Showing papers on "Absorption (logic) published in 2011"

Photocatalytic Activity of Bulk TiO 2 Anatase and Rutile Single Crystals Using Infrared Absorption Spectroscopy

Abstract: A systematic study on the photocatalytic activity of well-defined, macroscopic bulk single-crystal ${\mathrm{TiO}}_{2}$ anatase and rutile samples has been carried out, which allows us to link photoreactions at surfaces of well-defined oxide semiconductors to an important bulk property with regard to photochemistry, the life time of $e\mathrm{\text{\ensuremath{-}}}h$ pairs generated in the bulk of the oxides by photon absorption. The anatase (101) surface shows a substantially higher activity, by an order of magnitude, for CO photo-oxidation to ${\mathrm{CO}}_{2}$ than the rutile (110) surface. This surprisingly large difference in activity tracks the bulk $e\mathrm{\text{\ensuremath{-}}}h$ pair lifetime difference for the two ${\mathrm{TiO}}_{2}$ modifications as determined by contactless transient photoconductance measurements on the corresponding bulk materials.

Laser field absorption in self-generated electron-positron pair plasma.

Abstract: Recently, much attention has been attracted to the problem of limitations on the attainable intensity of high power lasers [A. M. Fedotov et al., Phys. Rev. Lett. 105, 080402 (2010)]. The laser energy can be absorbed by electron-positron pair plasma produced from a seed by a strong laser field via the development of the electromagnetic cascades. The numerical model for a self-consistent study of electron-positron pair plasma dynamics is developed. Strong absorption of the laser energy in self-generated overdense electron-positron pair plasma is demonstrated. It is shown that the absorption becomes important for a not extremely high laser intensity $I\ensuremath{\sim}{10}^{24}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$ achievable in the near future.

Nonlinear Atomic Response to Intense Ultrashort X Rays

Abstract: The nonlinear absorption mechanisms of neon atoms to intense, femtosecond kilovolt x rays are investigated. The production of ${\mathrm{Ne}}^{9+}$ is observed at x-ray frequencies below the ${\mathrm{Ne}}^{8+}$, $1{s}^{2}$ absorption edge and demonstrates a clear quadratic dependence on fluence. Theoretical analysis shows that the production is a combination of the two-photon ionization of ${\mathrm{Ne}}^{8+}$ ground state and a high-order sequential process involving single-photon production and ionization of transient excited states on a time scale faster than the Auger decay. We find that the nonlinear direct two-photon ionization cross section is orders of magnitude higher than expected from previous calculations.

Bulk sensitive x-ray absorption spectroscopy free of self-absorption effects

Abstract: We demonstrate a method of x-ray absorption spectroscopy (XAS) that is bulk sensitive, like traditional fluorescence yield measurements, but is not affected by self-absorption or saturation effects. This measure of XAS is achieved by scanning the incident photon energy through an absorption edge and using an energy-sensitive photon detector to measure the partial fluorescence yield (PFY). The x-ray emission from any element or core-hole excitation that is not resonant with the absorption edge under investigation is selected from the PFY. It is found that the inverse of this PFY spectrum, which we term inverse partial fluorescence yield (IPFY), is linearly proportional to the x-ray absorption cross-section without any corrections due to saturation or self-absorption effects. We demonstrate this technique on the Cu ${L}_{2,3}$ and Nd ${M}_{4,5}$ absorption edges of the high-${T}_{c}$ cuprate ${\mathrm{La}}_{1.475}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.125}{\mathrm{CuO}}_{4}$ by measuring the O ${K}_{\ensuremath{\alpha}}$ PFY and comparing the total electron yield, total fluorescence yield, and IPFY spectra.

Tin dioxide from first principles: Quasiparticle electronic states and optical properties

Abstract: The structural, electronic, and optical properties of the semiconducting oxide SnO${}_{2}$ are investigated using first-principles calculations. We employ the ${G}_{0}{W}_{0}$ formalism based on hybrid-functional calculations to compute the quasiparticle band structure and density of states for which we find good agreement with results from photoemission and two-photon absorption experiments. We also address open questions regarding the band ordering and band symmetries. In a second step we use our electronic structure as a starting point to calculate optical spectra by solving the Bethe-Salpeter equation including the electron-hole interaction. The dielectric tensor is predicted for a wide range of photon energies. Our results resolve the long-standing discrepancy between theory and experiment on the highly anisotropic onsets of absorption. The anisotropy can be explained in terms of dipole-allowed direct transitions in the vicinity of the valence-band maximum without having to invoke lower-lying valence bands.

BROADBAND TRANSMISSION SPECTROSCOPY OF THE SUPER-EARTH GJ 1214b SUGGESTS A LOW MEAN MOLECULAR WEIGHT ATMOSPHERE

Abstract: We use the Wide-field Infrared Camera (WIRCam) on the Canada-France-Hawaii Telescope to observe four transits of the super-Earth planet GJ 1214b in the near-infrared. For each transit, we observe GJ 1214 in two bands nearly simultaneously by rapidly switching the WIRCam filter wheel back and forth for the duration of the observations. By combining all our J-band ({approx}1.25 {mu}m) observations we find a transit depth, analogous to the planet-to-star radius ratio squared, in this band of (R{sub PJ} /R{sub *}){sup 2} = (1.338 {+-} 0.013)%-a value consistent with the optical transit depth reported by Charbonneau and collaborators. However, our best-fit combined K{sub s}-band ({approx}2.15 {mu}m) transit depth is deeper: (R{sub PKs} /R{sub *}){sup 2} = (1.438 {+-} 0.019)%. Formally, our K{sub s}-band transits are deeper than the J-band transits observed simultaneously by a factor of (R{sub PKs} /R{sub PJ}){sup 2} = 1.072 {+-} 0.018-a 4{sigma} discrepancy. The most straightforward explanation for our deeper K{sub s}-band transit depth is a spectral absorption feature from the limb of the atmosphere of the planet; for the spectral absorption feature to be this prominent, the atmosphere of GJ 1214b must have a large-scale height and a low mean molecular weight. That is, its atmospheremore » would have to be hydrogen/helium dominated and this planet would be better described as a mini-Neptune. However, recently published observations from 0.78 to 1.0 {mu}m, by Bean and collaborators, show a lack of spectral features and transit depths consistent with those obtained by Charbonneau and collaborators. The most likely atmospheric composition for GJ 1214b that arises from combining all these observations is less clear; if the atmosphere of GJ 1214b is hydrogen/helium dominated, then it must have either a haze layer that is obscuring transit-depth differences at shorter wavelengths or significantly different spectral features from what current models predict. Our observations disfavor a water-world composition, but such a composition will remain a possibility for GJ 1214b until observations reconfirm our deeper K{sub s}-band transit depth or detect features at other wavelengths.« less

Ultrafast screening and carrier dynamics in ZnO: Theory and experiment

Abstract: At carrier densities above the Mott density, Coulomb screening destroys the exciton resonance. This, together with band-gap renormalization and band filling, severely affects the optical spectra. We have experimentally studied these effects by ultrafast pump-probe reflectivity measurements on a ZnO single crystal at various wavelengths around the exciton resonance and in a broad carrier-density range. Theoretically, we determined the Mott density in ZnO to be $1.5\ifmmode\times\else\texttimes\fi{}{10}^{24}$ m${}^{\ensuremath{-}3}$ at 300 K. Taking a field-theoretical approach, we derived and solved the Bethe-Salpeter ladder equation and we computed the density-dependent reflectivity and absorption spectra. A carrier dynamics model has been developed, containing three-photon absorption, carrier cooling, and carrier trapping near the surface. The agreement between the theoretical reflectivity based on our model and the experimental data is excellent.

The First Observations of Low-redshift Damped Lyα Systems with the Cosmic Origins Spectrograph

Abstract: We report on the first Cosmic Origins Spectrograph observations of damped Ly{alpha} systems (DLAs) and sub-damped Ly{alpha} systems (sub-DLAs) discovered in a new survey of the gaseous halos of low-redshift galaxies. From observations of 37 sightlines, we have discovered three DLAs and four sub-DLAs. We measure the neutral gas density {Omega}{sub HI}, and redshift density d N/d z, of DLA and sub-DLA systems at z < 0.35. We find d N/dz = 0.25{sup +0.24}-{sub 0.14} and {Omega}{sub HI} = 1.4{sup +1.3}{sub -0.7} x 10{sup -3} for DLAs, and d N/d z = 0.08{sup +0.19}{sub -0.06} with {Omega}{sub HI} = 4.2{sup +9.6}{sub -3.5} x 10{sup -5} for sub-DLAs over a redshift path {Delta}z = 11.9. To demonstrate the scientific potential of such systems, we present a detailed analysis of the DLA at z{sub abs} = 0.1140 in the spectrum of SDSS J1009+0713. Profile fits to the absorption lines determine log N(H I) = 20.68 {+-} 0.10 with a metallicity determined from the undepleted element sulfur of [S/H] = -0.62 {+-} 0.18. The abundance pattern of this DLA is similar to that of higher z DLAs, showing mild depletion of the refractory elements Fe and Ti with [S/Fe] = +0.24 {+-} 0.22more » and [S/Ti] = +0.28 {+-} 0.15. Nitrogen is underabundant in this system with [N/H] = -1.40 {+-} 0.14, placing this DLA below the plateau of the [N/{alpha}] measurements in the local universe at similar metallicities. This DLA has a simple kinematic structure with only two components required to fit the profiles and a kinematic width of {Delta}v{sub 90} = 52 km s{sup -1}. Imaging of the QSO field with the Hubble Space Telescope/Wide Field Camera 3 reveals a spiral galaxy at very small impact parameter to the QSO and several galaxies within 10'', or 20 comoving kpc at the redshift of the DLA. Follow-up spectra with the Low Resolution Imaging Spectrometer on the Keck telescope reveal that none of the nearby galaxies are at the redshift of the DLA. The spiral galaxy is identified as the host galaxy of the QSO based on the near perfect alignment of the nucleus and disk of the galaxy as well as spectra of an H II region showing emission lines at the QSO redshift. A small feature appears 0.''70 from the nucleus of the QSO after point-spread function subtraction, providing another candidate for the host galaxy of the DLA system. Even with these supporting data, we are unable to unambiguously identify the host galaxy of the DLA, exemplifying some of the difficulties in determining DLA hosts even at low redshift.« less

Proposal for a nuclear gamma-ray laser of optical range.

Abstract: A possibility of the amplification of the 7.6 eV $\ensuremath{\gamma}$ radiation by the stimulated $\ensuremath{\gamma}$ emission of the ensemble of the $^{229m}\mathrm{Th}$ isomeric nuclei in a host dielectric crystal is proved theoretically. This amplification is a result of (1) the excitation of a large number of $^{229m}\mathrm{Th}$ isomers by laser radiation, (2) the creation of the inverse population of nuclear levels in a cooled sample owing to the interaction of thorium nuclei with the crystal electric field or with an external magnetic field, (3) the emission or absorption of the optical photons by thorium nuclei in the crystal without recoil, and (4) the nuclear spin relaxation through the conduction electrons of the metallic covering.

The supernova impostor impostor sn 1961v: spitzer shows that zwicky was right (again)

Abstract: SN 1961V, one of Zwicky's defining Type V supernovae (SNe), was a peculiar transient in NGC 1058 that has variously been categorized as either a true core-collapse SN leaving a black hole (BH) or neutron star (NS) remnant, or an eruption of a luminous blue variable star. The former case is suggested by its possible association with a decaying non-thermal radio source, while the latter is suggested by its peculiar transient light curve and its low initial expansion velocities. The crucial difference is that the star survives a transient eruption but not an SN. All stars identified as possible survivors are significantly fainter, L{sub opt} {approx} 10{sup 5} L{sub sun}, than the L{sub opt} {approx_equal} 3 x 10{sup 6} L{sub sun} progenitor star at optical wavelengths. While this can be explained by dust absorption in a shell of material ejected during the transient, the survivor must then be present as an L{sub IR} {approx_equal} 3 x 10{sup 6} L{sub sun} mid-infrared source. Using archival Spitzer observations of the region, we show that such a luminous mid-IR source is not present. The brightest source of dust emission is only L{sub IR} {approx_equal} 10{sup 5} L{sub sun} and does not correspond tomore » the previously identified candidates for the surviving star. The dust cannot be made sufficiently distant and cold to avoid detection unless the ejection energy, mass, and velocity scales are those of an SN or greater. We conclude that SN 1961V was a peculiar, but real, SN. Its peculiarities are probably due to enhanced mass loss just prior to the SN, followed by the interactions of the SN blast wave with this ejecta. This adds to the evidence that there is a population of SN progenitors that have major mass-loss episodes shortly before core collapse. The progenitor is a low metallicity, {approx}1/3 solar, high-mass, M{sub ZAMS} {approx}> 80 M{sub sun}, star, which means either that BH formation can be accompanied by an SN or that surprisingly high-mass stars can form an NS. We also report on the mid-IR properties of the two other SNe in NGC 1058, SN 1969L, and SN 2007gr.« less

Redshift in the optical absorption of ZnO single crystals in the presence of an intense midinfrared laser field.

Abstract: We report time-resolved electroabsorption of a weak probe in a $500\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ thick zinc-oxide crystal in the presence of a strong midinfrared pump in the tunneling limit. We observe a substantial redshift in the absorption edge that scales with the cube root of intensity up to $1\text{ }\text{ }\mathrm{TW}/{\mathrm{cm}}^{2}$ ($0.38\text{ }\text{ }\mathrm{eV}\text{ }{\mathrm{cm}}^{2/3}\text{ }{\mathrm{TW}}^{\ensuremath{-}1/3}$) after which it increases more slowly to 0.4 eV at a maximum applied intensity of $5\text{ }\text{ }\mathrm{TW}/{\mathrm{cm}}^{2}$. The maximum shift corresponds to more than 10% of the band gap. The change in scaling occurs in a regime of nonperturbative high-order harmonic generation where electrons undergo periodic Bragg scattering from the Brillouin zone boundaries. It also coincides with the limit where the electric field becomes comparable to the ratio of the band gap to the lattice spacing.

Temperature range of superconducting fluctuations above T c in YBa 2 Cu 3 O 7-δ single crystals

Abstract: Microwave absorption measurements in magnetic fields from 0 up to 16 T were used to determine the temperature range of superconducting fluctuations above the superconducting critical temperature ${T}_{c}$ in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\mathrm{\ensuremath{\delta}}}$. Measurements were performed on deeply underdoped, slightly underdoped, and overdoped single crystals. The temperature range of the superconducting fluctuations above ${T}_{c}$ is determined by an experimental method which is free from arbitrary assumptions about subtracting the nonsuperconducting contributions to the total measured signal and/or theoretical models to extract the unknown parameters. The superconducting fluctuations are detected in the $\mathit{ab}$ plane, and $c$-axis conductivity, by identifying the onset temperature ${T}^{\ensuremath{'}}$. Within the sensitivity of the method, this fluctuation regime is found only within a fairly narrow region above ${T}_{c}$. Its width increases from 7 K in the overdoped sample (${T}_{c}=89 \mathrm{K}$) to, at most, 23 K in the deeply underdoped sample (${T}_{c}=57 \mathrm{K}$), so that ${T}^{\ensuremath{'}}$ falls well below the pseudogap temperature ${T}^{*}$. Implications of these findings are discussed in the context of other experimental probes of superconducting fluctuations in the cuprates.

Time delay between photoemission from the 2p and 2s subshells of neon

Abstract: The $R$-matrix incorporating time (RMT) method is a method developed recently for solving the time-dependent Schr\"odinger equation for multielectron atomic systems exposed to intense short-pulse laser light. We have employed the RMT method to investigate the time delay in the photoemission of an electron liberated from a $2p$ orbital in a neon atom with respect to one released from a $2s$ orbital following absorption of an attosecond xuv pulse. Time delays due to xuv pulses in the range 76--105 eV are presented. For an xuv pulse at the experimentally relevant energy of 105.2 eV, we calculate the time delay to be $10.2\ifmmode\pm\else\textpm\fi{}1.3$ attoseconds (as), somewhat larger than estimated by other theoretical calculations, but still a factor of 2 smaller than experiment. We repeated the calculation for a photon energy of 89.8 eV with a larger basis set capable of modeling correlated-electron dynamics within the neon atom and the residual Ne${}^{+}$ ion. A time delay of $14.5\ifmmode\pm\else\textpm\fi{}1.5$ as was observed, compared to a $16.7\ifmmode\pm\else\textpm\fi{}1.5$ as result using a single-configuration representation of the residual Ne${}^{+}$ ion.

Coexistence of multiple regimes for near-field thermal radiation between two layers supporting surface phonon polaritons in the infrared

Abstract: We demonstrate the coexistence of different near-field thermal radiation regimes between two layers supporting surface phonon polaritons (SPhPs) in the infrared. These regimes exist when the distance of separation between the media $d$ is much smaller than the dominant emission wavelength. This coexistence is noticed after computations of the near-field radiative heat transfer coefficient ${h}_{r}$ for silicon carbide films using fluctuational electrodynamics and following an asymptotic analysis of ${h}_{r}$. We show that the emergence of these regimes is a function of a dimensionless variable $D$ defined as the ratio of the layer thickness $t$ to $d$. When $D$ $\ensuremath{\gg}$ 1 for both films, SPhPs dominating near-field radiant energy exchange do not couple within the layers, such that ${h}_{r}$ follows a ${d}^{\ensuremath{-}2}$ power law as for the case of two planar half-spaces. When $D$ $\ensuremath{\ll}$ 1 for both layers, the dominant SPhPs couple within the films, thus resulting in a splitting of the spectral distribution of flux into two distinct modes. Despite this splitting, the asymptotic expansion reveals that ${h}_{r}$ varies as ${d}^{\ensuremath{-}2}$ due to the fact that the spectral bands of high emission and absorption are essentially the same for both films. However, when both layers have a thickness of the order of a nanometer or less, a purely theoretical regime emerges where ${h}_{r}$ follows a ${d}^{\ensuremath{-}4}$ asymptote. Also, when one layer has $D$ $\ensuremath{\ll}$ 1 while the other one is characterized by $D$ $\ensuremath{\gg}$ 1, there is an important mismatch between the spectral bands of high emission and absorption of the films, thus resulting in a ${h}_{r}$ varying as ${d}^{\ensuremath{-}3}$. These various near-field thermal radiation regimes are finally summarized in a comprehensive regime map. This map provides a clear understanding of near-field thermal radiation regimes between two layers, which are particularly important for designing highly efficient nanoscale-gap thermophotovoltaic power generation devices.

Theoretical and Experimental Evidence for a New Post-Cotunnite Phase of Titanium Dioxide with Significant Optical Absorption

Abstract: We report the discovery of a post-cotunnite phase of ${\mathrm{TiO}}_{2}$ by both density-functional ab initio calculations and high-pressure experiments. A pressure-induced phase transition to a hexagonal ${\mathrm{Fe}}_{2}\mathrm{P}$-type structure (space group $P\overline{6}2m$) was predicted to occur at 161 GPa and 0 K and successfully observed by in situ synchrotron x-ray diffraction measurements at 210 GPa and 4000 K with a significant increase in opacity. This change in opacity is attributed to a reduction of band gap from 3.0 to 1.9 eV across the phase change. The ${\mathrm{Fe}}_{2}\mathrm{P}$-type structure is proved to be the densest phase in major metal dioxides.

K^- nuclear potentials from in-medium chirally motivated models

Abstract: A self-consistent scheme for constructing ${K}^{\ensuremath{-}}$ nuclear optical potentials from subthreshold in-medium $\overline{K}N$ $s$-wave scattering amplitudes is presented and applied to analysis of kaonic atoms data and to calculations of ${K}^{\ensuremath{-}}$ quasibound nuclear states. The amplitudes are taken from a chirally motivated meson-baryon coupled-channel model, both at the Tomozawa-Weinberg leading order and at the next to leading order. Typical kaonic atoms potentials are characterized by a real part $\ensuremath{-}\mathrm{Re}\phantom{\rule{0.222222em}{0ex}}{V}_{{K}^{\ensuremath{-}}}^{\mathrm{chiral}}=85\ifmmode\pm\else\textpm\fi{}5$ MeV at nuclear matter density, in contrast to half this depth obtained in some derivations based on in-medium $\overline{K}N$ threshold amplitudes. The moderate agreement with data is much improved by adding complex $\ensuremath{\rho}$- and ${\ensuremath{\rho}}^{2}$-dependent phenomenological terms, found to be dominated by ${\ensuremath{\rho}}^{2}$ contributions that could represent $\overline{K}NN\ensuremath{\rightarrow}YN$ absorption and dispersion, outside the scope of meson-baryon chiral models. Depths of the real potentials are then near 180 MeV. The effects of $p$-wave interactions are studied and found secondary to those of the dominant $s$-wave contributions. The in-medium dynamics of the coupled-channel model is discussed and systematic studies of ${K}^{\ensuremath{-}}$ quasibound nuclear states are presented.

Challenging the weak cosmic censorship conjecture with charged quantum particles

Abstract: Motivated by the recent attempts to violate the weak cosmic censorship conjecture for near-extreme black holes, we consider the possibility of overcharging a near-extreme Reissner-Nordstr\"om black hole by the quantum tunneling of charged particles. We consider the scattering of spin-0 and spin-$\frac{1}{2}$ particles by the black hole in a unified framework and obtain analytically, for the first time, the pertinent reflection and transmission coefficients without any small charge approximation. Based on these results, we propose some gedanken experiments that could lead to the violation of the weak cosmic censorship conjecture due to the (classically forbidden) absorption of small energy charged particles by the black hole. As for the case of scattering in Kerr spacetimes, our results demonstrate explicitly that scalar fields are subject to (electrical) superradiance phenomenon, while spin-$\frac{1}{2}$ fields are not. Superradiance impose some limitations on the gedanken experiments involving spin-0 fields, favoring, in this way, the mechanisms for creation of a naked singularity by the quantum tunneling of spin-$\frac{1}{2}$ charged fermions. We also discuss the implications that vacuum polarization effects and quantum statistics might have on these gedanken experiments. In particular, we show that they are not enough to prevent the absorption of incident small energy particles and, consequently, the formation of a naked singularity.

$F$-factors in hypergraphs via absorption

Abstract: Given integers $ n \ge k >l \ge 1 $ and a $k$-graph $F$ with $|V(F)|$ divisible by $n$, define $t_l^k(n,F)$ to be the smallest integer $d$ such that every $k$-graph $H$ of order $n$ with minimum $l$-degree $\delta_l(H) \ge d $ contains an $F$-factor. A classical theorem of Hajnal and Szemer\'{e}di implies that $t^2_1(n,K_t) = (1-1/t)n$ for integers $t$. For $k \ge 3$, $t^k_{k-1}(n,K_k^k)$ (the $\delta_{k-1}(H)$ threshold for perfect matchings) has been determined by K\"{u}hn and Osthus (asymptotically) and R\"{o}dl, Ruci\'{n}ski and Szemer\'{e}di (exactly) for large $n$. In this paper, we generalise the absorption technique of R\"{o}dl, Ruci\'{n}ski and Szemer\'{e}di to $F$-factors. We determine the asymptotic values of $t^k_1(n,K_k^k(m))$ for $k = 3,4$ and $m \ge 1$. In addition, we show that for $t>k = 3$ and $\gamma >0$, $ t^3_{2}(n,K_t^3) \le (1- \frac{2}{t^2-3t+4} + \gamma) n$ provided $n$ is large and $t | n$. We also bound $t^3_{2}(n,K_t^3)$ from below. In particular, we deduce that $t^3_2(n,K_4^3) = (3/4+o(1))n$ answering a question of Pikhurko. In addition, we prove that $t^k_{k-1}(n,K_t^k) \le (1- \binom{t-1}{k-1}^{-1} + \gamma)n$ for $\gamma >0$, $k \ge 6$ and $t \ge (3+ \sqrt5)k/2$ provided $n$ is large and $t | n$.

Electronic structure of the delafossite-type CuMO2 (M = Sc, Cr, Mn, Fe, and Co): Optical absorption measurements and first-principles calculations

Abstract: We compared the results of first-principles calculations with measured absorption spectra in thin films of delafossite-type Cu$M$O${}_{2}$ ($M$ $=$ Sc, Cr, Mn, Fe, and Co) taken at 10 K. Two optical transitions were found: one is associated with Cu3d $+$ O2$P$ \ensuremath{\rightarrow} Cu3${d}_{{\mathrm{z}}^{2}}$ $+$ 4$S$ observed in all of the Cu$M$O${}_{2}$ films, and the other is associated with Cu3d $+$ O2$P$ \ensuremath{\rightarrow} $M$3$d$ detected for $M$ $=$ Mn, Fe, and Co. The energy of the former transition showed an unexpected dependence on the atomic number of $M$. An abrupt change of this energy occurs for $M$ $=$ Cr, which is explained as a result of the interaction between the Cu3${d}_{{\mathrm{z}}^{2}}$ $+$ 4$s$ and $M$3$d$ bands.

Deformed Distance Duality Relations and Supernovae Dimming

Abstract: The basic cosmological distances are linked by the Etherington cosmic distance duality relation, $\eta (z) = D_{L}(z)(1+z)^{-2}/D_{A}(z) \equiv 1$, where $D_{L}$ and $D_{A}$ are, respectively, the luminosity and angular diameter distances. In order to test its validity, some authors have proposed phenomenological expressions for $\eta(z)$ thereby deforming the original Etherington's relation and comparing the resulting expressions with the available and future cosmological data. The relevance of such studies is unquestionable since any violation of the cosmic distance duality relation could be the signal of new physics or non-negligible astrophysical effects in the usually assumed perfectly transparent Universe. In this letter, we show that under certain conditions such expressions can be derived from a more fundamental approach with the parameters appearing in the $\eta(z)$ expression defining the cosmic absorption parameter as recently discussed by Chen and Kantowski. Explicit examples involving four different parametrizations of the deformation function are given. Based on such an approach, it is also found that the latest Supernova data can also be explained in the framework of a pure cold dark matter model (Einstein-de Sitter). Two different scenarios with cosmic absorption are discussed. Only if the cosmic opacity is fully negligible, the description of an accelerating Universe powered by dark energy or some alternative gravity theory must be invoked.

Phase resolved spectroscopic study of the isolated neutron star RBS 1223 (1RXS J130848.6+212708)

Abstract: The data from all observations of RBS 1223 (1RXS J130848.6+212708) conducted by XMM-Newton EPIC pn with the same instrumental setup in 2003-2007 were combined to form spin-phase resolved spectra. A number of complex models of neutron stars with strongly magnetized ($B_{pole} 10^{13}-10^{14}$ G) surface, with temperature and magnetic field distributions around magnetic poles, and partially ionized hydrogen thin atmosphere above it have been implemented into the X-ray spectral fitting package XSPEC for simultaneous fitting of phase-resolved spectra. A Markov-Chain-Monte-Carlo (MCMC) approach is also applied to verify results of fitting and estimating in multi parameter models. The spectra in different rotational phase intervals and light curves in different energy bands with high S/N ratio show a high complexity. The spectra can be parameterized with a Gaussian absorption line superimposed on a blackbody spectrum, while the light curves with double-humped shape show strong dependence of pulsed fraction upon the energy band (13%-42%), which indicates that radiation emerges from at least two emitting areas. A model with condensed iron surface and partially ionized hydrogen thin atmosphere above it allows us to fit simultaneously the observed general spectral shape and the broad absorption feature observed at 0.3 keV in different spin phases of RBS 1223. It allowed to constrain some physical properties of X-ray emitting areas, i.e. the temperatures ($T_{p1} ~ 105$ eV, $T_{p2} ~ 99$ eV), magnetic field strengths ($B_{p1}= B_{p2} ~ 8.6x10^{13}$G) at the poles, and their distributions parameters ($a_{1} 0.61, a_{2} 0.29$, indicating an absence of strong toroidal magnetic field component). In addition, it puts some constraints on the geometry of the emerging X-ray emission and gravitational redshift ($z=0.16^{+0.03}_{-0.01}$) of RBS 1223.

Identification of the V Al -O N defect complex in AlN single crystals

Abstract: In this Rapid Communication, we report positron annihilation results on in-grown and proton irradiation-induced vacancies and their decoration in aluminium nitride (AlN) single crystals. By combining positron lifetime and coincidence Doppler measurements with ab initio calculations, we identify in-grown ${V}_{\mathrm{Al}}\ensuremath{-}{\mathrm{O}}_{\mathrm{N}}$ complexes in the concentration range ${10}^{18}$ cm${}^{\ensuremath{-}3}$ as the dominant form of ${V}_{\mathrm{Al}}$ in the AlN single crystals, while isolated ${V}_{\mathrm{Al}}$ were introduced by irradiation. Further, we identify the UV absorption feature at around 360 nm that involves ${V}_{\mathrm{Al}}$.

Speed-dependent effects in the near-infrared spectrum of self-colliding H 2 O 18 molecules

Abstract: The absorption line shape of a given ${\mathrm{H}}_{2}$$^{18}\mathrm{O}$ vibration-rotation line at 1.38 $\ensuremath{\mu}$m was deeply investigated using a spectroscopic approach based upon the use of a pair of offset-frequency locked extended-cavity diode lasers. This dual-laser apparatus ensures extreme levels of accuracy in controlling and measuring any variation of the laser frequency around a given absolute reference. As a result, high levels of precision and accuracy were reached in the observation of a molecular absorption line shape in the near infrared. A variety of semiclassical models, accounting for Dicke narrowing and speed-dependent effects, were implemented and tested in order to describe the physical situation of self-colliding ${\mathrm{H}}_{2}$$^{18}\mathrm{O}$ molecules. Our study demonstrates that the molecular confinement alone is unable to explain entirely the departures from the Voigt profile and that the speed dependence of pressure-induced broadening and shift cannot be ignored, even in the case of pure water samples at relatively small pressures. The absorption spectrum was successfully interpolated using the uncorrelated version of the speed-dependent Galatry profile, with a hypergeometric dependence on the absorber speed for both pressure broadening and pressure shift parameters, thus reaching an agreement between theory and experiment at the level of $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$.

Donor-band ferromagnetism in cobalt-doped indium oxide

Abstract: Ferromagnetic cobalt-doped indium oxide, (In${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$O${}_{3}$, thin films with $x$ between 1.6% and 8.1% have been studied by x-ray, magnetic, and optical methods. Evidence gathered from x-ray diffraction and x-ray absorption fine-structure studies suggest that the Co atoms are substitutional within the In${}_{2}$O${}_{3}$ matrix. The magnetization of the films were found to consist of two components: a paramagnetic term that agrees with what is expected for paramagnetic cobalt ions and a temperature-dependent ferromagnetic hysteresis loop. The ferromagnetic component was too large to have been generated by the free carriers, implying that it originates from electrons bound in defect states associated with oxygen vacancies. This is confirmed by optical absorption and magneto-optical studies because the magnetic circular dichroism scales with concentration of cobalt and temperature in the same way as the measured magnetization.

Is there further evidence for spatial variation of fundamental constants

Abstract: Indications of spatial variation of the fine-structure constant, $\ensuremath{\alpha}$, based on study of quasar absorption systems have recently been reported [J. K. Webb, J. A. King, M. T. Murphy, V. V. Flambaum, R. F. Carswell, and M. B. Bainbridge, arXiv:1008.3907.]. The physics that causes this $\ensuremath{\alpha}$-variation should have other observable manifestations, and this motivates us to look for complementary astrophysical effects. In this paper we propose a method to test whether spatial variation of fundamental constants existed during the epoch of big bang nucleosynthesis and study existing measurements of deuterium abundance for a signal. We also examine existing quasar absorption spectra data that are sensitive to variation of the electron-to-proton mass ratio $\ensuremath{\mu}$ and $x={\ensuremath{\alpha}}^{2}\ensuremath{\mu}{g}_{p}$ for spatial variation.

Angular distribution of thick-target bremsstrahlung produced by electrons with initial energies ranging from 10 to 20 keV incident on Ag

Abstract: Experimental results are presented comparing the intensities of the bremsstrahlung produced by electrons with initial energies ranging from 10 to 20 keV incident on a thick Ag target, measured at forward angles in the range of 0\ifmmode^\circ\else\textdegree\fi{} to 55\ifmmode^\circ\else\textdegree\fi{}. When the data are corrected for attenuation due to photon absorption within the target, the results indicate that the detected radiation is distributed anisotropically only at photon energies $k$ that are approximately equal to the initial energy of the incident electrons ${E}_{0}$. The results of our experiments suggest that, as $k$/${E}_{0}$ \ensuremath{\rightarrow} 0, the detected radiation essentially becomes isotropic due primarily to the scattering of electrons within the target. A comparison to the theory of Kissel et al. [At. Data Nucl. Data Tables 28, 381 (1983)] suggests that the angular distribution of bremsstrahlung emitted by electrons incident on thick targets is similar to the angular distribution of bremsstrahlung emitted by electrons incident on free-atom targets only when $k$/${E}_{0}$ \ensuremath{\approx} 1. The experimental data also are in approximate agreement with the angular distribution predictions of the Monte Carlo program penelope.

Role of Ti 3d carriers in mediating the ferromagnetism of Co∶TiO2 anatase thin films.

Abstract: We study the surface and bulk electronic structure of the room-temperature ferromagnet $\mathrm{Co}\ensuremath{\mathbin:}{\mathrm{TiO}}_{2}$ anatase films using soft- and hard-x-ray photoemission spectroscopy with probe sensitivities of $\ensuremath{\sim}1$ and $\ensuremath{\sim}10\text{ }\text{ }\mathrm{nm}$, respectively. We obtain direct evidence of metallic ${\mathrm{Ti}}^{3+}$ states in the bulk, which get suppressed to give a surface semiconductor, thus indicating the difference in electronic structure between surface and bulk. X-ray absorption and resonant photoemission spectroscopy reveal ${\mathrm{Ti}}^{3+}$ electrons at the Fermi level (${E}_{F}$) and high-spin ${\mathrm{Co}}^{2+}$ electrons occurring away from ${E}_{F}$. The results show the importance of the charge neutrality condition: ${\mathrm{Co}}^{2+}+V_{\mathrm{O}}{}^{2\ensuremath{-}}+2{\mathrm{Ti}}^{4+}\ensuremath{\leftrightarrow}{\mathrm{Co}}^{2+}+2{\mathrm{Ti}}^{3+}$ (${V}_{\mathrm{O}}$ is oxygen vacancy), which gives rise to the elusive Ti $3d$ carriers mediating ferromagnetism via the Co $3d--\mathrm{O}$ $2p--\mathrm{Ti}$ $3d$ exchange interaction pathway of the occupied orbitals.

Anomalous x-ray scattering study of Ge x Se 1 − x glassy alloys across the stiffness transition composition

Abstract: Anomalous x-ray scattering experiments on glassy Ge${}_{x}$Se${}_{1\ensuremath{-}x}$ were carried out at energies close to the Ge and Se $K$ absorption edges at concentrations between $x=0.15$ and $0.333$ in order to explore the correlation between the atomic structures in short and intermediate ranges and the stiffness transition which appears at approximately $x=0.20$ in this glassy system. The partial structure factors ${S}_{ij}(Q)$ and the corresponding partial pair-distribution functions ${g}_{ij}(r)$ were obtained using reverse Monte Carlo modeling. Although the ${S}_{ij}(Q)$ and ${g}_{ij}(r)$ spectra seem to gradually change with $x$, some indications are found related to the stiffness transition, in particular in the intermediate-range structure. First, the preshoulder position in ${S}_{\mathrm{SeSe}}(Q)$ largely shifts toward lower $Q$ values in the intermediate phase concentration region of the stiffness transition, while the prepeak positions in ${S}_{\mathrm{GeGe}}(Q)$ and ${S}_{\mathrm{GeSe}}(Q)$ remain almost unchanged. Second, the Ge-Se-Se bond angles are distributed at $\ensuremath{\sim}$90${}^{\ensuremath{\circ}}$ when the transition region is approached with decreasing $x$. No appreciable portions of the Ge-Se-Se-Ge sequences, i.e., Ge(Se${}_{1/2}$)${}_{4}$ tetrahedra connected by the Se${}_{2}$ dimer, are found in the Ge${}_{0.20}$Se${}_{0.80}$ glass. Instead, there is experimental evidence for a phase-separation tendency between directly connected Ge(Se${}_{1/2}$)${}_{4}$ tetrahedra and Se${}_{n}$ ($n\ensuremath{\geqslant}3$) chains in the intermediate phase concentration region of the stiffness transition. This may be due to avoid large stress in the Se${}_{2}$ dimer bonds of the Ge-Se-Se bond angle.

VCSEL based Faraday rotation spectroscopy with a modulated and static magnetic field for trace molecular oxygen detection

Abstract: Faraday Rotation Spectroscopy (FRS) is a useful technique for quantification of paramagnetic trace gases with significantly higher sensitivity when compared to direct absorption techniques. Our prototype system based on the openPHOTONS sensor core measures the concentration of molecular oxygen (O2) in the A band using a 763-nm vertical cavity surface emitting laser. We provide detailed analysis of two measurement methods based on FRS using the same sensor configuration: one with a modulated magnetic field, and one with a static magnetic field in combination with wavelength modulation. Our spectra signal-to-noise ratios agree well with our simulations via modeling of the FRS signal. For alternating magnetic field, we achieve an equivalent minimum detectable absorption (MDA) of \(8.86\times 10^{-7}/\mathrm{Hz}^{\frac{1}{2}}\) resulting in a minimum detection limit of 30 ppmv⋅m/\(\mathrm{Hz}^{\frac{1}{2}}\) of O2, limited by detector noise and laser noise. For the same system configuration in the static field case, parasitic etalon fringes limited the MDA to \(4.8\times 10^{-6}/\mathrm{Hz}^{\frac{1}{2}}\). In both cases, we describe methods to improve signal-to-noise ratio based on our data and models.