Showing papers on "Absorption (logic) published in 1998"

Gauge Theory Correlators from Non-Critical String Theory

Abstract: We suggest a means of obtaining certain Green's functions in 3+1-dimensional ${\cal N} = 4$ supersymmetric Yang-Mills theory with a large number of colors via non-critical string theory. The non-critical string theory is related to critical string theory in anti-deSitter background. We introduce a boundary of the anti-deSitter space analogous to a cut-off on the Liouville coordinate of the two-dimensional string theory. Correlation functions of operators in the gauge theory are related to the dependence of the supergravity action on the boundary conditions. From the quadratic terms in supergravity we read off the anomalous dimensions. For operators that couple to massless string states it has been established through absorption calculations that the anomalous dimensions vanish, and we rederive this result. The operators that couple to massive string states at level $n$ acquire anomalous dimensions that grow as $2\left (n g_{YM} \sqrt {2 N} )^{1/2}$ for large `t Hooft coupling. This is a new prediction about the strong coupling behavior of large $N$ SYM theory.

Direct determination of the exciton binding energy of conjugated polymers using a scanning tunneling microscope

Abstract: We report scanning tunneling spectroscopy measurements of the threshold energy for injecting electrons or holes into thin, conjugated polymer films deposited on Au(111) substrates. Combining these results with optical absorption measurements, we estimate an exciton binding energy of ${E}_{b}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.36\ifmmode\pm\else\textpm\fi{}0.10\mathrm{eV}$ for poly[(2-methoxy-5-dodecyloxy)-1,4-phenylenevinylene-co-1,4-phenylenevinylene] and ${E}_{b}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.30\ifmmode\pm\else\textpm\fi{}0.10\mathrm{eV}$ for poly(9,9'-dioctylfluorene). In addition, we determine the alignment of the electronic levels of the polymers relative to the substrate.

Excitonic Effects and the Optical Absorption Spectrum of Hydrogenated Si Clusters

Abstract: We calculate the optical absorption spectrum of hydrogen-terminated silicon clusters by solving the Bethe-Salpeter equation for the two-particle Green{close_quote}s function using an {ital ab initio} approach. The one-particle Green{close_quote}s function and the electron-hole interaction kernel are calculated within the GW approximation for the electron self-energy operator. Very large exciton binding energies are observed. Our results for the one-particle properties and the optical absorption spectra of the clusters are in very good agreement with available experimental data. {copyright} {ital 1998} {ital The American Physical Society}

Where do we stand with solar neutrino oscillations

Abstract: We determine parameters for MSW and vacuum oscillations (active and sterile neutrinos) that are allowed by separate, and collective, imposition of the constraints from total event rates in the chlorine, GALLEX, SAGE, and SuperKamiokande experiments (504 days), the SuperKamiokande energy spectrum, and the SuperKamiokande zenith-angle dependence. The small mixing angle MSW solution is acceptable at 7% C.L. (8% for sterile \ensuremath{ u}'s) and the vacuum solution is acceptable at 6% C.L. The best-fit global MSW solution for active neutrinos is $\ensuremath{\Delta}{m}^{2}=5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}{\mathrm{eV}}^{2},$ ${\mathrm{sin}}^{2}2\ensuremath{\theta}=5.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ (and for sterile neutrinos $\ensuremath{\Delta}{m}^{2}=4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}{\mathrm{eV}}^{2},$ ${\mathrm{sin}}^{2}2\ensuremath{\theta}=7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}).$ For vacuum oscillations, the best-fit solution is $\ensuremath{\Delta}{m}^{2}=6.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}{\mathrm{eV}}^{2},$ ${\mathrm{sin}}^{2}2\ensuremath{\theta}=0.75.$ An arbitrary combination of undistorted (no oscillations) $pp,$ ${}^{7}\mathrm{Be},$ ${}^{8}\mathrm{B},$ and CNO neutrino fluxes is inconsistent with the combined data sets at the 3.5\ensuremath{\sigma} C.L., independent of astrophysical considerations. We use improved calculations of solar model fluxes, neutrino absorption cross sections and energy spectra, and a detailed evaluation of regeneration effects.

The Power Spectrum of Mass Fluctuations Measured from the Lyman-alpha Forest at Redshift z=2.5

Abstract: We measure the linear power spectrum of mass density fluctuations at redshift z=2.5 from the \lya forest absorption in a sample of 19 QSO spectra, using the method introduced by Croft et al. (1998). The P(k) measurement covers the range 2\pi/k ~ 450-2350 km/s (2-12 comoving \hmpc for \Omega=1). We examine a number of possible sources of systematic error and find none that are significant on these scales. In particular, we show that spatial variations in the UV background caused by the discreteness of the source population should have negligible effect on our P(k) measurement. We obtain consistent results from the high and low redshift halves of the data set and from an entirely independent sample of nine QSO spectra with mean redshift z=2.1. A power law fit to our measured P(k) yields a logarithmic slope n=-2.25 +/- 0.18 and an amplitude \Delta^2(k_p) = 0.57^{+0.26}_{-0.18}, where $\Delta^2$ is the contribution to the density variance from a unit interval of lnk and k_p=0.008 (km/s)^{-1}. Direct comparison of our mass P(k) to the measured clustering of Lyman Break Galaxies shows that they are a highly biased population, with a bias factor b~2-5. The slope of the linear P(k), never previously measured on these scales, is close to that predicted by models based on inflation and Cold Dark Matter (CDM). The P(k) amplitude is consistent with some scale-invariant, COBE-normalized CDM models (e.g., an open model with \Omega_0=0.4) and inconsistent with others (e.g., \Omega=1). Even with limited dynamic range and substantial statistical uncertainty, a measurement of P(k) that has no unknown ``bias factors'' offers many opportunities for testing theories of structure formation and constraining cosmological parameters. (Shortened)

Tunable polariton absorption of distributed feedback microcavities at room temperature

Abstract: We have demonstrated well-separated tunable polariton absorption for a semiconductor-cavity composite system in transmission measurements at room temperature. A distributed feedback microcavity of the fourth order is fabricated by spin coating a self-organized inorganic/organic multiple quantum wells, $({\mathrm{C}}_{6}{\mathrm{H}}_{5}{\mathrm{C}}_{2}{\mathrm{H}}_{4}{\mathrm{NH}}_{3}{)}_{2}{\mathrm{PbI}}_{4}$, on a corrugated quartz substrate with a period of about $0.7 \ensuremath{\mu}\mathrm{m}$. By changing the grating period or the incident angle, the absorption dips exhibit anticrossing behavior. Owing to the large excitonic oscillator strength of the material, the polariton mode splitting is as large as $100 \mathrm{meV}$ even at room temperature. At the normal incidence, an exciton and a light form a strongly coupled standing wave, which corresponds to a cavity polariton in Fabry-Perot semiconductor microcavities.

Crossover from large to small polarons across the metal-insulator transition in manganites

Abstract: We report Mn $K$-edge extended x-ray absorption fine structure spectra on ${\mathrm{La}}_{0.75}{\mathrm{Ca}}_{0.25}{\mathrm{MnO}}_{3}$ up to high momentum transfer across the metal-insulator $(M\ensuremath{-}I)$ transition. The data show compelling evidence for (i) large or intermediate Jahn-Teller polarons (IJTP), characterized by an anomalous longer Mn-O bond $(\ensuremath{\Delta}R\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.09\AA{})$ in the metallic phase $(Tl170\mathrm{K})$, and (ii) appearance of small JT polarons (SJTP) at $Tg170\mathrm{K}$, characterized by a longer Mn-O bond $(\ensuremath{\Delta}R\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.21\AA{})$, which coexist with the IJTP above the $M\ensuremath{-}I$ transition and has equal probability in the temperature range of colossal magnetoresistance.

Theory of optical absorption in diamond, Si, Ge, and GaAs

Abstract: We report first-principles calculations of ${\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})$ for diamond, Si, Ge, and GaAs that include the electron-hole interaction in detail. The only essential experimental input is the crystal structure of the materials. Comparison with reflectivity and ellipsometry measurements allows us to assess the validity of the approximations made in the calculation. We conclude that the approximation of singly excited electronic states and a statically screened electron-hole interaction is sufficient to understand the main features of the absorption spectra of these materials in the visible to near ultraviolet.

Comprehensive analysis of intermolecular charge-transfer excited states in C 60 and C 70 films

Abstract: We have investigated intermolecular charge transfer (CT) excited states and demonstrated their contribution to the excitation-relaxation and photocarrier generation mechanisms for both ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ polycrystalline films. This has been done (1) experimentally, using UV-visible absorption (Abs) and electroabsorption (EA), luminescence and its modulation by external electric-field and steady-state photoconductivity; (2) semiempirically, analyzing the Abs and EA spectra in order to determine the CT state energies and to estimate the variation of the average polarizability tensor and the dipole moment; and (3) theoretically, by performing calculations of polarization energies and electrostatic stabilization energies. The most significant CT states have been identified at approximately 2.43 and 3.50 eV for ${\mathrm{C}}_{60}$ films and at 2.26 and 3.60 eV for ${\mathrm{C}}_{70}$ films (data from the semiempirical analysis). Their properties have been discussed in terms of crystallographic and electronic structure, with special emphasis on their mixing with Frenkel states.

APM 08279+5255: an ultraluminous BAL quasar at a redshift z=3.87

Abstract: We report on the discovery of a highly luminous, broad absorption line quasar at a redshift of $z=3.87$ which is positionally coincident, within one arcsecond, with the IRAS FSC source F08279+5255. A chance alignment of the quasar and the IRAS source is extremely unlikely and we argue that the optical and FIR flux are different manifestations of the same object. With an R-band magnitude of 15.2, and an IRAS 60$\mum$ flux of $0.51\jy$, APM 08279+5255 is (apparently) easily the most intrinsically luminous object known, with $ L_{Bol}\sim5\times10^{15}L_{\odot}}$. Imaging suggests that gravitational lensing may play a role in amplifying the intrinsic properties of the system. The optical spectrum of the quasar clearly reveals the presence of three potential lensing galaxies, \mg absorption systems at $z=1.18$ and $z=1.81$, and a \ly absorption system at $z=3.07$. We estimate the total amplification of the optical component to be $\approx40$, but, due to the larger scale of the emitting region, would expect the infrared amplification to be significantly less. Even making the conservative assumption that all wavelengths are amplified by a factor 40, APM 08279+5255 still possesses a phenomenal luminosity of $\simgt 10^{14L_{\odot}}$, indicating that it belongs to a small, but significant population of high--redshift, hyperluminous objects with copious infrared emission.

Phase Transitions in Confined Gallium Droplets

Abstract: Phase transitions in submicrometric Ga droplets confined in epoxy resin are studied by combining energy-dispersive x-ray diffraction (EDXRD), x-ray absorption fine structure, and single-energy x-ray absorption. The restricted fluid is undercooled down to 150 K while the melting point is depressed down to 254 K. Melting and freezing are sharp processes occurring with temperature broadening of 1 and 10 K, respectively. EDXRD patterns are consistent with that of $\ensuremath{\beta}\ensuremath{-}\mathrm{Ga}$, while the stable phase at ambient conditions $\ensuremath{\alpha}\ensuremath{-}\mathrm{Ga}$ is not found to exist. Appearance of $\ensuremath{\gamma}\ensuremath{-}\mathrm{Ga}$ and $\ensuremath{\delta}\ensuremath{-}\mathrm{Ga}$ solid phases and relevance of present results to recent studies of Ga confined in porous glass are discussed.

Localized exciton and its stimulated emission in surface mode from single-layer InxGa1-xN

Abstract: Exciton localization in ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ was studied. At 2 K, the time-integrated photoluminescence (PL) spectrum showed a Stokes shift from the absorption shoulder and broadening at the lower photon energy side. Site-selectively excited PL measurements determined the mobility edge. The exciton relaxation processes were studied by use of time-resolved PL spectroscopy. The PL decay time increased with the decrease of the detection-photon energy, indicating the dynamical features of exciton localization. In addition, we observed localized exciton luminescence turned into stimulated emission just below the mobility edge.

Inclusion of local structure effects in theoretical x-ray resonant scattering amplitudes using ab initio x-ray-absorption spectra calculations

Abstract: Improved calculations of Bragg peak intensities near atomic resonance are obtained by including the effect of the local environment around the resonant atoms on the resonant scattering amplitudes $\ensuremath{\Delta}{f=f}^{\ensuremath{'}}{+if}^{\ensuremath{''}}.$ Theoretical absorption cross sections calculated by the ab initio x-ray-absorption code FEFF are used to obtain the imaginary part ${f}^{\ensuremath{''}}$ by extension of the optical theorem to nonforward scattering under the dipole approximation. The real part ${f}^{\ensuremath{'}}$ is obtained by a limited range Kramers-Kronig transform of the difference between ${f}^{\ensuremath{''}}$ based on FEFF and existing theoretical calculations of ${f}^{\ensuremath{''}}$ based on an isolated-atom model. The atomic part of $\ensuremath{\Delta}f$ calculated by FEFF for the resonant atom embedded in the local potential is assumed to have spherical symmetry; however, no restriction is placed on the spectral features due to multiple scattering of the intermediate-state virtual photoelectron. Bragg peak intensities calculated in the kinematic approximation using the FEFF-based $\ensuremath{\Delta}f$ are compared to intensities calculated using the isolated-atom $\ensuremath{\Delta}f$ and to experimental data for Cu metal and ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.8}$ at the Cu K absorption edge, and for ${\mathrm{UO}}_{2}$ at the U ${M}_{\mathrm{IV}}$ absorption edge.

Optical transitions in Mn 3 + -doped garnets

Abstract: The optical spectra of ${\mathrm{Mn}}^{3+}$-doped garnet crystals reveal a large Jahn-Teller stabilization energy of about $1900{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for the ${}^{5}E$ ground state, and smaller Jahn-Teller stabilization energies for the excited states, i.e., $\ensuremath{\approx}325{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for the ${}^{5}{T}_{2},$ and $\ensuremath{\approx}180{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for the ${}^{1}{T}_{2}$ level. The absorption spectra are dominated by the spin-allowed ${}^{5}{E}^{5}{T}_{2}$ transition. At low temperatures, the emission occurs from the ${}^{1}{T}_{2}$ level to the Jahn-Teller-split ground state and the ${}^{3}{T}_{1}$ intermediate level. With increasing temperature the ${}^{5}{T}_{2}$ level becomes thermally populated and the emission spectrum is dominated by the spin-allowed ${}^{5}{\stackrel{\ensuremath{\rightarrow}}{{T}_{2}}}^{5}E$ transition. The emission lifetime is nearly independent of the detection wavelength, but strongly dependent of the temperature and the host lattice. At 12 K the lifetime is $\ensuremath{\approx}6\mathrm{ms}$ for all crystals, while at room temperature it is between 1.1 ms for ${\mathrm{Mn}}^{3+}{:\mathrm{Y}}_{3}{\mathrm{Al}}_{5}{\mathrm{O}}_{12}$ and $l0.5\ensuremath{\mu}\mathrm{s}$ for ${\mathrm{Mn}}^{3+}{:\mathrm{G}\mathrm{d}}_{3}{\mathrm{Sc}}_{2}{\mathrm{Ga}}_{3}{\mathrm{O}}_{12}.$ The radiative lifetimes of the ${}^{1}{T}_{2}$ and ${}^{5}{T}_{2}$ levels were determined to be about 6 ms and 16 \ensuremath{\mu}s, respectively. Both the radiative and the nonradiative rate are temperature dependent due to the coupling of odd-parity and totally-symmetric phonons, and the thermalization of the ${}^{5}{T}_{2}$ level. The nonradiative decay is more pronounced for lower crystal-field strengths, because of the smaller ${}^{5}{T}_{2}{\ensuremath{-}}^{3}{T}_{1}$ energy gap and the higher population of the ${}^{5}{T}_{2}$ level. Excited-state absorption transitions arising from the energetically lower ${}^{1}{T}_{2}$ in higher lying singlet levels cover the entire spectral range of the emission; therefore laser oscillation at room temperature is unlikely in ${\mathrm{Mn}}^{3+}$-doped garnets.

Neutrino Transport in Strongly Magnetized Proto-Neutron Stars and the Origin of Pulsar Kicks: The Effect of Asymmetric Magnetic Field Topology

Abstract: In proto-neutron stars with strong magnetic fields, the cross section for νe (${u{{ν}}{"7016}}$ --> -->e) absorption on neutrons (protons) depends on the local magnetic field strength resulting from the quantization of energy levels for the e- (e+) produced in the final state. If the neutron star possesses an asymmetric magnetic field topology in the sense that the magnitude of magnetic field in the north pole is different from that in the south pole, then asymmetric neutrino emission may be generated. We calculate the absorption cross sections of νe and ${u{{ν}}{"7016}}$ --> -->e in strong magnetic fields as a function of the neutrino energy. These cross sections exhibit oscillatory behaviors that occur because new Landau levels for the e- (e+) become accessible as the neutrino energy increases. By evaluating the appropriately averaged neutrino opacities, we demonstrate that the change in the local neutrino flux caused by the modified opacities is rather small. To generate appreciable kick velocity (~300 km s-1) to the newly formed neutron star, the difference between the field strengths at the two opposite poles of the star must be at least 1016 G. We also consider the magnetic field effect on the spectral neutrino energy fluxes. The oscillatory features in the absorption opacities give rise to modulations in the emergent spectra of νe and ${u{{ν}}{"7016}}$ --> -->e.

Optical Absorption and Nonradiative Decay Mechanism of E ′ Center in Silica

Abstract: We report ab initio configuration interaction calculations on the optical transitions of the ${E}^{\ensuremath{'}}$ center, a hole trapped at an oxygen vacancy, $(---\mathrm{O}{)}_{3}{\mathrm{Si}}^{\ifmmode\bullet\else\textbullet\fi{}}$ ${}^{+}\mathrm{Si}(\mathrm{O}---{)}_{3}$, in silica. We found two competing excitation mechanisms: (1) promotion of one electron from an $\mathrm{O}(2p)$ valence band orbital to the singly occupied Si dangling bond; (2) charge transfer (CT) transition from $(---\mathrm{O}{)}_{3}{\mathrm{Si}}^{\ifmmode\bullet\else\textbullet\fi{}}$ to ${}^{+}\mathrm{Si}(\mathrm{O}---{)}_{3}$. The two excitations occur at similar energies, $\ensuremath{\approx}5.8--6\mathrm{eV}$ (5.85 eV in the experiment), but only the CT has a strong intensity. The excitation is followed by a complex nonradiative decay process which may explain the absence of luminescence for this center.

Adsorption energies and ordered structures of hydrogen on Pd(111) from density functional periodic calculations

Abstract: The full three-dimensional potential energy surface of H on Pd(111) has been calculated with periodic band-structure computations using the generalized gradient approximation of density-functional theory. The fcc hollow site was found to be most stable, followed by the hcp hollow site. Excellent agreement with experimental values of the adsorption energy and the vibrational frequencies was achieved. Subsurface occupation at low coverages and low temperatures is ruled out by our results, but there are no or very low barriers for hydrogen reaching the subsurface region from the molecular gas phase, thus direct absorption is feasible at high coverages. Different ordered structures of the adsorbed hydrogen were considered, and we found that two structures with $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}R30\ifmmode^\circ\else\textdegree\fi{}$ symmetry are most stable at low temperatures, in agreement with experiment. Results for the adsorption energies and effective hydrogen-hydrogen interactions showed that only fcc hollow sites were occupied on the ordered structures, also in agreement with experiment.

Angular momentum sum rules for x-ray absorption

Abstract: The sum rules for circular and linear x-ray dichroism, which relate the signals of the core absorption edges to the expectation values of the valence spin and orbital operators, are expressed in $\mathrm{jj}$-coupled operators. By including the cross terms between the $j=l\ifmmode\pm\else\textpm\fi{}1/2$ ground-state levels these sum rules are no longer restricted to $\mathrm{jj}$ coupling but are equally valid in intermediate coupling. The physical significance of these---usually very large---cross terms is discussed.

Effects of the isotopic composition on the fundamental gap of CuCl

Abstract: We have investigated the effects of isotopic composition on the band gap of CuCl on a series of samples made out of the stable isotopes ${}^{63}\mathrm{Cu}$, ${}^{65}\mathrm{Cu}$, ${}^{35}\mathrm{Cl}$, and ${}^{37}\mathrm{Cl}$. Besides specimens containing elements with the natural abundances, we have measured samples with monoisotopic sublattices as well as artificial mixtures of isotopes. With nonlinear (two-photon absorption, second-harmonic generation) and linear (luminescence) optical spectroscopy we find that the fundamental gap of CuCl increases by 364(18) \ensuremath{\mu}eV/amu when increasing the Cl mass. However, it decreases by 76(5) \ensuremath{\mu}eV/amu when increasing the Cu mass. Using a two-oscillator model for the lattice dynamics of CuCl we show that these rates are consistent with the anomalous increase of the band gap with increasing temperature. These effects can be traced back to the strong $p$-$d$ mixing in the copper halides. From the temperature dependence of the band gap of CuBr we also estimate the changes of its gap with isotopic composition.

Investigation of three-dimensional grain-boundary structures in oxides through multiple-scattering analysis of spatially resolved electron-energy-loss spectra

Abstract: Grain boundaries in oxide materials such as electroceramics, ferroelectrics, and high-${T}_{c}$ superconductors are known to dominate their overall bulk properties. The critical first step in a fundamental understanding of how they control the properties of the material is a determination of the atomic structure of the boundary. While this determination has traditionally been performed by transmission-electron microscopy, the images that are generated are only a two-dimensional projection of the atomic columns in the grain-boundary core. In addition, as the images are least sensitive to light elements, such as oxygen, the complete three-dimensional boundary structure is particularly difficult to determine. Employing electron-energy-loss spectroscopy in a scanning transmission-electron microscope, it is possible to obtain an oxygen K-edge spectrum that contains information on the three-dimensional electronic structure of the boundary. Using the multiple-scattering methodology, originally developed for x-ray absorption near-edge structure, this can be directly related to the local three-dimensional atomic structure. Contained in the spectrum is therefore all of the information needed to investigate the atomic scale structure-property relationships at grain boundaries. The application of the technique is demonstrated here for the 25\ifmmode^\circ\else\textdegree\fi{} [001] symmetric tilt boundary in ${\mathrm{SrTiO}}_{3}.$

Cold and dense cesium clouds in far-detuned dipole traps

Abstract: We study the behavior of cesium atoms confined in far-detuned laser traps and submitted to blue Sisyphus cooling. First, in a single focused yttrium aluminum garnet (YAG) beam, the atomic cloud has a rod shape with a $6\ensuremath{-}\ensuremath{\mu}\mathrm{m}$ transverse waist radius, a temperature of $2\ensuremath{\mu}\mathrm{K}$, and a transient density of 10${}^{12}{\phantom{\rule{0ex}{0ex}}\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{c}\mathrm{m}}^{3}$. For this sample, we have not detected any influence of photon multiple scattering on the atomic temperature, in contrast to previous measurements in isotropic samples. Second, the YAG laser is used to produce a $29\ensuremath{-}\ensuremath{\mu}\mathrm{m}$ period hexagonal optical lattice in which we directly observe the localization of the atoms by absorption imaging. Equilibrium densities on the order of ${10}^{13}{\phantom{\rule{0ex}{0ex}}\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{c}\mathrm{m}}^{3}$ are achieved in this structure.

Direct calculation of inverse-bremsstrahlung absorption in strongly coupled, nonlinearly driven laser plasmas

Abstract: Inverse-bremsstrahlung absorption in strongly coupled plasmas produced by high-intensity lasers is studied numerically. The simultaneous presence of high density and intensity makes it difficult to treat this problem with standard methods. A technique for modeling collisional plasmas is demonstrated which uses a hierarchical tree code---an accelerated molecular dynamics algorithm with an $N$ log $N$ computation time---adapted to model periodic, non-equilibrium two-component plasmas. Good agreement is found with standard theoretical results for classical, weakly coupled plasmas. In a series of further simulations, the dependence of the inverse-bremsstrahlung absorption coefficient on plasma coupling parameter, laser frequency, and the ratio of quiver to thermal velocity ${v}_{0}{/v}_{\mathrm{te}}$ is computed. An important outcome of this study is that the Langdon effect---a change of the velocity distribution function due to an imbalance of heating and equilibration rates---is verified in a direct microscopic particle simulation.

Excited-state optical transitions of excitons and biexcitons in a single In x Ga 1 − x As quantum disk

Abstract: The photoluminescence of ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum disks is investigated by microscope photoluminescence (PL) and PL excitation spectroscopies. The excited-state transitions of excitons and biexcitons are studied in terms of their dependence on excitation photon polarization. Their origins are analyzed on the basis of symmetry properties expected of excitons and biexcitons. Dominant excited-state resonances are ascribed to successive two-step absorptions of two photons of opposite optical orientation, one into an exciton excited state followed by another into a densely distributed biexciton excited state. Filling of the exciton state is found to lead to absorption from the exciton ground state to the biexciton excited state. Remaining excitation resonances with large oscillator strength are identified as two-photon absorption processes that directly create weakly correlated exciton pair states.

1s2p resonant inelastic x-ray scattering in alpha-Fe2O3

Abstract: We report experimental and theoretical results on the Fe K edge x-ray absorption spectrum and $1s2p$ resonant inelastic x-ray scattering (RIXS) spectra in $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}.$ The results are interpreted using an ${\mathrm{FeO}}_{6}^{9\ensuremath{-}}$ cluster model with intra-atomic multiplet coupling and interatomic covalency hybridization. The $1s2p$ RIXS is treated as a coherent second-order optical process. It is shown that the double-peak structure in the pre-edge region of Fe K absorption spectrum is due to the cubic crystal-field splitting, and that the intensity of the ${e}_{g} {(t}_{2g})$ component in the $1s2p$ resonant inelastic spectrum is enhanced by tuning the incident photon energy to the ${e}_{g} {(t}_{2g})$ component in the absorption spectrum.

Hydrogen atoms in KH 2 PO 4 crystals

Abstract: Hydrogen atoms have been produced at 77 K in single crystals of potassium dihydrogen phosphate (${\mathrm{KH}}_{2}{\mathrm{PO}}_{4}$ or KDP) using either 60-kV x rays or the fourth harmonic (266 nm) of a pulsed Nd:YAG (yttrium aluminum garnet) laser. The electron-paramagnetic-resonance spectrum from these hydrogen atoms exhibits a characteristic 500-G hyperfine splitting and is easily saturated with microwave power. These atoms occupy interstitial sites and are electron traps. The corresponding hole trap is the well-known $({\mathrm{HPO}}_{4}{)}^{\ensuremath{-}}$ hole center identified by its 31-G phosphorus hyperfine splitting. Both the hydrogen atoms and the hole centers thermally decay in the temperature range between 80 and 200 K. The observed displacement of protons by 266-nm photons provides direct evidence in support of the proton-transport mechanism recently proposed by Davis, Hughes, and Lee [Chem. Phys. Lett. 207, 540 (1993)] to explain laser-induced transient optical absorption at room temperature in KDP.

New precision measurement of the pionic deuterium s -wave strong interaction parameters

Abstract: The x rays of the pionic deuterium 2p-1s transition were measured with a high resolution crystal spectrometer including a cyclotron trap (a magnetic device to increase the pion stopping density) and a CCD (charge-coupled device) detector system. The 1s strong interaction shift ${\ensuremath{\epsilon}}_{1s}$ and total width ${\ensuremath{\Gamma}}_{1s}$ were determined from the position and line shape of the x-ray peak. The (complex) pionic deuterium s-wave scattering length ${a}_{{\ensuremath{\pi}}^{\ensuremath{-}}d}$ was deduced. Its real part was related to the pion-nucleon scattering lengths, and the isoscalar coupling constant for ${\ensuremath{\pi}}^{\ensuremath{-}}$ absorption was deduced from the imaginary part.

Effect of hydroxyl bonds on persistent spectral hole burning in Eu 3+ -doped BaO-P 2 O 5 glasses

Abstract: The effect of OH bonds on the optical absorption and persistent spectral hole burning (PSHB) properties was quantitatively analyzed in ${\mathrm{Eu}}^{3+}$-doped $\mathrm{BaO}\ensuremath{-}{\mathrm{P}}_{2}{\mathrm{O}}_{5}$ glasses. Glasses were prepared by melting the raw material at 600--$800\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ in which the OH content was changed. The hole was burned in the ${}^{7}{\stackrel{\ensuremath{\rightarrow}}{{F}_{0}}}^{5}{D}_{0}$ transition of the ${\mathrm{Eu}}^{3+}$ ions at 6 K and the dependence of the PSHB properties on temperature and time was measured. The bond covalency between ${\mathrm{Eu}}^{3+}$ and oxygens decreased and the hole depth linearly increased with increasing the OH content in the glass. The proposed model was that the hole was burned by the optically activated rearrangement of the OH bonds surrounding the ${\mathrm{Eu}}^{3+}$ ions. The hole burnt at 6 K was refilled with increasing time and temperature and an average thermal barrier height for the hole filling was $\ensuremath{\sim}140 \mathrm{meV},$ which was smaller than that for the ${\mathrm{Eu}}^{3+}$ ions doped in silica and silicate glasses.

Competition between lattice distortion and charge dynamics for the charge carriers of double-layered manganites

Abstract: We have investigated the characteristics of the charge carriers of the double-layered manganites, ${\mathrm{La}}_{2\ensuremath{-}2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}(x=0.35,$ 0.40), using polarized O $1s$ x-ray absorption spectroscopy. When the system undergoes the insulator to metal transition, the spectral weight is transferred to the absorption threshold, analogous to the Drude weight in optical absorption. The charge carriers in the metallic phase contain comparable amounts of in-plane and out-of-plane character. This result is due to the competition between the tetragonal lattice distortion and the charge dynamics that force the charge carriers out of and in the plane, respectively.

Transmission through a many-channel random waveguide with absorption

Abstract: We compute the statistical distribution of the transmittance of a random waveguide with absorption in the limit of many propagating channels. We consider the average and fluctuations of the conductance $T=\mathrm{tr}{t}^{\ifmmode\dagger\else\textdagger\fi{}}t$, where $t$ is the transmission matrix, the density of transmission eigenvalues $\ensuremath{\tau}$ (the eigenvalues of ${t}^{\ifmmode\dagger\else\textdagger\fi{}}t)$, and the distribution of the plane-wave transmittances ${T}_{a}$ and ${T}_{\mathrm{ab}}.$ For weak absorption (length $L$ smaller than the exponential absorption length ${\ensuremath{\xi}}_{a})$, we compute moments of the distributions, while for strong absorption $(L\ensuremath{\gg}{\ensuremath{\xi}}_{a})$, we can find the complete distributions. Our findings explain recent experiments on the transmittance of random waveguides by Stoytchev and Genack [Phys. Rev. Lett. 79, 309 (1997)].