Showing papers on "Absorption (logic) published in 2007"

Electronic structure of rare-earth nitrides using the LSDA + U approach: Importance of allowing 4 f orbitals to break the cubic crystal symmetry

Abstract: Electronic structure calculations were performed for the rare-earth (RE) nitrides in the rocksalt structure using density functional theory calculations within the $\mathrm{LSDA}+U$ approach (local spin density approximation with Hubbard-$U$ corrections). The $\mathrm{LSDA}+U$ method is implemented in the full-potential linearized muffin-tin orbital method and applied to the $4f$ as well as $5d$ states. Parameters $U$ and $J$ were determined from atomic calculations complemented with experimental photoemission and inverse photoemission data and optical absorption data for Gd pnictides. The solution for the density matrix of $f$ electrons is not unique and thus several configurations need to be investigated to determine the lowest energy state. A trivalent solution is found to have the lowest energy in all cases except $\mathrm{CeN}$, which was found to be tetravalent. Hund's second rule requires maximizing the orbital momentum component ${L}_{z}$, which breaks the cubic symmetry and lowers the total energy. We find Hund's second rule to be obeyed in all cases except $\mathrm{EuN}$ and $\mathrm{YbN}$, where a cubic symmetry solution has lower energy. In these cases, the divalent solution is also in competition with the trivalent solution. The symmetry breaking in most cases lowers the total energy and in some cases, those with two electrons or holes away from a closed or half-filled shell, is essential to remove $f$ states from the Fermi level. The spin magnetic moments are nearly integer, defined by the number of filled $4f$ states. The orbital magnetic moment is of comparable magnitude to the spin moment. Hund's third rule, according to which the orbital and spin moment are opposite to each other in the first half of the series but parallel to each other in the second half, is also found to be obeyed. Interestingly, this leads to zero net magnetic moment for $\mathrm{SmN}$.Apart from the few cases where $f$ states remain close to the Fermi level, the band structure is borderline semiconductor to semimetallic in most cases, a RE $5d$ conduction band minimum at $X$, and a $\mathrm{N}\phantom{\rule{0.2em}{0ex}}2p$ valence band maximum at $\ensuremath{\Gamma}$. The early members of the series before $\mathrm{GdN}$ (with the exception of $\mathrm{NdN}$) are slightly semimetallic in the majority spin channel only and are thus half-metals, while the later members after Gd have a small indirect gap. In $\mathrm{EuN}$ a complicated hybridization occurs between an $f$ level pinned at ${E}_{F}$ and the $\mathrm{N}\phantom{\rule{0.2em}{0ex}}2p$ states, leading to a metallic band structure. Above the Curie temperature of these ferromagnets, in the paramagnetic state, one expects an average of majority and minority spin gaps and thus an increase in the gap.

Pulsed-laser-induced dewetting in nanoscopic metal films : Theory and experiments

Abstract: Hydrodynamic pattern formation (PF) and dewetting resulting from pulsed-laser-induced melting of nanoscopic metal films have been used to create spatially ordered metal nanoparticle arrays with monomodal size distribution on $\mathrm{Si}{\mathrm{O}}_{2}∕\mathrm{Si}$ substrates. PF was investigated for film thickness $h\ensuremath{\leqslant}7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}l$laser absorption depth $\ensuremath{\sim}11\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, and different sets of laser parameters, including energy density $E$ and the irradiation time, as measured by the number of pulses $n$. PF was only observed to occur for $E\ensuremath{\geqslant}{E}_{m}$, where ${E}_{m}$ denotes the $h$-dependent threshold energy required to melt the film. Even at such small length scales, theoretical predictions for ${E}_{m}$ obtained from a continuum-level lumped parameter heat transfer model for the film temperature, coupled with the one-dimensional transient heat equation for the substrate phase, were consistent with experimental observations provided that the thickness dependence of the reflectivity of the metal-substrate bilayer was incorporated into the analysis. The model also predicted that perturbations in $h$ would result in intrinsic thermal gradients $\ensuremath{\partial}T∕\ensuremath{\partial}h$ whose magnitude and sign depend on $h$, with $\ensuremath{\partial}T∕\ensuremath{\partial}hg0$ for $hl{h}_{c}$ and $\ensuremath{\partial}T∕\ensuremath{\partial}hl0$ for $hg{h}_{c}\ensuremath{\approx}9\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. For the thickness range investigated here, the resulting thermocapillary effect was minimal since the thermal diffusion time ${\ensuremath{\tau}}_{H}$ is less than or equal to the pulse time. Consequently, the spacing between the nanoparticles and the particle diameter were found to increase as ${h}^{2}$ and ${h}^{5∕3}$, respectively, which is consistent with the predictions of the thin-film hydrodynamic (TFH) dewetting theory. PF was characterized by the appearance of discrete holes followed by bicontinuous or cellular patterns which finally consolidated into nanoparticles via capillary flow rather than via Rayleigh-like instabilities reported for low-temperature dewetting of viscous liquids. This difference is attributed to the high capillary velocities of the liquid metal arising from its relatively large interfacial tension and low viscosity as well as the smaller length scales of the liquid bridges in the experiments. The predicted liquid-phase lifetime ${\ensuremath{\tau}}_{L}$ was between 2 and $15\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$, which is much smaller than the dewetting time ${\ensuremath{\tau}}_{D}\ensuremath{\geqslant}25\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$ as predicted by the linear TFH theory. Therefore, dewetting required the application of multiple pulses. During the early stages of dewetting, the ripening rate, as measured by the rate of change of characteristic ordering length with respect to $n$, increased linearly with $E$ due to the linear increase in ${\ensuremath{\tau}}_{L}$ with increasing $E$ as predicted by the thermal model. The final nanoparticle spacing was robust, independent of $E$ and $n$, and only dependent on $h$ due to the relatively weak temperature dependence of the thermophysical properties of the metal (Co). These results suggest that fast thermal processing combined with the unique thermophysical parameters of metals can lead to a different pattern formation, including quenching of a wide range of length scales and morphologies.

Electronic structure and optical properties of ZnX ( X=O, S, Se, Te): A density functional study

Abstract: Electronic band structure and optical properties of zinc monochalcogenides with zinc-blende- and wurtzite-type structures were studied using the ab initio density functional method within the local-density approximation (LDA), generalized-gradient approximation, and $\mathrm{LDA}+U$ approaches. Calculations of the optical spectra have been performed for the energy range $0--20\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, with and without including spin-orbit coupling. Reflectivity, absorption and extinction coefficients, and refractive index have been computed from the imaginary part of the dielectric function using the Kramers-Kronig transformations. A rigid shift of the calculated optical spectra is found to provide a good first approximation to reproduce experimental observations for almost all the zinc monochalcogenide phases considered. By inspection of the calculated and experimentally determined band-gap values for the zinc monochalcogenide series, the band gap of ZnO with zinc-blende structure has been estimated.

X-ray nonlinear optical processes using a self-amplified spontaneous emission free-electron laser

Abstract: In contrast to the long-wavelength regime, x-ray nonlinear optical processes are characterized in general by sequential single-photon single-electron interactions. Despite this fact, the sequential absorption of multiple x-ray photons depends on the statistical properties of the radiation field. Treating the x rays generated by a self-amplified spontaneous emission free-electron laser as fully chaotic, a quantum-mechanical analysis of inner-shell two-photon absorption is performed. It is demonstrated that double-core-hole formation via x-ray two-photon absorption is enhanced by chaotic photon statistics. Numerical calculations using rate equations illustrate the impact of field chaoticity on x-ray nonlinear ionization of helium and neon for photon energies near $1\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$. In the case of neon, processes are discussed that involve up to seven photons. Assuming an x-ray coherence time of $2.6\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$, double-core-hole formation in neon is found to be statistically enhanced by about 30% at an intensity of ${10}^{16}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$.

The complexity of propositional proofs

Abstract: Propositional proof complexity is the study of the sizes of propositional proofs, and more generally, the resources necessary to certify propositional tautologies. Questions about proof sizes have connections with computational complexity, theories of arithmetic, and satisfiability algorithms. This is article includes a broad survey of the field, and a technical exposition of some recently developed techniques for proving lower bounds on proof sizes.

Iron-Plasma Transmission Measurements at Temperatures Above 150 eV

Abstract: Measurements of iron-plasma transmission at $156\ifmmode\pm\else\textpm\fi{}6\text{ }\text{ }\mathrm{eV}$ electron temperature and $6.9\ifmmode\pm\else\textpm\fi{}1.7\ifmmode\times\else\texttimes\fi{}{10}^{21}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$ electron density are reported over the 800--1800 eV photon energy range. The temperature is more than twice that in prior experiments, permitting the first direct experimental tests of absorption features critical for understanding solar interior radiation transport. Detailed line-by-line opacity models are in excellent agreement with the data.

Coherent instabilities in a semiconductor laser with fast gain recovery

Abstract: We report the observation of a coherent multimode instability in quantum cascade lasers (QCLs), which is driven by the same fundamental mechanism of Rabi oscillations as the elusive Risken-Nummedal-Graham-Haken (RNGH) instability predicted $40\phantom{\rule{0.3em}{0ex}}\text{years}$ ago for ring lasers. The threshold of the observed instability is significantly lower than in the original RNGH instability, which we attribute to saturable-absorption nonlinearity in the laser. Coherent effects, which cannot be reproduced by standard laser rate equations, can play therefore a key role in the multimode dynamics of QCLs, and in lasers with fast gain recovery in general.

Quantum state-resolved probing of strong-field-ionized xenon atoms using femtosecond high-order harmonic transient absorption spectroscopy.

Abstract: Femtosecond high-order harmonic transient absorption spectroscopy is used to resolve the complete $|j,m⟩$ quantum state distribution of ${\mathrm{Xe}}^{+}$ produced by optical strong-field ionization of Xe atoms at 800 nm. Probing at the Xe ${N}_{4/5}$ edge yields a population distribution ${\ensuremath{\rho}}_{j,|m|}$ of ${\ensuremath{\rho}}_{3/2,1/2}\ensuremath{\mathbin:}{\ensuremath{\rho}}_{1/2,1/2}\ensuremath{\mathbin:}{\ensuremath{\rho}}_{3/2,3/2}=75\ifmmode\pm\else\textpm\fi{}6\text{ }\mathrm{\text{:}}12\ifmmode\pm\else\textpm\fi{}3\text{ }\mathrm{\text{:}}13\ifmmode\pm\else\textpm\fi{}6%$. The result is compared to a tunnel ionization calculation with the inclusion of spin-orbit coupling, revealing nonadiabatic ionization behavior. The sub-50-fs time resolution paves the way for tabletop extreme ultraviolet absorption probing of ultrafast dynamics.

Paramagnetism of the Co sublattice in ferromagnetic Zn 1-x Co x O films

Abstract: Using the spectroscopies based on x-ray absorption, we have studied the structural and magnetic properties of ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{O}$ films ($x=0.1$ and 0.25) produced by reactive magnetron sputtering. These films show ferromagnetism with a Curie temperature ${T}_{C}$ above room temperature in bulk magnetization measurements. Our results show that the Co atoms are in a divalent state and in tetrahedral coordination, thus substituting Zn in the wurtzite-type structure of ZnO. However, x-ray magnetic circular dichroism at the $\mathrm{Co}\phantom{\rule{0.2em}{0ex}}{L}_{2,3}$ edges reveals that the $\mathrm{Co}\phantom{\rule{0.2em}{0ex}}3d$ sublattice is paramagnetic at all temperatures down to $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, both at the surface and in the bulk of the films. The $\mathrm{Co}\phantom{\rule{0.2em}{0ex}}3d$ magnetic moment at room temperature is considerably smaller than that inferred from bulk magnetization measurements, suggesting that the $\mathrm{Co}\phantom{\rule{0.2em}{0ex}}3d$ electrons are not directly at the origin of the observed ferromagnetism.

Surface chemistry and bonding configuration of ultrananocrystalline diamond surfaces and their effects on nanotribological properties

Abstract: We present a comprehensive study of surface composition and nanotribology for ultrananocrystalline diamond (UNCD) surfaces, including the influence of film nucleation on these properties. We describe a methodology to characterize the underside of the films as revealed by sacrificial etching of the underlying substrate. This enables the study of the morphology and composition resulting from the nucleation and initial growth of the films, as well as the characterization of nanotribological properties which are relevant for applications including micro-/nanoelectromechanical systems. We study the surface chemistry, bonding configuration, and nanotribological properties of both the topside and the underside of the film with synchrotron-based x-ray absorption near-edge structure spectroscopy to identify the bonding state of the carbon atoms, x-ray photoelectron spectroscopy to determine the surface chemical composition, Auger electron spectroscopy to further verify the composition and bonding configuration, and quantitative atomic force microscopy to study the nanoscale topography and nanotribological properties. The films were grown on $\mathrm{Si}{\mathrm{O}}_{2}$ after mechanically polishing the surface with detonation synthesized nanodiamond powder, followed by ultrasonication in a methanol solution containing additional nanodiamond powder. The $s{p}^{2}$ fraction, morphology, and chemistry of the as-etched underside are distinct from the topside, exhibiting a higher $s{p}^{2}$ fraction, some oxidized carbon, and a smoother morphology. The nanoscale single-asperity work of adhesion between a diamond nanotip and the as-etched UNCD underside is far lower than for a silicon-silicon interface ($59.2\ifmmode\pm\else\textpm\fi{}2$ vs $826\ifmmode\pm\else\textpm\fi{}186\phantom{\rule{0.3em}{0ex}}\mathrm{mJ}∕{\mathrm{m}}^{2}$, respectively). Exposure to atomic hydrogen dramatically reduces nanoscale adhesion to $10.2\ifmmode\pm\else\textpm\fi{}0.4\phantom{\rule{0.3em}{0ex}}\mathrm{mJ}∕{\mathrm{m}}^{2}$, at the level of van der Waals' interactions and consistent with recent ab initio calculations. Friction is substantially reduced as well, demonstrating a direct link between the surface chemistry and nanoscale friction. The proposed mechanism, supported by the detailed surface spectroscopic analysis, is the elimination of reactive (e.g., ${\mathrm{C}}^{*}$), polar (e.g., $\mathrm{C}\mathrm{O}$), and $\ensuremath{\pi}$-bonded $(\mathrm{C}\mathrm{C})$ surface groups, which are replaced by fully saturated, hydrogen-terminated surface bonds to produce an inert surface that interacts minimally with the contacting counterface.

Effect of Co and O defects on the magnetism in Co-doped ZnO: Experiment and theory

Abstract: The electronic structure of ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{O}$ ($x=002$, 006, and 010) diluted magnetic semiconductors is investigated using soft x-ray emission spectroscopy and first-principles calculations X-ray absorption and emission measurements reveal that most Co dopants are incorporated at the Zn sites and that free charge carriers are absent over a wide range of Co concentrations The excess Co interstitials appear in the samples with high Co concentration ($10\phantom{\rule{03em}{0ex}}\mathrm{at}\phantom{\rule{02em}{0ex}}%$) but are isolated without any direct exchange interaction with substitutional Co atoms The lack of free charge carriers and the direct Co-Co interactions is responsible for the absence of ferromagnetism in the samples First-principles calculations suggest that the exchange interaction between substitutional Co atoms induces only an antiferromagnetic coupling, and strong ferromagnetism in Co-doped ZnO requires not only free charge carriers but also the Co interstitials directly interacting with substitutional Co atoms

Measuring the configurational heat capacity of liquids.

Abstract: A high electric field impedance experiment on supercooled molecular liquids is employed to transfer energy to the slow modes by absorption from the field and detect the increase of their ``configurational temperature'', ${T}_{\mathrm{cfg}}$, via the change of the relaxation times. This allows us to determine the configurational heat capacity, which accounts for most of the excess heat capacity for stronger liquids, but for only half of the heat capacity step in the case of more fragile systems. It is also observed that ${T}_{\mathrm{cfg}}$ gradually approaches the phonon temperature on the structural relaxation time scale.

Selection rule for the optical absorption of graphene nanoribbons

Abstract: We demonstrate that the optical absorption of zigzag-edge graphene nanoribbons is qualitatively different from that of armchair nanotubes. Unlike the selection rule for nanotubes, when the incident beam is polarized along the longitudinal direction, the interband transitions at direct gaps are forbidden for graphene nanoribbons. This selection rule is due to the finite width of graphene nanoribbons. We also demonstrate that the edge states of graphene nanoribbons play an important role in the optical absorption. They are involved in many of the absorption peaks within optical range $(\ensuremath{\hbar}\ensuremath{\omega}l0.12\phantom{\rule{0.3em}{0ex}}\mathrm{a.u.})$ and have no contribution to the absorption peaks beyond optical range.

C iv absorption in damped and sub-damped Lyman-alpha systems: Correlations with metallicity and implications for galactic winds at z~2-3

Abstract: We present a study of $\ion{C}{iv}$ absorption in a sample of 63 damped Lyman- α (DLA) systems and 11 sub-DLAs in the redshift range $1.75\! -1 FWHM ), high signal-to-noise VLT/UVES spectra. The complex absorption line profiles show both narrow and broad $\ion{C}{iv}$ components, indicating the presence of both warm, photoionized and hot, collisionally ionized gas. We report new correlations between the metallicity (measured in the neutral-phase) and each of the $\ion{C}{iv}$ column density, the $\ion{C}{iv}$ total line width, and the maximum $\ion{C}{iv}$ velocity. We explore the effect on these correlations of the sub-DLAs, the proximate DLAs (defined as those within 5000 km s -1 of the quasar), the saturated absorbers, and the metal line used to measure the metallicity, and we find the correlations to be robust. There is no evidence for any difference between the measured properties of DLA $\ion{C}{iv}$ and sub-DLA $\ion{C}{iv}$. In 25 DLAs and 4 sub-DLAs, covering 2.5 dex in [Z/H], we directly observe $\ion{C}{iv}$ moving above the escape speed, where v esc is derived from the total line width of the neutral gas profiles. These high-velocity $\ion{C}{iv}$ clouds, unbound from the central potential well, can be interpreted as highly ionized outflowing winds, which are predicted by numerical simulations of galaxy feedback. The distribution of $\ion{C}{iv}$ column density in DLAs and sub-DLAs is similar to the distribution in Lyman Break galaxies, where winds are directly observed, supporting the idea that supernova feedback creates the ionized gas in DLAs. The unbound $\ion{C}{iv}$ absorbers show a median mass flow rate of ~22 ( r /40 kpc) $M_\odot$ yr -1 , where r is the characteristic $\ion{C}{iv}$ radius. Their kinetic energy fluxes are large enough that a star formation rate (SFR) of ~2 $M_\odot$ yr -1 is required to power them.

Towards an S-matrix Description of Gravitational Collapse

Abstract: Extending our previous results on trans-Planckian ($Gs \gg \hbar$) scattering of light particles in quantum string-gravity we present a calculation of the corresponding S-matrix from the region of large impact parameters ($b \gg G\sqrt{s}>\lambda_s$) down to the regime where classical gravitational collapse is expected to occur. By solving the semiclassical equations of a previously introduced effective-action approximation, we find that the perturbative expansion around the leading eikonal result diverges at a critical value $b = b_c = O(G\sqrt{s})$, signalling the onset of a new (black-hole related?) regime. We then discuss the main features of our explicitly unitary S-matrix -- and of the associated effective metric -- down to (and in the vicinity of) $b = b_c$, and present some ideas and results on its extension all the way to the $ b \to 0$ region. We find that for $b

Model of laser cooling in the Yb 3 + -doped fluorozirconate glass ZBLAN

Abstract: A quantitative description of optical refrigeration in ${\mathrm{Yb}}^{3+}$-doped ZBLAN glass in the presence of transition-metal and $\mathrm{OH}$ impurities is presented. The model includes the competition of radiative processes with energy migration, energy transfer to transition-metal ions, and multiphonon relaxation. Molecular dynamics calculations of pure ZBLAN and ZBLAN doped with transition-metal ions provide the structural information that, when combined with spectroscopic data, allows for the calculation of electric-dipole energy-transfer rates in the framework of the Dexter theory. The structural data is further used to extend the traditional energy-gap law to multiphonon relaxation via vibrational impurities. The cooling efficiency is sensitive to the presence of both $3d$ metal ions with absorption in the near infrared and high-frequency vibrational impurities such as $\mathrm{OH}$. The calculation establishes maximum impurity concentrations for different operating temperatures and finds ${\mathrm{Cu}}^{2+}$, ${\mathrm{Fe}}^{2+}$, ${\mathrm{Co}}^{2+}$, ${\mathrm{Ni}}^{2+}$, and $\mathrm{OH}$ to be the most problematic species. ${\mathrm{Cu}}^{2+}$ in particular has to be reduced to $l2\phantom{\rule{0.3em}{0ex}}\mathrm{ppb}$, and ${\mathrm{Fe}}^{2+}$, ${\mathrm{Co}}^{2+}$, ${\mathrm{Ni}}^{2+}$, and $\mathrm{OH}$ have to be reduced to $10--100\phantom{\rule{0.3em}{0ex}}\mathrm{ppb}$ for a practical $\mathrm{ZBLAN}:{\mathrm{Yb}}^{3+}$ optical cryocooler to operate at $100--150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

Robust population transfer by stimulated raman adiabatic passage in a Pr3+:Y2SiO5 crystal.

Abstract: We report on the experimental implementation of stimulated Raman adiabatic passage (STIRAP) in a ${\mathrm{Pr}}^{3+}\mathrm{\text{:}}{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}$ crystal. Our data provide clear and striking proof for nearly complete population inversion between hyperfine levels in the ${\mathrm{Pr}}^{3+}$ ions. The transfer efficiency was monitored by absorption spectroscopy. Time-resolved absorption measurements serve to monitor the adiabatic population dynamics during the STIRAP process. Efficient transfer is observed for negative pulse delays (STIRAP), as well as for positive delays. We identify the latter by an alternative adiabatic passage process.

Fougerite, a new mineral of the pyroaurite-iowaite group: Description and crystal structure

Abstract: Fougerite (IMA 2003-057) is a mixed M(II)-M(III) hydroxysalt of the green rust group, where M(II) can be Fe or Mg, and M(III) is Fe. The general structural formula is: ${[{\rm{Fe}}_{1 - }^{2 + }x{\rm{Fe}}_x^{3 + }{\rm{M}}{{\rm{g}}_y}{({\rm{OH}})_{2 + 2y}}]^{ + x}}{[x{\rm{/}}n{A^{ - n}}.m{{\rm{H}}_2}{\rm{O}}]^{ - x}}$ where A is the interlayer anion and n its valency, with 1/4 ≼ x/(1+y) ≼ 1/3 and m ≼ (1−x+y). The structure of green rusts and parent minerals can accommodate a variety of anions, such as OH−, Cl−, ${\rm{CO}}_3^{2 - },\;{\rm{SO}}_4^{2 - }$ . The structure of the mineral was studied by Mossbauer, Raman and X-ray absorption spectroscopies (XAS) at the FeK edge. Mossbauer spectra of the mineral obtained at 78 K are best fitted with four doublets: D1 and D2 due to Fe2+ (isomer shift δ ≈ 1.27 and 1.25 mm s−1, quadrupole splitting ΔEQ ≈ 2.86 and 2.48 mm s−1, respectively) and D3 and D4 due to Fe3+ (δ ≈ 0.46 mm s−1, ΔEQ ≈ 0.48 and 0.97 mm s−1, respectively). Microprobe Raman spectra obtained with a laser at 514.53 nm show the characteristic bands of synthetic green rusts at 427 and 518 cm−1. X-ray absorption spectroscopy shows that Mg is present in the mineral in addition to Fe, that the space group is and the lattice parameter a ≈ 0.30–0.32 nm. The mineral forms by partial oxidation and hydrolysis of aqueous Fe2+, to give small crystals (400–500 nm) in the form of hexagonal plates. The mineral is unstable in air and transforms to lepidocrocite or goethite. The name is for the locality of the occurrence, a forested Gleysol near Fougeres, Brittany, France. Its characteristic blue-green color (5BG6/1 in the Munsell system) has long been used as a universal criterion in soil classification to identify Gleysols. From a thermodynamic model of soil-solution equilibria, it was proposed that for the eponymous mineral, Fougeres-fougerite, OH− may be the interlayer anion. In other environments, the interlayer anion may be different, and other varieties of fougerite may exist. Fougerite plays a key role in the pathways of formation of Fe oxides.

Anomalous Stokes shift in CdSe nanocrystals

Abstract: We investigate the temperature dependence of exciton fluorescence, absorption, and Stokes shift for five sizes of high quality $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{S}$ nanocrystals from the perspective of the fine structure model and exciton-acoustic phonon coupling. The Stokes shift is found to have a weak temperature dependence for all sizes. Within the fine structure model, we use the temperature dependence of the Stokes shift to infer the upper level energy and oscillator strength. We also interpret our ensemble measurements using an exciton-acoustic phonon scattering model. We find that neither the fine structure nor exciton-acoustic phonon scattering is able to adequately explain our data in isolation and conclude that a comprehensive theory which includes the physics of both models on an equal footing is required.

A 0535+26 in the August/September 2005 outburst observed by RXTE and INTEGRAL

Abstract: Aims. In this Letter we present results from INTEGRAL and RXTE observations of the spectral and timing behavior of the High Mass X-ray Binary A 0535+26 during its August/September 2005 normal (type I) outburst with an average flux $F_{(5-100)}\,_{\mathrm{keV}} \sim400$ mCrab. The search for cyclotron resonance scattering features (fundamental and harmonic) is one major focus of the paper. Methods. Our analysis is based on data from INTEGRAL and RXTE Target of Opportunity Observations performed during the outburst. The pulse period is determined. X-ray pulse profiles in different energy ranges are analyzed. The broad band INTEGRAL and RXTE pulse phase averaged X-ray spectra are studied. The evolution of the fundamental cyclotron line at different luminosities is analyzed. Results. The pulse period P is measured to be 103.39315(5) s at MJD 53614.5137. Two absorption features are detected in the phase averaged spectra at $E_{1}\sim 45$ keV and $E_{2}\sim 100$ keV. These can be interpreted as the fundamental cyclotron resonance scattering feature and its first harmonic and therefore the magnetic field can be estimated to be $B\sim 4\times 10^{12}$ G.

Frequency degenerate and nondegenerate two-photon absorption spectra of semiconductor quantum dots

Abstract: The frequency degenerate and nondegenerate two-photon absorption (2PA) spectra of direct band gap semiconductor quantum dots are studied. Measuring the spectra for both cases in samples of CdSe and CdTe with different quantum dot sizes and size distributions, we observe that the 2PA spectra and the 2PA coefficient are size dependent, so that smaller dots have smaller 2PA even after taking into account the volume fraction. Theory considering the mixing of the hole bands, in a $\stackrel{P\vec}{k}∙\stackrel{P\vec}{p}$ model, explains the data quite well except for the smallest dots. A comparison with the parabolic band approximation is also shown.

Hopping and clustering of oxygen vacancies in SrTiO3 by anelastic relaxation

Abstract: The complex elastic compliance ${s}_{11}(\ensuremath{\omega},T)$ of $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ has been measured as a function of the O deficiency $\ensuremath{\delta}l0.01$. The two main relaxation peaks in the absorption are identified with hopping of isolated O vacancies over a barrier of $0.60\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and reorientation of pairs of vacancies involving a barrier of $1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The pair binding energy is $\ensuremath{\simeq}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, and indications for additional clustering, possibly into chains, is found already at $\ensuremath{\delta}\ensuremath{\sim}0.004$. The anisotropic component of the elastic dipole of an O vacancy is $\ensuremath{\Delta}\ensuremath{\lambda}=0.026$. The possible role of electrostatic repulsion between vacancies in slowing the kinetics for their aggregation is discussed.

Experimental demonstration of second-order processes in photonic crystal microcavities at submilliwatt excitation powers

Abstract: The far-field second-order radiation pattern from a wavelength-scale, InP-based photonic crystal microcavity that confines light in three dimensions is measured when excited on resonance by $300\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{W}$ of continuous-wave power from a laser diode. The measurements are accurately simulated using the finite-difference time-domain method, showing that both absorption and scattering play significant roles in determining the pattern of the radiation. The results show that the bulk second-order nonlinear susceptibility mediates the nonlinear process. In a separate set of experiments, a short-pulse laser is used to simultaneously populate two distinct modes of a similar microcavity. The detected second-order spectra show features due to the second harmonic generated by each mode, as well as the sum-frequency generation due to nonlinear intermode mixing. The key phenomena that determine the second-order response of these fundamentally small optical cavities are identified.

Electronic properties and optical spectra of Mo S 2 and W S 2 nanotubes

Abstract: Symmetry based calculations of the polarized optical absorption in single-wall $\mathrm{Mo}{\mathrm{S}}_{2}$ and $\mathrm{W}{\mathrm{S}}_{2}$ nanotubes are presented. Optical conductivity tensor for the individual tubes, using line group symmetry and density-functional tight binding implemented in POLSYM code, is numerically evaluated and its dependence on the diameter and chiral angle of the nanotubes is investigated. This minimal, full symmetry implementing algorithm enabled calculations of the optical response functions very efficiently and addressed the large diameter tubes and highly chiral tubes as well. It is predicted that, due to the symmetry transformation properties of the relevant electronic states, fluorescence is not expected in the metal dichalcogenide tubes. In accordance with the measurements, the calculations show redshift of the absorption peaks as the tube diameter increases. Also, it is found that curvature strain induces strong chiral angle dependence of the absorption spectra.

Electronic structure, bonding, charge distribution, and x-ray absorption spectra of the (001) surfaces of fluorapatite and hydroxyapatite from first principles

Abstract: Fluorapatite (FAP) and hydroxyapatite (HAP) are two very important bioceramic crystals. The (001) surfaces of FAP and HAP crystals are studied by ab initio density functional calculations using a supercell slab geometry. It is shown that in both crystals, the O-terminated (001) surface is more stable with calculated surface energies of 0.865 and $0.871\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕{\mathrm{m}}^{2}$ for FAP and HAP, respectively. In FAP, the two surfaces are symmetric. In HAP, the orientation of the OH group along the $c$ axis reduces the symmetry such that the top and bottom surfaces are no longer symmetric. It is revealed that the atoms near the surface and subsurface are significantly relaxed especially in the case of HAP. The largest relaxations occurred via the lateral movements of the O ions at the subsurface level. The electronic structures of the surface models in the form of layer-by-layer resolved partial density of states for all the atoms show systematic variation from the surface region toward the bulk region. The calculated Mulliken effective charge on each type of atom and the bond order values between cations (Ca, P) and anions (O, F) show different charge transfers and bond strength variations from the bulk crystal values. Electron charge density calculations show that the surfaces of both FAP and HAP crystals are mostly positively charged due to the presence of Ca ions at the surface. The positively charged surfaces have implications for the absorption on apatite surfaces of water and other organic molecules in an aqueous environment which are an important part of its bioactivity. The x-ray absorption near-edge structure (XANES) spectra ($\mathrm{Ca}\text{\ensuremath{-}}K$, $\mathrm{O}\text{\ensuremath{-}}K$, $\mathrm{F}\text{\ensuremath{-}}K$, $\mathrm{P}\text{\ensuremath{-}}K$, and $\mathrm{P}\text{\ensuremath{-}}{L}_{3}$ edges) of both the surface models and the bulk crystals are calculated and compared. The calculations use a supercell approach which takes into account the electron--core-hole interaction. It is shown that the site-specific XANES spectra show significant differences between atoms near the surface and in the bulk and are very sensitive to the local atomic environment of each atom. This information will be very valuable for characterizing the apatite materials and in the interpretation of experimental data. Comparisons of several sets of experimental data with the weighted sums of the calculated spectra at different sites for the same element show very good agreement.

Detection of optical trapping of CdTe quantum dots by two-photon-induced luminescence

Abstract: Nanometer-sized CdTe quantum dots (QDs) in ${\mathrm{D}}_{2}\mathrm{O}$ can be optically trapped by a high repetition-rate picosecond Nd:YLF laser with an input power as low as $100\phantom{\rule{0.3em}{0ex}}\mathrm{mW}$. A large two-photon absorption (TPA) cross section of ${10}^{4}--{10}^{5}\phantom{\rule{0.3em}{0ex}}\mathrm{GM}$ for CdTe QDs makes it possible to detect the trapping process with TPA induced luminescence. In ${\mathrm{D}}_{2}\mathrm{O}$, the luminescence intensity of CdTe QDs was found to increase with time, whereas an intensity decrease was clearly detected in ${\mathrm{H}}_{2}\mathrm{O}$. In addition, the optically trapped CdTe QDs were fixed on a hydrophilic glass substrate. Laser trapping of QDs may provide a basic technique for quantum information and biological applications.

Interpreting atomic-resolution spectroscopic images

Abstract: Core-loss electron energy loss spectroscopy is a powerful experimental tool with the potential to provide atomic-resolution information about electronic structure at defects and interfaces in materials and nanostructures. Interpretation, however, is nonintuitive. Comparison of experimental and simulated compositional maps in $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_{3}$ shows good agreement, apart from an overall scaling of image contrast, and shows that the shape and width of spectroscopic images do not show a simple variation with binding energy, as commonly assumed, or with the size of the orbital excited. For the low lying La ${N}_{4,5}$ edge with threshold at around $99\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, delocalization does not preclude atomic resolution, but reduces the image contrast. The image width remains comparable to that of the much higher lying O $K$ edge with threshold at around $532\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Both edges show a volcanolike feature, a dip at the column position not previously seen experimentally. In the case of the O $K$ edge, this represents an experimental verification of nonlocal inelastic scattering effects in electron energy loss spectroscopy imaging. In the case of the ${N}_{4,5}$ edge, the volcanolike feature is due to dynamical channeling and absorption of the probe through the specimen thickness. Simulation is therefore critical to the interpretation of atomic-resolution elemental maps.

Structural relaxation around substitutional Cr3+ in MgAl2O4

Abstract: The structural environment of a substitutional ${\mathrm{Cr}}^{3+}$ ion in a $\mathrm{Mg}{\mathrm{Al}}_{2}{\mathrm{O}}_{4}$ spinel has been investigated by Cr $K$-edge extended x-ray absorption fine structure and x-ray absorption near edge structure (XANES) spectroscopies. First-principles computations of the structural relaxation and of the XANES spectrum have been performed, with a good agreement with the experiment. The Cr-O distance is close to that in $\mathrm{Mg}{\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$, indicating a full relaxation of the first neighbors, and the second shell of Al atoms relaxes partially. These observations demonstrate that Vegard's law is not obeyed in the $\mathrm{Mg}{\mathrm{Al}}_{2}{\mathrm{O}}_{4}\text{\ensuremath{-}}\mathrm{Mg}{\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$ solid solution. Despite some angular site distortion, the local ${D}_{3d}$ symmetry of the $B$ site of the spinel structure is retained during the substitution of Cr for Al. Here, we show that the relaxation is accommodated by strain-induced bond buckling, with angular tilts of the Mg-centered tetrahedra around the Cr-centered octahedron. By contrast, there is no significant alteration of the angles between the edge-sharing octahedra, which build chains aligned along the three fourfold axes of the cubic structure.

Formation of Mn 3 O 4 (001) on MnO(001): Surface and interface structural stability

Abstract: X-ray absorption and photoemission spectroscopies, high-resolution electron energy loss spectroscopy, spot profile analysis low energy electron diffraction, and density functional theory calculations are employed to study the growth of (001) oriented ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$ surfaces on a Pd(100)-supported MnO(001) substrate, with the Hausmannite planar lattice constants aligned along the [110] direction of the underlying MnO(001) support. We show that despite the rather large lattice mismatch, abrupt interfaces may exist between rocksalt MnO and Hausmannite. We argue that this process is facilitated by the relatively low computed strain energy and we propose realistic models for the interface. An atop site registry between the Mn(O) atoms of the oxygen rich ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$ termination and the MnO(001) O(Mn) atoms underneath is found to be the energetically most favorable configuration. The significant planar expansion is accompanied by a large compression of the ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$ vertical lattice constant, yielding structural distortion of the O-Mn-O octahedral axis. Spot profile analysis low energy electron diffraction experiments show that the conversion reaction proceeds easily in both directions, thus indicating the reversible redox character of the transition.

Ferroelectric distortion in SrTiO 3 thin films on Si ( 001 ) by x-ray absorption fine structure spectroscopy: Experiment and first-principles calculations

Abstract: $\mathrm{Ti}\phantom{\rule{0.3em}{0ex}}K$ and $\mathrm{Ti}\phantom{\rule{0.3em}{0ex}}{L}_{2,3}$ x-ray absorption fine-structure near-edge spectra of ${\mathrm{SrTiO}}_{3}$ thin films grown coherently on $\mathrm{Si}(001)$ reveal the presence of a ferroelectric (FE) distortion at room temperature. This unique phase is a direct consequence of the compressive biaxial strain achieved by coherent epitaxial growth.