Pronounced Narrowing Research Articles

Intermixing in quantum-dot ensembles with sharp adjustable shells

State-filling spectroscopy is used to study the effects of alloy intermixing in quantum-dot (QD) ensembles having well-defined electronic shells. Rapid thermal annealing is performed on samples of self-assembled QDs grown with different intersublevel energy spacings. For InAs/GaAs QDs, the intersublevel is tuned between ∼90 and 25 meV. The intense and sharp shell structures observed in photoluminescence indicate unambiguously that the QDs retained their zero-dimensional density of states after the diffusion of the potential, which also causes strong blueshifts (up to ∼200 meV) and a pronounced narrowing of the inhomogeneously broadened emission (down to ∼12 meV).

Silicon and zinc telluride nanoparticles synthesized by low energy density pulsed laser ablation into ambient gases

The size distributions of Si and ZnTe nanoparticles produced by low energy density ArF (193 nm) pulsed laser ablation into ambient gases were measured as a function of the gas pressure, P, and target-substrate separation, Dts. For both Si and ZnTe, the largest nanoparticles were found closest to the ablation target, and the mean nanoparticle size decreased with increasing Dts. For Si ablation into He, the mean nanoparticle diameter did not increase monotonically with gas pressure but reached a maximum near P = 6 Torr. High resolution Z-contrast transmission electron microscopy and energy loss spectroscopy revealed that ZnTe nanoparticles consist of a crystalline core surrounded by an amorphous ZnO shell; growth defects and surface steps are clearly visible in the crystalline core. A pronounced narrowing of the ZnTe nanocrystal size distribution with increasing Dts also was found. The results demonstrate that the size of laser-ablated nanoparticles can be controlled by varying the molecular weight and pressure of an ambient gas and that nanometer-scale particles can be synthesized. Larger aggregates of both ZnTe and Si having a “flakelike” or “weblike” structure were formed at the higher ambient gas pressures; for ZnTe these appear to be open agglomerates of much smaller (∼10 nm) particles.

Electrooptical properties of InP epitaxial layers grown with rare-earth admixture

The influence of rare-earth (RE) elements (Ho, Er and Nd) addition during the LPE growth, on electrooptical properties of InP layers is reported. Temperature dependent Hall effect and capacitance-voltage curves show quite a dramatic impact of Er and Nd on shallow impurity and free-carrier concentrations: they were decreased by as much as three orders of magnitude. The background concentration of both donors and acceptors decrease with increasing of RE content in the melt up to concentrations of about 0.25 wt.%. Low-temperature photoluminescence (PL) spectra have been measured for various levels of excitation power and temperature. The major manifestation of the RE admixture is the pronounced narrowing of PL curves and the corresponding appearence of fine spectral features. Temperature and excitation dependences of shallow acceptor related bands of InP:Nd layers are discussed in more detail.

An unfortunate consequence of being tickled

and some involvement of the left thalamic body in the territory of the left middle cerebral artery. Axial and coronal T2 arterial images showed pronounced narrowing of the left internal carotid artery with a double signal consistent with post-traumatic dissection. Non-invasive magnetic resonance angiography showed a pronounced narrowing of the left internal carotid artery (figure) extending from the base of the skull posteriorly to the carotid siphon. Some perfusion of the left middle cerebral artery was seen via a crossover from the right side by the right anterior communicating and anterior cerebral arteries and also from the left posterior communicating artery. All neurological signs resolved on warfarin treatment which was continued for 9 months and did not recur after warfarin was stopped. Magnetic resonance angiographic abnormalities of the left internal carotid artery and the reduced flow on Doppler persisted. When last seen in March, 1997, she was well, with no neurological sequelae.

Two-photon excitation processes and linewidths of surface and image states on Cu(111)

The occupied surface state and the first image state on Cu(111) have been observed by high-resolution, two-photon photoemission as a function of photon energy. A pronounced narrowing of the linewidth is found for the resonant excitation from the occupied surface state into the first image state. The product of two Lorentzians convoluted with a Gaussian leads to a satisfactory description of the complete series of spectra. The intrinsic linewidth of the first image state on Cu(111) is obtained as 85 ± 10 meV, in agreement with recent time-resolved measurements. The occupied surface state has an intrinsic linewidth of 65 ± 15 meV, as also observed in one-photon photoemission experiments.

Pharyngeal narrowing/occlusion during central sleep apnea

We hypothesized that subatmospheric intraluminal pressure is not required for pharyngeal occlusion during sleep. Six normal subjects and six subjects with sleep apnea or hypopnea (SAH) were studied during non-rapid-eye-movement sleep. Pharyngeal patency was determined by using fiber-optic nasopharyngoscopy during spontaneous central sleep apnea (n = 4) and induced hypocapnic central apnea via nasal mechanical ventilation (n = 10). Complete pharyngeal occlusion occurred in 146 of 160 spontaneously occurring central apneas in patients with central sleep apnea syndrome. During induced hypocapnic central apnea, gradual progressive pharyngeal narrowing occurred. More pronounced narrowing was noted at the velopharynx relative to the oropharynx and in subjects with SAH relative to normals. Complete pharyngeal occlusion frequently occurred in subjects with SAH (31 of 44 apneas) but rarely occurred in normals (3 of 25 apneas). Resumption of inspiratory effort was associated with persistent narrowing or complete occlusion unless electroencephalogram signs of arousal were noted. Thus pharyngeal cross-sectional area is reduced during central apnea in the absence of inspiratory effort. Velopharyngeal narrowing consistently occurs during induced hypocapnic central apnea even in normal subjects. Complete pharyngeal occlusion occurs during spontaneous or induced central apnea in patients with SAH. We conclude that subatmospheric intraluminal pressure is not required for pharyngeal occlusion to occur. Pharyngeal narrowing or occlusion during central apnea may be due to passive collapse or active constriction.

Structure of the Variscan belt beneath the British and Armorican overstep sequences

Interpretation of gravity and magnetic data from southern England and of deep seismic data from the English Channel, Celtic Sea, and Paris basin provides a new picture of the buried Hercynian belt between France and England. The main structure corresponds to a mobile belt of crustal thickening that developed between the Midlands block, a northern block that had detached from the Armorican block prior to Hercynian time, and the Armorican block. The frontal collision between the two blocks recorded during the late Carboniferous was responsible for a pronounced narrowing of the belt between the Isle of Wight and Normandy. During collision, structural wedges were ejected in the channel and possibly also in the Hercynian basement beneath the Paris basin.

The electronic structure of liquid alkali metals: calculation of photoemission spectra

The authors present a first-principles calculation of the atomic structure (pair correlation function, static structure factor), the electronic structure (total and partial densities of states), and of the photoemission intensities of liquid Li and Na. They find that in liquid Li the electronic density of states shows a strong structure-induced 'Brillouin-kink' which leads to a pronounced narrowing of the width of the occupied band compared to the prediction of the free electron model. The DOS of liquid Na on the other hand is close to the free electron parabola. The origin of the difference is in the strong p component in the electron-ion potential in Li, but not in Na. The observed deviation of the photoemission spectrum of Na from a free electron form is explained in terms of the partial photoionisation cross-sections. A detailed comparison with experiment shows that the accuracy of the theoretical predictions is limited only by the local density approximation for the many-electron interactions.

Structural control of conjugation in functionalized polythiophenes

A comparative analysis of the electrochemical and optical properties of 3-substituted poly- (thiophenes) has been carried out on four polymer structures, poly(3-methylthiophene) (PMT), poly(3- isoamylthiophene) (PiAT), poly(3-nonylthiophene) (PNT), and poly(3-(3,6-dioxyheptyl)thiophene) (PDHT). The spectroelectrochemical properties of these polymers have been investigated by cyclic voltammetry (CV), derivative cyclic voltammetric absorptometry (DCVA), and spectroelectrochemistry. CV shows that the symmetry of the anodic and cathodic waves increases from PMT to PDHT; furthermore in this latter case, a second redox system appears in the CV. The DCVA responses of the polymers indicate that the oxidation potential decreases in the order PiAT > PMT > PNT > PDHT whereas the increasing symme- try of the curves suggests an increasing electrochemical reversibility. UV-visible absorption spectra have been recorded in situ at various applied potentials. In the undoped state, the absorption maximum of the polymers shifts bathochromically from PiAT to PDHT and the absorption spectrum of PDHT exhibits several defined structures resembling those previously observed at low temperature on poly(3- alkylthiophenes). Upon electrochemical doping, the spectroscopic behavior of PiAT, PNT, and PDHT differs from that of PMT by a more intense absorption at 1.5 eV at the intermediate doping level and by a more pronounced narrowing of the band gap with the appearance of metallic-like behavior at high applied potential. These effects that are particularly evident in PDHT are interpreted by an extension of the effective mean conjugation length resulting from the stabilization of more planar conformations.

Dynamic-structure-factor measurements on a model Lorentz gas.

A model system for the Lorentz gas can be made [Eder, Chen, and Egelstaff, Proc. Phys. Soc. London 89, 833 (1966); McPherson and Egelstaff, Can. J. Phys. 58, 289 (1980)] by mixing small quantities of hydrogen with an argon host. For neutron-scattering experiments the large H-to-Ar cross section ratio (\ensuremath{\sim}200) makes the argon relatively invisible. Dynamic-structure-factor [S(Q,\ensuremath{\omega}) for ${\mathrm{H}}_{2}$] measurements at room temperature have been made on this system using the IN4 spectrometer at the Institute Laue Langevin, Grenoble, France. Argon densities between 1.9 and 10.5 atoms/${\mathrm{nm}}^{3}$ were used for 0.4<Q<5 A${\r{}}^{\mathrm{\ensuremath{-}}1}$. Additional measurements were made with a He gas host at densities of 4 and 10.5 atoms/${\mathrm{nm}}^{3}$; helium is relatively invisible also compared to hydrogen. These experiments are described, and some examples of the results are presented to show the qualitative effects observed. The principle observation is a pronounced narrowing of S(Q,\ensuremath{\omega}) as a function of \ensuremath{\omega} as the argon density is increased. This effect is large at low Q and decreases with increasing Q, and also decreases substantially when helium is used in place of argon. In addition, the shape of S(Q,\ensuremath{\omega}) is more complex than can be accommodated within a simple model, but slightly less complicated than a computer simulation so showing the significance of multiple-collision processes.

Ga1−xAlxAs purification during its liquid phase epitaxial growth in the presence of Yb

The influence of Yb added to the melt, on the near-band-gap emission of Ga1−xAlxAs grown by liquid phase epitaxy, is reported. No characteristic Yb3+ emission is found, but a pronounced narrowing of the bound-exciton spectrum is observed for Yb concentrations in the Ga melt below 100 ppm. A similar effect is seen in conduction band to acceptor transitions, for which the smallest linewidth observed is limited by fluctuations in composition and agrees well with the currently accepted 66:34 band-offset partition between the conduction and valence bands of GaAs/Ga1−xAlxAs heterojunctions in the direct band-gap range.

Influence of coupling of wells on spontaneous emission line shape in GaAs/GaAlAs multiple quantum wells

The results of a comparative study of the low-temperature luminescence line shape of molecular beam epitaxially grown GaAs/GaAlAs coupled and uncoupled quantum wells (QW’s) are presented. A pronounced spectral narrowing of the emission line with increasing coupling of QW’s is found. A detailed model for spontaneous emission line shapes for real superlattices is established: The superlattice density of states is calculated in the framework of envelope function approximation. Interface roughness yields Gaussian distribution of layer thicknesses resulting in a broadened density of states. The variance of this distribution function is a measure of interface quality. The reduction of the variance with increased coupling is explained by an enhanced excitonic averaging mechanism in superlattices.

Vibrational characteristics of acceptor-hydrogen complexes in silicon

Acceptor-hydrogen complexes for the group III acceptors, B, Al, and Ga, in Si have been studied with low-temperature infrared spectroscopy. The Si-H stretching band narrows dramatically upon cooling to low temperature thereby aiding the detection of the vibrations of the Al and Ga acceptor-H complexes. The frequency 2201 cm−1 we have measured for the Al-H complex is in reasonable agreement with the prediction made by G. G. DeLeo and W. B. Fowler [Phys. Rev. B 31, 6861 (1985)] (2220 cm−1 for a 〈111〉 interstitial configuration for the H). Assignment of the new vibrational bands is confirmed by isotopic substitution. The strength of the absorption provides evidence that the passivation is not the result of compensation alone and that a major fraction of the passivated acceptors result in acceptor-H complexes. A new, low-energy excitation of the acceptor-hydrogen complexes gives rise to a sideband to the main stretching vibration and explains the pronounced energy shift and narrowing of the spectra upon cooling to He temperature.

Pulmonary function in relapsing polychondritis.

The pulmonary mechanics and bronchoscopic findings in 5 patients with relapsing polychondritis were studied to evaluate the mechanism of obstruction. Two of the patients did not have clinical symptoms referable to the respiratory tract: pulmonary function was normal in 1 patient and was suggestive of mild restriction in the other. Three patients had dyspnea; pulmonary function studies revealed expiratory and inspiratory obstruction in all 3. The maximal flow-static recoil curves demonstrated that the expiratory obstruction was due predominantly to airway abnormality and not to loss of elastic recoil forces of the lung. The bronchoscopic appearance of the extrathoracic airway during quiet breathing did not accurately reflect pulmonary function results. One patient had a visually normal upper airway but greatly reduced maximal inspiratory flow rates, whereas another patient had pronounced narrowing of the upper airway but only a modest reduction of maximal inspiratory flow rates. The bronchoscopic appearance of the intrathoracic airway during quiet breathing did relate well to maximal expiratory flow rates. We conclude that in our group of patients the predominant mechanism of expiratory obstruction in relapsing polychondritis is due to airway abnormality. Although the bronchoscopic and radiographic findings are useful, spirometry is more important in determining functional abnormality.

Dynamics of concentration fluctuations in a heterocoordinated binary liquid alloy

The quasielastic part of the scattering law has been measured for the concentration fluctuations, ${S}_{\mathrm{cc}}(q,E)$, of a binary liquid with compound-forming tendency, ${\mathrm{Li}}_{0.80}$${\mathrm{Pb}}_{0.20}$. In this neutron scattering experiment on a time-of-flight (TOF) spectrometer (resolution 0.28 meV), a range of momentum transfers $q$ of $0.27<q<2.5$ ${\mathrm{\AA{}}}^{\ensuremath{-}1}$ was studied which includes the prominent first peak of ${S}_{\mathrm{cc}}(q)$; the energy range studied varies from -1... + 1 to 0... + 16 meV, with increasing $q$ values. The sample was measured at 1.04, 1.13. and $1.20{T}_{\mathrm{melt}}$. From the TOF spectra, the self-diffusion constant of $^{7}\mathrm{Li}$ and the interdiffusion constant for ${\mathrm{Li}}_{0.80}$${\mathrm{Pb}}_{0.20}$ were extracted. The latter, which reveals a pronounced narrowing of the quasielastic width around the peak of ${S}_{\mathrm{cc}}(q)$, can be interpreted to yield a lifetime of Li-Pb agglomerates. An extension of a relation which connects self- and interdiffusion constants is discussed. A simple description of elastic, i.e., $E=0$, scattering is achieved. Good fits to the data are obtained using a Mori description; although the LO mode is not seen in the spectra, some hint as to its characteristic frequency can be derived. The fits require a sum of two relaxation terms for which the widths and relative strength are obtained as functions of $q$. The fast mechanism may contain an indication for coupling of concentration fluctuations to conduction electrons.

Spinless Anderson Model with a Local Phonon Mode

AbstractThe electron‐phonon coupling in the spinless Anderson model is considered within the perturbation theory. The second‐order theory including the finite lifetime for phonons leads to some narrowing of the density of states for higher values of the ratio of the electron‐phonon coupling constant and virtual level halfwidth, λ/Δ ≈ 1. The simplified fourth‐order theory (V = 0) shows that very pronounced narrowing is expected (λ/Δ < 1) and there is no change in the height of the central peak.

Coherent Anti-Stokes Raman Spectroscopic Modeling For Combustion Diagnostics

The ability to model the CARS (coherent anti-Stokes Raman spectroscopy) spectra of combustion molecules is an important part of the process of extracting diagnostic information from CARS signals; measurements of temperature or species concentration usually require that the experimental signatures be analyzed with the aid of computer-synthesized spectra. This dependence on theoretical models arises from spectral interference effects unique to CARS; because it is a nonlinear optical process whose signal strength depends on the squared modulus of the third-order electric susceptibility, the spectral shape is affected by interferences between neighboring Raman transitions and between the resonant and nonresonant contributions to the susceptibility. This paper will review the status of modeling the spectra of molecules important in combustion, such as N2, H2O, and CO2. It will be shown that accurate modeling generally requires highly precise knowledge of line positions and reasonable estimates of Raman linewidths, and the sources of these data will be discussed. In many practical applications, pressures well above atmo-spheric are encountered, and the phenomenon of collisional narrowing can become important. This effect, which has its origins in rotationally inelastic energy transfer between adjacent J-states, gives rise to a pronounced narrowing of signatures with increasing pressure. The status of modeling this effect in N2, H2O, and CO2 will be described, and it will be seen that good agreement with experiment can be achieved using either the Gordon rotational diffusion model or phenomenological models for inelastic energy transfer rates.

Fluorescence studies of the calcium binding to whiting (Gadus merlangus) parvalbumin.

The calcium binding by parvalbumin of whiting (Gadus merlangus) has been studied using tryptophanyl fluorescence characteristics. Titration of Ca2+-free parvalbumin with Ca2+ leads to a very pronounced blue shift, narrowing and intensification of the fluorescence spectrum. These spectral changs proceed in two stages reflecting the existence of at least three forms which can be interpreted as (a) the protein without Ca2+, (b) with one Ca2+ and (c) with two bound Ca2+ ions/molecule. The fluorescence of these forms has been identified and the fluorescence spectra measured at varied Ca2+ concentrations were resolved into three components corresponding to these spectral forms. The dependence of the relative concentration of the three fomrs on Ca2+ concentrations agree well with the two-step binding of Ca2+ to parvalbumin: Protein + Ca in equilibrium K1 protein x Ca; Protein x Ca + Ca in equilibrium K2 Ca x protein x Ca. The equilibrium binding constants K1 and K2 obtained by the computer fit are approximately 5 X 10(8) M-1 and 6 X 10(6) M-1. This scheme and the K1 and K2 value are in a good agreement with the independent experimental data resulting from EGTA titration of Ca2+-saturated parvalbumin and pH titratin of parvalbumin in the presence of EGTA and CA2+.

Narrow energy distribution of output electrons from a modified single channel electron multiplier

New methods which reduce the unfavorable energy spread of output electrons from a single channel electron multiplier are presented. A modified output electrode produces a pronounced narrowing effect on the energy distribution without much affect on the gain. A simplified analysis of the field inside the channel shows that the fixed channel wall potential near the output end is responsible for the narrow energy distributions obtainable.

Direct observation of superexchange in a disordered molecular solid

A novel observation of magnetic superexchange interactions in a disordered molecular solid is reported. Coordinated electrical conductivity and electron spin resonance measurements are made on controlled, variable concentrations of the molecule trip- p-tolylamine and its paramagnetic radical cation, molecularly dispersed in a polymer matrix. For a given concentration of spins in the concentration range 10 19cm -3 < N s <5× 10 20 cm -3, pronounced exchange narrowing is discovered as the density of neutral molecules is systematically increased. The temperature dependences of the ESR linewidth and the conductivity eliminate motional narrowing due to diffusive hopping of spins as the mechanism, since in these systems the paramagnetic ions are also the source of mobile charge.