bluuebunny/crossref_metadata_2025 huggingface dataset:
Concordance between fasting and 2‐h post‐glucose challenge criteria for the diagnosis of diabetes mellitus and glucose intolerance in high risk individuals
Authors: Drzewoski J. , Czupryniak L.
Published: July 2002

SUMMARY

Aims  To compare the new fasting with the 2‐h post glucose challenge diagnostic criteria for diabetes mellitus in a high‐risk Central European population.

Methods  The results of the 75‐g oral glucose tolerance tests (OGTT) performed between 1st January 1990 and 31st December 1998 in patients at high risk of glucose metabolism disturbance were analysed.

Results  From 1554 patients with OGTT results available for the study, 1360 (759 women and 601 men, aged 65.5 ± 6.9 years, body mass index 28.2 ± 4.5 kg/m2) were included into the study. With the use of the post‐challenge criteria, 41.3% of the analysed population had diabetes or impaired diabetes tolerance (IGT), whereas with the new fasting system only 16.6% would have been be diagnosed with any type of glucose intolerance. Diabetes was significantly more often diagnosed with the post‐challenge criteria than with the fasting ones: 16.2 vs. 5.3% (P < 0.0001). The subjects with diabetes diagnosed upon fasting glucose value were significantly younger than the subjects with diabetes diagnosed according to the 2‐h glucose challenge: 65.7 ± 6.2 vs. 68.8 ± 7.0 years, respectively (P < 0.01). The sensitivity of the new criteria for the diagnosis of diabetes was 18.2%, and specificity 97.2%. A total of 77.8% of IGT cases would have been diagnosed as having normal glucose metabolism according to the fasting glucose. The sensitivity of the new criteria for the diagnosis of impaired glucose tolerance (IGT or impaired fasting glucose) was 14.6%, and specificity 89.8%. The overall kappa statistic (κ) was low: 0.211 (95% confidence interval 0.149–0.27).

Conclusions  The new lower fasting criteria might be too insensitive to identify a large proportion of individuals with diabetes or impaired glucose intolerance, particularly in a high‐risk population.
Microsatellite Analysis of Population Structure in the Endangered Hawaiian Monk Seal
Authors: Kretzmann Maria B. , Gemmell Neil J. , Meyer Axel
Published: July 2002

Abstract: The endangered Hawaiian monk seal breeds at six locations in the northwestern Hawaiian Islands. To determine whether significant genetic differentiation exists among these sites, we used microsatellite loci to examine the monk seal population structure at the five largest breeding colonies. Of 27 loci isolated from other seal species, only 3 were polymorphic in an initial screening of one individual from each breeding site. Only two alleles were found at each of these 3 loci in samples of 46–108 individuals. This extremely low variation is consistent with other measures of genetic variability in this species and is probably the result of a recent severe population bottleneck, combined with a long‐term history of small population sizes. Although the smallest monk seal subpopulation in this study ( Kure Atoll) showed some evidence of heterozygote deficit, possibly indicative of inbreeding, the next smallest ( Pearl and Hermes Reef) had an apparent excess of heterozygous individuals. Genetic differentiation was detected between the two subpopulations at extreme ends of the range ( Kure and French Frigate Shoals). This trend was significant only at the microsatellite locus for which we had the largest sample size ( Hg6.3: RST = 0.206, p = 0.002; allelic goodness of fit Gh = 15.412, p < 0.005). French Frigate Shoals is the source population for translocated animals that have been released primarily at Kure Atoll. Differentiation between these sites consisted of allele frequency differences (with the same allele predominant in each location at all three loci), rather than the preservation of alternative alleles. Although the translocations have had positive demographic effects, we recommend continued genetic monitoring of both the source and recipient populations because translocated individuals are now entering the breeding population.
CAUSE AND CLINICAL–NEUROPHYSIOLOGICAL PICTURE OF PERONEAL NEUROPATHY
Authors: Aprile I. , Padua L. , Meloni A. , D'amico P. , Tonali P.
Published: July 2002

We retrospectively analysed 40 patients with peroneal neuropathy (PN) referred to our EMG laboratory during the last 4 years. Aim of the study was to evaluate the incidence and causes of PN. Moreover, the relationship between causes and clinical‐neurophysiological picture was assessed and a follow‐up was performed.

Preliminary results showed that, in most cases, association between PN and surgical event was present (27,5%): unexpectedly, the higher occurrence of PN was observed in patients who underwent operation in the hip region (prosthesis). In 20% of cases, a damaging event was not identified. Erroneous postures (kneeling and crossing leg) caused PN in 12% of cases. Prolonged allurement was related in 10% of PN (50% of these in intensive care unit admission ).

PN was not significantly related to gender. The age range of the patients was extremely wide: from 7 to 80 years (mean 50 years). In more than 50% of cases, neuroapraxic block was demonstrated to neurophysiological evaluation.
Experimental evidence of both interstitial- and vacancy-assisted diffusion of Ge in Si
Authors: Fahey P. , Iyer S. S. , Scilla G. J.
Published: July 2002

We present the first experimental identification of the diffusion mechanisms of Ge in Si. Using thermal nitridation reactions to create either excess self-interstitials or vacancies, it is established that under equilibrium conditions at 1050 °C Ge diffusion takes place by both substitutional-interstitial interchange and vacancy mechanisms, with comparable contributions from each. If previous conjectures that Ge diffusion in Si is similar to Si self-diffusion are correct, our findings support the idea that Si self-diffusion takes place by both interstitial and vacancy mechanisms.
Amorphization processes in self-ion-implanted Si: Dose dependence
Authors: Motooka T. , Holland O. W.
Published: July 2002

The structural transformation in self-ion-implanted Si has been investigated using Raman spectroscopy and Rutherford backscattering spectrometry. The crystal Si Raman peak at 520 cm−1 decreased, broadened, and shifted toward lower wave numbers as the 100 keV Si+ dose was increased from 0.8 to 8.5×1014 cm−2. These peak shifts can be attributed to uniaxial lattice expansion in the direction normal to the Si surface and they are substantially larger than those predicted in uniaxially strained crystal Si. The results suggest that accumulated defects generated by ion bombardment not only expand the crystal Si lattice but also reduce the force constant which in turn increases plasticity and finally gives rise to amorphization of the Si lattice.
In situ determination of free-carrier concentrations by reflectance difference spectroscopy
Authors: Tanaka H. , Colas E. , Kamiya I. , Aspnes D. E. , Bhat R.
Published: July 2002

We determine types and concentrations of free carriers in GaAs layers under organometallic chemical vapor deposition growth conditions from the linear electro-optic structure observed near 3 eV in reflectance-difference spectroscopy. The sensitivity is about 1017 cm−3 at 400 °C and 1018 cm−3 at 600 °C, sufficient to measure common doping levels at or near growth temperatures. We observed the transition between n- and p-type doping during atomic layer epitaxy of a carbon-doped p-type layer on an n-type substrate at 470 °C.
Reduced flux motion via flux creep annealing in high- J
c single-crystal Y1Ba2Cu3O7
Authors: Thompson J. R. , Sun Yang Ren , Malozemoff A. P. , Christen D. K. , Kerchner H. R. , Ossandon J. G. , Marwick A. D. , Holtzberg F.
Published: July 2002

We investigated the stabilization of magnetic flux in a high-temperature superconductor (a proton-irradiated Y1Ba2Cu3O7 crystal), by operating with subcritical current density J. Using the thermal history to obtain an induced current density J≤Jc, we observed a drastically reduced relaxation rate dM/dt (M=magnetization), after ‘‘flux creep annealing.’’ The results show that the field gradient ∼J∼M determined the relaxation rate, independent of the sample’s H-T history, in agreement with recent theory.
Formation of titanium and zirconium nitrides by mechanical alloying
Authors: Calka A.
Published: July 2002

Using a novel ball mill with controlled ball movement, titanium and zirconium nitrides were synthesized by simple milling, at room temperature, of titanium and zirconium elemental powders in molecular nitrogen atmosphere. The structural evolution during milling is studied using x-ray diffractometry.
Measurements of particle size kinetics from nanometer to micrometer scale in a low-pressure argon-silane radio-frequency discharge
Authors: Boufendi L. , Plain A. , Blondeau J. Ph. , Bouchoule A. , Laure C. , Toogood M.
Published: July 2002

The time evolution of the size of particles produced in a silane-argon low-pressure radio-frequency discharge has been determined in the nanometer to the micrometer range using transmission electron microscopy. Highly ‘‘monodisperse’’ distributions are observed and their temporal evolution is followed. These studies are combined with laser light scattering measurements to obtain particle densities, which are of the order of 107 cm−3. For short plasma durations (Δt<5 s), a fast (10 nm/s) linear increase in diameter is measured.
New polymeric material containing the tricyanovinylcarbazole group for photorefractive applications
Authors: Tamura K. , Padias A. B. , Hall H. K. , Peyghambarian N.
Published: July 2002

We report on the synthesis and characterization of a new, completely polymeric material with a large electro-optic coefficient and photoconductivity. A methacrylic ester polymer containing the tricyanovinylcarbazole group with an alkylene spacer was synthesized. With 49 mW laser power, the photoconductivity of the polymer is 9.8×10−10 Ω−1 cm−1 at an applied electric field of 1.4×106 V/cm. Its electro-optic coefficient is 6.1 pm/V. The quantum efficiency of photocarrier generation and the photorefractive figure of merit were calculated. This electro-optic polymer is photoconductive and has photocarrier trap sites without any added sensitizer or photocarrier transport reagent, making it a suitable candidate for photorefractive applications.
Spectroscopic ellipsometric measurements of the dielectric function of germanium dioxide films on crystal germanium
Authors: Hu Y. Z. , Zettler J.-Th. , Chongsawangvirod S. , Wang Y. Q. , Irene E. A.
Published: July 2002

From spectroscopic ellipsometry measurements in the 1.5–5.7 eV photon energy range we determined the complex dielectric function of thermally grown germanium dioxide in the 1.0–6.3 eV range. A Kramers–Kronig consistent dispersion formula utilizing an exponential-shaped optical band edge was used in conjunction with both previously published far ultraviolet absorbance data for amorphous GeO2 and our spectra. These measurements show that ε2 for GeO2 can be regarded to be zero in the range of E<5.5 eV, which differs from a previous report. Using these new optical results for the investigation of oxide growth, we find that GeO2 grows via a parabolic growth law with a growth constant, kp=1.2×10−19 m2 s−1 at 550 °C.
Minority hole mobility in n+GaAs
Authors: Lovejoy M. L. , Melloch M. R. , Lundstrom M. S. , Ahrenkiel R. K.
Published: July 2002

The minority hole diffusivity, or equivalently the hole mobility, was measured in n+GaAs with the zero-field time-of-flight technique. The minority hole mobility was measured for the donor doping range of 1.3×1017 cm−3 to 1.8×1018 cm−3 and was found to vary from 235 to 295 cm2/V s. At the lower doping level, the minority hole mobility is comparable to the corresponding majority hole mobility, but at 1.8×1018 cm−3 the minority hole mobility was 30% higher than the majority carrier hole mobility. These results have important implications for the design of devices such as solar cells and pnp-heterojunction bipolar transistors.
Self-pumped phase conjugation with a new mechanism in KTa1−xNbxO3:Fe crystals
Authors: Lian Yingwu , Gao Hong , Ye Peixian , Guan Qingcai , Wang Jiyang
Published: July 2002

A new model is proposed to explain the origin of the self-pumped phase conjugation (SPPC) that we have observed in KTa1−xNbxO3:Fe crystals. In this model SPPC is generated by both four-wave mixing and backscattering processes rather than by only one of them. The advantage of this kind of phase conjugator is also discussed.
Scanning tunneling microscope measurement of insulator surfaces
Authors: Heike Seiji , Wada Yasuo , Kondo Seiichi , Lutwyche Mark , Murayama Ken , Kuroda Hiroshi
Published: July 2002

The possibility of measuring insulator surfaces with a scanning tunneling microscope (STM) is demonstrated. The mechanism is attributed to the conduction caused by electron beam assisted carrier generation in the insulator. A scanning electron microscope is used as the electron source. A 10-nm-thick and 100-nm-wide line-and-space patterned silicon dioxide layer formed on silicon substrate is observed by the STM only when an electron beam is directed at the sample. Tunneling spectroscopy results indicate a band gap of about 5 eV, which is attributed to that of silicon dioxide of about 8 eV.
Digital controller for a magnetic suspension system
Authors: Scudiere M. B. , Willems R. A. , Gillies G. T.
Published: July 2002

An active magnetic suspension system controlled digitally by a microprocessor has been built and operated in our laboratories. The control algorithm has integral, proportional, and derivative feedback loops. The remainder of the system including the sensor, power amplifier, and suspension coils, is of standard design. This developmental system has been used successfully to suspend small steel spheres and small rotors up to 190 g in weight, and shows promise of having wide application.
Estimation of HgCdTe band-gap variations by differentiation of the absorption coefficient
Authors: Ariel V. , Garber V. , Rosenfeld D. , Bahir G.
Published: July 2002

A new simple nondestructive method for band-gap estimation in HgCdTe layers is presented in this work. The theoretical approach is based on a qualitative empirical model that assumes two different regions of absorption in HgCdTe. The border between the two regions, which indicates the approximate value of the band gap, is determined by differentiation of the absorption coefficient in respect to the photon energy. The approach is verified by experimental measurements on several HgCdTe layers grown by different techniques. First, the transmission is measured by Fourier transform infrared (FTIR) spectroscopy at room temperature; then, the measured data is smoothed and the absorption coefficient extracted. The absorption coefficient is then differentiated twice in respect to the photon energy which allows estimation of the average band gap and band-gap variations within the HgCdTe layer. It appears that this simple procedure can assist in monitoring the quality of HgCdTe layers and help predicting the cutoff wavelength of HgCdTe photodiodes.
Measurement of current-density changes during sawtooth activity in a tokamak by far-infrared polarimetry (invited)
Authors: Soltwisch H.
Published: July 2002

Simultaneous measurements of plasma-induced phase shifts and Faraday rotation angles along nine vertical chords by means of a far-infrared interferometer/polarimeter have been used for some time to determine the electron density profile and the poloidal magnetic field distribution in the TEXTOR tokamak. While the sensitivity of the interferometric subsystem is sufficient to clearly detect sawtooth oscillations of the density profile, the noise level of the polarimetric signals usually obscures the appearance of sawteeth. However, owing to the high reproducibility of the sawtooth phenomenon under certain quasistationary discharge conditions, coherent-signal-averaging techniques can be applied to extract sawtooth-correlated modulations of the Faraday rotation angles. After correction for density-induced changes, these data represent the variation of the poloidal field pattern during a sawtooth cycle. Their analysis yields a periodic shift of the magnetic axis by about 1 mm, in good agreement with equilibrium calculations. For the axial current density a modulation amplitude of about 8% at an average central safety factor q0≂0.77 is found.
Corona-preionized nitrogen laser with variable pulse width
Authors: Hariri A. , Tarkashvand M. , Karami A.
Published: July 2002

A simple method of producing pulsed coherent radiation at 337.1 nm with variable pulse durations of 2.0–3.5 ns using a small transversely excited N2 laser is described. For laser operation in a moderately broad range of gas pressure (up to 1 atm), a weak surface corona discharge has been utilized as a preionizer. The electrical excitation is a flat plate Blumlein circuit with the main electrodes fixed at a height of 35 mm above the corona streamers. The laser produces an output peak power in excess of 70 kW at 350 Torr gas pressure and pulse repetition rate of 10–50 Hz.
Langmuir probe array for the DIII-D divertor
Authors: Buchenauer D. , Hsu W. L. , Smith J. P. , Hill D. N.
Published: July 2002

Two prototype Langmuir probes (based on a JET design) have been installed near the outer strike point on the divertor floor in the DIII-D tokamak. Results from these graphite probes were used to design a full array of Langmuir probes for the DIII-D divertor and an additional set of probes for the bias ring of the Advanced Divertor program. The design allows for the projected area of the probes to be relatively insensitive to the magnetic field angle in the divertor region, while projecting the intercepted heat flux over a much larger area. The prototype probes have survived the full separatrix plasma flux during both L- and H-mode operation. I/V characteristics during high power neutral beam operation exhibit electron temperatures approaching 50 eV and large negative floating potentials near the outer separatrix strike point. Edge localized modes cause an increase in the ion flux as large as a factor of 10 above the H-mode level. As a result, a new set of faster, higher voltage electronics will be used for the full array. The electronics will be capable of floating either with the divertor floor or with the bias ring.
Lasing operation up to 200 K in the wavelength range of 570–590 nm by GaInP/AlGaInP double-heterostructure laser diodes on GaAsP substrates
Authors: Tanaka T. , Uchida K. , Ishitani Y. , Minagawa S.
Published: July 2002

Short-wavelength stimulated emission from a GaInP/AlGaInP double-heterostructure (DH) grown on GaAs0.6P0.4 substrates, where lattice-matched Ga0.7In0.3P is the active layer with Γ band-gap energy beyond 2.1 eV is investigated. Laser oscillation is attained at a wavelength below 590 nm. This shows that the DH attains sufficient carrier confinement for lasing even though the minimum Γ band-gap energy is close to that in the X band. By applying high-reflectivity coating on both facets of the cavity to decrease the optical mirror loss, we achieve lasing operation by the DH devices under pulsed current injection up to 200 K. The device exhibits threshold currents of 115 mA at 77 K and 380 mA at 200 K, and an output power level up to 0.3 mW. The oscillation wavelength is 577 nm at 77 K and 588 nm at 200 K when the current is injected at 1.2 times the threshold.
Using diode lasers for atomic physics
Authors: Wieman Carl E. , Hollberg Leo
Published: July 2002

We present a review of the use of diode lasers in atomic physics with an extensive list of references. We discuss the relevant characteristics of diode lasers and explain how to purchase and use them. We also review the various techniques that have been used to control and narrow the spectral outputs of diode lasers. Finally we present a number of examples illustrating the use of diode lasers in atomic physics experiments.
A fast, time averaging data acquisition system for the PC-AT bus
Authors: Ambrosetti Roberto , Ricci Domenico
Published: July 2002

A data acquisition system designed for magnetic resonance instrumentation is presented. Its performance may be suitable for a broad set of applications where repetitive and noisy signals must be time-averaged in order to increase the signal-to-noise ratio. The system is built from easily available, low cost parts on a standard board for the PC-AT (16-bit) bus. 12-bit data acquisition and 32-bit time averaging take place in the memory of the PC under control of its CPU. The sustained data rate, including on-line signal averaging, can be as high as 250 kHz (500 kHz with interlaced data acquisitions). Timing is accomplished by on-board software programmable hardware timers. The CPU is then free for other tasks during the time, if any, between acquisition scans. Thus on-line display of the time-averaged data and their FT spectrum may be performed without impairing the timing accuracy, which is limited by the 10-MHz board clock to 100 ns. All functions of the interface are controlled by machine-language routines, callable from within quickbasic programs.
Room temperature persistent spectra hole burning in Sm2+-doped silicate glasses prepared by the sol-gel process
Authors: Nogami Masayuki , Abe Yoshihiro , Hirao Kazuyuki , Cho D. H.
Published: July 2002

Persistent spectra hole burning is observed at room temperature in the excitation spectrum for the 7F0→5D0 transition of the Sm2+ ions that are doped in aluminosilicate glass. Glass having the composition of Sm2+-doped Al2O3⋅9SiO2 has been prepared by the sol-gel processing of metal alkoxides and the reaction with H2 gas at 800 °C. Sol-gel derived glass enables Sm to be doped in the Sm2+ state of which the 4f6 (7F0)→4f55d band does not overlap with the 5D0→7F0 line. The hole width and depth, which are burned by the DCM dye laser, are ∼15 cm−1 and ∼10% of the total intensity, respectively, at 20 °C.
8–13 μm InAsSb heterojunction photodiode operating at near room temperature
Authors: Kim J. D. , Kim S. , Wu D. , Wojkowski J. , Xu J. , Piotrowski J. , Bigan E. , Razeghi M.
Published: July 2002

p +-InSb/π-InAs1−xSbx/n+-InSb heterojunction photodiodes operating at near room temperature in the 8–13 μm region of infrared (IR) spectrum are reported. A room-temperature photovoltaic response of up to 13 μm has been observed at 300 K with an x≊0.85 sample. The voltage responsivity-area product of 3×10−5 V cm2/W has been obtained at 300 K for the λ=10.6 μm optimized device. This was close to the theoretical limit set by the Auger mechanism, with a detectivity at room temperature of ≊1.5×108 cm Hz1/2/W.
Stabilization and precise calibration of a continuous-wave difference frequency spectrometer by use of a simple transfer cavity
Authors: Riedle E. , Ashworth S. H. , Farrell J. T. , Nesbitt D. J.
Published: July 2002

A novel, simple, and inexpensive calibration scheme for a continuous-wave difference frequency spectrometer is presented, based on the stabilization of an open transfer cavity by locking onto the output of a polarization stabilized HeNe laser. High frequency, acoustic fluctuations of the transfer cavity length are compensated with a piezoelectric transducer mounted mirror, while long term drift in cavity length is controlled by thermal feedback. A single mode Ar+ laser, used with a single mode ring dye laser in the difference frequency generation of 2–4 μm light, is then locked onto a suitable fringe of this stable cavity, achieving a very small long term drift and furthermore reducing the free running Ar+ linewidth to about 1 MHz. The dye laser scan provides tunability in the difference frequency mixing process, and is calibrated by marker fringes with the same stable cavity. Due to the absolute stability of the marker cavity, precise frequency determination of near infrared molecular transitions is achieved via interpolation between these marker fringes. It is shown theoretically that the residual error of this scheme due to the dispersion of air in the transfer cavity is quite small, and experimentally that a frequency precision on the order of 1 MHz per hour is routinely obtained with respect to molecular transitions.
Projection x-ray microscope powered by highly charged ions
Authors: Marrs R. E. , Schneider D. H. , McDonald J. W.
Published: July 2002

Recombination of slow highly charged ions at the surface of a target foil can be used as a source of x rays for a projection x-ray microscope. In a first test of this concept, a low emittance beam of Ar18+ and Ar17+ ions from an electron beam ion trap was focused with einzel lenses to a 20 μm full width at half maximum spot on a beryllium target foil. The 3 keV x rays from radiative deexcitation of the ions were used to obtain a magnified image of an electroformed nickel mesh with 20 μm resolution by projection onto a CCD camera. Prospects for substantial improvements in resolution and intensity are discussed.
Thickness and conductivity determination of thin nonmagnetic coatings on ferromagnetic conductive substrates using surface coils
Authors: Ptchelintsev A. , de Halleux B.
Published: July 2002

An eddy current technique for determining parameters of a thin nonmagnetic conductive coating on a ferromagnetic conductive sheet metal is reported. Under several conditions the electrical impedance of a coil is shown to be a function of two independent quantities: the substrate permeability-to-conductivity ratio and the coating thickness-conductivity product. Thus, thickness or conductivity of the coating can be determined independently of the variations of magnetic and electrical properties of the substrate. Simple analytic formulae for the electrical impedance of circular and rectangular surface coils are obtained and applied to predict the behavior of split wound surface coils. The performance of the method was tested in the frequency range 20–100 kHz on 1.5 mm thick low carbon steel sheets coated with 15–45 μm thick aluminum layers and on two series of 0.75–2.0 mm thick hot dip galvanized low carbon steel sheets of 9–20 μm coating thickness. The experiment was carried out using printed surface coils of rectangular and circular shape. Agreement between theory and experiment is excellent. The mathematical inversion carried out using the obtained formulae and the Newton–Raphson method takes about 450 ms on a Pentium 133 MHz PC. Discrepancies between the eddy current thicknesses and those obtained by other techniques were typically below 1 μm. An uncertainty in the thickness or conductivity determination better than 2% is obtained. The method has an extremely low sensitivity to variations in the substrate permeability. A large change in the substrate permeability, implying large changes in the coil electrical impedance, does not significantly influence the determined coating thickness. When the permeability change is about a factor of 10, additional errors in the thickness determination typically do not exceed 0.5 μm.
Harmonic detection of multipole moments and absolute calibration in a simple, low-cost vibrating sample magnetometer
Authors: Krishnan R. V. , Banerjee A.
Published: July 2002

The design and fabrication of a low-cost, highly sensitive, fully automated vibrating sample magnetometer for dc magnetization measurements is described. Harmonic detection of magnetic multipole moments is demonstrated experimentally in an inhomogeneously magnetized nickel sample at room temperature and in a gadolinium sample across the magnetic phase transition. A method is presented to nullify the earth’s effective field even without any shielding. More significant, the setup uses only one lock-in amplifier for signal detection, vibration excitation, and temperature measurement and control. First principles calculations give an absolute determination of the instrumental conversion factor that agrees with the experimental calibration factor. The setup is stable from 77 to 300 K, the overall sensitivity is ⩽5×10−6 emu, and the reproducibility is ⩽1%.
Simple scheme for tunable frequency offset locking of two lasers
Authors: Schünemann U. , Engler H. , Grimm R. , Weidemüller M. , Zielonkowski M.
Published: July 2002

We present a scheme for stabilizing the difference frequency of two independent lasers. The scheme is based on simple electronics and makes use of the frequency-dependent phase shift experienced by a signal when it propagates through a delay line of coaxial cable. The stabilized difference frequency can be tuned over a wide range. Difference frequencies in the radio-frequency domain (100 MHz–10 GHz) can be controlled with long-term stability of better than 1 MHz.
Femtosecond pump-probe near-field optical microscopy
Authors: Nechay B. A. , Siegner U. , Achermann M. , Bielefeldt H. , Keller U.
Published: July 2002

We have developed an instrument for optically measuring carrier dynamics in thin-film materials with ∼150 nm lateral resolution, ∼250 fs temporal resolution, and high sensitivity. This is achieved by combining ultrafast pump-probe laser spectroscopic techniques, which measure carrier dynamics with femtosecond-scale temporal resolution, with the nanometer-scale lateral resolution of near-field scanning optical microscopes (NSOMs). We employ a configuration in which carriers are excited by a far-field pump laser pulse and locally measured by a probe pulse sent through a NSOM tip and transmitted through the sample in the near field. A novel detection system allows for either two-color or degenerate pump and probe photon energies, permitting greater measurement flexibility over earlier published work. The capabilities of this instrument are proven through near-field degenerate pump-probe studies of carrier dynamics in GaAs/AlGaAs single quantum well samples locally patterned by focused-ion-beam (FIB) implantation. We find that lateral carrier diffusion across the nanometer-scale FIB pattern plays a significant role in the decay time of the excited carriers within ∼1 μm of the implanted stripes, an effect which could not have been resolved with a far-field system.
Electron field emission from ion-implanted diamond
Authors: Zhu W. , Kochanski G. P. , Jin S. , Seibles L. , Jacobson D. C. , McCormack M. , White A. E.
Published: July 2002

Diamond films and islands grown by chemical vapor deposition were implanted with boron, sodium, and carbon ions at doses of 1014–1015/cm2. This structural modification at the subsurface resulted in a significant reduction of the electric field required for electron emission. The threshold field for producing a current density of 10 mA/cm2 can be as low as 42 V/μm for the as-implanted diamond compared to 164 V/μm for the high quality p-type diamond. When the ion-implanted samples were annealed at high temperatures in order to anneal out the implantation-induced defects, the low-field electron emission capability of diamond disappeared. These results further confirm our earlier findings about the role of defects in the electron emission from undoped or p-type doped diamond and indicate that the improved emission characteristics of as-implanted diamond is due to the defects created by the ion implantation process.
Efficiency enhancement of microcavity organic light emitting diodes
Authors: Jordan R. H. , Dodabalapur A. , Slusher R. E.
Published: July 2002

We report microcavity efficiency enhancement of organic electroluminescent devices based on the hole transporter bis(triphenyl) diamine and the electron transporter and light emitter tris(8-hydroxyquinoline) aluminum. Microcavity organic light emitting diodes are described that emit four times the light measured in the forward direction (spectrally integrated), or almost twice the total light (spectrally and spatially integrated) of a noncavity organic light-emitting diode for identical electrical drive conditions.
7000 h continuous wave operation of multiple quantum well laser on Si at 50 °C
Authors: Yamada T. , Tachikawa M. , Sasaki T. , Mori H. , Kadota Y.
Published: July 2002

A 1.55 μm multiple quantum well laser heteroepitaxially grown on Si substrate operates under the severe aging condition of the light output of 5 mW/facet at 50 °C. The laser has been operating for more than 7000 h. The threshold current and the slope efficiency of the laser on Si at 20 °C are 32 mA and 0.19 W/A, respectively. The maximum operation temperature is above 80 °C.
The effect of surface nucleation on the evolution of crystalline microstructure during solid phase crystallization of amorphous Si films on SiO2
Authors: Ryu Myung-Kwan , Hwang Seok-Min , Kim Tae-Hoon , Kim Ki-Bum , Min Seok-Hong
Published: July 2002

The effect of surface nucleation on the evolution of crystalline microstructure during the solid phase crystallization (SPC) of an amorphous Si (a-Si) film, deposited by low pressure chemical vapor deposition (LPCVD) on SiO2, has been investigated. The surface nucleation phenomenon was observed by suppressing the interface (a-Si/SiO2) nucleation by the incorporation of oxygen atoms during the initial deposition period of a-Si. It was found that the surface-nucleated polycrystalline Si (poly-Si) had equiaxial grains with the size of about 3–5 μm, while interface-nucleated one had elliptical grains with the size of about 0.3–1 μm.
Huge thermopower of porous Y2O3
Authors: Koumoto K. , Seo W. S. , Ozawa S.
Published: July 2002

Porous Y2O3 ceramic was found to show huge thermopower values up to −50 mV/K at 900–1000 K in vacuum, but not in air. Such huge thermopowers may be generated by the electron gas emitted from the internal surfaces of the pores and are associated with some unknown effects. The nondimensional thermoelectric figure of merit, ZT, of this porous ceramic was as large as ∼0.95 at ∼950 K, and hence, it can be used as a thermoelectric material.
Continuous-wave and high-power pulsed operation of index-coupled distributed feedback quantum cascade laser at λ≈8.5 μm
Authors: Gmachl Claire , Capasso Federico , Faist Jérôme , Hutchinson Albert L. , Tredicucci Alessandro , Sivco Deborah L. , Baillargeon James N. , Chu S. N. George , Cho Alfred Y.
Published: July 2002

High performance index-coupled quantum cascade distributed feedback (QC-DFB) lasers operating at λ≈8.5 μm are reported. Reliable dynamic single-mode emission with a side mode suppression ratio ⩾30 dB is obtained. The continuous single-mode tuning range is 140 nm. In pulsed operation a record high peak output power of 60 mW at 300 K is achieved. We further report on the first continuous-wave QC-DFB lasers. These devices display an output power of 10 mW at 120 K.
Green photoluminescence from Er-containing amorphous SiN thin films
Authors: Zanatta A. R. , Nunes L. A. O.
Published: July 2002

Green light emission at room temperature was achieved from nonhydrogenated amorphous silicon–nitrogen (a-SiN) thin films. The films were deposited by cosputtering a silicon target covered with metallic erbium platelets in an Ar+N2 atmosphere. According to the deposition conditions, the nitrogen concentration [N] reached ∼40 at. % rendering an optical gap of approximately 3.5 eV while the Er concentration [Er] was estimated to be ∼10 at. % in the present films. The high [Er] associated to the optical band gap allows the direct excitation of Er3+ ions. This optical excitation is more efficient at low temperatures as a consequence of the reduction in nonradiative processes, and when exciting the samples with the 488.0 nm line of an Ar+ laser which is in resonance with the F47/2Er3+ energy level. In addition to light emission at ∼520 and ∼545 nm, transitions in the infrared energy region could be easily verified in as-deposited samples.
Role of localized and extended electronic states in InGaN/GaN quantum wells under high injection, inferred from near-field optical microscopy
Authors: Vertikov A. , Nurmikko A. V. , Doverspike K. , Bulman G. , Edmond J.
Published: July 2002

We report on spatially resolved optical measurements of high-quality InGaN/GaN multiple quantum wells under conditions of direct high optical injection (>1019 cm−3) using near-field optical microscopy in the collection mode. The spectral dependence of the spatial distribution of the photoluminescence indicates that the range of In-composition fluctuations reaches the 100-nm lateral scale. The spectra are dependent on the carrier injection level and reveal significant state filling effect. We sketch tentative conclusions about the In-cluster size distribution in terms of contributions to the radiative processes that involve localized and extended states, respectively, in the regime of electron–hole (e–h) pair densities at which present diode lasers operate.
Magnetic field dependence of quasiparticle losses in a superconductor
Authors: Ullom J. N. , Fisher P. A. , Nahum M.
Published: July 2002

We have studied the propagation of quasiparticles in a superconducting film which is penetrated by a magnetic field. We inject quasiparticles into an Al film at 75 mK using a normal–insulator–superconductor tunnel junction and measure the flux of quasiparticles which diffuse into an adjoining thermal detector. A magnetic field applied perpendicular to the plane of the film produces regions of reduced energy gap which trap quasiparticles. We find a significant reduction in the quasiparticle flux for fields as small as 2 G and explain our measurements with a model that incorporates inelastic scattering rates in Al and the characteristics of field penetration.
In situ texture monitoring for growth of oriented cubic boron nitride films
Authors: Litvinov Dmitri , Clarke Roy
Published: July 2002

We report evidence for oriented growth of pure-phase cubic boron nitride on silicon (100) substrates. The films are deposited at high temperatures (up to 1200 °C) by reduced-bias ion-assisted sputtering. The growth technique produces highly textured c-BN films with relatively large grain size (∼1000 Å) and reduced residual stress as the bias voltage is decreased. We have been able to grow thick (up to 2 μm) cubic boron nitride films containing 100% of the cubic phase with the (001) crystallographic axis of c-BN oriented perpendicular to the surface of the film. We show how reflection high-energy electron diffraction applied to texture monitoring in polycrystalline films can be used as an in situ process control technique that allows texture identification and quantitative characterization of its angular spread.
Band alignment at the organic-inorganic interface
Authors: Koller G. , Blyth R. I. R. , Sardar S. A. , Netzer F. P. , Ramsey M. G.
Published: July 2002

The band alignment of the bithiophene interface with a diverse range of substrates has been determined by a combination of ultraviolet photoemission and work function measurements. Not only is vacuum level alignment clearly shown to be invalid but also any sort of linear relationship between band alignment and substrate work function is shown not to be the case. Rather, the alignment is determined by the interface dipole, which is specific to the interaction at the inorganic-organic interface. The interface dipoles, which always appear, while dominated by the first monolayer interaction, are completed after two to three monolayers. As the ionization potentials of the films are shown to be constant, it is argued that a simple work function measurement, for an organic film on a particular substrate, quantifies the band alignment.
Piezoreflectance study of silicon carbon nitride nanorods
Authors: Hsieh C. H. , Huang Y. S. , Kuo P. F. , Chen Y. F. , Chen L. C. , Wu J.-J. , Chen K. H. , Tiong K. K.
Published: July 2002

Detailed piezoreflectance (PzR) measurements of quasialigned silicon carbon nitride nanorods in the temperature range between 15 and 400 K were performed. The direct band-to-band transition energies Egd at various temperatures were determined accurately through line shape fit of the experimental PzR spectra. The temperature dependence of Egd is analyzed by the Varshni equation [Y. P. Varshni, Physica, (Amsterdam) 34, 149 (1967)] and an expression containing the Bose–Einstein occupation factor for phonons proposed by L. Viña, S. Logothetidis, and M. Cardona [Phys. Rev. B 30, 1979 (1984)]. The parameters that describe the temperature dependence of Egd are evaluated and discussed.
Fluctuation formulas in molecular-dynamics simulations with the weak coupling heat bath
Authors: Morishita Tetsuya
Published: July 2002

The statistical mechanical ensemble associated with molecular-dynamics (MD) simulations with the weak coupling (WC) thermostat proposed by Berendsen et al. [J. Chem. Phys. 81, 3684 (1984)] is analyzed. Using a generalized Liouville equation, an approximate equilibrium distribution function in the configurational space for the WC thermostat is obtained. From this distribution function, fluctuation formulas associated with the constant volume specific heat Cv, pressure coefficient γv, and isothermal compressibility βT are obtained. The validity of the derived distribution function and the fluctuation formulas were examined in a Lennard-Jones fluid. Cv and γv calculated from the formulas for the WC thermostat agree very well with those obtained in the canonical or microcanonical MD.
Annealing of nonhydrogenated amorphous carbon films prepared by filtered cathodic arc deposition
Authors: Monteiro O. R. , Ager J. W. , Lee D. H. , Yu Lo R. , Walter K. C. , Nastasi M.
Published: July 2002

The effect of postdeposition annealing to 600 °C in vacuum on the hardness, modulus, film stress, and sp3 content of nonhydrogenated amorphous carbon was studied. The films were deposited using a filtered cathodic arc source and the film properties were varied by controlling the substrate bias. In films with an initial sp3 content of greater than 80% a significant decrease (from 10.5 GPa to less than 2 GPa) in the level of compressive stress was observed without deterioration of the mechanical properties. In films with an initial sp3 content of 50%, the stress and hardness remained constant. Films with lower sp3 content showed a significant increase in the level of stresses and only a marginal decrease in hardness. The evolution of stresses and hardness with annealing is discussed in light of a proposed microstructure of these amorphous films.
Performance enhancement of metal-hydride switchable mirrors using Pd/AlOx composite cap layers
Authors: van Gogh A. T. M. , van der Molen S. J. , Kerssemakers J. W. J. , Koeman N. J. , Griessen R.
Published: July 2002

A drastic improvement of the optical properties and lifetime of switchable mirrors is obtained by placing a thin AlOx buffer layer between the Pd cap layer and the optically active, rare earth layer. The buffer layer lowers the minimum necessary Pd thickness to ∼1 nm, resulting in a ≈20% increase of the maximum transmittance. The optimal Pd and Al layer thicknesses are determined for the YHx and LaHx system using a powerful combination of optical and matrix film techniques. The AlOx buffer is shown to be superior to the native oxide layers YOx and LaOx. The buffer layer is essential for lanthanum, which is a particularly vulnerable, but fundamentally very important material. Using this composite cap layer, we have been able to switch LaHx films several times.
Photodetachment diagnostic techniques for measuring negative ion densities and temperatures in plasmas
Authors: Bacal M.
Published: July 2002

Photodetachment diagnostic techniques can help determine densities and temperatures of negative ions in a variety of scientific devices in which these ions are one of the major charged particle species. This method has been extensively used in the development of hydrogen negative ion sources as well as other devices. In order to obtain spatial resolution, a photodetachment diagnostic technique is used with an electrostatic probe that detects the currents of photodetached electrons.
Structural disorder in the high-energy milled magnesium ferrite
Authors: Šepelák V. , Baabe D. , Litterst F. J. , Becker K. D.
Published: July 2002

The structural and magnetic evolution in magnesium ferrite (MgFe2O4) caused by high-energy milling are investigated by Mössbauer spectroscopy. It is found that the nanostructural state of the milled MgFe2O4 is characterized by a mechanically induced cation redistribution between tetrahedral (A) and octahedral [B] sites. The reduced concentration of iron ions at (A) sites in the mechanically treated samples leads to the variation in the number of magnetic and nonmagnetic (A)-site ions as nearest neighbors of the Fe3+[B] ions. This results in a broad distribution of magnetic hyperfine fields at the [B] sites. In addition to the local magnetic fields B(6), B(5), and B(4) characteristic of nonactivated ferrite and corresponding to Fe3+[B] ions with n=6, 5, and 4 nearest (A)-site iron neighbors, respectively, the distribution curves of mechanically treated samples show additional components at smaller magnetic fields. The weight of the B(6) field decreases with increasing milling time, and the B(5) field becomes the most probable hyperfine field component in the distribution curve of the mechanically activated samples. The degree of inversion in MgFe2O4 is calculated from the probabilities of the different [B]-site surroundings as well as from the Mössbauer subspectral areas. Excellent agreement is obtained in the two independent procedures for the determination of the cation distribution. This enables us to separate from the [B]-site magnetic field distribution profile the contribution arising from the mechanically induced “new” nearest-neighbor (A)-site configuration with n=3 nearest (A)-site iron neighbors. Taking into account the nanoscale nature of the mechanically activated MgFe2O4, the observed spin canting, which increases with increasing milling time, is attributed to the noncollinear spin structure of the near-surface atoms. In strongly activated ferrite, the magnetic hyperfine splitting breaks down totally and the Mössbauer spectrum is dominated by a superparamagnetic relaxation effect.
Electron Interactions With CF3I
Authors: Christophorou L. G. , Olthoff J. K.
Published: July 2002

Low-energy electron collision data for the plasma processing gas CF3I are sparse. Limited cross section data are available only for total and differential elastic electron scattering, electron-impact ionization, and electron attachment. These data are assessed, synthesized, and discussed in this paper. There is a need for confirming measurements for some of these data, and for measurements of cross sections for the other main electron–collision processes for which no data exist. There are presently no data available for the electron transport, ionization, and attachment coefficients of this molecule. © 2001 by the U.S. Secretary of Commerce on behalf of the United States. All rights reserved.
Diffusion of boron in silicon carbide: Evidence for the kick-out mechanism
Authors: Bracht H. , Stolwijk N. A. , Laube M. , Pensl G.
Published: July 2002

We report diffusion experiments of implanted boron (B) in nitrogen-doped 4H- and aluminum-doped 6H-SiC which were performed at temperatures between 1700 and 1800 °C. Transient enhanced B diffusion caused by implantation damage was effectively suppressed by annealing of the B-implanted samples at 900 °C prior to the diffusion anneal. Concentration profiles of B measured with secondary ion mass spectrometry are accurately described on the basis of the kick-out mechanism. This provides strong evidence that Si self-interstitials mainly mediate B diffusion.