremath{\approx}{12}^{3}$).

Role of vacancies in the high-temperature pseudodisplacive phase transition in GeTe

Abstract: To date, the high-temperature phase transition R3m$\text{\ensuremath{-}}Fm\phantom{\rule{0.83328pt}{0ex}}\overline{3}\phantom{\rule{0.83328pt}{0ex}}m$ in GeTe is commonly believed to be second-order displacive with an anomalous volume contraction at the phase transition temperature, ${T}_{PT}$, from diffraction measurements. Three main results are here reported: (i) the phase transition is accompanied by latent heat absorption, (ii) in the high-resolution x-ray-diffraction powder pattern the cubic phase appears before the merging of the rhombohedral peaks, and (iii) the cubic phase possesses a larger amount of germanium vacancies than the rhombohedral phase. From results (i)--(iii) we conclude that the phase transition is not purely second-order displacive and that the volume contraction observed at ${T}_{PT}$ can be explained with the formation of vacancies in the cubic phase.

First Spectroscopic Study of a Young Quasar

Abstract: The quasar lifetime $t_{\rm\,Q}$ is one of the most fundamental quantities for understanding quasar evolution and the growth of supermassive black holes (SMBHs), but remains uncertain by several orders of magnitude. In a recent study we uncovered a population of very young quasars ($t_{\rm Q}\lesssim10^4-10^5$ yr), based on the sizes of their proximity zones, which are regions of enhanced Ly$\alpha$ forest transmission near the quasar resulting from its own ionizing radiation. The presence of such young objects poses significant challenges to models of SMBH formation, which already struggle to explain the existence of SMBHs at such early cosmic epochs. We conduct the first comprehensive spectroscopic study of the youngest quasar known, $\rm SDSS\,J1335+3533$ at $z=5.9012$, whose lifetime is $t_{\rm Q}<10^4$ yr ($95\%$ confidence). A careful search of our deep optical and near-infrared spectra for HI and metal absorption lines allows us to convincingly exclude that its small proximity zone results from an associated absorption system rather than a short lifetime. We use the MgII emission line to measure its black hole mass $M_{\rm BH}=(4.09\pm0.58)\times 10^9 M_{\odot}$, implying an Eddington ratio of $0.30\pm0.04$ -- comparable to other co-eval quasars. We similarly find that the relationship between its black hole mass and dynamical mass are consistent with other $z\sim6$ quasars. The only possible anomaly associated with youth are its weak emission lines, but larger samples are needed to shed light on a potential causal connection. We discuss the implications of short lifetimes for various SMBH growth scenarios, and argue that future observations of young quasars with JWST could distinguish between them.

Large disparity between optical and fundamental band gaps in layered In 2 Se 3

Abstract: ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ is a semiconductor material that can be stabilized in different crystal structures (at least one 3D and several 2D layered structures have been reported) with diverse electrical and optical properties. This feature has plagued its characterization over the years, with reported band gaps varying in an unacceptable range of 1 eV. Using first-principles calculations based on density functional theory and the HSE06 hybrid functional, we investigate the structural and electronic properties of four layered phases of ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$, addressing their relative stability and the nature of their fundamental band gaps, i.e., direct versus indirect. Our results show large disparities between fundamental and optical gaps. The absorption coefficients are found to be as high as those in direct-gap III-V semiconductors. The band alignment with respect to conventional semiconductors indicate a tendency to $n$-type conductivity, explaining recent experimental observations.

Efficient orbital angular momentum transfer between plasmons and free electrons

Abstract: Free electrons can efficiently absorb or emit plasmons excited in a thin conductor, giving rise to multiple energy peaks in the transmitted electron spectra separated by multiples of the plasmon energy. When the plasmons are chiral, this can also give rise to transfer of orbital angular momentum (OAM). Here, we show that large amounts of OAM can be efficiently transferred between chiral plasmons supported by a thin film and free electrons traversing it. Under realistic conditions, our predictive simulations reveal efficient absorption of a number $\ensuremath{\ell}\ensuremath{\gg}1$ of chiral plasmons of vorticity $m\ensuremath{\gg}1$, resulting in an OAM transfer $\ensuremath{\ell}m\ensuremath{\hbar}\ensuremath{\gg}\ensuremath{\hbar}$. Our work supports the use of chiral plasmons sustained by externally illuminated thin films as a way of generating high-vorticity electrons, resulting in a remarkably large fraction of kinetic energy associated with motion along the azimuthal direction, perpendicular to the incident beam.

Excitonic structure of the optical conductivity in MoS 2 monolayers

Abstract: We investigate the excitonic spectrum of ${\mathrm{MoS}}_{2}$ monolayers and calculate its optical absorption properties over a wide range of energies. Our approach takes into account the anomalous screening in two dimensions and the presence of a substrate, both cast by a suitable effective Keldysh potential. We solve the Bethe-Salpeter equation using as a basis a Slater-Koster tight-binding model parameterized to fit the ab initio ${\mathrm{MoS}}_{2}$ band structure calculations. The resulting optical conductivity is in good quantitative agreement with existing measurements up to ultraviolet energies. We establish that the electronic contributions to the $C$ excitons arise not from states at the $\mathrm{\ensuremath{\Gamma}}$ point, but from a set of $\mathbit{k}$ points over extended portions of the Brillouin zone. Our results reinforce the advantages of approaches based on effective models to expeditiously explore the properties and tunability of excitons in TMD systems.

Interference of Single Photons Emitted by Entangled Atoms in Free Space

Abstract: The generation and manipulation of entanglement between isolated particles has precipitated rapid progress in quantum information processing. Entanglement is also known to play an essential role in the optical properties of atomic ensembles, but fundamental effects in the controlled emission and absorption from small, well-defined numbers of entangled emitters in free space have remained unobserved. Here we present the control of the emission rate of a single photon from a pair of distant, entangled atoms into a free-space optical mode. Changing the length of the optical path connecting the atoms modulates the single-photon emission rate in the selected mode with a visibility $V=0.27\ifmmode\pm\else\textpm\fi{}0.03$ determined by the degree of entanglement shared between the atoms, corresponding directly to the concurrence ${\mathcal{C}}_{\ensuremath{\rho}}=0.31\ifmmode\pm\else\textpm\fi{}0.10$ of the prepared state. This scheme, together with population measurements, provides a fully optical determination of the amount of entanglement. Furthermore, large sensitivity of the interference phase evolution points to applications of the presented scheme in high-precision gradient sensing.

Constraining the H$_2$ column density distribution at z$\sim$3 from composite DLA spectra

Abstract: We present the detection of the average H_2 absorption signal in the overall population of neutral gas absorption systems at z∼ 3 using composite absorption spectra built from the Sloan Digital Sky Survey-III damped Lyman α catalogue. We present a new technique to directly measure the H_2 column density distribution function $$f_{\rm H_2}(N)$$ from the average H_2 absorption signal. Assuming a power-law column density distribution, we obtain a slope $$\beta = -1.29 \pm 0.06(\rm stat) \pm 0.10 (\rm sys)$$ and an incidence rate of strong H_2 absorptions [with N(H_2) ≳ 10^18 cm^−2] to be $$4.0 \pm 0.5(\rm stat) \pm 1.0 (\rm sys)\, \hbox{ per cent}$$ in H i absorption systems with N(H i) ≥10^20 cm^−2. Assuming the same inflexion point where $$f_{\rm H_2}(N)$$ steepens as at z = 0, we estimate that the cosmological density of H_2 in the column density range $$\log N(\rm H_2) ({\rm cm}^{-2})= 18{\text{--}}22$$ is $${\sim } 15\hbox{ per cent}$$ of the total. We find one order of magnitude higher H_2 incident rate in a sub-sample of extremely strong damped Lyman α absorption systems (DLAs) [$$\log N(\rm{H\,\small {I}}) ({\rm cm}^{-2}) \ge 21.7$$], which, together with the derived shape of $$f_{\rm H_2}(N)$$, suggests that the typical H i–H_2 transition column density in DLAs is log N(H)(cm^−2) ≳ 22.3 in agreement with theoretical expectations for the average (low) metallicity of DLAs at high-z.

Verification of detailed balance for γ absorption and emission in Dy isotopes

Abstract: The photoneutron cross sections of $^{162,163}\mathrm{Dy}$ have been measured for the first time in an energy region from the neutron threshold (${S}_{n}$) up to $\ensuremath{\approx}13\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$. The $(\ensuremath{\gamma},n)$ reaction was induced with quasimonochromatic laser Compton-scattered $\ensuremath{\gamma}$ rays, produced at the NewSUBARU laboratory. The corresponding $\ensuremath{\gamma}$-ray strength functions ($\ensuremath{\gamma}\mathrm{SF}$) have been calculated from the photoneutron cross sections. The data are compared to reanalyzed $\ensuremath{\gamma}\mathrm{SFs}$ of $^{160\text{--}164}\mathrm{Dy}$, which are measured below ${S}_{n}$. The excellent agreement with the photoneutron data at ${S}_{n}$ confirms the principle of detailed balance. Thus, a complete $\ensuremath{\gamma}\mathrm{SF}$ is established covering in total the energy region of $1\ensuremath{\le}{E}_{\ensuremath{\gamma}}\ensuremath{\le}13$ MeV. These mid-shell well-deformed dysprosium isotopes all show scissors resonances with very similar structures. We find that our data predict the same integrated scissors strength as $(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ data when integrated over the same energy range, which shows that the scissors mode very likely is consistent with the generalized Brink hypothesis. Finally, using the $\ensuremath{\gamma}\mathrm{SFs}$ as input in the reaction code talys, we have deduced radiative neutron-capture cross sections and compared them to direct measurements. We find a very good agreement within the uncertainties, which gives further support to the experimentally determined $\ensuremath{\gamma}\mathrm{SFs}$.

Results from EDGES High-band. II. Constraints on Parameters of Early Galaxies

Abstract: We use the sky-average spectrum measured by EDGES High-Band ($90-190$ MHz) to constrain parameters of early galaxies independent of the absorption feature at $78$~MHz reported by Bowman et al. (2018). These parameters represent traditional models of cosmic dawn and the epoch of reionization produced with the 21cmFAST simulation code (Mesinger & Furlanetto 2007, Mesinger et al. 2011). The parameters considered are: (1) the UV ionizing efficiency ($\zeta$), (2) minimum halo virial temperature hosting efficient star-forming galaxies ($T^{\rm min}_{\rm vir}$), (3) integrated soft-band X-ray luminosity ($L_{\rm X\,<\,2\,keV}/{\rm SFR}$), and (4) minimum X-ray energy escaping the first galaxies ($E_{0}$), corresponding to a typical H${\rm \scriptstyle I}$ column density for attenuation through the interstellar medium. The High-Band spectrum disfavors high values of $T^{\rm min}_{\rm vir}$ and $\zeta$, which correspond to signals with late absorption troughs and sharp reionization transitions. It also disfavors intermediate values of $L_{\rm X\,<\,2\,keV}/{\rm SFR}$, which produce relatively deep and narrow troughs within the band. Specifically, we rule out $39.4<\log_{10}\left(L_{\rm X\,<\,2\,keV}/{\rm SFR}\right)<39.8$ ($95\%$ C.L.). We then combine the EDGES High-Band data with constraints on the electron scattering optical depth from Planck and the hydrogen neutral fraction from high-$z$ quasars. This produces a lower degeneracy between $\zeta$ and $T^{\rm min}_{\rm vir}$ than that reported in Greig & Mesinger (2017a) using the Planck and quasar constraints alone. Our main result in this combined analysis is the estimate $4.5$~$\leq \log_{10}\left(T^{\rm min}_{\rm vir}/\rm K\right)\leq$~$5.7$ ($95\%$ C.L.). We leave for future work the evaluation of $21$~cm models using simultaneously data from EDGES Low- and High-Band.

Low-Threshold Lasing and Coherent Perfect Absorption in Generalized P T -Symmetric Optical Structures

Abstract: A necessary, but not sufficient, condition for parity-time ($P\phantom{\rule{0}{0ex}}T$) symmetry to hold in an optical system is that spatially separated gain and loss must be exactly balanced. This study introduces generalized $P\phantom{\rule{0}{0ex}}T$-symmetric optical structures that can have asymmetric and unbalanced gain-loss profiles, yet offer similar scattering properties and phase transitions as traditional $P\phantom{\rule{0}{0ex}}T$-symmetric ones. The concept of general $P\phantom{\rule{0}{0ex}}T$ symmetry may help to reduce the threshold gain in recently discovered $P\phantom{\rule{0}{0ex}}T$-enabled applications, such as the coherent-perfect-absorber laser and exceptional-point dynamics, and will facilitate optical and photonic devices by offering greater design freedom.

The Evolution of the HeII-Ionizing Background at Redshifts 2.3<z<3.8 Inferred from a Statistical Sample of 24 HST/COS HeII Ly$\alpha$ Absorption Spectra

Abstract: We present measurements of the large-scale (~40 comoving Mpc) effective optical depth of HeII Ly$\alpha$ absorption, $\tau_{\rm eff}$, at 2.54 3 HeII sightlines at z>3.5, and study the redshift evolution and sightline-to-sightline variance of $\tau_{\rm eff}$ in 24 HeII sightlines. We confirm an increase of the median $\tau_{\rm eff}$ from ~2 at z=2.7 to >5 at z>3, and a scatter in $\tau_{\rm eff}$ that increases with redshift. The z>3.5 HeII absorption is predominantly saturated, but isolated narrow ($\Delta v 2.74 the variance in $\tau_{\rm eff}$ significantly exceeds expectations for a spatially uniform UV background, but is consistent with a fluctuating radiation field sourced by variations in the quasar number density and the mean free path in the post-reionization IGM. We develop a method to infer the approximate median HeII photoionization rate $\Gamma_{\rm HeII}$ of a fluctuating UV background from the median $\tau_{\rm eff}$, finding a factor ~5 decrease in $\Gamma_{\rm HeII}$ between z~2.6 and z~3.1. At z~3.1 a $\Gamma_{\rm HeII}=\left[9.1^{+1.1}_{-1.2}\,\mathrm{(stat.)}\,^{+2.4}_{-3.4}\,\mathrm{(sys.)}\right]\times10^{-16}\,\mathrm{s}^{-1}$ corresponds to a median HeII fraction of ~2.5%, indicating that our data probe the tail end of HeII reionization.