bluuebunny/crossref_metadata_2025 huggingface dataset:
Detection of Cr impurities in GaN by room temperature cathodoluminescence spectroscopy
Authors: Toth M. , Phillips M. R.
Published: July 2002

Trace levels of Cr impurities in epitaxial GaN grown on sapphire substrates were investigated using cathodoluminescence (CL) spectroscopy. CL emissions characteristic of Cr in an octahedral crystal field were observed from β−Ga2O3 overlayers produced on GaN by post-growth thermal annealing in dry O2. Cr luminescence was also observed from the sapphire substrates, a likely source of the Cr contaminant. The presented results illustrate the use of β−Ga2O3 overlayers as high sensitivity indicators of the presence of Cr in GaN.
Second-harmonic imaging of poled silica waveguides
Authors: Arentoft Jesper , Pedersen Kjeld , Bozhevolnyi Sergey I. , Kristensen Martin , Yu Ping , Nielsen Christian B.
Published: July 2002

Electric-field poled silica-based waveguides are characterized by measurements of second-harmonic generation (SHG) and of the linear electro-optic effect (LEO). A SHG scanning technique allowing for high-resolution imaging of poled devices is demonstrated. Scans along the direction of the poling field show that the second-order optical nonlinearity is located near the interface between differently doped glass layers. Both SHG and LEO measurements indicate that the ratio between the main elements of the second-order nonlinear optical susceptibility tensor, χ33(2) and χ31(2), is significantly smaller than three.
Threshold voltage of excimer-laser-annealed polycrystalline silicon thin-film transistors
Authors: Angelis C. T. , Dimitriadis C. A. , Farmakis F. V. , Brini J. , Kamarinos G. , Miyasaka M.
Published: July 2002

Based on experimental studies of n-channel excimer-laser-annealed polycrystalline silicon thin-film transistors with gate ratio width/length varying from 0.5 to 2.5, we propose a reliable method to determine the threshold voltage Vt from linear extrapolation of the transconductance to zero. The results reveal that the determined values of Vt are independent of the device geometry and the applied drain voltage in the linear region, in contrast with the drain current linear extrapolation method. The values of Vt are correlated with the density of the total trap states derived from the subthreshold gate swing voltage.
Si
1−x
Ge
x
/
Si
resonant-cavity-enhanced photodetectors with a silicon-on-oxide reflector operating near 1.3 μm
Authors: Li Cheng , Yang Qinqing , Wang Hongjie , Zhu Jialian , Luo Liping , Yu Jinzhong , Wang Qiming , Li Yongkang , Zhou Junming , Lin Chenglu
Published: July 2002

We report on a Si1−xGex/Si multiple quantum-well resonant-cavity-enhanced (RCE) photodetector with a silicon-on-oxide reflector as the bottom mirror operating near 1.3 μm. The breakdown voltage of the photodetector is above 18 V and the dark current density at 5 V reverse bias is 12 pA/μm2. The RCE photodetector shows enhanced responsivity with a clear peak at 1.285 μm and the peak responsivity is measured around 10.2 mA/W at a reverse bias of 5 V. The external quantum efficiency at 1.3 μm is measured to be 3.5% under reverse bias of 16 V, which is enhanced three- to fourfold compared with that of a conventional p-i-n photodetector with a Ge content of 0.5 reported in 1995 by Huang et al. [Appl. Phys. Lett. 67, 566 (1995)].
Modal gain and internal optical mode loss of a quantum dot laser
Authors: Herrmann E. , Smowton P. M. , Summers H. D. , Thomson J. D. , Hopkinson M.
Published: July 2002

The modal gain spectra and internal optical mode loss of a semiconductor laser structure containing a single layer of InGaAs quantum dots have been measured independently and directly as a function of current density. The quantum dot gain exhibits no obvious polarization dependence. The maximum modal gain of (11±4) cm−1 obtained from the ground state of a single layer of quantum dots is in this case insufficient for lasing operation since the internal optical mode loss measured on the same sample is (11±4) cm−1. As expected laser emission is not observed from the dot ground state, but from the excited dot state or from the wetting layer depending on device length.
Kinetics of hard sphere and chain adsorption into circular and elliptical pores
Authors: Skoulidas Anastasios I. , Sholl David S.
Published: July 2002

Transport of molecules through microporous crystals can be influenced by transport limitations at the mouths of pores. In the limit of high temperatures, the kinetics of adsorption into micropores is dominated by steric effects. To assess the role of molecular shape and pore size on pore mouth adsorption kinetics, discontinuous molecular dynamics has been used to simulate a variety of hard-sphere molecules adsorbing into slit, circular, and elliptical pores. Particular attention has been given to pores with length scales similar to zeolites. By simulating a range of straight chain, branched, and cyclic molecules, the selectivities that can arise due to differences in adsorption kinetics as functions of molecular size and shape have been probed.
The dependence of intermolecular interactions upon valence coordinate excitation: The υHF=4 levels of ArHF
Authors: Chuang Cheng-Chi , Klemperer William
Published: July 2002

The valence state dependence of the Ar–HF interaction potential is extended to υHF=4. Three new ArHF (υHF=4) states, (4000), (4100), and (4110), are observed between 14 780 and 14 880 cm−1 using intracavity laser induced fluorescence. The term values and rotational constants of these states are the following: (4000) ν0=14 783.603 23(30) cm−1, B=0.103 606 8(68) cm−1; (4100) ν0=14 867.419 06(70) cm−1, B=0.102 612(27) cm−1; and (4110) ν0=14 875.048 30(39) cm−1, B=0.103 217(19) cm−1, respectively. The spectral red shifts of ArHF (υ000) dramatically increase from 9.654 cm−1 at υ=1 to 48.024 cm−1 at υ=4. The rotational constant of ArHF(4000) increases essentially linearly with HF valence excitation, becoming 1.3% (40 MHz) greater than that observed at υ=0. At υ=4, the outer classical turning point of HF is extended by 0.4 Å from re, and there is no evidence for Ar–H repulsion. The spectral red shift for linear hydrogen bonded Ar–HF(υ000) indicates a strong enhancement of binding energy upon HF valence bond excitation, while the rotational constant reveals an almost surprising decrease in heavy atom separation. Both the T-shaped ArHF(υ110) and antilinear Ar–FH(υ100), however, show very little dependence of binding energy upon υHF valence excitation. These observations are in good accord with the ab initio intermolecular potential surface.
Development of a Fourier-transform ion cyclotron resonance mass spectrometer-ion mobility spectrometer
Authors: Bluhm Brian K. , Gillig Kent J. , Russell David H.
Published: July 2002

In an effort to incorporate ion-molecule reaction chemistry with ion mobility measurements we designed and constructed a novel instrument that combines a Fourier-transform ion cyclotron resonance (ICR) mass spectrometer with an ion mobility drift cell and a time-of-flight mass spectrometer. Measured mobilities for Ar+ and CO+ in helium are in excellent agreement with accepted literature values demonstrating that there are no adverse effects from the magnetic field on ion mobility measurements. Drift cell pressure, extracted from the measured mobility of Ar+ in helium, indicate that a pressure of ∼0.25 Torr is achieved in the present configuration. There are significant technological challenges associated with combining ICR and ion mobility that occurred during construction of this instrument, such as differential pumping and aperture alignment are presented.
Can the visual molecular configuration in computer simulations locate solid–fluid phase boundaries? The case of C60
Authors: Hasegawa M. , Ohno K.
Published: July 2002

The Monte Carlo method is used to explicitly show that solid–fluid phase boundaries determined by the visual appearance and disappearance of a solidlike structure is quite different from that expected in the bulk system and predicted by a full free-energy analysis. This feature of the computer simulations is almost self-evident and may be understood by the argument that the phase coexistence in the finite system is inevitably suppressed by the non-negligible interfacial energy compared with that in the infinite (bulk) system. One of the discrepancies among the recent simulation studies for the high-temperature phase behavior of a model C60 is found to be the consequence of overlooking the above feature of computer simulations.
Origin of the wavelength-dependence of effective trap density in photorefractive BaTiO3:Ce
Authors: Song Hongwei , Dou S. X. , Ye Peixian
Published: July 2002

The wavelength dependence of the total effective trap density, Neff, in a photorefractive BaTiO3:Ce crystal is experimentally determined by two-beam coupling. The result shows that Neff decreases with increasing wavelength. The deep-shallow-trap model for charge transport well fits to the experimental data, indicating that the model can quantitatively explain the variation of Neff with wavelength in BaTiO3:Ce. It is concluded that the remarkable variation of the ratio of the photo ionization cross section of the deep center to that of the shallow trap with wavelength, and the higher density of the shallow traps than that of the uncompensated deep centers are significant to the variation of Neff with wavelength.
Theory of energy gain in a laser-amplifier based on a photon-branched chain reaction: Auto-wave amplification mode under the condition of input signal focusing
Authors: Letfullin Renat R. , George Thomas F.
Published: July 2002

Huge energy gain is detected theoretically in a pulsed chemical laser amplifier based on a photon-branched chain reaction initiating in a gaseous disperse medium composed of H2-F2-O2-He and Al particles by focused external infrared radiation. It is shown that this effect is observed due to the possibility of the ignition of the laser–chemical reaction into an initial small focal volume of an active medium. It then spreads out of this minimal volume spontaneously in the auto-wave regime without external power sources and subsequently fills the whole volume of the laser cavity with a high intensive electromagnetic field as self-supporting cylindrical photon-branching zones formed by the paths of the rays inside the unstable telescopic cavity. Calculations show that the ignition of an auto-wave photon-branched chain reaction under the condition of external signal focusing strongly reduces the input pulse energy necessary for initiation up to ∼10−8 J, and thereby allows a huge value of the energy gain of ∼1011. The observed effect of this huge laser energy gain makes it possible to construct a self-contained laser with kilojoule output energy, which can be initiated by a very weak source signal.
Thermodynamic analysis of III–V semiconductor alloys grown by metalorganic vapor phase epitaxy
Authors: Asai Toshihiro , Dandy David S.
Published: July 2002

A thermodynamic analysis has been applied to systematically study III–V semiconductor alloy deposition, including nitrides grown by metalorganic vapor phase epitaxy. The predicted solid compositions of a number of ternary and quaternary alloys, including AlxGa1−xPyAs1−y, are compared with experimental data. For phosphorus-containing alloys, introduction of a parameter f representing incomplete PH3 pyrolysis yields good agreement with experimental data. It is shown that the input mole fraction of the group III metalorganic sources influences the incorporation of P into the solid for these alloys. Solid composition is also calculated for nitride alloys as a function of inlet gas concentration. To date, thermodynamic models have been applied solely to predict N solubility limits for nitride alloys where mixing occurs on the group V sublattice. The present model is used to predict N solid compositions in ternary and quaternary alloys, and it is demonstrated that these values are below the theoretical solubility limits for In-containing nitrides. The role of H2 in the carrier gas is investigated for III–N–V, III–III–N–V, and III–N–V–V systems.
Microscope system for use in high magnetic fields
Authors: Valles James M. , Wasserman Sarah , Denegre James M. , Mowry Kimberly L.
Published: July 2002

We describe a microscope system, with video output, that operates within the confines and the high magnetic field of the 195 mm wide bore, 20 T resistive magnet at the National High Magnetic Field Laboratory. The accessibility of this system allows the user to vary the field of view and focus in situ and to produce images with up to 10 micron resolution. It allows for temperature regulation of samples to within 1 °C and in situ sample manipulation. Images of a biological specimen, Xenopus laevis embryos, in high magnetic field are presented.
A technique for thermoelectric power measurements at high pressure in an octahedral multianvil press
Authors: Polvani D. A. , Fei Y. , Meng J. F. , Badding J. V.
Published: July 2002

We report a method for measuring the thermoelectric power of samples compressed in an octahedral multianvil pressure apparatus. Results are presented for the thermoelectric power at 300 K of doped Sb1.5Bi0.5Te3 up to 9 GPa and Nd0.45Ce2.55Pt3Sb4 up to 4.5 GPa.
Properties of random state manifolds with applications to intramolecular vibrational redistribution
Authors: Dietz W. , Fischer S. F.
Published: July 2002

We investigate spectral properties of random manifolds, which consist of a bright state coupled to mutually uncoupled dark states for an equal but otherwise arbitrary distribution of the couplings. Both Poisson and Wigner distributions of the energy spacings of the dark states are taken into account. The Poisson spacing model is solved exactly. The average spectrum is Lorentzian. The average dilution factor comes out to be a function alone of the mean coupling strength normalized to the mean neighbor spacing of the dark states. A simple expression for the explored fraction of the available phase space is obtained. Numerical studies indicate that the normalized coupling even controls the whole distribution of the dilution factor for the Poisson model. For weak mean coupling strength a secondary peak occurs in this distribution for both the Poisson and the Wigner model. A perturbational analysis shows that this peak leads back to accidental resonances of the bright state with single dark states. A simple tier model is suggested for treating the intermixing of vibrational dark basis states in molecules. Results are compared with experimental data.
Submillimeter resolution in one-dimensional position measurements of γ-ray photons by using a CsI(Tl) scintillator coupled to a linear array of silicon drift detectors
Authors: Fiorini C. , Labanti C. , Perotti F.
Published: July 2002

We present the one-dimensional (1D) position and energy resolution results obtained by a γ-ray detector based on a single CsI(Tl) scintillator coupled to a linear array of silicon drift detectors (SDDs). The present prototype has been realized in order to investigate the basic performances of this new architecture in view of the realization of Anger cameras for 2D imaging in nuclear medicine, based on the use of SDDs instead of photomultiplier tubes. The SDD provides a high value of quantum efficiency to the scintillation light, typical of a silicon photodetector, and is moreover characterized by a lower value of electronics noise with respect to conventional silicon photodiodes, thanks to the low value of output capacitance. At 122 keV the present detector shows a position resolution better than 0.5 mm FWHM, and an energy resolution of about 13% FWHM. The experimental setup is described and the most significant experimental results are presented.
Implementation of a photonic automatic gain control system for correcting gain variations in the Green Bank Telescope fiber optic system
Authors: White Steven D.
Published: July 2002

The application of a photonic automatic gain control system, over a 3 km loop of single-mode fiber for the 100 m Green Bank Telescope in order to correct the effects of polarization state changes, is described. Commercially available photodetector conversion gains are polarization mode dependent, which produce microwave gain changes on the order of 10−2. With the selection of the least polarization sensitive photodetector, the gain changes were reduced to less than the measurement limit of 10−4. The implementation of the control loop with a digital feedback loop is detailed, along with measured results and correction limit explanations. Effects of temperature on the quadrature point and corrections for small temperature deviations with a direct current bias circuit is also described.
Interpretation and deperturbation of the Λ-type doubling in the a 3Π state of GaH
Authors: Sbata Mouna , Schamps Joël
Published: July 2002

The unusual J dependence of the Λ doubling splitting that separates e and f levels in the a 3Π state of GaH has been interpreted using a deperturbation study carried out in an effective Hamiltonian point of view and based on a case (c) partition of the Hamiltonian: rather than limiting the zeroth-order part of the Hamiltonian to electronic and vibrational motions, spin-orbit interaction is also included, leaving outside just those effects linked with nuclear rotational motion. Thus the reasons for the peculiar behavior of the Λ splitting come out in a simple and appealing way. Essentially, with such a procedure, the variation with increasing rotation of the Λ-doubling splitting, i.e., the variations of the energy differences between e and f levels, are directly connected with fairly large differences between the rotational parameters (including mandatorily first and second order centrifugal distorsion) occurring in the off-diagonal Coriolis matrix elements within the 3Π block. To a lesser extent, smaller differences between the e and f rotational constants in the diagonal elements also contribute to the pattern of splittings. The consistency of the resulting values of the main fitted difference parameters (3Π0 e/f splitting at null rotation and first-order diagonal and off-diagonal rotational constant differences) is examined on the basis of experimental data on the one hand and new ab initio calculations on the other. Correct orders of magnitude are obtained although rotational constant differences but one are experimentally larger than expected theoretically from a model restricted to the four electronic states dissociating into ground state products.
Low temperature, postgrowth self-doping of CdTe single crystals due to controlled deviation from stoichiometry
Authors: Lyahovitskaya V. , Chernyak L. , Greenberg J. , Kaplan L. , Cahen David
Published: July 2002

Careful analysis of the Cd–Te pressure-temperature-composition phase diagram, shows a deviation of CdTe stoichiometry only in the Te-depletion direction between 450 and 550 °C. Combined control over the semiconductor composition, via intrinsic defects, and over the atmosphere and cooling rate can, therefore, yield a process for intrinsic doping of CdTe at these relatively low temperatures. We present results that support this. Quenching of CdTe, following its annealing in Te atmosphere at 400–550 °C, leads to p-type conductivity with a hole concentration of ∼2×1016 cm−3. Slow cooling of the samples, after 550 °C annealing in Te or in vacuum, increases the hole concentration by one order of magnitude, as compared to quenching at the same temperature. We explain this increase by the defect reaction between Te vacancies and Te interstitials. Annealing in Cd at 400–550 °C leads to n-type conductivity with an electron concentration of ∼2×1016 cm−3. Annealing at 450–550 °C in the equilibrium atmosphere, provided by adding CdTe powder, gives n-type material.
Stress induced electrical noise in stainless steel
Authors: Segales Lyndon D. , Gaines James R. , Misra Anupam K. , Rocheleau Richard E.
Published: July 2002

Stainless steel wires under tensile stress displayed narrow band peaks in the low frequency region of the power spectrum that were ∼104 times the background Johnson noise. The electrical response to stress was analyzed in terms of the root mean squared (rms) voltage (Vrms) of the noise signal over a 13 Hz bandwidth. The information obtained from such measurements is independent of other nondestructive testing methods and is therefore a useful probe of the stress-strain dynamics in metals and alloys. The rms voltage initially increases exponentially with the applied stress but subsequently decreases near the point of failure. This voltage is also approximately proportional to the square root of the sample resistance (Rs1/2). It is proposed that this noise signal is due to the fluctuations in the piezoresistivity as a result of stress induced reorientation of defects, similar to the Zener relaxation of solute atoms in a solid solution.
Intermultiplet interactions in normal and local mode molecules in the algebraic force-field expansion approach
Authors: Sako Tokuei , Yamanouchi Kaoru , Iachello Francesco
Published: July 2002

The algebraic force-field expansion recently proposed [T. Sako, K. Yamanouchi, and F. Iachello, Chem. Phys. Lett. 299, 35 (1999)] is applied to fit the experimental vibrational term values of H2O and SO2 in the electronic ground X̃ 1A1 state. The comparison of results of least-squares fits by the algebraic force-field expansion with those by the conventional force-field expansion shows that the convergence of the algebraic model is much faster than that of the conventional model and this rapid convergence becomes more significant when the Hamiltonian is expressed in local coordinates rather than in normal coordinates. It is also demonstrated that coordinate-space vibrational wave functions can be constructed directly through the experimental-level energy fit by the algebraic Hamiltonian expansion. From the nodal patterns of the vibrational wave functions constructed with the optimized Hamiltonian parameters of SO2, the bifurcation of the wave functions characteristic of the local-mode doublet states are identified in a vibrationally highly excited energy region. It is shown that the local-mode structure of the normal-mode limit molecule SO2 has the same origin as that for the local-mode limit molecule H2O.
Temperature dependence of low-frequency noise in Al–Al2O3–Al single-electron transistors
Authors: Kenyon M. , Lobb C. J. , Wellstood F. C.
Published: July 2002

We have measured the temperature dependence of the charge noise power spectral density Sq in two-junction Al–Al2O3–Al single-electron transistors at temperatures from 85 mK to 4 K. Although individual Lorentzians are often visible, the noise spectra are dominated by excess low-frequency noise with a power-law dependence on frequency f where Sq∝1/fβ and β≃1. Below about 0.5 K, Sq is weakly dependent on the temperature T. Above 1 K, the charge noise Sq increases with T, and at 4 K Sq≈10−4 e2/Hz at 1 Hz, about a factor of 100 greater than at 85 mK.
Luminescence quantum yield of molecular aggregates and excitons in α-sexithienyl thin films at variable temperature
Authors: Mei P. , Murgia M. , Taliani C. , Lunedei E. , Muccini M.
Published: July 2002

The energy transport and photoluminescence quantum yield in vacuum sublimed thin films of the α-sexithienyl (T6) model system, are investigated as a function of temperature in the range 30–300 K. The emission from the intrinsic bulk excitons and from the molecular aggregate states, is identified in the photoluminescence spectrum at each temperature. The corresponding absolute quantum yield of photoluminescence is measured with a home-built experimental apparatus based on an integrating sphere, which allows photoluminescence and electroluminescence quantum yield measurements in the temperature range 5–400 K. The photoluminescence quantum yield of the molecular aggregates placed below the exciton band span from 0.1% to 5% in the temperature range 300–30 K. In the same temperature range, the quantum efficiency of the intrinsic excitons increases by a factor of 2 from 0.4% to 1%. Therefore, in organic thin films both the spectral and quantum yield properties depend on the relative concentration of molecular aggregates formed during the growth process. We reconcile in this way the wealth of photoluminescence spectra and quantum yield values reported in the literature for T6 films grown under different conditions and with different thickness, i.e., containing different concentration of aggregate states.
Supercritical-fluid cell with device of variable optical path length giving fringe-free terahertz spectra
Authors: Saitow Ken-ichi , Nishikawa Keiko , Ohtake Hideyuki , Sarukura Nobuhiko , Miyagi Hiroshi , Shimokawa Yuji , Matsuo Hitoshi , Tominaga Keisuke
Published: July 2002

An optical cell suitable for supercritical fluids was constructed for measurements of far infrared absorption spectra with terahertz radiation as a light source. It was designed to withstand temperature up to 400 K and pressure up to 15 MPa. The cell has two characteristic devices; one is diamond windows set in the Brewster angle to the incident far infrared light and the other is a variable optical path length from 30 μm to 20 mm under high pressure conditions. Using the cell, fringe-free spectra of CHF3 ranging from low-density gaseous states to high-density supercritical ones were measured in a low-energy region of 10–100 cm−1.
Fast digital method of critical current measurement of superconducting tunnel junction
Authors: Vávra Ondrej , Gaži Štefan
Published: July 2002

This article describes a new computerized method of the fast measurement of the critical current IC of superconducting weak links. The method is based on the electronic transformation of the current to the time measurement. Sinusoidal current is also supplied which simplifies the filtration of the mains noise as well as helps to suppress a common mode voltage and inductive voltages. The instrument allows for the measurement of the critical current with a 1 μV voltage criterion. As one period of sinusoidal current is sufficient to determine IC, the measurement time is of the order of milliseconds for the current supply at a frequency range of 300–1000 Hz. The instrument has been tested by the measurement of the critical current of Josephson junctions as functions of magnetic field or temperature.
Reduction of the source size of gas-puff z-pinch plasmas using an inductive pulsed power system
Authors: Imasaka K. , Kawazoe K. , Kawauchi K. , Hara S. , Suehiro J. , Hara M.
Published: July 2002

A small scale inductive pulsed power (IPP) generator with copper wire fuses as an opening switch has been applied to an argon gas-puff z-pinch plasma which can be used as a point source for x-ray lithography. The IPP generator with fuses supplies a driving current of 60 kA in 400 ns to the z-pinch plasma. Without fuses, the generator serves as a conventional fast bank (FB) system which supplies z-pinch current of 60 kA in 4.5 μs. Characteristics of the z-pinch implosions in IPP and FB systems are investigated and compared. The result obtained shows that the intensity of soft x rays emitted from hot spots in both systems is decreased gradually with longer delay time, which is the time difference between energizing a gas-puff actuator and a primary current trigger. Dual axis pinhole imagery shows that the hot spot size is 0.8 mm in diameter in both systems. However, the average radial displacement of hot spots is 0.9 mm in the FB system and it is reduced to 0.4 mm in the IPP system. Optical framing camera images have confirmed that the radial distribution of hot spots is due to kink instability. The framing images also show that the imploding plasma in the FB system which has a pinch size of 2.2 mm in diameter can be further compressed to 1.3 mm. The IPP system can suppress the kink instability of pinch plasma and increases plasma column uniformity along the z axis. This suggests that the IPP system can improve the stability of the pinched plasma so as to minimize the end-on source size for x-ray lithography applications.
Electrical conduction in epitaxial BeSeTe/Si
Authors: Clark Kevin , Maldonado Eduardo , Barrios Pedro , Spencer Gregory F. , Bate Robert T. , Kirk Wiley P.
Published: July 2002

Epitaxial wide-band gap II–VI beryllium chalcogenide semiconductor films BeTe, BeSe, and BeSeTe were grown on arsenic-terminated silicon substrates. Electrical characteristics of Schottky contacts to the films were measured. Current-temperature measurements at elevated temperatures showed conduction band offsets of 1.2 eV for the BeSe0.41Te0.59/As/Si and 1.3 eV for the BeSe/As/Si heterostructures. At room temperature, the current density through BeSe/Si and BeSe0.41Te0.59/Si films was mid-10−9 A/cm2 at 0.1 MV/cm, similar to previously reported values for ZnS/Si, while BeTe/Si films had orders of magnitude higher current density possibly due to interfacial recombination.
Photoluminescence stability of a cyanoterphenyl chromophore in liquid crystalline polymeric systems
Authors: Sánchez C. , Alcalá R. , Cases R. , Oriol L. , Piñol M.
Published: July 2002

Polarized photoluminescence as well as birefringence measurements have been performed in different films of side-chain liquid crystal polyacrylates prepared by in situ photopolymerization. The polymeric systems contain a cyanoterphenyl chromophore covalently bound as a side group. The stability of the luminescence has been studied as a function of the composition and crosslinking of the polymer films. Although both the thermal quenching and the optical bleaching of the luminescence depend on the composition of the polymer matrix they seem to be independent of the crosslinking degree. Polymer films showing a stable luminescence with a high dichroic ratio have been obtained in copolymers combining mono and diacrylates having the same mesogenic core, with a low content (10%) of the diacrylate (crosslinking agent).
On the role of coherence in the transition from kinetics to dynamics: Theory and application to femtosecond unimolecular reactions
Authors: Møller Klaus B. , Henriksen Niels E. , Zewail Ahmed H.
Published: July 2002

We consider the relation between observed pump–probe signals in the femtosecond regime and the kinetics of unimolecular reactions, that is, the exponential decay of reactants and the exponential rise of the product population, respectively. It is shown that the signals cannot be fully accounted for within standard approaches of unimolecular decay, conventionally used in the past, since interference effects between the quasi-bound vibrational states within the bandwidth of the pump laser cannot be neglected. When these effects are included, all features of the signals can be accounted for. We apply this theoretical treatment of coherent interference to examine the dynamics and kinetics of the quasi-bound transition configurations, and relate them to the decay rates of individual quasi-bound resonance states. The signals show multi-exponential behavior, reflecting the different decay rates of the resonance states, with an average rate constant (within the bandwidth of the pump laser) which can be extracted directly from the signals. The persistence of coherence is evident in the observed signals. The predissociation of NaI is used as a prototype for numerical illustration. © 2000 American Institute of Physics.
Near interface H+ trapping and its influence on H+ transport in hydrogenated Unibond buried oxides
Authors: Macfarlane P. J. , Stahlbush R. E.
Published: July 2002

The effects of applied oxide field and H+ concentration on the transport of mobile protons across a buried oxide layer are examined in hydrogenated Unibond material. The H+ travel slower when moving from the substrate Si/SiO2 to the top Si/SiO2 interfaces than in the opposite direction. The effect of the self-field due to H+ in the oxide is insignificant even when that field is larger than the applied field. These results suggest that detrapping from defects near the Si/SiO2 interfaces plays a critical role in the H+ transport.
A simple and efficient laser beam trap using a highly absorbing glass plate at Brewster incidence
Authors: Lintz M. , Papoyan A. V.
Published: July 2002

Laser beam traps based on the use of highly absorbing neutral density glass plates at Brewster incidence have been implemented. These simple and inexpensive devices reach residual light levels on the order of 10−9 with no particular difficulty. Better rejection efficiencies require good quality polishing of the absorbing glass plates.
A new scanning slit rheometer for electro-rheological fluids
Authors: Shin S. , Cho Y. I.
Published: July 2002

A new scanning slit rheometer is presented for the rheological property measurement of electro-rheological (ER) fluids with Bingham-plastic model, based on a single measurement of liquid height variation with respect to time. The apparatus consisting mainly of an overhead reservoir, an ER slit, connecting and rising tubes, and a video camera connected to a computer is described with an experimental procedure. The present method overcomes one of the major drawbacks of the conventional viscometer, the inability to produce the yield stress of the ER fluid in an absolute zero shear rate range. The results with the present rheometer are compared with those obtained from a commercially available rheometer, indicating an excellent agreement.
S-Cam: A spectrophotometer for optical astronomy: Performance and latest results
Authors: Rando N. , Verveer J. , Andersson S. , Verhoeve P. , Peacock A. , Reynolds A. , Perryman M. A. C. , Favata F.
Published: July 2002

S-Cam is a cryogenic optical camera for ground-based astronomy designed around a 6×6 array of superconducting tunnel junctions. It has been conceived as a technology demonstrator, aimed at proving the potential of a new generation of single photon counting detectors for ground-based telescopes and as a possible precursor to a future space-based instrumentation. The camera is based on a 6×6 array of Ta–Al Josephson junctions, operating at about 350 mK and individually read out. For each detected photon, the absorption position, the arrival time, and the corresponding energy are measured. This allows for recording an image and simultaneously obtaining spectrophotometric information from the observed objects. In this article we provide an overview of the latest cryogenic detector performance, an up-to-date description of the S-Cam system, and a summary of the results obtained both during testing at ESTEC and during actual observations at the William Herschel Telescope in La Palma (Canary Islands, Spain). An example of the novel astronomical data obtained during the latest observation campaigns (December 1999 and April 2000) is also described.
Electrical isolation and transparency in ion-irradiated p-InGaP/GaAs/InGaAs structures
Authors: Danilov I. , Pataro L. L. , de Castro M. P. P. , Frateschi N. C.
Published: July 2002

He + -ion irradiation was applied for electrical isolation of p-In0.49Ga0.51P in InGaP/GaAs/InGaAs structures. Sheet resistance of approximately 1×106 Ω/□ was achieved with doses above 1×1013 cm−2 at 100 keV. Thermal stability of isolation was maintained for annealing temperatures up to 500 °C. Photoluminescence results show that InGaP transparency to InGaAs/GaAs quantum-well emission is closely related to sheet resistance changes in the irradiated structure.
Effects of postannealing on the electrical properties of Ta2O5 thin films deposited on TiN/T
Authors: Chang Chich-Shang , Liu Tzu-Ping , Wu Tai-Bor
Published: July 2002

The effects of postannealing on the structural and leakage current characteristics of Ta2O5 thin films deposited on a TiN/Ti bottom electrode by chemical-vapor deposition at 350 °C were investigated. The Ta2O5 film of 50 nm thickness shows an amorphous phase and smooth surface morphology after annealing in N2O plasma at 350 °C, but the film treated with rapid thermal annealing in nitrogen (RTN) at 750 °C prior to N2O plasma annealing at 350 °C has an orthorhombic structure and rugged surface morphology. In addition, the TiSi phase also appears in the specimen of RTN-treated Ta2O5 film. Moreover, the residual carbon can be effectively eliminated and the oxygen deficiency can be well compensated by the N2O plasma annealing for both amorphous and crystalline Ta2O5 films. The I–V relation of the amorphous Ta2O5 films is asymmetric with respect to the biasing polarity, which indicates that the leakage current is primarily dominated by the Schottky emission. On the other hand, the crystalline Ta2O5 film has a symmetrical I–V relation, suggesting that the non-Ohmic leakage in the crystalline Ta2O5 film is related to the Poole–Frenkel effect. The barrier heights φB for the non-Ohmic leakage in the amorphous and crystalline Ta2O5 films, as evaluated from the temperature dependence of the I–V relation, are 0.52 and 0.82 eV, respectively.
Far-field characteristics of mid-infrared angled-grating distributed feedback lasers
Authors: Vurgaftman I. , Bewley W. W. , Bartolo R. E. , Felix C. L. , Jurkovic M. J. , Meyer J. R. , Yang M. J. , Lee H. , Martinelli R. U.
Published: July 2002

The far-field emission characteristics of mid-infrared angled-grating distributed-feedback (α-DFB) lasers with W active regions are calculated using a self-consistent beam-propagation formalism that is more general than previous analyses. The theoretical projections are compared with the results of an experimental study of optically pumped α-DFB devices. Near-diffraction-limited beam quality is obtained both theoretically and experimentally for pump stripes ⩽50 μm wide. While simulations employing the theoretical linewidth enhancement factor of 1.7 for the homogeneously-broadened W-laser gain spectrum predict that the good beam quality should be retained for stripes as wide as ≈200 μm, the data indicate a much more rapid degradation. That finding can be reproduced only by assuming that inhomogeneous broadening increases the structure’s linewidth enhancement factor to ≈5. The experiments and theory also yield a steering of the output beam to off-normal angles as large as 6° when temperature tuning shifts the gain peak away from the grating resonance.
A filter bank system for scattered spectrum analysis in collective Thomson scattering diagnostic on JT-60
Authors: Lee S. , Kondoh T. , Yonemoto Y. , Miura Y.
Published: July 2002

In JT-60, a collective Thomson scattering (CTS) diagnostic system based on a pulsed CO2 laser (wavelength 10.6 μm, beam energy 15 J, pulse width 1 μs) has been developed to measure ion temperature and velocity distribution of fast ions to demonstrate the feasibility of measurements of confined alpha particles. A high power pulsed CO2 laser and heterodyne receiver system using a quantum-well infrared photodetector (QWIP) has been developed and installed in the diagnostic room. In order to resolve frequency distribution of pulsed scattered light (1 μs), we developed a wideband filter bank system (0.4–4.5 GHz) for scattered spectrum analysis in JT-60 CTS diagnostic. The detector bandwidth and frequency resolution are both determined by the calculation results of expected scattered spectrum. In order to realize wideband multichannel detection, cascade connected seventh order simultaneous-Chebyshev (elliptic) bandpass filters were adopted. Signal to noise ratio based on the noise equivalent power of the QWIP receiver and laser pulse length are also evaluated.
The infrared-ultraviolet dispersed fluorescence spectrum of acetylene: New classes of bright states
Authors: Hoshina Kennosuke , Iwasaki Atsushi , Yamanouchi Kaoru , Jacobson Matthew P. , Field Robert W.
Published: July 2002

Single rotational levels of ungerade vibrational levels, 2ν3′+ν6′ and 3ν3′+ν6′ (both with bu symmetry), in the Ã 1Au electronically excited state of acetylene were excited by an IR-UV double resonance scheme via the ν3″ fundamental level in the X̃ 1Σg+ state, and the rotationally resolved dispersed fluorescence (DF) spectra were recorded at 3.2–4.5 cm−1 resolution. The term values of the new ungerade levels were determined within an accuracy of 0.56 cm−1(1σ) through careful calibration achieved by frequency standard atomic Fe and Hg lines. A total of 111 new ungerade vibrational levels with Σu+, Σu−, and Δu symmetry below 10 000 cm−1 was identified in the high-resolution IR-UV-DF spectra, which provide access to new classes of X̃ 1Σg+ bright states: (i) (0,v2″,0,v4″1,1−1)Σu+, (0,v2″,0,v4″1,11)Δu, and (0,v2″,0,v4″3,1−1)Δu, which are the Franck–Condon (FC) bright levels from the nν3′+ν6′ (n=2,3) levels in the Ã 1Au state; (ii) (0,v2″,0,v4″−1,11)Σu− levels which appear through the a-axis Corioris interaction between nν3′+ν6′ and nν3′+ν4′ (n=2,3) in the Ã 1Au state; and (iii) (0,v2″,1,v4″0,0)Σu+ and (0,v2″,1,v4″2,0)Δu levels which gain transition intensity from the Duschinsky effect associated with the bent-linear Ã–X̃ transition. All observed ungerade term values and previously determined gerade and ungerade term values below 10 000 cm−1 were fitted by two effective model Hamiltonians, i.e., a pure-bend effective Hamiltonian and a stretch–bend effective Hamiltonian. The stretch–bend effective Hamiltonian is expressed in terms of 31 Dunham expansion parameters and 11 anharmonic resonance parameters associated with (i) five stretch–bend anharmonic resonances; (ii) one stretch–stretch and two bend–bend Darling–Dennison resonances; and (iii) one vibrational l resonance. The parameters in this Hamiltonian were determined from a least-squares fit of 287 vibrational term values (111 new ungerade levels, 128 levels from absorption, 1 level from stimulated Raman, 13 levels from stimulated emission pumping (SEP), and 34 levels from UV-DF spectroscopy) below 10 000 cm−1 with a standard deviation of σ=1.21 cm−1. The FC patterns for the v4″=odd ungerade levels, (0,v2″,0,v4″,1), in the IR-UV-DF spectra were derived, and the nodes along the v4″ trans-bend mode were found at v4″=11 via the 2ν3′+ν6′ upper state, and at v4″=9 and 15 via the 3ν3′+ν6′ upper state, which is consistent with the ν3′ dependence of the FC patterns observed in previous UV-DF studies.
Heat capacity of the ternary compounds RENiC2 (RE=Dy, Ho, Er, and Y)
Authors: Long Y. , Zheng C. Z. , Luo J. L. , Cheng Z. J. , He Y. S.
Published: July 2002

The heat capacities of the ternary compounds RENiC2 (RE=Dy, Ho, Er, and Y) were studied in the temperature range between 2 and 30 K. From the heat capacity curves, the ordering temperatures are found to be 7.5, 3.0, and 8.1 K for DyNiC2, HoNiC2, and ErNiC2, respectively. Also a small peak is found at 4.7 K for DyNiC2. The heat capacity of YNiC2 was well-fitted with an average Debye temperature of 456 K in the temperature range between 2 and 30 K. The magnetic heat capacities and the changes of magnetic entropy for RENiC2 (RE=Dy, Ho, and Er) indicate a strong influence of the crystal field on these compounds.
Spectrometer for stochastic solid-state nuclear magnetic resonance spectroscopy
Authors: Hirsch O. , Scheler G. , Jäger C.
Published: July 2002

A home-built spectrometer for solid-state nuclear magnetic resonance (NMR) spectroscopy with stochastic excitation is described. An overview of the instrument is given and selected components are explained in more detail. These are the control unit and the module for the pulse generation. Furthermore, a static probe with crossed coils for the transmitter and receiver circuits and the data processing part of the spectrometer software are described. The pulse unit can generate pulse trains with up to 219 pulses. Also, pulse-width-modulated sequences with automatic pulse centering can be produced. Using the crossed coil, probe decoupling between transmitter and receiver of more than 80 dB is possible. Several examples of NMR measurements are shown, which include selective excitation in solids and the acquisition of static H2 solid state NMR spectra with a spectral width of up to 185 kHz.
Experimental self-focusing of a single laser pulse in photorefractive Bi12TiO20
Authors: Wolfersberger D. , Fressengeas N. , Maufoy J. , Kugel G.
Published: July 2002

A single 5 ns laser pulse is experimentally shown to self-focus in electrically biased Bi12TiO20. Systematic measurements have been conducted in order to characterize the physics of fast photorefractive self-focusing. The influence of the externally applied electric field, the incident energy density, the input beam waist and polarization state are reported.
Electromigration voiding in nanoindented, single crystal Al lines
Authors: Dwyer V. M. , Ismail W. S. Wan
Published: July 2002

We consider the interpretation of some theoretical and experimental work regarding electromigration voiding in nanoindented, single crystal aluminum lines. A recently suggested voiding criterion of a critical accumulated flux divergence is found, in fact, to be identical to the widely accepted critical stress criterion. The inclusion of the stress dependence of the atomic diffusion coefficient is shown to be vital when the steady state is characterized by J≠0, such as in the case of a void growing at a constant rate. It is found, for example, that the stress required for steady void growth, within single crystal Al lines, is probably significantly smaller than previously suggested.
Photoluminescence of erbium oxide nanocrystals/TiO2/γ-glycidoxypropyltrimethoxysilane (GLYMO) composite sol–gel thin films derived at low temperature
Authors: Que Wenxiu , Zhou Y. , Lam Y. L. , Zhou J. , Chan Y. C. , Kam C. H. , Gan L. H. , Deen G. Roshan
Published: July 2002

Erbium (III) oxide nanocrystals prepared using the inverse microemulsion technique were dispersed in sol–gel TiO2/γ-glycidoxypropyltrimethoxysilane (GLYMO) composite thin films at low temperature. X-ray diffraction analysis and transmission electron microscopy observation show that the erbium (III) oxide nanoparticles have a nanocrystalline structure and their size is in the range from 5 to 30 nm. A relatively strong room-temperature photoluminescence (PL) was observed at 1.531 μm with a full width at half-maximum (FWHM) of 22 nm. The shape, peak position, and FWHM of the PL curve from the composite thin films are quite comparable to those prepared by other methods. The PL peak of the composite thin film reached a maximum intensity after a heat treatment at 300 °C. Fourier transform infrared results show that water and hydroxyl groups are present in the films and are responsible for the luminescence peak intensity reduction of the films heated at higher temperature.
Two-dimensional energy dispersive x-ray diffraction at high pressures and temperatures
Authors: Ma Yanzhang , Mao Ho-kwang , Hemley Russell J. , Gramsch Stephen A. , Shen Guoyin , Somayazulu Maddury
Published: July 2002

Diffraction studies at extreme pressure-temperature conditions encounter intrinsic difficulties due to the small access angle of the diamond anvil cell and the high background of the diffraction peaks. Energy-dispersive x-ray diffraction is ideal for overcoming these difficulties and allows the collection and display of diffracted signals on the order of seconds, but is limited to one-dimensional information. Materials at high pressures in diamond anvil cells, particularly during simultaneous laser heating to temperatures greater than 3000 K often form coarse crystals and develop preferred orientation, and thus require information in a second dimension for complete analysis. We have developed and applied a diamond cell rotation method for in situ energy-dispersive x-ray diffraction at high pressures and temperatures in solving this problem. With this method, we can record the x-ray diffraction as a function of χ angle over 360°, and we can acquire sufficient information for the determination of high P–T phase diagrams, structural properties, and equations of state. Technical details are presented along with experimental results for iron and boron.
Resonant markers for noninvasive, three-dimensional dynamic bone histomorphometry with x-ray microtomography
Authors: Kinney J. H. , Ryaby J. T.
Published: July 2002

A technique was developed to determine the spatial localization of radio-opaque resonant markers in newly formed bone by synchrotron x-ray microtomography. The resolution of the technique shows potential for three-dimensional tomographic mapping of bone mineral deposition, as an index of bone formation.