Showing papers on "Absorption (logic) published in 2008"

Bosonic super-WIMPs as keV-scale dark matter

Abstract: We consider models of light superweakly interacting cold dark matter, with $\mathcal{O}(10--100)\text{ }\text{ }\mathrm{keV}$ mass, focusing on bosonic candidates such as pseudoscalars and vectors. We analyze the cosmological abundance, the $\ensuremath{\gamma}$ background created by particle decays, the impact on stellar processes due to cooling, and the direct-detection capabilities in order to identify classes of models that pass all the constraints. In certain models, variants of photoelectric (or axioelectric) absorption of dark matter in direct-detection experiments can provide a sensitivity to the superweak couplings to the standard model which is superior to all existing indirect constraints. In all models studied, the annual modulation of the direct-detection signal is at the currently unobservable level of $O({10}^{\ensuremath{-}5})$.

Photosystem II: The machinery of photosynthetic water splitting

Abstract: This review summarizes our current state of knowledge on the structural organization and functional pattern of photosynthetic water splitting in the multimeric Photosystem II (PS II) complex, which acts as a light-driven water: plastoquinone-oxidoreductase. The overall process comprises three types of reaction sequences: (1) photon absorption and excited singlet state trapping by charge separation leading to the ion radical pair \( {\text{P}}680^{ + \bullet } {\text{Q}}_{\text{A}}^{ - \bullet } \left( { \overset{\wedge}{=}{\text{P}}_{\text{D1}}^{ + \bullet } {\text{Q}}_{\text{A}}^{ - \bullet } } \right) \) formation, (2) oxidative water splitting into four protons and molecular dioxygen at the water oxidizing complex (WOC) with \( {\text{P}}680^{ + \bullet } \) as driving force and tyrosine YZ as intermediary redox carrier, and (3) reduction of plastoquinone to plastoquinol at the special QB binding site with \( {\text{Q}}_{\text{A}}^{ - \bullet } \) acting as reductant. Based on recent progress in structure analysis and using new theoretical approaches the mechanism of reaction sequence (1) is discussed with special emphasis on the excited energy transfer pathways and the sequence of charge transfer steps: \( ^{1} \left( {\text{RC-PC}} \right)^{ *} {\text{Q}}_{\text{A}} \to {\text{P}}_{{{\text{D}}2}} {\text{P}}_{{{\text{D}}1}} {\text{Chl}}_{{{\text{D}}1}}^{ + \bullet } {\text{Pheo}}_{{{\text{D}}1}}^{ - \bullet } {\text{Q}}_{\text{A}} \to {\text{P}}_{{{\text{D}}2}} {\text{P}}_{{{\text{D}}1}}^{ + \bullet } {\text{Chl}}_{{{\text{D}}1}} {\text{Pheo}}_{{{\text{D}}1}}^{ - \bullet } {\text{Q}}_{\text{A}} \to {\text{P}}_{{{\text{D}}2}} {\text{P}}_{{{\text{D}}1}}^{ + \bullet } {\text{Chl}}_{\text{D1}} {\text{Pheo}}_{\text{D1}} {\text{Q}}_{\text{A}}^{ - \bullet } , \) where 1(RC-PC)* denotes the excited singlet state 1P680* of the reaction centre pigment complex. The structure of the catalytic Mn4OXCa cluster of the WOC and the four step reaction sequence leading to oxidative water splitting are described and problems arising for the electronic configuration, in particular for the nature of redox state S3, are discussed. The unravelling of the mode of O–O bond formation is of key relevance for understanding the mechanism of the process. This problem is not yet solved. A multistate model is proposed for S3 and the functional role of proton shifts and hydrogen bond network(s) is emphasized. Analogously, the structure of the QB site for PQ reduction to PQH2 and the energetic and kinetics of the two step redox reaction sequence are described. Furthermore, the relevance of the protein dynamics and the role of water molecules for its flexibility are briefly outlined. We end this review by presenting future perspectives on the water oxidation process.

Evidence of orbital reconstruction at interfaces in ultrathin La0.67Sr0.33MnO3 films.

Abstract: Linear dichroism (LD) in x-ray absorption, diffraction, transport, and magnetization measurements on thin ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ films grown on different substrates, allow identification of a peculiar interface effect, related just to the presence of the interface. We report the LD signature of preferential $3d\ensuremath{-}{e}_{g}(3{z}^{2}\ensuremath{-}{r}^{2})$ occupation at the interface, suppressing the double exchange mechanism. This surface orbital reconstruction is opposite to that favored by residual strain and is independent of dipolar fields, the chemical nature of the substrate and the presence of capping layers.

Theoretical and experimental examination of the intermediate-band concept for strain-balanced (In,Ga)As/Ga(As,P) quantum dot solar cells

Abstract: The intermediate-band solar cell (IBSC) concept has been recently proposed to enhance the current gain from the solar spectrum whilst maintaining a large open-circuit voltage. Its main idea is to introduce a partially occupied intermediate band (IB) between the valence band (VB) and conduction band (CB) of the semiconductor absorber, thereby increasing the photocurrent by the additional $\text{VB}\ensuremath{\rightarrow}\text{IB}$ and $\text{IB}\ensuremath{\rightarrow}\text{CB}$ absorptions. The confined electron levels of self-assembled quantum dots (QDs) were proposed as potential candidates for the implementation of such an IB. Here we report experimental and theoretical investigations on ${\text{In}}_{y}{\text{Ga}}_{1\ensuremath{-}y}\text{As}$ dots in a ${\text{GaAs}}_{1\ensuremath{-}x}{\text{P}}_{x}$ matrix, examining its suitability for acting as IBSCs. The system has the advantage of allowing strain symmetrization within the structure, thus enabling the growth of a large number of defect-free QD layers, despite the significant size mismatch between the dot material and the surrounding matrix. We examine the various conditions related to the optimum functionality of the IBSC, in particular those connected to the optical and electronic properties of the system. We find that the intensity of absorption between QD-confined electron states and host CB is weak because of their localized-to-delocalized character. Regarding the position of the IB within the matrix band gap, we find that, whereas strain symmetrization can indeed permit growth of multiple dot layers, the current repertoire of ${\text{GaAs}}_{1\ensuremath{-}x}{\text{P}}_{x}$ barrier materials, as well as ${\text{In}}_{y}{\text{Ga}}_{1\ensuremath{-}y}\text{As}$ dot materials, does not satisfy the ideal energetic locations for the IB. We conclude that other QD systems must be considered for QD-IBSC implementations.

Absorption of short laser pulses on solid targets in the ultrarelativistic regime.

Abstract: We report the first direct measurements of total absorption of short laser pulses on solid targets in the ultrarelativistic regime. The data show an enhanced absorption at intensities above ${10}^{20}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$, reaching 60% for near-normal incidence and 80%--90% for 45\ifmmode^\circ\else\textdegree\fi{} incidence. Two-dimensional particle-in-cell simulations demonstrate that such high absorption is consistent with both interaction with preplasma and hole boring by the intense laser pulse. A large redshift in the second harmonic indicates a surface recession velocity of 0.035c.

Observational $5-20\mu$m Interstellar Extinction Curves Toward Star-Forming Regions Derived from Spitzer IRS Spectra

Abstract: Using \emph{Spitzer} Infrared Spectrograph observations of G0--M4 III stars behind dark clouds, I construct $5-20\mu$m empirical extinction curves for $0.3\leq A_K 1$, the curve exhibits lower contrast between the silicate and absorption continuum, developes ice absorption, and lies closer to the \citet{wd01} $R_V=5.5$ case B curve, a result which is consistent with that of \citet{flaherty07} and \citet{chiar07}. Recently work using \emph{Spitzer} Infrared Array Camera data by \citet{chapman08} independently reaches a similar conclusion, that the shape of the extinction curve changes as a function of increasing $A_K$. By calculating the optical depths of the $9.7\mu$m silicate and 6.0, 6.8, and 15.2 $\mu$m ice features, I determine that a process involving ice is responsible for the changing shape of the extinction curve and speculate that this process is coagulation of ice-mantled grains rather than ice-mantled grains alone.

Infrared Renormalization Group Flow for Heavy Quark Masses

Abstract: A short-distance heavy-quark mass depends on two parameters: the renormalization scale $\ensuremath{\mu}$ and a scale $R$ controlling the absorption of infrared fluctuations. The radius for perturbative corrections that build up the mass beyond its pointlike definition in the pole scheme is $\ensuremath{\sim}1/R$. Treating $R$ as a variable gives a renormalization-group equation. $R$ evolution improves the stability of conversion between short-distance mass schemes, allowing us to avoid large logs and the renormalon. $R$ evolution can also be used to study IR renormalons without using bubble chains, yielding a convergent sum rule for the coefficient of the $O({\ensuremath{\Lambda}}_{\mathrm{QCD}})$ renormalon ambiguity of the pole mass.

Stringent null constraint on cosmological evolution of the proton-to-electron mass ratio.

Abstract: We present a strong constraint on variation of the proton-to-electron mass ratio $\ensuremath{\mu}$ over cosmological time scales using molecular hydrogen transitions in optical quasar spectra. Using high quality spectra of quasars $\mathrm{Q}0405\ensuremath{-}443$, $\mathrm{Q}0347\ensuremath{-}383$, and $\mathrm{Q}0528\ensuremath{-}250$, variation in $\ensuremath{\mu}$ relative to the present day value is limited to $\ensuremath{\Delta}\ensuremath{\mu}/\ensuremath{\mu}=(2.6\ifmmode\pm\else\textpm\fi{}3.0)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$. We reduce systematic errors compared to previous works by substantially improving the spectral wavelength calibration method and by fitting absorption profiles to the forest of hydrogen Lyman $\ensuremath{\alpha}$ transitions surrounding each ${\mathrm{H}}_{2}$ transition. Our results are consistent with no variation, and inconsistent with a previous $\ensuremath{\approx}4\ensuremath{\sigma}$ detection of $\ensuremath{\mu}$ variation involving $\mathrm{Q}0405\ensuremath{-}443$ and $\mathrm{Q}0347\ensuremath{-}383$. If the results of this work and those suggesting that $\ensuremath{\alpha}$ may be varying are both correct, then this would tend to disfavor certain grand unification models.

LiNbO3 ground- and excited-state properties from first-principles calculations

Abstract: The atomic and electronic structure, zone center phonon frequencies, and optical absorption of $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}$ are calculated from first principles. The structural and vibrational properties predicted from density functional theory are in good agreement with experiment and earlier theoretical work. The electronic band-structure and optical properties are found to be very sensitive to quasiparticle and electron-hole attraction effects, which are included here using the $GW$ approach and by solving the Bethe-Salpeter equation, respectively. We predict the fundamental gap to be more than $1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ larger than the $3.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ frequently cited for ferrolectric $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_{3}$ and calculate optical absorption spectra in good agreement with experiment.

Mechanism of the Far-Infrared Absorption of Carbon-Nanotube Films

Abstract: The far-infrared conductivity of single-wall carbon-nanotube ensembles is dominated by a broad absorption peak around 4 THz whose origin is still debated. We observe an overall depletion of this peak when the nanotubes are excited by a short visible laser pulse. This finding excludes optical absorption due to a particle-plasmon resonance and instead shows that interband transitions in tubes with an energy gap of $\ensuremath{\sim}10\text{ }\text{ }\mathrm{meV}$ dominate the far-infrared conductivity. A simple model based on an ensemble of two-level systems naturally explains the weak temperature dependence of the far-infrared conductivity by the tube-to-tube variation of the chemical potential.

Electronic structures of hexagonal R Mn O 3 ( R = Gd , Tb, Dy, and Ho) thin films: Optical spectroscopy and first-principles calculations

Abstract: We investigated the electronic structure of multiferroic hexagonal $R\mathrm{Mn}{\mathrm{O}}_{3}$ ($R=\mathrm{Gd}$, Tb, Dy, and Ho) thin films using both optical spectroscopy and first-principles calculations One of the difficulties in explaining the electronic structures of hexagonal $R\mathrm{Mn}{\mathrm{O}}_{3}$ is that they exist in nature with limited rare earth ions (ie, $R=\mathrm{Sc}$, Y, and Ho-Lu), so a systematic study in terms of the different $R$ ions has been lacking Recently, our group succeeded in fabricating hexagonal $R\mathrm{Mn}{\mathrm{O}}_{3}$ ($R=\mathrm{Gd}$, Tb, and Dy) using the epitaxial stabilization technique [Adv Mater (Weinheim Ger) 18, 3125 (2006)] Using artificially stabilized hexagonal $R\mathrm{Mn}{\mathrm{O}}_{3}$, we extended the optical spectroscopic studies on the hexagonal multiferroic manganite system We observed two optical transitions located near 17 and $23\phantom{\rule{03em}{0ex}}\mathrm{eV}$, in addition to the predominant absorption above $5\phantom{\rule{03em}{0ex}}\mathrm{eV}$ With the help of first-principles calculations, we attributed the low-lying optical absorption peaks to interband transitions from the oxygen states hybridized strongly with different Mn orbital symmetries to the $\mathrm{Mn}\phantom{\rule{02em}{0ex}}3{d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ state As the ionic radius of the rare earth ion increased, we observed a systematic increase of the lowest peak position, which became more evident when compared with previously reported results We explained this systematic change in terms of a flattening of the $\mathrm{Mn}{\mathrm{O}}_{5}$ triangular bipyramid

On-site interband excitations in resonant inelastic x-ray scattering from Cu 2 O

Abstract: The electronic structure of cuprite $({\mathrm{Cu}}_{2}\mathrm{O})$ has been studied by high-resolution x-ray photoemission (XPS), x-ray absorption (XAS), and resonant x-ray emission spectroscopies (XES) supported by band structure calculations using a hybrid exchange approximation to density functional theory. A pronounced loss feature at about $4.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ due to on-site interband excitation has been identified in resonant inelastic x-ray scattering from ${\mathrm{Cu}}_{2}\mathrm{O}$ close to the ${L}_{3}$ $(\mathrm{Cu}\phantom{\rule{0.2em}{0ex}}2{p}_{3∕2})$ core threshold. Although ${\mathrm{Cu}}_{2}\mathrm{O}$ nominally has a filled upper valence band of $\mathrm{Cu}\phantom{\rule{0.2em}{0ex}}3d$ states and an empty conduction band of $\mathrm{Cu}\phantom{\rule{0.2em}{0ex}}4s$ states, the band structure calculations show that there is substantial $3d$ character in the conduction band and that the inelastic loss is dominated by on-site $3d$ to $3d$ excitation conforming to the selection rule $\ensuremath{\Delta}l=0$ rather than $3d$ to $4s$ transitions with $\ensuremath{\Delta}l=\ensuremath{-}2$. However, unlike in previous work, these transitions do not arise from ligand field splitting of the $\mathrm{Cu}\phantom{\rule{0.2em}{0ex}}3d$ states but rather from on-site $3d\text{\ensuremath{-}}4s$ hybridization which introduces $3d$ character into the conduction band. Comparison between XPS, XES, and XAS data shows that Cu ${L}_{3}$ XAS is dominated by a core exciton lying $0.65\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the bottom of the conduction band and that inelastic scattering is only observed for photon energies below that required to excite the core electron into the conduction band.

Hydrothermal synthesis, between 75 and 150°c, of high-charge, ferric nontronites

Abstract: High-charge nontronites were synthesized at 75, 90, 100, 110, 125, and 150°C from a silicoferrous starting gel with Si2FeNa2O6.nH2O composition. This gel was oxidized in contact with air and then hydrothermally treated, for a period of 4 weeks, under equilibrium water pressure. The synthesized nontronites were similar to each other, regardless of the synthesis temperature. Their structural formula, obtained from chemical analysis, X-ray diffraction (XRD), and Fourier transform infrared (FTIR), Mossbauer, and X-ray absorption fine structure spectroscopies is: $\left( {{\rm{S}}{{\rm{i}}_{3.25}}{\rm{Fe}}_{0.75}^{3 + }} \right){\rm{Fe}}_2^{3 + }{{\rm{O}}_{10}}{\left( {{\rm{OH}}} \right)_2}{\rm{N}}{{\rm{a}}_{0.75}}$ . A strictly ferric end-member of the nontronite series was therefore synthesized for the first time. The uncommon chemistry of the synthesized nontronites, notably the high level of Fe-for-Si substitution, induced particular XRD, FTIR, and differential thermal analysis-thermogravimetric analysis data. The ethylene glycol expandability of the synthetic nontronites was linked to their crystallinity and depended on the nature of the interlayer cation, moving from smectite to vermiculite-like behavior. As the synthesis temperature increased, the crystallinity of the synthesized clays increased. The nontronite obtained at 150°C had the ‘best crystallinity’, which cannot be improved by increasing synthesis time or temperature.

Size dependence of L2,3 branching ratio and 2p core-hole screening in x-ray absorption of metal clusters.

Abstract: Resonant $2p$ x-ray absorption spectra of size-selected transition metal ions and clusters consisting of $1\ensuremath{\le}n\ensuremath{\le}200$ atoms are reported. Remnants of atomic multiplet splitting in ${L}_{2,3}$-edge x-ray absorption can be resolved up to the trimer, above which the overall line shape is already bulklike. A strong cluster size dependence of the ${L}_{2,3}$ branching ratio was found for titanium, vanadium, and cobalt clusters. While $3d$ electron delocalization increases with cluster size, the apparent $2p$ spin-orbit splitting remains constant within the error bars. The size dependence of the ${L}_{2,3}$ branching ratio can be attributed to cluster size specific screening of the $2p--3d$ Coulomb interaction by $3d/4s$ valence electrons.

Bound-bound transitions in hydrogenlike ions in Debye plasmas

Abstract: Plasma screening effects on the properties of bound-bound transitions of hydrogenlike ions imbedded in Debye plasmas are investigated The electron eigenenergies and wave functions are determined by numerically solving the scaled Schr\"odinger equation with a Debye potential by the fourth-order symplectic integration scheme The scaled spectral properties of hydrogenlike ions in the plasma, including the transition frequencies, absorption oscillator strengths, radiative transition probabilities, as well as the line intensities of the Lyman and Balmer series, are presented for a wide range of plasma screening parameters While for the $\ensuremath{\Delta}n\ensuremath{ e}0$ transitions the oscillator strengths and spectral line intensities decrease with increasing the plasma screening, those for the $\ensuremath{\Delta}n=0$ transitions rapidly increase The lines associated with the $\ensuremath{\Delta}n\ensuremath{ e}0$ transitions are redshifted, whereas those for $\ensuremath{\Delta}n=0$ transitions are blueshifted Comparison of present results with those of other authors, when available, is made The results reported here should be useful in the interpretation of spectral properties of hydrogenlike ions in laboratory and astrophysical Debye plasmas

Dynamics of femtosecond laser-induced melting of silver

Abstract: We use optical third-harmonic generation to measure the melting dynamics of silver following femtosecond laser excitation. The dynamics reveals an unusual two-step process that is associated with the extreme electronic temperatures and very short time and length scales. In the first, which lasts a few picoseconds, the electron and phonon systems begin to equilibrate, and a thin surface layer undergoes melting. Heat conduction during this period is strongly suppressed by electron scattering from $d$-band excitations. In the second stage, the surface region remains above the melting temperature for a surprisingly long time, 20--30 ps, with the melt front propagating into the bulk at a velocity of $\ensuremath{\approx}350\text{ }\text{m}\text{ }{\text{s}}^{\ensuremath{-}1}$. In this stage, the electron and phonon systems again fall out of equilibrium and conduction of heat away from the surface region is now limited by the weak electron-phonon $(e\text{\ensuremath{-}}p)$ coupling. From our model calculation, we propose that the melt depths in noble metals irradiated by femtosecond lasers are limited to thicknesses on the order of two to three times of the optical-absorption depth of the light.

Temperature dependence of the structural parameters of gold nanoparticles investigated with EXAFS

Abstract: The ${L}_{3}$ edge of Au nanoparticles, having sizes ranging from $2.4\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}5.0\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, have been investigated by x-ray absorption fine structure spectroscopy in the temperature range of $20--300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Data were recorded at the European Synchrotron Radiation Facility with a very good signal to noise ratio. To achieve a very high accuracy in the determination of the first shell distance, a very careful data analysis was performed also taking into account the presence of asymmetry effects. In all samples, the temperature dependence of the first neighbor distance results is different from that of the macrocrystalline counterpart. In the largest size samples, a reduction of the thermal expansion was found, whereas in the smallest ones, the presence of a crossover from an initial thermal expansion to a thermal contraction was observed. Calculations based on a simple model show that localization effects that increase as the nanoparticle size decreases can explain the reported thermal effects.

Evidence of substitutional Co ion clusters in Zn 1-x Co x O dilute magnetic semiconductors

Abstract: Structural studies become increasingly important for understanding the nature of magnetic properties in dilute magnetic semiconductors (DMSs) [K. Ando, Science 312, 1883 (2006)]. In this work, x-ray absorption fine structure (XAFS) at both Co and $\text{O}\text{ }K$ edges is used to investigate the Co occupation sites and distributions in ${\text{Zn}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{O}$ thin films with low Co concentrations $(x=0.005,0.02)$. $\text{Co}\text{ }K$-edge XAFS analysis indicates unambiguously that the doped Co ions are substantially incorporated into the ZnO host lattice. However, the $\text{O}\text{ }K$-edge XAFS spectral features are strikingly different from those of ZnO and ${\text{Zn}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{O}$ reported in the literature so far, and can be reproduced by the assembling of substitutional Co ions around intervening oxygen atoms. This provides experimental evidence for numerous theoretical predications that the substitutional Co ions in ZnO tend to gather together. It is expected that similar phenomenon can also be observed by XAFS in a number of other ZnO-, GaN-, and ${\text{TiO}}_{2}$-based DMSs for which the clustering of substitutional dopants have been previously predicted.

X-ray Linear Dichroism in cubic compounds: the case of Cr3+ in MgAl2O4

Abstract: The angular dependence (x-ray linear dichroism) of the $\text{Cr}\text{ }K$ pre-edge in ${\text{MgAl}}_{2}{\text{O}}_{4}:{\text{Cr}}^{3+}$ spinel is measured by means of x-ray absorption near-edge structure spectroscopy and compared to calculations based on density functional theory (DFT) and ligand field multiplet (LFM) theory. We also present an efficient method, based on symmetry considerations, to compute the dichroism of the cubic crystal starting from the dichroism of a single substitutional site. DFT shows that the electric dipole transitions do not contribute to the features visible in the pre-edge and provides a clear vision of the assignment of the $1s\ensuremath{\rightarrow}3d$ transitions. However, DFT is unable to reproduce quantitatively the angular dependence of the pre-edge, which is, on the other side, well reproduced by LFM calculations. The most relevant factors determining the dichroism of $\text{Cr}\text{ }K$ pre-edge are identified as the site distortion and $3d\text{\ensuremath{-}}3d$ electronic repulsion. From this combined DFT, LFM approach is concluded that when the pre-edge features are more intense than 4% of the edge jump, pure quadrupole transitions cannot explain alone the origin of the pre-edge. Finally, the shape of the dichroic signal is more sensitive than the isotropic spectrum to the trigonal distortion of the substitutional site. This suggests the possibility to obtain quantitative information on site distortion from the x-ray linear dichroism by performing angular dependent measurements on single crystals.

State preparation by optical pumping in erbium-doped solids using stimulated emission and spin mixing

Abstract: Erbium-doped solids are potential candidates for the realization of a quantum memory for photons at telecommunication wavelengths. The implementation of quantum memory proposals in rare-earth-ion-doped solids require spectral tailoring of the inhomogeneous absorption profile by efficient population transfer between ground-state levels (spin polarization) using optical pumping. In this paper we investigate the limiting factors of efficient optical pumping between ground-state Zeeman levels in an erbium-doped ${\mathrm{Y}}_{2}\mathrm{Si}{\mathrm{O}}_{5}$ crystal. We introduce two methods to overcome these limiting factors: stimulated emission using a second laser and spin mixing using radio frequency excitation. Both methods significantly improve the degree of spin polarization. Population transfer between two Zeeman levels with less than 10% of the total population in the initial ground state is achieved, corresponding to a spin polarization greater than 90%. In addition, we demonstrate spectral tailoring by isolating a narrow absorption peak within a large transparency window.

Dependence of the BALQSO fraction on Radio Luminosity

Abstract: We find that the fraction of classical Broad Absorption Line quasars (BALQSOs) among the FIRST radio sources in the Sloan Data Release 3, is 20.5^{+7.3}_{-5.9}% at the faintest radio powers detected (L_{\rm 1.4 GHz}~10^{32} erg/s), and rapidly drops to <8% at L_{\rm 1.4 GHz}~3*10^{33} erg/s. Similarly, adopting the broader Absorption Index (AI) definition of Trump et al. (2006) we find the fraction of radio BALQSOs to be 44^{+8.1}_{-7.8}% reducing to 23.1^{+7.3}_{-6.1}% at high luminosities. While the high fraction at low radio power is consistent with the recent near-IR estimates by Dai et al. (2008), the lower fraction at high radio powers is intriguing and confirms previous claims based on smaller samples. The trend is independent of the redshift range, the optical and radio flux selection limits, or the exact definition of a radio match. We also find that at fixed optical magnitude, the highest bins of radio luminosity are preferentially populated by non-BALQSOs, consistent with the overall trend. We do find, however, that those quasars identified as AI-BALQSOs but \emph{not} under the classical definition, do not show a significant drop in their fraction as a function of radio power, further supporting independent claims for which these sources, characterized by lower equivalent width, may represent an independent class with respect to the classical BALQSOs. We find the balnicity index, a measure of the absorption trough in BALQSOs, and the mean maximum wind velocity to be roughly constant at all radio powers. We discuss several plausible physical models which may explain the observed fast drop in the fraction of the classical BALQSOs with increasing radio power, \emph{although no one is entirely satisfactory}. (abridged).

Fundamental photophysics and optical loss processes in Si-nanocrystal-doped microdisk resonators

Abstract: We report a detailed analytical, numerical, and experimental study of microdisk resonators doped with nanometer sized silicon quantum dots (nanocrystals). An intuitive analytical ray-optics-based model is developed and used to capture the behavior of the quality factor $(Q)$ as a function of the disk size and the attenuation coefficient. Two regimes in the behavior of $Q$ with the disk size establish a simple design rule for optimizing the performance of these cavities. The validity of our analytical model is verified by full-vectorial finite element method calculations of the microcavity modes. Based on the predictions of the analytical and numerical calculations, we have fabricated microdisk resonators with diameters ranging between 2 and $8\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$. $Qg{10}^{3}$ are obtained for disk radii as small as $4\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$---highest observed for Si-nanocrystal-doped microdisk resonators. The fundamental limit on $Q$ is estimated by quantifying all of the potential optical loss processes through a careful analysis which includes the effects of nanocrystal size distribution. Our theoretical calculations match well with experiments and reveal that the line-edge roughness scattering and radiation loss can be minimized sufficiently to enable study and quantification of more fundamental optical loss processes of this material due to band-to-band absorption, Mie scattering, and free-carrier absorption in the Si nanocrystals. Using the experimental $Q$'s and the mode volumes, we predict the maximum low-temperature Purcell enhancement factor in our structures on the order of 6 and with some design improvements enhancements up to 50 can be realized.

Primordial black holes in the Dark Ages: Observational prospects for future 21cm surveys

Abstract: We consider the signatures of a population of primordial black holes (PBHs) in future observations of 21cm radiation from neutral hydrogen at high redshift. We focus on PBHs in the mass range $5 \times 10^{10} kg \lesssim M_{PBH} \lesssim 10^{14} kg$, which primarily influence the intergalactic medium (IGM) by heating from direct Hawking radiation. Our computation takes into account the black hole graybody factors and the detailed energy dependence of photon and e+/- absorption by the IGM. We find that for black holes with initial masses between $5 \times 10^{11} kg \lesssim M_{PBH} \lesssim 10^{14} kg$, the signal mimics that of a decaying dark matter species. For black holes in the range $5 \times 10^{10} kg \lesssim M_{PBH} \lesssim 5 \times 10^{11} kg$, the late stages of evaporation produce a characteristic feature in the 21cm brightness temperature that provides a unique signature of the black hole population. If no signal is observed, then 21cm observations will provide significantly better constraints on PBHs in the mass range $5 \times 10^{10} kg \lesssim M_{PBH} \lesssim 10^{12} kg$ than are currently available from the diffuse $\gamma$-ray background.

Orbital hybridization and spin polarization in the resonant 1s photoexcitations of a-Fe2O3

Abstract: A spin-selective and polarization dependent study at the $K$ absorption pre-edge in hematite $(\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3})$ was performed by means of $1s3p$ resonant x-ray emission spectroscopy on a single crystal. The experimental results can be modeled using a band structure approach [local density approximation (LDA + $U$)]. The spin-selective spectra in connection with the calculations firmly establish metal-oxygen and metal-metal orbital hybridizations. A phase shift in the polarization dependent absorption cross section is observed between the first two absorption features showing directionality in the orbital hybridization.

Efficient positronium laser excitation for antihydrogen production in a magnetic field

Abstract: Antihydrogen production by charge exchange reaction between positronium (Ps) atoms and antiprotons requires an efficient excitation of Ps atoms up to high-$n$ levels (Rydberg levels). In this study it is assumed that a Ps cloud is produced within a relatively strong uniform magnetic field $(1\phantom{\rule{0.3em}{0ex}}\mathrm{T})$ and with a relatively high temperature $(100\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. Consequently, the structure of energy levels are deeply modified by Zeeman and motional Stark effects. A two-step laser light excitation, the first one from ground to $n=3$ and the second from this level to a Rydberg level, is proposed and the physics of the problem is discussed. We derive a simple formula giving the absorption probability with substantially incoherent laser pulses. A 30% population deposition in high-$n$ states can be reached with feasible lasers suitably tailored in power and spectral bandwidth.

Structural and magnetic properties of epitaxial Fe/CoO bilayers on Ag(001)

Abstract: We have investigated the magnetic coupling between a metallic ferromagnet and an oxidic antiferromagnet in epitaxial single-crystalline $\mathrm{Fe}∕\mathrm{Co}\mathrm{O}$ bilayers on Ag(001) using x-ray absorption spectroscopy. Absorption spectra taken from bilayers with different amounts of deposited Fe show only a weak indication for the formation of Fe oxide at the $\mathrm{Fe}∕\mathrm{Co}\mathrm{O}$ interface. From the spectral shape, it is concluded that an FeO type of oxide is formed. X-ray magnetic circular dichroism (XMCD) measurements exhibit a sizable induced ferromagnetic signal at the $\mathrm{Co}\phantom{\rule{0.2em}{0ex}}{L}_{2,3}$ absorption edges, corresponding to an interface layer of 1.1 ML of CoO in which the Co magnetic moments couple with the Fe moments. The angular dependence of the Fe XMCD and Co x-ray magnetic linear dichroic signals at the ${L}_{2,3}$ edges shows that the orientation of the Co and Fe spins is parallel along the crystallographic ⟨110⟩ directions.

Electronic spectra and crystal field analysis of Yb^ in SrCl_2

Abstract: The $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ emission spectrum and absorption spectrum between 27 000 and $30\phantom{\rule{0.2em}{0ex}}000\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ have been recorded and assigned for $\mathrm{Sr}{\mathrm{Cl}}_{2}:{\mathrm{Yb}}^{2+}$ (0.05 and $1\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$), where the divalent ion is situated at a site of ${\mathrm{O}}_{h}$ symmetry with eightfold coordination to chloride ions. In emission, two broad bands are observed at room temperature and these are assigned to orbitally forbidden and allowed transitions from $4{f}^{13}5d$ to the $4{f}^{14}$ ground state $^{1}S_{0}$. Nearly all of the transitions, except for some of the highest energy absorption bands, are spin forbidden in the sense that the transition final states are dominated by spin triplet components, while the initial state is a pure spin singlet. The spectral intensities in the absorption spectrum have been fitted satisfactorily, following the crystal field calculation for $4{f}^{13}5d$ in which four parameters were freely varied. There is no need to cite the presence of ${\mathrm{Yb}}^{2+}$ ions at several sites in order to explain the observed absorption bands, as has been done previously, but we invoke the coparticipation of the (low-lying) $4{f}^{13}6s$ configuration. The rationale for the parameter values is explained in detail as are the effects of parameter variations. The calculated radiative lifetime for the orbitally allowed emission transition is in agreement with that measured at low temperature $(\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s})$ and the nonradiative rate from the luminescent level has been modeled as a function of temperature.

Vibrational stability and electronic structure of a B 80 fullerene

Abstract: We investigate the vibrational stability and the electronic structure of the proposed icosahedral fullerenelike cage structure of ${\text{B}}_{80}$ [N. G. Szwacki, A. Sadrzadeh, and B. I. Yakobson, Phys. Rev. Lett. 98, 166804 (2007)], by an all electron density-functional theory using polarized Gaussian basis functions containing 41 basis functions per atom. The vibrational analysis of ${\text{B}}_{80}$ indicates that the icosahedral structure is vibrationally unstable with seven imaginary frequencies. The equilibrium structure has ${T}_{h}$ symmetry and a smaller gap of 0.96 eV between the highest occupied and the lowest unoccupied molecular orbital energy levels compared to the icosahedral structure. The static dipole polarizability of a ${\text{B}}_{80}$ cage is $149\text{ }{\text{\AA{}}}^{3}$, and the first ionization energy is 6.4 eV. The ${\text{B}}_{80}$ cage has rather large electron affinity of 3 eV making it a useful candidate as electron acceptor if it is synthesized. The infrared and Raman spectra of the highly symmetric structure are characterized by a few absorption peaks.

HD as a probe for detecting mass variation on a cosmological time scale.

Abstract: The strong electronic absorption systems of the B{sup 1}{sigma}{sub u}{sup +}-X{sup 1}{sigma}{sub g}{sup +} Lyman and the C{sup 1}{pi}{sub u}-X{sup 1}{sigma}{sub g}{sup +} Werner bands can be used to probe possible mass-variation effects on a cosmological time scale from spectra observed at high redshift, not only in H{sub 2} but also in the second most abundant hydrogen isotopomer HD. High resolution laboratory determination of the most prominent HD lines at extreme ultraviolet wavelengths is performed at an accuracy of {delta}{lambda}/{lambda}{approx}5x10{sup -8}, forming a database for comparison with astrophysical data. Sensitivity coefficients K{sub i}=dln{lambda}{sub i}/dln{mu} are determined for HD from quantum ab initio calculations as a function of the proton-electron mass ratio {mu}. Strategies to deduce possible effects beyond first-order baryon/lepton mass ratio deviations are discussed.