Showing papers on "Absorption (logic) published in 1983"

Dielectric functions and optical parameters of Si, Ge, GaP, GaAs, GaSb, InP, InAs, and InSb from 1.5 to 6.0 eV

Abstract: We report values of pseudodielectric functions $〈\ensuremath{\epsilon}〉=〈{\ensuremath{\epsilon}}_{1}〉+i〈{\ensuremath{\epsilon}}_{2}〉$ measured by spectroscopic ellipsometry and refractive indices $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{n}=n+ik$, reflectivities $R$, and absorption coefficients $\ensuremath{\alpha}$ calculated from these data. Rather than correct ellipsometric results for the presence of overlayers, we have removed these layers as far as possible using the real-time capability of the spectroscopic ellipsometer to assess surface quality during cleaning. Our results are compared with previous data. In general, there is good agreement among optical parameters measured on smooth, clean, and undamaged samples maintained in an inert atmosphere regardless of the technique used to obtain the data. Differences among our data and previous results can generally be understood in terms of inadequate sample preparation, although results obtained by Kramers-Kronig analysis of reflectance measurements often show effects due to improper extrapolations. The present results illustrate the importance of proper sample preparation and of the capability of separately determining both ${\ensuremath{\epsilon}}_{1}$ and ${\ensuremath{\epsilon}}_{2}$ in optical measurements.

Optical absorption from polarons in a model of polyacetylene

Abstract: We present a theoretical calculation, within a continuum electron-phonon-coupled model, of the optical absorption due to polarons in polyacetylene [${(\mathrm{CH})}_{x}$]. Our results can be applied to both trans- and cis-${(\mathrm{CH})}_{x}$, as well as potentially to other polymers (polypyroles, polyparaphenylenes) in which polarons are present. We establish that the essential signature of polaron absorption is the existence of three separate absorption peaks in the gap with qualitatively different features. For trans-${(\mathrm{CH})}_{x}$, we compare and contrast this structure with that from the kink solitons, which are expected to dominate the optical absorption at all but the lowest doping levels. For cis-${(\mathrm{CH})}_{x}$ and related polymers, we discuss polaron (and multipolaron) absorption and the relation of polarons to possible excitons in these materials. Finally, we evaluate briefly the existing experimental situation regarding optical absorption in polyacetylene and indicate possible future experiments that could confirm the existence of polarons in ${(\mathrm{CH})}_{x}$ and similar polymers.

Electron-spin-resonance studies of pyrrole polymers: Evidence for bipolarons

Abstract: ESR measurements have been performed on samples of neutral polypyrrole, polypyrrole-perchlorate, oxygen-doped polypyrrole, and poly($\ensuremath{\beta}$,${\ensuremath{\beta}}^{\ensuremath{'}}$-dimethylpyrrole perchlorate). A narrow Lorentzian line is observed in oxidized, highly conducting films, but no correlation between conductivity and either linewidth or intensity is found. Electrochemically cycled films, which remain highly conducting, show little or no detectable ESR absorption. It is deduced that the ESR signal seen in the as-prepared material does not arise from the current-carrying species, but rather from accidental neutral $\ensuremath{\pi}$-radical defects. The absence of paramagnetism in the metallic state is discussed within the framework of bipolaron formation.

Oxygen-bonding environments in glow-discharge-deposited amorphous silicon-hydrogen alloy films

Abstract: This paper presents the results of a systematic study of oxygen incorporation in $a\ensuremath{-}\mathrm{Si}:\mathrm{H}$ alloys produced by the glow-discharge decomposition of Si${\mathrm{H}}_{4}$, ${\mathrm{H}}_{2}$, and ${\mathrm{O}}_{2}$. We identify four oxygen-related absorption bands, at 2090, 980, 780, 500 ${\mathrm{cm}}^{\ensuremath{-}1}$, and show that the absorption strength in each band scales linearly with the oxygen concentration. We demonstrate that oxygen can increase the solubility of hydrogen in $a\ensuremath{-}\mathrm{Si}$ in the monohydride bonding geometry. The features identified above are shown to be characteristic of a bonding site in which the oxygen and hydrogen atoms are bonded to the same silicon atom. We find no features in the infrared absorption that are associated with bonding configurations having OH groups. In films containing both oxygen and polysilane bonding, as evidenced by the doublet absorption at 845 and 890 ${\mathrm{cm}}^{\ensuremath{-}1}$, we find no evidence for bonding sites in which a substantial fraction of the silicon atoms have one oxygen and two hydrogen neighbors.

Simple Rules for Discontinuities in Finite Temperature Field Theory

Abstract: The discontinuity, or imaginary part, of the self-energy function at $T\ensuremath{ e}=0$ is found to be of the form $\ensuremath{\Gamma}={\ensuremath{\Gamma}}_{d}\ensuremath{\mp}{\ensuremath{\Gamma}}_{i}$ for bosons and fermions, respectively. The generalized decay rate ${\ensuremath{\Gamma}}_{d}$ and inverse decay rate ${\ensuremath{\Gamma}}_{i}$ are recognizable as integrals over phase space of amplitudes squared, weighted with certain statistical factors that account for the possibility of particle absorption from the medium or particle emission into the medium. Nonequilibrium statistical mechanics shows that $\ensuremath{\Gamma}$ gives precisely the rate at which the single-particle distribution function approaches the equilibrium form.

Theory of defects in vitreous silicon dioxide

Abstract: Models for defects in Si${\mathrm{O}}_{2}$ fall into the two basic categories of "vacancy-bridge" and valence-alternation models. We have calculated the local electronic structure of the main defects in each model, using the tight-binding and recursion methods. The localization of each level is found and compared to that measured by ESR for the paramagnetic centers. The silicon dangling bond, the neutral oxygen vacancy, and the positively charged oxygen vacancy (${E}^{\ensuremath{'}}$ center) all give deep states near mid-gap. The Si---Si bond gives a bonding state in the lower gap and an antibonding state near the conduction-band minimum. The positive, threefold-coordinated oxygen site $\mathrm{O}_{3}^{}{}_{}{}^{+}$(${\mathrm{Si}}_{3}$) gives a state bound only by its Coulombic field. In general, all positively charged centers possess a "shallow" bound state 1-2 eV below the conduction-band minimum. Such shallow states account for the prevalence of optical absorption around 7.6 eV in Si${\mathrm{O}}_{2}$. The nonbridging oxygen introduces states just above the valence-band maximum. The peroxyl bridge and radical give states both at mid-gap and in the lower part of the gap. A broad absorption band around 5-6 eV is associated with the peroxyl radical, for the first time. It is suggested that valence-alternation defects must still be present in $v\ensuremath{-}\mathrm{S}\mathrm{i}{\mathrm{O}}_{2}$, but at a much lower concentration, of order ${10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, than previously supposed, due to a higher valence-alternation creation energy in Si${\mathrm{O}}_{2}$ than in $a\ensuremath{-}\mathrm{S}\mathrm{e}$ or $a\ensuremath{-}{\mathrm{As}}_{2}{\mathrm{Se}}_{3}$.

Multiply charged ions induced by multiphoton absorption in rare gases at 0.53 \mu{}m

Abstract: Up to quadruply charged ions are induced in Xe atoms by a 50-psec laser pulse at 0.53 \ensuremath{\mu}m in the ${10}^{12}$ W ${\mathrm{cm}}^{\ensuremath{-}2}$ range. The mechanism of the formation of ${\mathrm{Xe}}^{2+}$ ions is elucidated. In the lowest intensity range, ${\mathrm{Xe}}^{2+}$ ions are produced by direct 15-photon absorption from the ground state of the atoms, while at a higher intensity ${\mathrm{Xe}}^{2+}$ ions are produced by a stepwise process via ${\mathrm{Xe}}^{+}$ ions. These two processes take place at distinctly different intensities. A kinetic model using rate equations affords a good fit with experimental results. Moreover, for the first time it was possible to obtain the multiphoton ionization cross sections related to the two-electron removal from Xe atoms, as well as the one-electron removal from ${\mathrm{Xe}}^{+}$ ions.

Magnetic and magneto-optic properties of lead- and bismuth-substituted yttrium iron garnet films

Abstract: The saturation magnetization ${M}_{s}$, the uniaxial anisotropy ${K}_{u}$, the optical absorption $\ensuremath{\alpha}$, the Faraday rotation ${\ensuremath{\theta}}_{F}$, and the Faraday ellipticity ${\ensuremath{\psi}}_{F}$ of epitaxial garent films of composition ${\mathrm{Y}}_{3\ensuremath{-}x}{\mathrm{Bi}}_{x}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ and ${\mathrm{Y}}_{3\ensuremath{-}y}{\mathrm{Pb}}_{y}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ have been investigated for $x\ensuremath{\le}1.7$ and $y\ensuremath{\le}0.25$. The magnetostriction constants ${\ensuremath{\lambda}}_{100}$,${\ensuremath{\lambda}}_{111}$ and the cubic anisotropy ${K}_{1}$ were studied on flux-grown crystals for $x\ensuremath{\le}1$. The temperature dependence of ${M}_{s}$, ${K}_{1}$, ${K}_{u}$, ${\ensuremath{\lambda}}_{100}$, ${\ensuremath{\lambda}}_{111}$, and ${\ensuremath{\theta}}_{F}$, ${\ensuremath{\psi}}_{F}$ at 633 nm has been measured in the range $4.2 \mathrm{K}\ensuremath{\le}T\ensuremath{\le}{T}_{c}$. The concentration dependence of these properties is linear. In particular, the contribution of the bismuth and lead to the Faraday rotation $\frac{\ensuremath{\Delta}{\ensuremath{\theta}}_{F}}{x}$ and $\frac{\ensuremath{\Delta}{\ensuremath{\theta}}_{F}}{y}$ at $\ensuremath{\lambda}=633$ nm turned out to be -25 400 and -18 500 deg ${\mathrm{cm}}^{\ensuremath{-}1}$ at $T=4.2$ K and -20 600 and -18 400 deg ${\mathrm{cm}}^{\ensuremath{-}1}$ at $T=295$ K, respectively. The temperature dependence of ${\ensuremath{\theta}}_{F}$ and ${\ensuremath{\psi}}_{F}$ can be described in terms of the sublattice magnetizations inferred from the fit of the molecular-field theory to the measured saturation magnetization. The extracted magneto-optical coefficients reveal a nonlinear concentration dependence. The magnitude of the growth-induced anisotropy is essentially controlled by the supercooling of the melt for both the lead- and bismuth-substituted films. The temperature dependence of ${K}_{u}^{g}$ is discussed in terms of the single-ion theory.

Anomalous Collision-Free Multiple Ionization of Atoms with Intense Picosecond Ultraviolet Radiation

Abstract: Collisionless multiphoton absorption, resulting in multiple atomic ionization and exhibiting anomalously strong coupling, has been studied in the region spanning atomic number $Z=2$ (He) to $Z=92$ (U). The highest ion state identified is ${\mathrm{U}}^{10+}$, corresponding to absorption of 99 quanta (\ensuremath{\sim}633 eV). Models of stepwise ionization using standard theoretical techniques are incapable of describing these results. A mode of interaction involving radiative coupling to a collective motion of an atomic shell is proposed.

Renormalization and radiative corrections at finite temperature

Abstract: We discuss the renormalization prescription for fermions at finite temperature and describe the procedure for calculating radiative corrections. Novel features arise, such as a lack of explicit Lorentz invariance, $\frac{1}{{k}^{2}}$ singularities, and the absorption of soft photons from the background heat bath. The methods are illustrated by explicit calculation of the electron renormalization and the radiative corrections to $H\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}$ (with $H$ being spin zero) in finite-temperature QED.

Dynamical behavior of the susceptibility around the freezing temperature in (Eu,Sr)S

Abstract: New measurements of the ac susceptibility in zero field for a (${\mathrm{Eu}}_{0.2}$${\mathrm{Sr}}_{0.8}$)S single crystal are reported. For the first time in a spin-glass we can resolve the frequency dependence of the inphase component ${\ensuremath{\chi}}^{\ensuremath{'}}$ (dispersion) as well as the out-of-phase component ${\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}$ (absorption). From these studies a clear change in the dynamic behavior of the system around the freezing temperature is found, which can be related to large ferromagnetic clusters. The susceptibility behavior is described in terms of a wide distribution of relaxation times characterizing the intercluster interactions.

Magnetic and magneto-optical properties of bismuth-substituted lutetium iron garnet films.

Abstract: Epitaxially grown bismuth-substituted lutetium iron garnet films of composition ${\mathrm{Lu}}_{3\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Bi}}_{\mathrm{x}}$${\mathrm{Fe}}_{5}$${\mathrm{O}}_{12}$ and (Lu,Y${)}_{3\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Bi}}_{\mathrm{x}}$${\mathrm{Fe}}_{5}$${\mathrm{O}}_{12}$ have been investigated with respect to the bismuth-induced changes of the saturation magnetization ${M}_{s}$, the Curie temperature ${T}_{C}$, the growth-induced anisotropy ${K}_{u}^{g}$, the optical absorption \ensuremath{\alpha}, and the Faraday rotation ${\ensuremath{\theta}}_{F}$. The composition and temperature dependence of ${M}_{s}$ and ${\ensuremath{\theta}}_{F}$ at \ensuremath{\lambda}=633 nm have been measured in the range 0\ensuremath{\le}x\ensuremath{\le}2.17 and 4.2 K\ensuremath{\le}T\ensuremath{\le}${T}_{C}$, respectively, revealing basically the same dependences on x as observed for bismuth-substituted yttrium and gadolinium iron garnet films. In contrast to the latter compositions, ${K}_{u}^{g}$ turns out to be much smaller in lutetium-bismuth iron garnet films and is significantly increased by small yttrium additions. The temperature dependence of ${\ensuremath{\theta}}_{F}$ can be described in terms of the sublattice magnetizations using constant magneto-optical coefficients. The measured temperature dependence of ${K}_{u}^{g}$ is compared with calculated results based on the single-ion theory.

Collapse of the 4f orbital for Xe-like ions

Abstract: The effect of $4f$ orbital collapse on the spectra of Xe-like ions in the region of $4d\ensuremath{\rightarrow}nf,\ensuremath{\epsilon}f$ excitations is studied with the use of a term-dependent Hartree-Fock technique. We find that the collapse of the $4f$ orbital is strongly term dependent, and that the appearance of intense absorption lines in the observed ${\mathrm{Ba}}^{2+}$ spectrum is due to the partial collapse of the $4f$ orbital in the $4d\ensuremath{\rightarrow}f^{1}P$ channel. We also show that after the $4f$ orbital is completely collapsed for high degrees of ionization along the isoelectronic sequence, the bulk of absorption oscillator strengths is concentrated at the $4{d}^{9}4f^{1}P$ level so that the only strong line in the spectrum is again the $4d\ensuremath{\rightarrow}4f^{1}P$ transition.

Point defects in α − Al 2 O 3 : Mg studied by electrical conductivity, optical absorption, and ESR

Abstract: Total and partial ionic and electronic $c$-axis dc conductivities were measured for single-crystal ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$:8 ppm Mg as a function of oxygen pressure and temperature. The sample is unsaturated at $Tg1360$ \ifmmode^\circ\else\textdegree\fi{}C but is saturated in equilibrium with a spinel precipitate phase at lower temperatures. The results are analyzed to give ionic mobility, thermodynamic parameters of oxidation reduction of ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$, and the solubility of MgO as a function of temperature. Oxidized samples are grayish purple due to the presence of a broad absorption band with maximum at 480 nm. Discoloration can occur by reduction (at high temperature) or by precipitation (at $Tl1000$ \ifmmode^\circ\else\textdegree\fi{}C); both proceed from the surface, involving migration of oxygen out of or of Al into the crystal. An explanation for the rapid diffusion regulating precipitation below 1000 \ifmmode^\circ\else\textdegree\fi{}C is proposed. Recoloration at low temperature proceeds without a front and possibly involves oxygen diffusing into the sample along dislocations or subgrain boundaries. Hole-release studies at low temperature indicate that the hole binding energy in the defect ${\mathrm{Mg}}_{\mathrm{Al}}^{x}$, ${E}_{a}\ensuremath{\le}0.68$ eV. The combined results of the present work suggest that free holes move as large, rather than small, polarons and that ${E}_{a}=0.68$ eV.

Temperature-dependent nuclear magnetic resonance in CuIn X 2 ( X = S , S e , T e ) chalcopyrite-structure compounds

Abstract: The NMR spectra of $^{63}\mathrm{Cu}$ and $^{115}\mathrm{In}$ in CuIn${\mathrm{S}}_{2}$, CuIn${\mathrm{Se}}_{2}$, and CuIn${\mathrm{Te}}_{2}$ have been measured between room temperature and 550\ifmmode^\circ\else\textdegree\fi{}C. The effects of motional narrowing and of phase transitions on the shape and width of the absorption spectra were evaluated. In all three compounds, the absorption signal of $^{63}\mathrm{Cu}$ shows motional narrowing. For ${\mathrm{Cu}}_{1.00}$${\mathrm{In}}_{1.00}$${\mathrm{S}}_{2.00}$ the mean jump frequency of the ${\mathrm{Cu}}^{+}$ ions, $\ensuremath{ u}({\mathrm{Cu}}^{+})$, is $2\ifmmode\times\else\texttimes\fi{}{10}^{13}\mathrm{exp}[\ensuremath{-}\frac{(1.25\ifmmode\pm\else\textpm\fi{}0.10 \mathrm{eV})}{\mathrm{kT}}]$ ${\mathrm{s}}^{\ensuremath{-}1}$. For both CuIn${\mathrm{Se}}_{2}$ and CuIn${\mathrm{Te}}_{2}$, $\ensuremath{ u}({\mathrm{Cu}}^{+})$ is about 3\ifmmode\times\else\texttimes\fi{}${10}^{3}$ ${\mathrm{s}}^{\ensuremath{-}1}$ at 350\ifmmode^\circ\else\textdegree\fi{}C. In copper-deficient ${\mathrm{Cu}}_{0.96}$${\mathrm{In}}_{1.02}$${\mathrm{S}}_{2.00}$ no motional narrowing of the $^{63}\mathrm{Cu}$ absorption was observed up to 550\ifmmode^\circ\else\textdegree\fi{}C. However, the $^{63}\mathrm{Cu}$ line shape in CuIn${\mathrm{S}}_{2}$ depends on the copper-to-indium ratio. A line-shape analysis as a function of composition shows that, at 450\ifmmode^\circ\else\textdegree\fi{}C, CuIn${\mathrm{S}}_{2}$ exists in the composition range between ${\mathrm{Cu}}_{1.00}$${\mathrm{In}}_{1.00}$${\mathrm{S}}_{2}$ and ${\mathrm{Cu}}_{0.85}$${\mathrm{In}}_{1.05}$${\mathrm{S}}_{2}$. For specimens with [Cu]/[In]g1, ${\mathrm{Cu}}_{2}$S contributes to the absorption signal. In a separate study of the $^{63}\mathrm{Cu}$ absorption in ${\mathrm{Cu}}_{2}$S, the hexagonal-to-cubic transformation was observed at 440\ifmmode^\circ\else\textdegree\fi{}C. The activation energy for $\ensuremath{ u}({\mathrm{Cu}}^{+})$ in cubic ${\mathrm{Cu}}_{2}$S is 0.15 eV. The phase-transformation temperatures for the three $\mathrm{CuIn}{X}_{2}$ compounds (where $X$ is a group-VI element) were determined by differential thermal analysis. A previously unnoticed transformation was found in CuIn${\mathrm{Se}}_{2}$ at 665\ifmmode^\circ\else\textdegree\fi{}C. Finally, the quadrupole coupling constant ${\ensuremath{\chi}}_{q}$ ($^{63}\mathrm{Cu}$) of a number of $\mathrm{Cu}Z{X}_{2}$ compounds (where $Z$ is a group-III element) is shown to be a linear function of the tetragonal distortion parameter $\ensuremath{\delta}=2\ensuremath{-}\frac{c}{a}$ with $|{\ensuremath{\chi}}_{q}|=1.50+90\ensuremath{\delta}$ MHz. The value for $\ensuremath{\delta}=0$ is of the same order of magnitude as the result of a simple model of a point charge on a cubic lattice.

K-shell excitation of D 2 O and H 2 O ice: Photoion and photoelectron yields

Abstract: The yield of photoelectrons and photoions from condensed ${\mathrm{H}}_{2}$O and ${\mathrm{D}}_{2}$O ice near and above the oxygen $K$ edge is reported. Results of studies in the near-edge region indicate that Rydberg excitations, which are quenched in the bulk, survive, to some extent, on the surface. Analysis of the extended x-ray absorption fine structure of the photoelectron yield above the edge yields an oxygen-oxygen scattering phase shift in good agreement with theory.

Potassium absorption into the graphite (0001) surface: Intercalation

Abstract: Evaporation of potassium onto the graphite (0001) surface results in intercalation when potassium is evaporated from a hot molecular-beam source onto a graphite sample with high surface step density. Low-energy electron-diffraction (LEED) and Auger-electron spectroscopy (AES) studies indicate that after initial absorption of potassium from a hot source, the graphite surface structure became $\frac{A}{\mathrm{ABAB}}\ensuremath{\cdots}$ stacking (where "/" indicates potassium), with a disordered potassium layer intercalated between two graphite layers which have been shear shifted and expanded to a separation of 5.35 \AA{}. Further adsorption of potassium onto the sample surface results in $\frac{A}{\frac{A}{\mathrm{ABAB}}}\ensuremath{\cdots}$ stacking with 5.35 \AA{} separation between each of the three top graphite layers, indicating the formation of a stage-1-like intercalate within the surface region although with disordered potassium. These structures indicate that potassium intercalates into the graphite (0001) surface in a layer-by-layer fashion.

High-pressure optical spectra of condensed oxygen

Abstract: Optical-absorption spectra (1.5-3.5 eV) of the fluid and three solid phases of ${\mathrm{O}}_{2}$ have been recorded up to 40 GPa near 298 K. The principal feature of the spectra of the fluid and rhombohedral $\ensuremath{\beta}\ensuremath{-}{\mathrm{O}}_{2}$ is the $2^{1}\ensuremath{\Delta}_{g}\ensuremath{\leftarrow}2^{3}\ensuremath{\Sigma}_{g}^{\ensuremath{-}}$ band; in $\ensuremath{\beta}\ensuremath{-}{\mathrm{O}}_{2}$, it is polarized perpendicular to the $c$ axis of the hexagonal cell. The energies of this and other transitions among the $^{3}\ensuremath{\Sigma}_{g}^{\ensuremath{-}}$, $^{1}\ensuremath{\Delta}_{g}$, and $^{1}\ensuremath{\Sigma}_{g}^{+}$ states increase with increasing pressure. The spectra and visual observations of the transformation between orthorhombic and $\ensuremath{\epsilon}\ensuremath{-}{\mathrm{O}}_{2}$, whose structure has not been determined, suggest that $\ensuremath{\epsilon}\ensuremath{-}{\mathrm{O}}_{2}$ has a layered structure. Absorption spectra of orthorhombic and $\ensuremath{\epsilon}\ensuremath{-}{\mathrm{O}}_{2}$ for light propagating perpendicular to the layers are dominated by a moderately strong band which moves from the uv through the visible with increasing pressure and a very strong ultraviolet band. These bands can be assigned as excitations from ${\ensuremath{\pi}}^{4}\ensuremath{-}{\ensuremath{\pi}}^{*2}$ states to ${\ensuremath{\pi}}^{3}\ensuremath{-}{\ensuremath{\pi}}^{*3}$ states which are mixed with intermolecular-charge-transfer excitations. Both bands move toward longer wavelengths at high pressures. At pressures above 13 GPa, absorption features for light propagating in the planes of the $\ensuremath{\epsilon}$-phase structure can be assigned as $2^{1}\ensuremath{\Delta}_{g}\ensuremath{\leftarrow}2^{3}\ensuremath{\Sigma}_{g}^{\ensuremath{-}}$. However, between 10 and 13 GPa, this feature is absent, and the pressure dependence of the Raman spectrum also is unusual. Both observations suggest that, between these pressures, the molecules reorient with respect to the crystallographic axes.

Far-infrared conductivity and anomalous below-gap absorption in superconducting granular NbN

Abstract: The far-infrared (10-100 ${\mathrm{cm}}^{\ensuremath{-}1}$) complex conductivity of superconducting granular NbN films has been determined from reflection and transmission data. Moderately granular films, with ${R}_{\ensuremath{\square}}=150$ and $200 \frac{\ensuremath{\Omega}}{\ensuremath{\square}}$, followed Leplae-modified Bardeen-Cooper-Schrieffer---theory predictions with no absorption below the gap frequency ${\ensuremath{\omega}}_{g}$. The derived values of $\frac{2\ensuremath{\Delta}}{k{T}_{c}}$ were 3.8 and 4.0, slightly below the results for homogeneous films. Highly granular films, with ${R}_{\ensuremath{\square}}\ensuremath{\ge}500 \frac{\ensuremath{\Omega}}{\ensuremath{\square}}$, displayed dc resistive tails and evidence of Josephson coupling. The real part of the far-infrared conductivity was BCS-type for frequencies above ${\ensuremath{\omega}}_{g}$ but showed an anomalous absorption below ${\ensuremath{\omega}}_{g}$. This excess absorption may arise from two-dimensional fluctuations in the form of vortices or from normal conducting regions mixed in with the superconducting grains.

Magneto-optical properties of gallium-substituted yttrium iron garnets

Abstract: The optical absorption $\ensuremath{\alpha}$, the Faraday rotation ${\ensuremath{\theta}}_{F}$, and the Faraday ellipticity ${\ensuremath{\psi}}_{F}$ of epitaxial and flux-grown garnets of composition ${{\mathrm{Y}}_{3}}[{\mathrm{Fe}}_{2\ensuremath{-}x}{\mathrm{Ga}}_{x}]({\mathrm{Fe}}_{3\ensuremath{-}y}{\mathrm{Ga}}_{y}){\mathrm{O}}_{12}$ with $0\ensuremath{\le}x+y\ensuremath{\le}2.95$ have been investigated. The wavelength dependence of $\ensuremath{\alpha}$ and ${\ensuremath{\theta}}_{F}$ was studied in the range $500l\ensuremath{\lambda}l1100$ nm at $T=295$ K, revealing a strong reduction with increasing gallium content except for wavelengths between 600 and 700 nm. In this range the variation on $x+y$ for both ${\ensuremath{\theta}}_{F}$ and $|{\ensuremath{\psi}}_{F}|$ exhibits a maximum depending on temperature. Both quantities approximately approach zero at $x+y\ensuremath{\approx}3$. The temperature dependence of ${\ensuremath{\theta}}_{F}$ and ${\ensuremath{\psi}}_{F}$ was measured in the range $4.2\mathrm{K}\ensuremath{\le}T\ensuremath{\le}{T}_{C}$ at $\ensuremath{\lambda}=633$ nm. It can be described in terms of the sublattice magnetizations which were inferred from the fit of the molecular field theory to the measured saturation magnetization. The magneto-optical coefficients have been determined as a function of the gallium content.

Exciton and magnon-sideband absorption in the pyroelectric antiferromagnet BaMn F 4

Abstract: Polarized absorption and luminescence spectra of the pyroelectric antiferromagnet BaMn${\mathrm{F}}_{4}$ are measured in the temperature range 4.5-300 K. Single-exciton bands associated with ${\mathrm{Mn}}^{2+}$ ions, as well as double-exciton bands, are observed. The temperature dependence of the integrated intensities of both bands suggests that exchange-assisted electric dipole transitions are dominantly involved. It is shown for the single-exciton band corresponding to the $^{6}A_{1g}\ensuremath{\rightarrow}^{4}T_{2g}$ (II) transition that the fine structure observed consists of an electric-dipole -allowed pure exciton band and hot- and cold-magnon sidebands. A three-magnon hot band is found for the first time. The analyses of optical anisotropy in the pure exciton band indicate that the ${\mathrm{Mn}}^{2+}$ ion is displaced from a nearly cubic position along the $a$-axis direction by the pyroelectric moment due to ${\mathrm{Ba}}^{2+}$ ions. The observed temperature dependences of the peak positions and of the half-widths of the magnon sidebands are consistent with the appearance of three-dimensional spin ordering below ${T}_{N}$, although BaMn${\mathrm{F}}_{4}$ is a predominantly two-dimensional antiferromagnet. An emission band observed at 600 nm is discussed involving short-range exciton transfer from ${\mathrm{Mn}}^{2+}$ to impurity-perturbed ${\mathrm{Mn}}^{2+}$ ions.

Collision effects in velocity-selective optical pumping of sodium

Abstract: We report on a quantitative experimental investigation of velocity-changing collisions by means of velocity-selective optical pumping (VSOP). We have calculated the VSOP line shape for an atom with hyperfine structure with the use of two phenomenological kernels for the collision effects: the Keilson-Storer kernel, and a two-term kernel consisting of a broad Keilson-Storer part and a narrower Gaussian component. Corrections were included to account for the finite absorption in the sample and the backward reflection of the pumping beam. The experiments were carried out in sodium vapor with neon as the perturber gas. The ${D}_{1}$ line of sodium was used for optical pumping, and the orientation of the ground state was detected. Free parameters of the theory were determined by fitting the predicted line shapes to experimental curves. The Keilson-Storer kernel proved unsatisfactory, but the two-term kernel reproduced well the observed line shapes over the entire collision profiles in the neon pressure range 0-57 mtorr. In an independent experiment using rapidly modulated VSOP we also measured directly the cross section of velocity-changing collisions: $\ensuremath{\sigma}=(1.13\ifmmode\pm\else\textpm\fi{}0.10)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ ${\mathrm{cm}}^{2}$. The large weight obtained for the narrow Gaussian from the fits, as well as the collision cross section which is three times as large as the cross section deduced from tabulated gas kinetic radii, may indicate the presence of collisions with relatively small velocity changes in addition to hard-sphere encounters.

L-edge x-ray absorption resonances in palladium silicides and palladium metal

Abstract: The $\mathrm{Pd}{L}_{2,3}$ white line absorption has been measured for Pd metal, ${\mathrm{Pd}}_{2}$Si, and PdSi to probe the variations in occupation of the $\mathrm{Pd}4d$ states due to the rehybridization of the $d$ band in the silicides. A reduction of the density of occupied states of $d$ character in ${\mathrm{Pd}}_{2}$Si and PdSi with respect to Pd metal is found, in agreement with partial-density-of-states calculations for the hybrid-bonded silicides.

Two-photon excitation of Ho 3 + in the Ca F 2 , Sr F 2 , and Cd F 2 lattices

Abstract: The observation of two-photon absorption by a very efficient up-converted emission is reported for ${\mathrm{Ho}}^{3+}$ in Ca${\mathrm{F}}_{2}$, Sr${\mathrm{F}}_{2}$, and Cd${\mathrm{F}}_{2}$ with both cw- and pulse-laser excitations. The two-photon excitation spectra are found to be highly structured and consist of two bands: The band $A$ is excited by the dye-laser photons of frequencies 15 400-15 700 ${\mathrm{cm}}^{\ensuremath{-}1}$: The band $B$ is excited by the dye-laser photons of frequencies 15 800-16 400 ${\mathrm{cm}}^{\ensuremath{-}1}$. No one-photon absorption is observed for ${\mathrm{Ho}}^{3+}$ in the above lattices at twice these frequencies. Studies of concentration dependence, temperature dependence, and dependence on the pulse repetition rate are used for the assignment of the band. The investigation shows that the two bands $A$ and $B$ arise from two different modes of two-photon excitation. The band $A$ is assigned to arise from a sequential (stepwise) two-photon excitation $^{5}I_{8}\ensuremath{\rightarrow}^{5}F_{5}\ensuremath{\rightarrow}^{5}D_{2}$. The band $B$ is assigned to arise from the direct two-photon absorption $^{5}I_{8}\ensuremath{\rightarrow}^{3}L_{8}$. Our results are in agreement with the prediction of the Judd-Ofelt theory of lanthanide transition intensities.

Small-angle neutron scattering study of anisotropic growth morphology and irreversible photodensification in a -Ge Se 3 films

Abstract: The anisotropic growth morphology of obliquely evaporated amorphous Ge${\mathrm{Se}}_{3}$ thin films has been studied by small-angle neutron scattering Films deposited at 80\ifmmode^\circ\else\textdegree\fi{} to the normal contain ellipsoidal voids 240 \AA{} in length and 120 \AA{} in diameter inclined at 70\ifmmode^\circ\else\textdegree\fi{} to the normal, and which account for 47% of the volume Absorption of band-gap light reduces the level and anisotropy of the scattering The ellipsoidal voids collapse to form smaller spherical voids distributed isotropically

Observation of Gamow-Teller strength distribution in the reaction /sup 71/Ga(p,n) /sup 71/Ge for application to solar-neutrino detection p

Abstract: The reaction $^{71}\mathrm{Ga}(p,n)^{71}\mathrm{Ge}$ has been studied at ${E}_{p}=35$ MeV with a high-resolution time-of-flight system. Individual cross sections to the states at 0.0, 0.175, and 0.500 MeV, which are directly related to solar-neutrino absorption in a proposed $^{71}\mathrm{Ga}$ detector, have been determined. The cross sections for the 0.0- and 0.175-MeV states are nearly equal, contrary to systematics in this mass region. As a result the absorption of solar neutrinos is estimated to be \ensuremath{\gtrsim}40% greater than previously calculated.

Measurement of quasielastic scattering of pions from o-16 at energies around the delta (1232) resonance

Abstract: Inelastic pion scattering from $^{16}\mathrm{O}$ has been studied by measuring the spectrum of pions down to 40 MeV at five angles between 30\ifmmode^\circ\else\textdegree\fi{} and 134\ifmmode^\circ\else\textdegree\fi{} for three incident pion energies: 114, 163, and 240 MeV. The spectra are dominated by a broad peak due to quasielastic scattering from a single nucleon in the nucleus. The systematics of the spectra are discussed with emphasis on the pion-nucleus interaction dynamics including the effect of absorption. The partial ${\ensuremath{\pi}}^{+}$-$^{16}\mathrm{O}$ cross sections for all nonradiative channels are estimated.NUCLEAR REACTIONS $^{16}\mathrm{O}(\ensuremath{\pi}, {\ensuremath{\pi}}^{\ensuremath{'}})$ $E=114, 163, 240$ MeV. $\ensuremath{\theta}=30\ensuremath{-}134\ifmmode^\circ\else\textdegree\fi{}$, full inelastic spectrum measured; $\ensuremath{\sigma}(\ensuremath{\theta})$. Discussion of nuclear medium effects on quasifree processes, $\ensuremath{\Delta}$ propagation.

Spin-Orbit Splitting of \(\varGamma\) Exciton Bands in RbI and KI

Abstract: The exciton absorption bands in RbI and KI are studied by means of magnetic circular dichroism spectra, and their effective g -values are obtained. From the sign of the g eff -values, it is concluded that the lowest two sharp absorption bands, E 1 and E 2 , are (\(\varGamma^{-}_{8}\); \(\varGamma^{+}_{6}\)) and (\(\varGamma^{-}_{6}\); \(\varGamma^{+}_{6}\)) excitons, respectively, which are split by the spin-orbit interaction of iodine 5 p electrons. The higher energy bands, E 3 and E 4 , are probably the exciton bands produced by excitation of halogen 5 p electrons to the d -like conduction bands at X and L points in the Brillouin zone.

Chemistry of the Phytochrome Photoconversions

Abstract: The photoconversions of phytochrome, P $\_r$ \rightleftharpoons P $\_{fr}$ , occur both in vivo and in vitro. Structural differences between P $\_r$ and P $\_{fr}$ are discussed for chromophore and apoprotein. The chemical structure of the P $\_r$ chromophore has been established. The P $\_{fr}$ chromophore was recently demonstrated to be the 15E isomer. The red shift of absorption to 730 nm in native P $_{fr}$ is discussed as interaction between chromophore and apoprotein. The nature of this interaction is still unknown. Small changes in the apoprotein surface are of particular interest because they could be part of the signal chain in photoperception.

Selective Photoexcitation of Atoms and Molecules

Abstract: The first stage of any selective laser photoprocess is the selective absorption of one or several laser photons by a desired atom or molecule (in the case of intermolecular selectivity) or by a molecular vibration or functional group (in the case of intramolecular selectivity) In other words, the rate of laser photoexcitation of A (molecules, bonds, etc) must be much higher than that for the rest, designated B It is possible to define the magnitude of the photoexcitation selectivity (sometimes called radiative selectivity) as the ratio of the probability of exciting A to a state with the required energy Eexc to the probability of finding B in a state with the same energy: $$S_{rad} (A/B) = {{P_A (E_{exc} )} \over {P_B (E_{exc} )}}$$ (21)