bluuebunny/crossref_metadata_2025 huggingface dataset:
Melt velocity measurement by a noncontacting electromagnetic probe
Authors: Suh J. W. , Lee Z. H.
Published: July 2002

A novel method of the velocity measurement of melt was developed on the basis of electromagnetic principles. Two sensors were designed to measure the velocity of melt without direct contact. One, the electromagnetic velocity sensor (EMVS) consists of an electromagnet and a magnetic probe, and the other, the permanent magnet velocity sensor (PMVS) consists of a ring-shaped permanent magnet and a magnetic probe. These velocity sensors were tested with an aluminum cylinder and a Wood metal melt. The magnetic flux densities obtained by the velocity sensors were proportional to the rotating speed of the aluminum cylinder up to 1.5 m/s and the stirring speed of the Wood metal melt up to 0.8 m/s. The calculated magnetic-flux density near the velocity sensor was in good agreement with the measured one. The distribution of the magnetic field of the EMVS was more favorable in obtaining a larger sensor signal than the PMVS.
Direct temperature monitoring for semiconductors in plasma immersion ion implantation
Authors: Tian Xiubo , Chu Paul K.
Published: July 2002

In situ temperature monitoring is extremely important in plasma immersion ion implantation (PIII) of semiconductors. For instance, the silicon wafer must be heated to 600 °C or higher in separation by plasma implantation of oxygen, and in the PIII/ion-cut process, the wafer temperature must remain below 300 °C throughout the experiment. In this article, we present a thermocouple-based direct temperature measurement system for planar samples such as silicon wafers. In order to ensure reliable high-voltage operation and overall electrical isolation, the thermocouple assembly and wires are integrated into the sample chuck and feedthrough. Hydrogen plasma immersion ion implantation is performed in silicon to demonstrate the effectiveness and reliability of the device. Our experimental results indicate that instrumental parameters such as implantation voltage, pulse duration, and pulsing frequency affect the sample temperature to a different extent. The measured temperature rise is higher than that predicted by a theoretical model based on the Child–Langmuir law. The discrepancy is attributed to the finite-sample size and the nonplanar, conformal plasma sheath.
Theoretical prediction of GaN lasing and temperature sensitivity
Authors: Fang W. , Chuang S. L.
Published: July 2002

We present a theoretical prediction of the threshold current density and the temperature sensitivity of a GaN laser operating in the intrinsic band-to-band transition. We calculate the material gain and spontaneous emission spectrum for unintentionally doped bulk GaN under carrier injection. All stimulated and spontaneous emission calculations are compared to those of bulk GaAs. The transparency carrier density of GaN is found to be more than four times that of GaAs, and the momentum matrix element for optical transitions in bulk GaN is estimated to be about one-third the value in GaAs. In addition, the differential gain is approximately four times smaller in GaN. These differences are attributed to the larger effective masses of the electrons and holes in GaN. The calculated characteristic temperature T0 of the threshold current density for a GaN laser ranges from 185 to 220 K, which agrees well with the recently observed data from optical pumping experiments.
Asymmetric Y-junction wavelength demultiplexer based on segmented waveguides
Authors: Weissman Z. , Nir D. , Ruschin S. , Hardy A.
Published: July 2002

An asymmetric Y-junction wavelength demultiplexer, with a segmented branch, was realized in Ti:LiNbO3. The device was fabricated in a single-step lithographical process. Wavelength demultiplexing operation in the range 633 nm<λ<834 nm is demonstrated, with extinction ratios up to 30 dB. Insertion loss vary between a negligible value (far away from cutoff) up to 7 dB (near cutoff). Possible applications of the device are considered.
Structure and magnetic anisotropy of epitaxial fcc-Co(110) and hcp-Co(11̄00) films
Authors: Lo C. K. , Liou Y. , Chang C. P. , Klik I. , Yao Y. D. , Huang J. C. A.
Published: July 2002

Fcc-Co(110) and hcp-Co(11̄00) films of 200 Å thickness were grown on MgO(110) and bcc-Cr(211)/MgO(110), respectively, by the molecular beam epitaxy method. Reflection high energy diffraction was used to in situ characterize the crystal structure. Co films grown directly on MgO(110) were pseudomorphic fcc structure. Twofold symmetrical hcp-Co(11̄00) films on top of the bcc-Cr(211)/MgO(110) were grown and confirmed. The magneto-optical Kerr effect (MOKE) was used to investigate the magnetic anisotropy of these films. The magnetization of these samples was found to be in-plane. The magnetization of fcc-Co films has cubic symmetry with texture induced uniaxial anisotropy. For hcp-Co(11̄00)/bcc-Cr(211) bilayer films, the magnetization was strongly anisotropic, but independent of the thickness of the Cr layer.
Enhanced optical properties of in situ passivated near-surface AlxGa1−xAs/GaAs quantum wells
Authors: Lipsanen H. , Sopanen M. , Taskinen M. , Tulkki J. , Ahopelto J.
Published: July 2002

An epitaxial method for in situ passivation of epitaxial AlxGa1−xAs/GaAs surfaces is reported. The deposition of an ultrathin InP layer (about one monolayer) on the surface of AlxGa1−xAs/GaAs structures by metalorganic vapor phase epitaxy results in drastically reduced surface recombination. The effect is studied by low-temperature photoluminescence of near-surface Al0.22Ga0.78As/GaAs quantum wells where the top barrier thickness is varied from 0 to 50 nm. At the thicknesses of ≤5 nm, the intensity from passivated samples is more than four orders of magnitude larger than that obtained from unpassivated structures. For a passivated surface quantum well where InP is deposited directly onto the GaAs quantum well, we observe a blueshift of 15 meV and an intensity reduction of only a factor of 10 as compared to the luminescence from a quantum well placed at a depth of 50 nm from the surface.
Transmission electron microscopy investigation of tin sub-oxide nucleation upon SnO2 deposition on silicon
Authors: Alfonso Claude , Charaï Ahmed , Armigliato Aldo , Narducci Dario
Published: July 2002

Transmission electron microscopy analysis of tin dioxide films grown by aerosol-assisted chemical vapor deposition onto oxidized or etched silicon displayed the formation of a sub-oxide phase that was identified as Sn2O3. Such a phase is observed to disappear upon heat treatment, and is believed to be one of the factors responsible for the instability of tin dioxide films used as gas sensing layers.
A low-noise series-array Josephson junction parametric amplifier
Authors: Yurke B. , Roukes M. L. , Movshovich R. , Pargellis A. N.
Published: July 2002

We have obtained parametric gain at 19 GHz from a distributed Josephson junction parametric amplifier whose active gain medium consists of a series array of 1000 Josephson junctions embedded in a coplanar waveguide. When cooled to 1.7 K the amplifier provides 16 dB gain in a mode where the internally generated double sideband noise referred to input is 0.5±0.1 K. This noise is consistent with Nyquist noise generated from the losses. An instantaneous bandwidth of 125 MHz has been observed with a peak gain of 12 dB. The 3 dB compression point with a peak gain of 14.6 dB is -90.5 dB and the dynamic range is 38 dB.
Photorefractive effect in a photoconductive electro-optic carbazole trimer
Authors: Wang Liming , Zhang Yadong , Wada Tatsuo , Sasabe Hiroyuki
Published: July 2002

We present an experimental study of the photorefractive effect in a multifunctional carbazole trimer sensitized with 2,4,7-trinitro-9-fluorenone. The trimer consists of a central carbazole ring which is conjugated with two 6-dicyanovinylcarbazoles through ethynylene groups. It exhibits intrinsically photocarrier transporting, electro-optic, film-forming, and room-temperature poling properties. Photorefractive effect at a wavelength of 633 nm was demonstrated by measuring the phase shift of refractive index and asymmetric energy transfer in a two-beam coupling experiment. A large net photorefractive gain coefficient (Γ-α) of 76/cm was obtained in a sensitizer-optimized material. A four-wave mixing experiment was carried out to measure the magnitude of index modulation and dynamics of the grating formation.
Optimum tunnel barrier in ferromagnetic–insulator–ferromagnetic tunneling structures
Authors: Moodera Jagadeesh S. , Gallagher Elizabeth F. , Robinson Keziah , Nowak Janusz
Published: July 2002

Al 2 O 3 tunnel barriers I, formed by the oxidization of Al metal of various thicknesses between two ferromagnetic (FM) films were investigated to understand the influence of overlayer metal Al on the junction magnetoresistance (JMR). The optimum thickness of Al was observed to lie in the range of 1–1.6 nm to achieve good JMR in FM–I–FM junctions. Additionally, such junctions can be used to study the magnetic proximity effect in ferromagnet/normal metal bilayer systems.
Self-organized superlattice formation in II–IV and III–V semiconductors
Authors: Barabási Albert-László
Published: July 2002

There is extensive recent experimental evidence of spontaneous superlattice (SL) formation in various II–VI and III–V semiconductors. Here we propose an atomistic mechanism responsible for SL formation, and derive a relation predicting the temperature, flux, and miscut dependence of the SL layer thickness. Moreover, the model explains the existence of a critical miscut angle below which no SL is formed, in agreement with results on ZnSeTe, and predicts the formation of a platelet structure for deposition onto high symmetry surfaces, similar to that observed in InAsSb.
Luminescent observation of multiphoton ionization-fragmentation of chromate ions adsorbed on a disperse SiO2 surface
Authors: Glinka Yuri D.
Published: July 2002

Direct evidence for the multiphoton ionization with subsequent fragmentation of the chromate ions (CrO42−)adsorbed on a disperse SiO2 surface is provided by luminescence measurements using the Nd-doped yttrium aluminum garnet pulsed laser as a source of excitation. Experiments were done with the fundamental laser frequency (λ=1.064 μm; τp=20 ns). The multiphoton absorption proceeds through an intermediate three-photon resonance state. The luminescence response results from recombination of photoelectrons with ionized CrO42− ions and ionized surface oxygendeficient centers. The dynamics of this process include autoionization, which is stimulated by intermolecular perturbation in overexcited states, fragmentation of chromate ions, and annealing of the oxygen-deficient centers.
Crystal structure and electronic transport property of perovskite manganese oxides with a fixed tolerance factor
Authors: Sun J. R. , Rao G. H. , Liang J. K.
Published: July 2002

The structural and electronic transport properties of ABO3-type compounds La2/3−xRxCa1/3MnO3(R=Pr, Nd, Sm, Eu, Gd, Tb, Dy, Y, Er, and Tm) with a fixed tolerance factor of t=0.91137 is studied. Similar structure deformation, characterized by the nearly constant average Mn–O, A–O distances and Mn–O–Mn bond angles, is observed in different compounds. The metal–semiconductor transition shows a strong R dependent feature. The transition temperature decreases monotonously from 187 to 77.6 K as R varies from Tm to Pr, with a corresponding maximum resistivity ranging from 2.34 to 6.17×104 Ω cm. Very different magnetoresistance effects are also observed in this series of compounds. By assuming the presence of inhomogeneous local lattice distortions due to ions with different sizes at A sites and their mismatch with B ions, the lattice effects can be understood qualitatively.
Near-infrared sideband generation induced by intense far-infrared radiation in GaAs quantum wells
Authors: Černe J. , Kono J. , Inoshita T. , Sherwin M. , Sundaram M. , Gossard A. C.
Published: July 2002

GaAs quantum wells are simultaneously illuminated with near-infrared (NIR) radiation at frequency ωnir and intense far-infrared (FIR) radiation from a free-electron laser at ωfir. Magnetic fields up to 9 T are applied. Strong and narrow sidebands are observed at ωsideband=ωnir±2ωfir. The intensity of the sidebands is enhanced when either ωsideband or ωnir is near the onset of NIR absorption in the quantum well, or when ωfir is near the free-electron cyclotron frequency. We attribute these sidebands to four-wave mixing of NIR and FIR photons whose energies differ by more than a factor of 100.
Free-space electro-optics sampling of mid-infrared pulses
Authors: Wu Q. , Zhang X.-C.
Published: July 2002

We report on the coherent detection of ultra-broadband mid-infrared electromagnetic pulses using a 30-μm-thick ZnTe electro-optic sensor. The detected frequency spectrum exceeds 37 THz, extending from microwave to the mid-infrared. The frequency response can be further improved by reducing the sensor thickness to 10 μm.
Long-wavelength (≈15.5 μm) unipolar semiconductor laser in GaAs quantum wells
Authors: Gauthier-Lafaye O. , Boucaud P. , Julien F. H. , Sauvage S. , Cabaret S. , Lourtioz J.-M. , Thierry-Mieg V. , Planel R.
Published: July 2002

A unipolar semiconductor laser emitting in the mid-infrared spectral region is demonstrated. The laser scheme relies on a simple three-level system in GaAs/AlGaAs asymmetric coupled quantum wells. Population inversion between excited states is achieved by optical pumping of electrons from the ground state with a CO2 laser. Long-wavelength (≈15.5 μm) laser emission is demonstrated. The laser is operated in the pulsed regime up to a temperature of 110 K and with an output peak power ≈0.4 W at 77 K. Unipolar quantum well semiconductor lasers based on this principle are capable of covering the long wavelength mid-infrared spectral region above 12 μm.
GaInNAs/GaAs multiple quantum wells grown by gas-source molecular beam epitaxy
Authors: Xin H. P. , Tu C. W.
Published: July 2002

GaInNAs/GaAs multiple quantum wells (MQWs) with different N composition were successfully grown on semi-insulating GaAs substrate by gas-source molecular beam epitaxy. A nitrogen radical beam source was used to incorporate N into GaInAs layers. High resolution x-ray rocking curves measurements indicate that the N composition in GaInNAs layer was increased from 0.009 to 0.03 with increasing N2 flow rate. Photoluminescence (PL) measurements show that the PL wavelength red shifts with increasing N composition in GaInNAs layer. For a 7-period Ga0.7In0.3N0.02As0.98/GaAs MQW, a PL peak at 1.3 μm wavelength at room temperature has been successfully obtained. The band offset ΔEc for Ga0.7In0.3NxAs1−x/GaAs enlarges quickly from 0.26 eV to 0.56 eV with increasing N concentration from 0% to 3%.
Luminescence spectra from InGaN multiquantum wells heavily doped with Si
Authors: Deguchi T. , Shikanai A. , Torii K. , Sota T. , Chichibu S. , Nakamura S.
Published: July 2002

A systematic study has been carried out on emission spectra of heavily Si-doped InGaN multiquantum wells with different degree of potential fluctuation of InGaN in the lateral plane by the use of the various excitation sources. It is demonstrated that the quantum-confined Stark effect due to the piezoelectric field plays no serious role in optical spectra under appropriate doping conditions and, then, the degree of potential fluctuation of InGaN alloys is clearly reflected in spontaneous emission spectra.
Separation of density and viscosity influence on liquid-loaded surface acoustic wave devices
Authors: Herrmann F. , Hahn D. , Büttgenbach S.
Published: July 2002

Love-mode sensors are reported for separate measurement of liquid density and viscosity. They combine the general merits of Love-mode devices, e.g., ease of sensitivity adjustment and robustness, with a highly effective procedure of separate determination of liquid density and viscosity. A model is proposed to describe the frequency response of the devices to liquid loading. Moreover, design rules are given for further optimization and sensitivity enhancement.
Lateral current spreading in ridge waveguide laser diodes
Authors: Achtenhagen Martin , Hardy Amos
Published: July 2002

Lateral current spreading is experimentally and theoretically investigated in ridge waveguide laser diodes having various residual guide thickness outside the ridge region. It is found that a critical residual thickness exists below which the lasers emit in a single mode with a low threshold current. Above this critical value, the threshold rises rapidly and the lasers oscillate simultaneously in the two lowest order lateral modes. This critical thickness can be used to experimentally determine an average doping level of the upper waveguide layer. This doping level permits the control of the threshold current and series resistance.
Extensions of the Stoney formula for substrate curvature to configurations with thin substrates or large deformations
Authors: Freund L. B. , Floro J. A. , Chason E.
Published: July 2002

Two main assumptions which underlie the Stoney formula relating substrate curvature to mismatch strain in a bonded thin film are that the film is very thin compared to the substrate, and the deformations are infinitesimally small. Expressions for the curvature–strain relationship are derived for cases in which these assumptions are relaxed, thereby providing a basis for interpretation of experimental observations for a broader class of film–substrate configurations.
Experimental demonstration of a binary wire for quantum-dot cellular automata
Authors: Orlov A. O. , Amlani I. , Toth G. , Lent C. S. , Bernstein G. H. , Snider G. L.
Published: July 2002

Experimental studies are presented of a binary wire based on the quantum-dot cellular automata computational paradigm. The binary wire consists of capacitively coupled double-dot cells charged with single electrons. The polarization switch caused by an applied input signal in one cell leads to the change in polarization of the adjacent cell and so on down the line, as in falling dominos. Wire polarization was measured using single islands as electrometers. Experimental results are in very good agreement with the theory and confirm there are no metastable states in the wire.
Characterization and elimination of dry etching damaged layer in Pt/Pb(Zr0.53Ti0.47)O3/Pt ferroelectric capacitor
Authors: Lee June Key , Kim Tae-Young , Chung Ilsub , Desu Seshu B.
Published: July 2002

The damage of Pb(Zr0.53Ti0.47)O3 thin film due to dry etching process was characterized in terms of the microstructure and electrical properties. The damaged layer seems to be amorphous and the thickness of the damaged layer is about 10 nm. The existence of such a layer in Pt/Pb(Zr0.53Ti0.47)O3/Pt ferroelectric capacitor tends to increase the coercive voltage and the leakage current. The damaged layer was not fully reverted to perovskite phase by the thermal annealing. With the wet cleaning treatment, however, the damaged layer was successfully removed thereby revealing significantly improved electrical properties.
Temperature tuning of the stop band in transmission spectra of liquid-crystal infiltrated synthetic opal as tunable photonic crystal
Authors: Yoshino Katsumi , Shimoda Yuki , Kawagishi Yoshiaki , Nakayama Keizo , Ozaki Masanori
Published: July 2002

Nematic liquid crystals and smectic liquid crystals can be infiltrated into a three-dimensional periodic array of interconnected nanosize voids in opal films prepared by sedimentation of SiO2 spheres of various diameters. The optical stop band in the transmission spectra of opals shifts drastically by the infiltration of liquid crystals. The stop band is also found to shift at the phase transition points with changing temperature. This effect enables the tuning of optical properties of opals as a prototype tunable photonic crystal. This phenomenon can also be used as a measurement method for the refractive index.
Terahertz optical gain based on intersubband transitions in optically pumped semiconductor quantum wells: Coherent pump–probe interactions
Authors: Liu Ansheng , Ning C. Z.
Published: July 2002

Terahertz optical gain due to intersubband transitions in optically pumped semiconductor quantum wells (QWs) is calculated nonperturbatively. We solve the pump-field-induced nonequilibrium distribution function for each subband of the QW system from a set of rate equations that include both intrasubband and intersubband relaxation processes. The gain arising from population inversion and stimulated Raman processes is calculated in a unified manner. We show that the coherent pump and signal wave interactions contribute significantly to the tetrahertz gain. Because of the optical Stark effect and pump-induced population redistribution, optical gain saturation at larger pump intensities is predicted.
Strong nonlinear optical response in epitaxial liftoff single-crystal LiNbO3 films
Authors: Radojevic A. M. , Levy M. , Kwak H. , Osgood R. M.
Published: July 2002

We report on optical frequency mixing in epitaxial liftoff thin films of single-crystal LiNbO3 integrated onto heterogeneous planar glass platforms. These films are found to have a nonlinear optical response comparable to that of the bulk. Second-harmonic generation is investigated as a function of crystal orientation, ion implantation, and modal and temperature dispersion. Ion implantation-induced shifts in the refractive indices are shown to be useful for achieving phase matching.
Time-resolved photoluminescence in self-assembled InAs/GaAs quantum dots under strictly resonant excitation
Authors: Paillard M. , Marie X. , Vanelle E. , Amand T. , Kalevich V. K. , Kovsh A. R. , Zhukov A. E. , Ustinov V. M.
Published: July 2002

We investigate the carrier dynamics in self-assembled InAs/GaAs quantum dots under strictly resonant excitation of the ground state. The spectral selectivity of the resonant excitation allows us to study the physical properties of a class of dots characterized by an energy distribution comparable to the excitation laser spectrum. We detect no Stokes shift of the photoluminescence (PL) line. The PL decay time yields a straightforward determination of the radiative recombination time.
Dynamic response of 1.3-μm-wavelength InGaAs/GaAs quantum dots
Authors: Zhang L. , Boggess Thomas F. , Deppe D. G. , Huffaker D. L. , Shchekin O. B. , Cao C.
Published: July 2002

The temperature-dependent dynamic response of 1.3-μm-InGaAs/GaAs quantum dots is investigated using time-resolved photoluminescence upconversion with subpicosecond temporal resolution for excitation in either the GaAs bulk region surrounding the dots or within the wetting layer. Relaxation to the quantum-dot ground state occurs on a time scale as short as 1 ps, while radiative lifetimes as short as 400 ps are measured. The influence of nonradiative recombination is observed only for temperatures above 250 K. At temperatures below 77 K, an increase in the relaxation time and lifetime is observed when carriers are injected into the bulk GaAs region versus excitation into the wetting layer, which suggests that diffusion in the bulk GaAs region influences both the relaxation rate and the recombination rate.
Birefringence compensation in single solid-state rods
Authors: Fluck R. , Hermann M. R. , Hackel L. A.
Published: July 2002

Various methods for compensating birefringence depolarization in solid-state rods are theoretically and experimentally analyzed and compared. Gaussian and flat top beam profiles are investigated. The efficiency in depolarization loss reduction using different techniques is discussed in terms of beam profile, rod fill factor, and thermal heat load. In Nd:yttrium–aluminum–garnet, the depolarization loss can be efficiently reduced below 5% with a compensating quarter-waveplate, up to 20 W heat load for a flat top beam and up to 70 W for a gaussian beam.
Dense arrays of ordered GaAs nanostructures by selective area growth on substrates patterned by block copolymer lithography
Authors: Li R. R. , Dapkus P. D. , Thompson M. E. , Jeong W. G. , Harrison C. , Chaikin P. M. , Register R. A. , Adamson D. H.
Published: July 2002

GaAs has been selectively grown in a hexagonally ordered array of nanometer-scale holes with a density as high as ∼1011/cm2 by metalorganic chemical vapor deposition. This array of holes was created using block copolymer lithography, in which a thin layer of diblock copolymer was used as an etching mask to make dense holes in a 15-nm-thick SiNx film. These selectively grown nanoscale features are estimated to be 23 nm in diameter with narrow lateral size and height distributions as characterized by field-emission scanning electron microscopy and tapping mode atomic force microscopy. The narrow size distribution and uniform spatial position of the nanoscale dots we report offer potential advantages over self-assembled dots grown by the Stranski–Krastanow mode.
High-energy-resolution x-ray optics with refractive collimators
Authors: Chumakov A. I. , Rüffer R. , Leupold O. , Barla A. , Thiess H. , Asthalter T. , Doyle B. P. , Snigirev A. , Baron A. Q. R.
Published: July 2002

We have tested the concept of a high-resolution x-ray monochromator with a refractive collimator as an optical element. Two options were examined, where the refractive collimator was included either instead of, or in addition to the first crystal of the high-resolution monochromator. The first approach offers an easy means of improving the energy resolution of conventional optical schemes by few orders of magnitude while still accepting the entire angular divergence of the primary beam. The second approach improves the performance of existing devices with very high-energy resolution and simplifies the design of future optical schemes.
Room temperature InAsSb photovoltaic midinfrared detector
Authors: Rakovska A. , Berger V. , Marcadet X. , Vinter B. , Glastre G. , Oksenhendler T. , Kaplan D.
Published: July 2002

An InAs0.91Sb0.09 p-i-n photovoltaic midinfrared detector grown by molecular beam epitaxy and operating at room temperature is presented. An R0A of 1.05 Ω cm2 at 250 K and 0.12 Ω cm2 at 295 K has been achieved, resulting in a detectivity of 4.5×109 cmHz/W at 3.39 μm and 250 K. The quality of the active region material ensures a sufficiently low generation-recombination current. Room temperature performances are limited by the diffusion of holes from the active region through the confining barriers.
Conjecture on the interlacing of zeros in complex Sturm–Liouville problems
Authors: Bender Carl M. , Boettcher Stefan , Savage Van M.
Published: July 2002

The zeros of the eigenfunctions of self-adjoint Sturm–Liouville eigenvalue problems interlace. For these problems interlacing is crucial for completeness. For the complex Sturm–Liouville problem associated with the Schrödinger equation for a non-Hermitian 𝒫𝒯-symmetric Hamiltonian, completeness and interlacing of zeros have never been examined. This paper reports a numerical study of the Sturm–Liouville problems for three complex potentials, the large-N limit of a −(ix)N potential, a quasiexactly-solvable −x4 potential, and an ix3 potential. In all cases the complex zeros of the eigenfunctions exhibit a similar pattern of interlacing and it is conjectured that this pattern is universal. Understanding this pattern could provide insight into whether the eigenfunctions of complex Sturm–Liouville problems form a complete set.
Spectral resolution for a five-element, filtered, x-ray detector array using the method of Backus and Gilbert
Authors: Fehl D. L. , Biggs F. , Chandler G. A. , Stygar W. A.
Published: July 2002

The generalized method of Backus and Gilbert (BG) is described and applied to the inverse problem of obtaining the spectrum from a five-channel, filtered array of x-ray detectors. This diagnostic is routinely fielded on the Z facility at Sandia National Laboratories to study soft x-ray photons (∼100–2300 eV), emitted by high density Z-pinch plasmas. The BG method defines spectral resolution limits on the system of response functions that are in good agreement with a classical unfold method, based on a histogram representation of the source spectrum. The resolution so defined is independent of the source spectrum. For noise-free, simulated data the BG approximating function is also in reasonable agreement with the source spectrum (150 eV blackbody) and the unfolded spectrum. This function may be used as an initial trial function for iterative methods or a regularization model.
Potential-dependence of CO adlayer structures on Pt(111) electrodes in acid solution: Evidence for a site selective charge transfer
Authors: Akemann Walther , Friedrich Kaspar Andreas , Stimming Ulrich
Published: July 2002

The oxidation of carbon monoxide on a Pt(111) electrode surface is studied under transient and stationary reaction conditions in a 0.1 M HClO4 aqueous solution as a function of the applied electrode potential. The optical response is investigated by optical second harmonic generation (SHG) at 280 nm and infrared absorption spectroscopy (IRAS) starting at low potentials where the electrode is not reactive [e.g., 0.1 V vs the reversible hydrogen electrode (RHE)] towards more positive potentials where CO oxidation is initiated. For transient reaction conditions, in the absence of CO dissolved in solution, the oxidation starts at about 0.5 V vs. RHE and consists of a fast oxidation of about 10% of the adlayer and of a second reaction with slower kinetics involving the removal of the complete CO layer (overlayer stripping). Under steady-state conditions in CO-saturated solution the CO adlayer is stable up to 0.9 V vs RHE. At 0.63 V an overlayer phase transition is indicated by a 20% increase of the isotropic component of the second harmonic (SH) intensity (pp-polarization). The same potential region for this phase transition in the adlayer structure is derived from IRAS spectra after correcting for the effect of the thin layer electrolyte in IRAS measurements. The disappearance of hollow sites, the appearance of bridge sites, as well as an increased occupation of on-top sites at 0.63 V is interpreted as corresponding to the adlayer phase transition. The observations are consistent with a transition from the c(2×2) to the (√19×√19) adlayer structure of CO. The high sensitivity of SHG with regard to structural phase transitions of the CO adlayer is explained by distinct charge transfer contributions to the second-order surface susceptibility at different coordination sites.
Decay of elastic waves in alumina
Authors: Marom H. , Sherman D. , Rosenberg Z.
Published: July 2002

The dynamic response of alumina under shock compression was studied using planar impact experiments with different tile thicknesses. Stress-time measurements were made with manganin gauges backed by different backing materials in order to optimize gauge response. The results show an apparent decay in the Hugoniot elastic limit with propagation distance. However, further analysis reveals that this phenomenon is probably a measurement artifact, resulting from the relatively slow response times of manganin gauges.
Effect of Sb doping on the thermoelectric properties of Ti-based half-Heusler compounds, TiNiSn1−xSbx
Authors: Bhattacharya S. , Pope A. L. , Littleton R. T. , Tritt Terry M. , Ponnambalam V. , Xia Y. , Poon S. J.
Published: July 2002

Half-Heusler alloys (MgAgAs type) with the general formula MNiSn where M is a group IV transition metal (Hf, Zr, or Ti) are currently under investigation for potential thermoelectric materials. These materials exhibit a high negative thermopower (−40 to −250 μV/K) and low electrical resistivity values (0.1–8 mΩ cm) both of which are necessary for a potential thermoelectric material. Results are presented in this letter regarding the effect of Sb doping on the Sn site (TiNiSn1−xSbx). The Sb doping leads to a relatively large power factor of (0.2–1.0) W/m K at room temperature for small concentrations of Sb. These values are comparable to that of Bi2Te3 alloys, which are the current state-of-the-art thermoelectric materials. The power factor is much larger at T≈650 K where it is over 4 W/m K making these materials very attractive for potential power generation considerations.
Steady and oscillatory thermocapillary convection generated by a bubble
Authors: Kassemi M. , Rashidnia N.
Published: July 2002

In this article, we study steady and oscillatory thermocapillary and natural convective flows generated by a bubble on a heated solid surface. The dynamic characteristics of the time-dependent convection are captured using a combined numerical-experimental approach. The index of refraction fringe distribution patterns constructed numerically by taking an inverse Abel transform of the computed temperature fields are compared directly to the experimental Wollaston prism (WP) interferograms for both steady-state and oscillatory convection. The agreement between numerical predictions and experimental measurements is excellent in all cases. It is shown that below the critical Marangoni number, steady-state conditions are attainable. With increasing Ma, there is a complete transition from steady state up to a final nonperiodic fluctuating flow regime through several complicated symmetric and asymmetric oscillatory states. The most prevalent oscillatory mode corresponds to a symmetric up and down fluctuation of the temperature and flow fields associated with an axially traveling wave. Careful examination of the numerical results reveals that the origin of this class of convective instability is closely related to an intricate temporal coupling between large-scale thermal structures which develop in the fluid in the form of the cold return flow and the temperature sensitive surface of the bubble. Gravity and natural convection play an important role in the formation of these thermal structures and the initiation of the oscillatory convection. Consequently, at low-g, the time evolution of the temperature and flow fields around the bubble are very different from their 1-g counterparts for all Marangoni numbers.
Three-dimensional Marangoni–Bénard flows in square and nearly square containers
Authors: Bergeon A. , Henry D. , Knobloch E.
Published: July 2002

Numerical continuation is used to follow branches of steady solutions to the three-dimensional Marangoni–Bénard problem in a zero gravity environment. The upper surface of the fluid is heated by a constant heat flux while the bottom is maintained at a constant temperature. Instability arises due to temperature-dependent surface tension effects but surface deflection is ignored. Containers with square and nearly square cross sections and no-slip boundary conditions are analyzed, and the results interpreted in terms of predictions from equivariant bifurcation theory.
The determination of absolutely calibrated spectra from laser produced plasmas using a transmission grating spectrometer at the Nike laser facility
Authors: Weaver J. L. , Holland G. , Feldman U. , Seely J. F. , Brown C. M. , Serlin V. , Deniz A. V. , Klapisch M.
Published: July 2002

A transmission grating spectrometer has been developed that is capable of recording time-resolved spectra from laser-irradiated target foils in the wavelength range from 10 to 150 Å. Since the primary goal is to quantify the radiation flux from the resulting plasma, a detailed investigation has been conducted to evaluate the estimation of the absolute radiation intensity. Time-integrated spectra were observed on photographic film using a 5000 l/mm grating and clearly show the superposition of four diffraction orders. The deconvolution of the observed spectrum is based on a quantitative description of the spectrometer’s performance using the calibrated transmission efficiencies of the grating’s diffraction orders. This deconvolution procedure can provide an estimate of the absolute spectral intensity with a residual error of ∼20%. Results from a high spectral resolution grazing-incidence spectrometer have been used in the development of this method. A streak camera (not absolutely calibrated) was also used with the transmission grating spectrometer to record the time-resolved spectra.
Multiple quantum magic-angle spinning using rotary resonance excitation
Authors: Vosegaard Thomas , Florian Pierre , Massiot Dominique , Grandinetti Philip J.
Published: July 2002

We have discovered rotary resonances between rf field strength, ω1, and magic-angle spinning (MAS) frequency, ωR, which dramatically enhance the sensitivity of triple quantum preparation and mixing in the multiple-quantum MAS experiment, particularly for quadrupolar nuclei having low gyromagnetic ratios or experiencing strong quadrupole couplings. Triple quantum excitation efficiency minima occur when 2ω1=nωR, where n is an integer, with significant maxima occurring between these minima. For triple quantum mixing we observe maxima when ω1=nωR. In both preparation and mixing the pulse lengths required to reach maxima exceed one rotor period. We have combined these rotary resonance conditions into a new experiment called FASTER MQ-MAS, and have experimentally demonstrated a factor of 3 enhancement in sensitivity in comparison to conventional MQ-MAS.
Surface morphology of ion-beam-irradiated rutile single crystals
Authors: Ishimaru Manabu , Hirotsu Yoshihiko , Li Fuxin , Sickafus Kurt E.
Published: July 2002

Ion-beam-induced structural changes in rutile (TiO2) have been examined in detail using transmission electron microscopy (TEM). Single crystals of rutile with (100) orientation were irradiated with 360 keV Xe ions at room temperature to a fluence of 4×1015/cm2. In addition to a buried damage layer induced by projectile ions, a damage layer was formed near the surface (thickness about 10 nm). Nanobeam electron diffraction measurements and high-resolution TEM observations revealed that the surface damage layer consists of rutile crystallites with different orientations compared to the original single crystal. We discuss damage recovery processes in ion irradiated rutile within the context of our results as well as previous observations.
Dependence of the refractive index of AlxGa1−xN on temperature and composition at elevated temperatures
Authors: Tisch U. , Meyler B. , Katz O. , Finkman E. , Salzman J.
Published: July 2002

The refractive index of hexagonal AlxGa1−xN at room temperature and its temperature dependence at elevated temperatures have been determined with high accuracy by spectroscopic ellipsometry. Measurements have been conducted on samples with aluminum molar fractions ranging from 0% to 65% and at temperatures between 290 and 580 K. The refractive index in the transparent spectral region has been determined as a function of photon energy, using the Kramers–Kronig relations with suitable approximations, and applying a multilayer model. An analytical expression for the composition and temperature dependent refractive index in the transparent region, above room temperature, has been obtained. The refractive index has been found to increase with increasing temperature. The shift of the refractive index is strongest for GaN and decreases for AlGaN with increasing aluminum molar fraction. The impact on the properties of GaN based waveguides is illustrated by a slab waveguide calculation.
Atomic ordering in the LaFe0.5Mn0.5O3 solid solution film
Authors: Ueda K. , Muraoka Y. , Tabata H. , Kawai T.
Published: July 2002

LaFe 0.5 Mn 0.5 O 3 solid solution films have been formed on SrTiO3 (111) substrates using a pulsed laser deposition technique and their magnetic properties have been examined. The films showed ferromagnetic (or ferromagnetic) behavior with a Curie temperature of 380 K and the saturation magnetization was estimated to be about 1.5μB per magnetic ion site (B site). The x-ray photoemission spectra indicated that this behavior was due to the partial ordering of magnetic ions (Fe and Mn ions) which is caused by the charge disproportion between Fe and Mn ions under the film formation conditions.
Transient conduction in multidielectric silicon–oxide–nitride–oxide semiconductor structures
Authors: Bachhofer H. , Reisinger H. , Bertagnolli E. , von Philipsborn H.
Published: July 2002

The voltage- and time-dependence of the tunneling currents in polysilicon–oxide–nitride–oxide semiconductor structures have been investigated. Electron and hole contributions were separated using a shallow junction technique. The standard tunneling model for charge injection was successfully applied to describe the observed threshold voltage shifts. For both positive and negative gate voltages, the time-dependence of the current density through the tunneling oxide is given by a simple analytical equation. This equation is characterized by an initial time constant and an asymptotic t−1-dependence. At large programming times the current density follows the t−1-dependence, independent of the tunneling oxide thickness and applied voltage. Under positive polarity (write) electrons are injected from the substrate. Under negative polarity (erase) and previous injection electron back-tunneling rather than hole injection is dominant at the beginning of erasing. At the end of erasing, steady-state conduction can be dominated either by electrons or holes, depending on the applied voltage.
Vacuum–ultraviolet absorption and fluorescence spectroscopy of CF2H2, CF2Cl2, and CF2Br2 in the range 8–22 eV
Authors: Seccombe D. P. , Chim R. Y. L. , Tuckett R. P. , Jochims H. W. , Baumgärtel H.
Published: July 2002

The vacuum–ultraviolet (VUV) absorption and fluorescence spectroscopy of CF2X2 (X=H, Cl, Br) in the range 190–690 nm is reported. Tunable vacuum–UV radiation in the range 8–22 eV from synchrotron sources at either Daresbury, U.K. or BESSY1, Germany is used to excite the titled molecules. Fluorescence excitation spectra, with undispersed detection of the fluorescence, were recorded at Daresbury with a resolution of 0.1 nm. VUV absorption spectra at a resolution of 0.08 nm, and dispersed emission spectra with an optical resolution of 8 nm were recorded at BESSY1. Action spectra, in which the VUV energy is scanned with detection of the fluorescence at a specific wavelength, were also recorded at BESSY1 with a resolution of 0.3 nm; appearance energies for production of a particular emitting state of a fragment are then obtained. Using the single-bunch mode of BESSY1, lifetimes of all emitting states that fall in the range ∼3–80 ns have been measured. The peaks in the VUV absorption spectra of CF2X2 are assigned to Rydberg transitions. For CF2H2 below 11 eV, there is good agreement between the absorption and the fluorescence excitation spectra, whereas above 11 eV and for the whole range 8–22 eV for CF2Cl2 and CF2Br2 there is little similarity. This suggests that photodissociation to emitting states of fragment species represent minor channels. In the range 8–15 eV, emission is due mainly to CF2 Ã 1B1–X̃ 1A1 and weakly to CFX Ã 1A″−X̃ 1A′. These products form by photodissociation of Rydberg states of CF2X2, and the thresholds for their production, therefore, relate to energies of the Rydberg states of the parent molecule. For CF2H2 below 11.18 eV CF2 Ã 1B1 can only form with H2, whereas for CF2Cl2 and CF2Br2 it is not possible to say whether the other products are 2X or X2. For energies above ∼15 eV, emission is due to diatomic fragments; CF B 2Δ and A 2Σ+, CCl A 2Δ, CH B 2Σ− and A 2Δ, Cl2 and Br2 D′ 2 3Πg, and possibly CBr A 2Δ. From their appearance energies, there is evidence that with the exception of CF B 2Δ/CF2H2 where the ground state of HF must form, the excited state of CF, CCl, or CH forms in association with three atoms. Our results yield no information whether the three bonds in CF2X2* break simultaneously or sequentially. We suggest that the anomalous behavior of CF2H2, in forming H–H or H–F bonds in unimolecular photofragmentation processes, relates to the small size of the hydrogen atom, and hence, the unimportance of steric effects in the tightly constrained transition state. In no cases is emission observed from excited states of either the CF2X free radical or the parent molecular ion, CF2X2+.
Critical thickness for transformation of epitaxially stabilized cubic AlN in superlattices
Authors: Kim I. W. , Li Quan , Marks L. D. , Barnett S. A.
Published: July 2002

The epitaxial stabilization and transformation of cubic AlN layers in AlN/VN and AlN/TiN superlattices, grown by reactive sputtering on MgO (001), is described. In AlN/VN, the critical AlN thickness lC for transformation from cubic to hexagonal increased from ≈3.0 to >4 nm when the VN superlattice layer thickness was increased from 2.0 to 6.0 nm. The effect of lattice mismatch was observed by comparing AlN/VN (mismatch=1.46%) and AlN/TiN (mismatch=3.84%). The lC values were smaller, 2–2.5 nm, for the larger mismatch AlN/TiN system. The dependence of lC on the lattice mismatch and stabilizing layer thickness is discussed based on models of epitaxial stabilization.
Complete asymptotic expansions of the Fermi–Dirac integrals Fp(η)=1/Γ(p+1)∫∞[εp/(1+eε−η)]dε
Authors: Garoni T. M. , Frankel N. E. , Glasser M. L.
Published: July 2002

The complete asymptotic expansions, that is to say expansions which include any exponentially small terms lying beyond all orders of the asymptotic power series, are calculated for the Fermi–Dirac integrals. We present two methods to accomplish this, the first in the complex plane utilizing Mellin transforms and Hankel’s representation of the gamma function, and the second on the real line using the known asymptotic expansions of the confluent hypergeometric functions. The complete expansions of Fp(η) are then used to investigate the effect that these traditionally neglected exponentially small terms have on physical systems. It is shown that for a 2 dimensional nonrelativistic ideal Fermi gas, the subdominant exponentially small series becomes dominant.
Effect of argon on the thermal expansion of fullerite C60 at helium temperatures
Authors: Aleksandrovskii A. N. , Gavrilko V. G. , Esel’son V. B. , Manzhelii V. G. , Udovidchenko B. G. , Maletskiy V. P. , Sundqvist B.
Published: July 2002

The linear thermal expansion of compacted Ar-doped fullerite C60(ArxC60) is investigated at 2–12 K using a dilatometric method. The thermal expansion of ArxC60 is also studied after partial desaturation of argon from fullerite. It is revealed that argon doping resulted in a considerable change of the temperature dependence of the thermal expansion of fullerite. An explanation of the observed effects is proposed.
Adsorption of linear hydrocarbons in zeolites: A density-functional investigation
Authors: Benco Lubomir , Demuth Thomas , Hafner Jürgen , Hutschka François , Toulhoat Herve
Published: July 2002

An extensive first-principles periodical study of adsorption properties of linear hydrocarbons in zeolites is presented. The applicability of density-functional theory to weak interactions is inspected within both local-density (LDA) and generalized-gradient (GGA) approaches for C1 to C6 linear hydrocarbons. The LDA adsorption energies are due to the overbinding ∼2.5 times larger than the GGA values. A compact diagram is constructed showing the increase of the adsorption energy with the length of the adsorbed molecule and with the concentration of acid sites in the zeolite support. The flow of the electron density induced by the adsorption indicates that the adsorption on the acid site is realized through the hydrogen bonding between the OH group and the CH3 group. The pattern of the reconstructed bonding, however, is more complex than that of the simple hydrogen bond. The regions of redistributed electron density within the adsorbed molecule are spread over the whole CH3 group and the adjacent C–C bond. The off-centering of the reconstructed regions from atomic positions is in good agreement with recent C13 measurements, showing only slight variation of chemical shifts with the hydrocarbon length for both proton-free and the protonated forms of zeolites.