bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic force microscopy of domain wall stray fields on single-crystal iron whiskers
Authors: Göddenhenrich T. , Lemke H. , Hartmann U. , Heiden C.
Published: July 2002

Using a capacitively controlled force microscope we have imaged typical domain wall configurations like 90° closure structures and subdivided 180° wall segments in single-crystal iron whiskers. Differences in wall contrast between 90° and 180° domain walls are clearly observed. The effect of tip-to-sample distance on lateral resolution and wall contrast in magnetic force microscopy is shown.
Quantum size effects on photoluminescence in ultrafine Si particles
Authors: Takagi H. , Ogawa H. , Yamazaki Y. , Ishizaki A. , Nakagiri T.
Published: July 2002

Visible photoluminescence was observed in ultrafine Si particles at room temperature. Transmission electron microscopy revealed that Si microcrystallites were embedded in a Si oxide matrix for the sample which emitted the light. The emission energy depended on crystallite size in the range from 2.8 to 5 nm. The inverse relation between emission energy and the square of the crystallite size indicates that carrier confinement in the Si microcrystallites causes this photoluminescence phenomenon.
Photorefractive self-defocusing
Authors: Segev Mordechai , Ophir Yoav , Fischer Baruch
Published: July 2002

Self-defocusing of a Gaussian light beam in a bulk photorefractive BaTiO3 crystal is demonstrated. This is a low light intensity process which results from the coupling and power transfer between the spatial frequency components of the beam. We use this effect in photorefractive waveguide structure for all-optical switching with very low light powers.
Computer simulation of relativistic multiresonator cylindrical magnetrons
Authors: Chan Hei-Wai , Chen Chiping , Davidson Ronald C.
Published: July 2002

The relativistic multiresonator magnetron is analyzed in cylindrical geometry, using the two-dimensional particle-in-cell simulation code magic. Detailed comparisons are made between the simulation results and the experiments by A. Palevsky and G. Bekefi [Phys. Fluids 22, 986 (1979)] using the A6 magnetron configuration. Within a constant scale factor, the computer simulations show a similar dependence of microwave power on magnetic field, with dominant excitations in the π and 2π modes. In the preoscillation regime, the electron flow in the simulations differs substantially from the ideal Brillouin flow model. In the nonlinear regime, the saturation is dominated by the formation of spokes.
Low temperature and selective growth of β-SiC using the SiH2Cl2/i-C4H10/HCl/H2 gas system
Authors: Ohshita Yoshio
Published: July 2002

β-SiC is grown on a silicon substrate by the chemical vapor deposition method using the SiH2Cl2/i-C4H10/H2/HCl gas system. Stoichiometric β-SiC films are obtained with high growth rate at a low temperature of 900 °C. Highly (111) oriented β-SiC polycrystal is grown on a Si(111) substrate. Moreover, using the above-mentioned gas system, β-SiC selective growth is attained on a Si substrate, with no nucleation on the SiO2 area. This letter discusses the i-C4H10 effects and selective growth condition.
Excitation intensity dependence of photoluminescence in Ga0.52In0.48P
Authors: DeLong M. C. , Taylor P. C. , Olson J. M.
Published: July 2002

The excitation intensity dependence of the photoluminescence (PL) from Ga0.52In0.48P grown by organometallic vapor phase epitaxy on GaAs substrates has been investigated as a function of epitaxial layer growth temperature and substrate orientation. It is well known that the degree of ordering and the band-gap energy of this material are functions of growth conditions. We report here on a PL emission which shifts rapidly with excitation intensity. The rate of emission shift is also a function of growth conditions including substrate orientation. There is, however, no significant correlation between the band-gap energy and the rate of emission shift in Ga0.52In0.48P. This PL shift is explained in terms of band filling of potential fluctuations that are associated with a microstructure consisting of ordered domains within a disordered matrix.
Band offset in InP/Ga0.47In0.53As heterostructures
Authors: Nag B. R. , Mukhopadhyay Sanghamitra
Published: July 2002

Energy levels in InP/Ga0.47In0.53As quantum wells are calculated after reformulating the energy-dependent effective mass to be used for taking into account the energy-band nonparabolicity of both constituents. The required value of the ratio of the conduction-band and valence-band discontinuities is found to be close to 2/3, in agreement with the value found by other methods. The value of the nonparabolicity factor is also found to be the same as that used in earlier transport studies.
Single-mode polymer waveguide modulator
Authors: Chen Ray T. , Sadovnik Lev , Jannson Tomasz , Jannson Joanna
Published: July 2002

We report herein the first single-mode polymer waveguide modulator which can be formed on any surface regardless of its conductivity and refractive index. These include semiconductor, conductor, and insulator surfaces. The tunability of the refractive index of the polymer film allows us to shift the guiding layer from a stepped index profile to a graded index profile. The phase-matching condition for optical power transfer is achieved through current-induced index modulation. Thirty-six dB extinction ratio, which includes 3 dB absorption loss and 33 dB phase-matched cross coupling, was observed with a current injection density of ∼1.8 μA/μm2. Unlike the conventional symmetrical dual channel coupler, the disparity of the collinear waveguide pair provides us with a much larger dynamic range of waveguide dimension suitable for generating the required phase-matching condition and thus easing the requirement of waveguide fabrication.
Vacuum ultraviolet transitions from rare-gas alkali ionic excimers (XeRb)+ and (KrRb)+ by electron beam excitation
Authors: Xing Da , Ueda Ken-ichi , Takuma Hiroshi
Published: July 2002

Vacuum UV fluorescences from ionic rare-gas alkali excimer molecules were observed by electron beam excitation. A gas mixture of xenon or krypton with a hot vapor of rubidium was excited to obtain a diffuse emission band centered at 164 nm from (XeRb)+ and 133 nm from (KrRb)+ molecular ions. The observed emission bands are assigned to the 21Σ+→11Σ+ transition of (XeRb)+ and (KrRb)+, respectively.
Current-induced defect creation and recovery in hydrogenated amorphous silicon
Authors: Street R. A.
Published: July 2002

Measurements of metastable defects induced by a forward bias current in a-Si:H p-i-n devices are reported. The defect density increases approximately as the square root of time, reaching saturation at long inducing times. Current-induced defect annihilation is also observed, in which the current causes a reduction in the defect density. The kinetics of creation and recovery are discussed, as well as the implications for models of metastable defects.
Bound exciton luminescence in ZnSe under hydrostatic pressure
Authors: Shan W. , Hays J. M. , Yang X. H. , Song J. J. , Cantwell E. , Aldridge J.
Published: July 2002

We report pressure-dependent photoluminescence (PL) measurements on ZnSe single crystals grown by the seeded physical vapor phase transport technique. Two kinds of ZnSe samples, the as-grown and Zn-extracted single crystals were used in this study. The Id1 line is the predominant spectral feature for the as-grown samples and shifts with pressure at a rate of 7.3 meV/kbar. The I3 emission dominates the PL spectra of the Zn-extracted samples and has a pressure coefficient of 6.6 meV/kbar, which is inconsistent with the assumption of the I3 line originating from the recombination of the excitons bound to ionized donors.
High-responsivity photoconductive ultraviolet sensors based on insulating single-crystal GaN epilayers
Authors: Khan M. Asif , Kuznia J. N. , Olson D. T. , Van Hove J. M. , Blasingame M. , Reitz L. F.
Published: July 2002

We report on the fabrication and characterization of photoconductive ultraviolet detectors based on insulating single-crystal GaN. The active layer (GaN) was deposited over basal-plane sapphire substrates using a unique switched atomic-layer-epitaxy process. The sensors were measured to have a responsivity of 2000 A/W at a wavelength of 365 nm under a 5-V bias. The responsivity remained nearly constant for wavelengths from 200 to 365 nm and dropped by three orders of magnitude within 10 nm of the band edge (by 375 nm). We estimate our sensors to have a gain of 6×103 (for wavelength 365 nm) and a bandwidth in excess of 2 kHz. The photosignal exhibited a linear behavior over five orders of incident optical power, thereby implying a very large dynamic range for these GaN-based ultraviolet sensors.
Raman scattering of InGaAs/InP grown by uniform radial flow epitaxy
Authors: Feng Z. C. , Allerman A. A. , Barnes P. A. , Perkowitz S.
Published: July 2002

Uniform radial flow epitaxy, a novel growth technique, has been used to grow InGaAs films on InP. Epitaxial layers above and below the critical thickness for the onset of slip were grown. We used Raman spectroscopy to characterize the quality of epitaxial layers, determine alloy composition, and measure the strain. Raman spectra from both pseudomorphic (strained) and relaxed (unstrained) InGaAs films were obtained at 300 and 80 K. The difference in the frequencies of their GaAs-like longitudinal optical phonons was used to calculate stress for the strained InGaAs/InP, leading to a direct formula for the evaluation of the layer stress.
Doping mechanisms of tin-doped indium oxide films
Authors: Shigesato Yuzo , Hayashi Yasuo , Haranoh Takeshi
Published: July 2002

The polycrystalline Sn-doped In2O3 (ITO) films with different Sn concentrations of 0–6.7 wt% (SnO2 wt%) were prepared by postannealing the amorphous ITO films deposited by highly dense plasma-assisted EB evaporation at the low substrate temperature. These films were confirmed to show the high crystallinity and homogeneous Sn distribution by x-ray diffraction (XRD), scanning electron microscopy, and electron spectroscopy for chemical analysis. The detail XRD pattern of 1.9 wt% Sn concentration film showed the doublet peaks, indicating that the film structure was the mixture of two different lattice parameters (LP); one was +0.18% larger and another was −0.13% smaller than the In2O3 LP, which could be attributed to the interstitial and the substitutional Sn atoms, respectively.
Growth of a delta-doped silicon layer by molecular beam epitaxy on a charge-coupled device for reflection-limited ultraviolet quantum efficiency
Authors: Hoenk Michael E. , Grunthaner Paula J. , Grunthaner Frank J. , Terhune R. W. , Fattahi Masoud , Tseng Hsin-Fu
Published: July 2002

We have used low-temperature silicon molecular beam epitaxy to grow a δ-doped silicon layer on a fully processed charge-coupled device (CCD). The measured quantum efficiency of the δ-doped backside-thinned EG&G Reticon CCD is in agreement with the reflection limit for light incident on the back surface in the spectral range of 260–600 nm. The 2.5 nm silicon layer, grown at 450 °C, contained a boron δ-layer with surface density ∼2×1014 cm−2. Passivation of the surface was done by steam oxidation of a nominally undoped 1.5 nm Si cap layer. The UV quantum efficiency was found to be uniform and stable with respect to thermal cycling and illumination conditions.
Transport critical current densities of silver clad Bi-Pb-Sr-Ca-Cu-O tapes at liquid helium and hydrogen temperatures
Authors: Haldar P. , Hoehn J. G. , Rice J. A. , Walker M. S. , Motowidlo L. R.
Published: July 2002

Using improved processing conditions, we have succeeded in increasing the transport critical current density (Jc) of powder-in-tube fabricated silver sheathed Bi-Pb-Sr-Ca-Cu-O superconducting tapes. Jc values at zero field now exceed 1.6×105 A/cm2 (Ic∼164 A) at 4.2 K and 1.3×105 A/cm2 (Ic∼133 A) at 20.2 K for short tape samples. At 77 K the same samples exhibit values greater than 3.0×104 A/cm2 (Ic∼32 A). They continue to carry 6.5×104 A/cm2 at 4.2 K and 4.8×104 A/cm2 at 20.2 K with applied magnetic fields as high as 20 T. Long lengths of tape were also fabricated and measured by winding the composites into small spiral or pancake coils. The small pancake coils carried 4.8×104 A/cm2 at 4.2 K and 5.2×103 A/cm2 at 77 K, zero field. More recently, 1 m lengths have been measured with Jc’s as high as 1.0×104 A/cm2 at 77 K, zero field. Detailed transport measurements at liquid helium and hydrogen temperatures with applied magnetic fields up to 20 T are reported for short and long lengths of monocore tapes.
Electrical profiling of Si(001) p-n junctions by scanning tunneling microscopy
Authors: Yu E. T. , Johnson M. B. , Halbout J.-M.
Published: July 2002

Potential distributions across Si(001)p-n junctions have been studied using cross-sectional scanning tunneling microscopy, spectroscopy, and potentiometry. A clear transition between p- and n-type material can be seen across each junction, and variations in the energy of the conduction-band edge can be detected with a spatial resolution of better than 100 Å. Current-voltage characteristics have been measured in both unbiased and electrically biased structures, and measurements under both conditions are consistent with calculated potential distributions.
High power and high-temperature operation of GaInP/AlGaInP strained multiple quantum well lasers
Authors: Mannoh M. , Hoshina J. , Kamiyama S. , Ohta H. , Ban Y. , Ohnaka K.
Published: July 2002

High power and high-temperature operation of transverse-mode stabilized 690 nm AlGaInP strained multiple quantum well lasers is described. Three 0.5% biaxially compressive strained GaInP (8 nm) wells were employed in the active region. Low threshold current of 36 mA and very high output power of 60 mW at high temperature up to 100 °C were obtained with 700 μm long lasers, whose facets were coated with antireflection–reflection films. This is, to the best of our knowledge, the first report that an AlGaInP laser has reached a cw output power of 60 mW at a high temperature of 100 °C. Very low degradation rate at 50 °C with 35 mW output power was confirmed.
Silver-gold compound metal island films prepared by using a two-step evaporation method
Authors: Baba Kazutaka , Okuno Toshiaki , Miyagi Mitsunobu
Published: July 2002

A silver-gold compound metal island film prepared by using a two-step evaporation method is demonstrated for the first time. Silver and gold islands have been completely alloyed by heating the film at 300 °C. The resonance wavelength of the compound metal island film exists between those of pure silver and pure gold island films. It is shown that the resonance wavelength can easily be controlled by selection of the mixture ratio of silver and gold.
Radial flow effects in a multidimensional, three-moment fluid model of radio frequency glow discharges
Authors: Young Fongray F. , Wu Chwan-Hwa
Published: July 2002

For the first time, a two-dimensional self-consistent nonequilibrium fluid model is used in simulations of rf glow discharges to evaluate the quantitative effects of the radial and axial flow dynamics inside a cylindrically symmetric geometry. Electrons are modeled with a three-moment nonequilibrium model and ions are modeled with a nonequilibrium single-moment model which includes an ionic effective electric field. The nonuniform plasma density profiles and the radial sheath width variation with various gas pressures are discussed. These resulting radially nonuniform plasma density profiles are an important issue for plasma processing.
Carbon precipitation in silicon: Why is it so difficult?
Authors: Taylor W. J. , Tan T. Y. , Gösele U.
Published: July 2002

It is well-established that oxygen precipitation in silicon occurs readily and is further facilitated by the presence of carbon. In contrast, carbon precipitation in silicon appears to be a difficult process which takes place only in the presence of a sufficiently high supersaturation of oxygen or silicon self-interstitials. It is suggested that a high interface energy of carbon precipitates in conjunction with the volume decrease associated with carbon precipitation or agglomeration allows one to understand the precipitation behavior of carbon in silicon.
Electroreflectance polarization study of valence-band states in ordered Ga0.5In0.5P
Authors: Kanata Takashi , Nishimoto Masahiko , Nakayama Hiroshi , Nishino Taneo
Published: July 2002

Anisotropic optical transitions in an ordered Ga0.5In0.5P alloy semiconductor have been studied by electroreflectance (ER) polarization spectroscopy. The atomic ordering of column-III sublattices causes a splitting of the valence-band maximum into two doubly degenerated levels at k=0. The ER spectra reveal signals originated from the Γ6c-Γ4v, Γ5v and Γ6c-Γ6v transitions caused by the ordering. The [110] and [11̄0] ER signals due to the Γ6c-Γ6v transition show strong anisotropic characters of their intensity and line shape. On the other hand, the signal due to the Γ6c-Γ4v, Γ5v transition changes only in the intensity by the polarization direction. The ER intensities measured at various polarization angles follow the theoretically derived trends based on the selection rule for electronic-dipole transitions in the ordered crystal.
Positively charged states of a hydrogen atom in p-type InP
Authors: Cho Hoon Young , Choi Won Chel , Min Suk-Ki
Published: July 2002

It has been demonstrated that atomic hydrogen drifts as a charged state in p-type InP and the presence of a high-electric field strongly affects the dissociation of the hydrogen-acceptor complex. During reverse-bias anneal experiments on the n+-p diode, it is confirmed that a charged hydrogen is accelerated out the high-field region below the breakdown voltage. The dissociation frequencies dependent on the applied bias voltage increase from 5.6×10−6 to 2.3×10−5 s−1 at 150 °C as the bias voltage is increased from 3 to 9 V. The dissociation energies calculated from the first-order kinetics are in the ranges of 1.58–1.40 eV, at 3–7 V annealing. It is proposed that atomic hydrogen in Zn-doped p-type InP exposed to the plasma hydrogen could be positively charged and strongly passivates the charged Zn acceptor, and also the hydrogen of the hydrogen-Zn acceptor complex can be released with the help of minority carriers or/and the loss of the charged hydrogen atom by the electric field.
Hydrogenated microcrystalline silicon films produced at low temperature by the hot wire deposition method
Authors: Dusane R. O. , Dusane Suvarna R. , Bhide V. G. , Kshirsagar S. T.
Published: July 2002

In this letter we report the synthesis of hydrogenated microcrystalline silicon at low temperature and without hydrogen dilution of the silane gas by the hot wire method. These films are characterized by higher dark conductivity and larger band gap compared to hydrogenated amorphous silicon. Microcrystallinity in these films is clearly established from the sharp crystalline TO-like peak in the first-order Raman spectra. The crystallite size and its volume fraction show a critical dependence on the silane flow rate.
Self-organized growth of regular nanometer-scale InAs dots on GaAs
Authors: Moison J. M. , Houzay F. , Barthe F. , Leprince L. , André E. , Vatel O.
Published: July 2002

The deposition of InAs on GaAs proceeds first by two-dimensional (2D) growth and above a 1.75-monolayer coverage by the formation of single-crystal dots on a residual 2D wetting layer. By atomic force microscopy measurements, we show that the first dots formed are in the quantum size range (height 30 Å, half-base 120 Å), that the dispersion on their sizes is remarkably low (±10%), and that they are located fairly regularly (interdot distance 600 Å). Upon further growth, density and shapes do not change but sizes increase up to double values before coalescence occurs. Self-organized growth in strained structures is then shown to be a simple and efficient way of building regular quantum dots.
Improved process for liquid phase deposition of silicon dioxide
Authors: Chou Jenq-Shiuh , Lee Si-Chen
Published: July 2002

An improved process for liquid phase deposition of silicon dioxide (SiO2) at 50 °C is successfully developed. Contrary to conventional processes, silicic acid (SiO2:xH2O) is used instead of SiO2 powder to saturate hydrofluorosilicic acid at 30 °C. This will shorten the period required for solution preparation to 3 h. Water is now taking the role of boric acid (or Al) to reduce the concentration of HF in the solution and leads to deposition of oxide. The corresponding growth rate of SiO2 is about 50 nm per hour, larger than that reported in literature with boric acid concentration of 0.010 M. When this oxide is used to fabricate the metal-oxide-semiconductor (MOS) capacitor, the flatband voltage turns out to be 0.277 V and the achieved oxide breakdown field strength is 9 MV/cm.
Spatio-temporal measurement of excited Xe(1s4) atoms in a discharge cell of a plasma display panel by laser spectroscopic microscopy
Authors: Tachibana Kunihide , Kosugi Naoki , Sakai Tetsuo
Published: July 2002

Dynamic behavior of excited Xe atoms in the 1s4 state was measured in a micro-discharge cell of a color plasma display panel by a laser absorption technique combined with an optical microscope. The maximum absorption in a pulsed discharge with a peak current of 160 μA amounted 6% in one pass of the laser beam through the panel of 200-μm gap. This gives the path averaged density of 6.7×1012 cm−3. Measured spatio-temporal characteristics of the excited atoms are discussed in a comparison with a one-dimensional simulation by a fluid model.
Magnetoresistance in Ni80Fe20/Cu/Ni80Fe20/Fe50Mn50 spin valves with low coercivity and ultrahigh sensitivity
Authors: Rijks Th. G. S. M. , de Jonge W. J. M. , Folkerts W. , Kools J. C. S. , Coehoorn R.
Published: July 2002

We present magnetoresistance measurements on Ni80Fe20/Cu/Ni80Fe20/Fe50Mn50 spin valves with crossed anisotropies: the easy magnetization direction of the unbiased Ni80Fe20 layer is perpendicular to the exchange biasing field which effectively works on the second Ni80Fe20 layer due to the interaction with the Fe50Mn50 layer. The hysteresis in the low-field magnetoresistance is less than 0.03 kA/m, which is ten times smaller than the hysteresis for similar materials with parallel anisotropies. The sensitivity (∂R/R∞)/∂H ranges from 8% to 18%/(kA/m), depending on the preparational method. To our knowledge this combination of very high sensitivity and very low coercivity has not been reported before.
Breakdown of thermionic emission theory for quantum wells
Authors: Tsai Chin-Yi , Eastman Lester F. , Lo Yu-Hwa , Tsai Chin-Yao
Published: July 2002

Carriers escape from quantum wells into barriers via carrier-polar optical phonon absorption is theoretically studied in multisubband quantum well structures. We find that carriers in each subband have their own minimum escape time when the energy difference between the band edges of the subband and the barrier matches the energy of a longitudinal optical phonon. Compared to the calculations from classical thermionic emission theory, we find that the thermionic emission theory is no longer valid when the width or the depth of quantum wells is small.
Homogeneous and inhomogeneous linewidths of excitons in partially ordered Ga0.52In0.48P
Authors: Grossmann P. , Feldmann J. , Göbel E. O. , Thomas P. , Arent D. J. , Bertness K. A. , Olson J. M.
Published: July 2002

We report picosecond four-wave mixing experiments on Ga0.52In0.48P grown by organometallic vapor phase epitaxy on GaAs substrates. The spectral behavior of the homogeneous linewidth in the range of the inhomogeneously broadened band gap excitonic resonance is found to be different for a more disordered as compared to a partially ordered structure. Whereas the former shows the normal alloy behavior, the behavior of the partially ordered sample supports the assumption that its structure consists of ordered domains with varying degrees of order. This means, in particular, that the main origin of the inhomogeneous broadening is different for the disordered and ordered case. In addition, a polarization dependence of the four-wave mixing signal is only observed for the more disordered sample.
3.9-μm InAsSb/AlAsSb double-heterostructure diode lasers with high output power and improved temperature characteristics
Authors: Choi H. K. , Turner G. W. , Liau Z. L.
Published: July 2002

Double-heterostructure diode lasers emitting at ∼3.9 μm have exhibited pulsed operation at temperatures up to 170 K and cw operation up to 105 K, with single-ended cw output power of 30 mW at 70 K. The laser structure, grown on GaSb substrates by molecular-beam epitaxy, has an InAsSb active layer and AlAsSb cladding layers. The lowest pulsed threshold current density is 36 A/cm2 obtained at 60 K. The characteristic temperature is 20 K over the entire temperature range.
Enhanced transfer efficiency in AlGaAs asymmetric planar microcavities
Authors: Nishikawa T. , Kakimura T. , Lee Y. , Yamanishi M. , Ogura I. , Ishihara T.
Published: July 2002

Enhanced transfer efficiency of spontaneous emission, which is the quantum efficiency of the collection of total spontaneously emitted photons, has been revealed with asymmetric AlGaAs planar microcavities containing single GaAs quantum wells at 40 K. The obtained transfer efficiency is as high as ∼15%. An effect of photon reabsorption on the transfer efficiency is also discussed.
Heteroepitaxial growth of BaTiO3 films on Si by pulsed laser deposition
Authors: Lee Myung-Bok , Kawasaki Masashi , Yoshimoto Mamoru , Koinuma Hideomi
Published: July 2002

Dielectric BaTiO3 films were heteroepitaxially grown on Si (100) substrates in cube-on-cube manner by employing a conductive TiN buffer layer. An epitaxial TiN film was grown by KrF pulsed excimer laser deposition. BaTiO3 film was grown epitaxially on the TiN film at a substrate temperature higher than 600 °C and in an oxygen pressure less than 1 mTorr. This epitaxial BaTiO3/TiN layer on Si was observed to have sharp interfaces and little interdiffusion of constituent atoms by secondary ion mass spectrometry. The surface of the BaTiO3 film had a root-mean-square roughness less than 1 nm as observed by atomic force microscopy. A TiN/BaTiO3/TiN trilayer structure was formed and the dielectric property was measured for this metal-insulator-metal type capacitor. The BaTiO3 film showed a high relative dielectric constant of 803 at the frequency of 1 MHz and a low leakage current less than 10−8 A/cm2 at 2 V.
Analysis of the critical current density in grain boundary Josephson junctions on a nanometer scale
Authors: Froehlich O. M. , Schulze H. , Beck A. , Mayer B. , Alff L. , Gross R. , Huebener R. P.
Published: July 2002

The critical current Ic of YBa2Cu3O7−δ bicrystal grain boundary Josephson junctions (GBJs) have been measured in high magnetic fields up to B=12 T. Even at the highest applied magnetic fields, Ic varied periodically with varying magnetic field. By comparing the envelope of the measured Ic(B) dependencies to model calculations we obtain information on the spatial distribution of the supercurrent density jc on a length scale down to below 1 nm. Our analysis gives clear evidence that jc has spatial inhomogeneities on all length scales down to 1 nm with a probability distribution p(a) for the characteristic length scale a of the inhomogeneities.
Application of single electron tunneling: Precision capacitance ratio measurements
Authors: Clark Alan F. , Zimmerman Neil M. , Williams Edwin R. , Amar A. , Song Dian , Wellstood F. C. , Lobb C. J. , Soulen R. J.
Published: July 2002

A metrological application is reported of the single electron tunneling (SET) phenomena: a precise measurement of the ratio of two cryogenic capacitors. The measurement used a superconducting SET electrometer as the null detector for a capacitance bridge. A 3-ppm level of imprecision has been achieved in the measurement of the capacitance ratio from 100 to 1000 Hz. Further improvements can be made in the attempt to obtain an imprecision of 10−8 at lower frequencies, sufficient for the metrological measurement of capacitance or the fine-structure constant using a SET pump.
Coulomb blockade at 77 K in nanoscale metallic islands in a lateral nanostructure
Authors: Chen W. , Ahmed H. , Nakazoto K.
Published: July 2002

Clear Coulomb blockade effects have been observed in lateral metal nanostructures fabricated by combined electron beam lithography and ionized beam deposition. Islands between 2 and 3 nm in diameter were formed in a 25 nm gap for which the effective tunneling capacitance is calculated to be 0.2 aF. Clear Coulomb gaps and staircases were observed at 77 K in current–voltage characteristics. The staircases can still be seen at room temperature but the gaps tend to a nonlinearity in I–V characteristics around zero bias voltage. The total device area is about 0.01 μm2.
Temperature dependent hole fluence to breakdown in thin gate oxides under Fowler–Nordheim electron tunneling injection
Authors: Satake Hideki , Toriumi Akira
Published: July 2002

The dielectric breakdown mechanism in thin gate oxides is discussed based on the temperature dependence of hole fluence to dielectric breakdown through gate oxides. It has been demonstrated for the first time that the hole fluence to dielectric breakdown is not a constant value for different oxide fields but has a strong oxide field dependence at low temperatures. It has been also found that the dominant dielectric breakdown mechanism changes at around 150 K. At higher temperatures than 150 K, the dielectric breakdown mechanism in silicon-dioxide is not dependent on the oxide field. On the other hand, the dielectric breakdown at lower than 150 K is dominated by the mechanism strongly dependent on the oxide field.
Substrate temperature and monolayer coverage effects on epitaxial ordering of InAs and InGaAs islands on GaAs
Authors: Solomon G. S. , Trezza J. A. , Harris J. S.
Published: July 2002

For InAs Stranski–Krastanov (SK) island growth on GaAs by molecular-beam epitaxy, we show that the in-plane island diameter varies exponentially with the growth temperature over the range of 390–540 °C. A transition region in SK growth between isolated island growth and island coalescing is investigated as functions of growth temperature and equivalent InAs layer-by-layer monolayer (ML) coverage in order to extend the isolated island regime for quantum confinement applications. InAs islands of 150 Å in diameter have been grown. Growth of In0.5Ga0.5As islands indicates an increased 2D epitaxial region before island nucleation and a decreased island concentration compared to growth of InAs islands.
Helium-filled proportional counter for low-temperature operation (4.2–300 K) and its application to cryogenic resonance-electron Mössbauer spectroscopy
Authors: Isozumi Yasuhito , Kishimoto Shunji , Katano Rintaro , Takekoshi Hidekuni
Published: July 2002

The operation of a proportional counter filled with helium gas has been investigated at low temperatures down to 5 K. It has been found for the first time that a helium-filled counter cooled at temperatures below 30 K works well in the proportional region, but with low gas gains (≤100). The operation at low temperatures (≤30 K) is attributed to the complete removal of impurities from helium gas as well as the extreme depression of the reaction frequency of metastable heliums. The absence of impurities results in eliminating the Penning effect, i.e., an ionization process of impurity atoms or molecules by collisions with metastable helium, which spreads electron avalanches in both space and time if helium gas contains a slight amount of impurities (1–103 ppm). Continuous discharges caused by the photoeffect of ultraviolet photons, which are mostly produced in decay channels of metastable helium, are thermally quenched at low temperatures below 30 K. The helium-filled counter provides us with a new method to detect nuclear radiations at very low temperatures. The present counter technique has been successfully applied to the resonance-electron Mössbauer spectroscopy near liquid-helium temperature (∼5 K).
Thermal expansion and piezoelectric response of PZT Channel 5800 for use in low-temperature scanning tunneling microscope designs
Authors: Simpson A. M. , Wolfs W.
Published: July 2002

The thermal expansion of the ceramic piezoelectric material PZT Channel 5800 between 2 and 300 K is found to be anisotropic, being negative parallel to the poling direction and positive normal to this direction. The piezoelectric response is temperature dependent, decreasing significantly as the temperature is reduced.
Using the fast Fourier transform to determine the period of a physical oscillator with precision
Authors: Goldblum C. E. , Ritter R. C. , Gillies G. T.
Published: July 2002

Measuring the period of torsion pendulums with precision has long been a formidable challenge in gravitation experiments, particularly those measuring the Newtonian gravitational constant G. An alternative method to fitting the position signal of the pendulum to a sine wave is the use of the power spectrum generated by the fast Fourier transform (FFT) as the source of information from which the period of oscillation can be determined. There are, however, known limitations to the use of a FFT to measure the period of a physical oscillator with precision. These limitations include two effects due to the finiteness of the duration of the sinusoidal data record and one effect due to the uncertainty of the starting phase of the oscillator relative to the window imposed by this duration. We have done a phenomenological study of the FFT using a desktop computer to imitate a precision oscillator having the physical characteristics of a finite damping constant and drift in the zero potential-energy position. Also, we have taken extensive data with a torsion pendulum, and analyzed them in this way. These studies show that for a real oscillator, such as the classical torsion pendulum, the FFT is a useful tool for determining the period of oscillation with the precision usually associated with larger, more complex, fitting algorithms. With good signal-to-noise ratio and under conditions appropriate to a torsion pendulum, the FFT method can measure the frequency or period to five parts in 106 or better.
Soft-x-ray camera for internal shape and current-density measurements on a noncircular tokamak
Authors: Fonck R. J. , Jaehnig K. P. , Powell E. T. , Reusch M. , Roney P. , Simon M. P.
Published: July 2002

A two-dimensional, tangentially viewing, soft-x-ray pinhole camera has been fabricated to provide internal shape measurements on the PBX-M tokamak. It consists of a scintillator at the focal plane of a foil-filtered pinhole camera, which is, in turn, fiber-optically coupled to an intensified framing video camera (Δt≥3 ms). Automated data acquisition is performed on a stand-alone image processing system, and data archiving and retrieval take place on an optical disk video recorder. The derivation of the poloidal emission distribution from the measured image is done by fitting to model profiles.
500 °C infrared absorption cell
Authors: Robinson A. M. , Billing M. , Garand D.
Published: July 2002

A high-temperature IR-transmitting absorption cell has been designed and tested. The cell is heated by utilizing the cell wall as the heating element. A PTFE (Teflon) gasket double-window assembly allows temperatures as high as 500 °C to be attained. Temperature uniformity of 4 °C is observed, and preliminary absorption results by CO2 at 10 μm are presented.
Use of quartz-halogen lamps as the reference source for spectroradiometric temperature measurements
Authors: Paul Phillip H. , Self Sidney A.
Published: July 2002

Spectroradiometric temperature measurement techniques require a stable high-temperature reference source. It is essential that the brightness temperature of this reference source be calibrated against an accepted standard. We show that the optimal choice for the operating temperature of the reference source is slightly higher than the medium under test. We consider the use of miniature quartz-halogen lamps, as an alternative to the traditional tungsten ribbon lamp, as the reference source for emission–absorption pyrometry. The use of subminiature quartz-halogen lamps can extend the optimum operating condition by over 500 K and are less prone to lamp envelope fouling than tungsten vacuum lamps.
Powder aspirator for shock tube studies of heterogeneous reactions
Authors: Cheng M. T. , Kirsch M. J. , Lester T. W.
Published: July 2002

The application of single-pulse shock tube techniques in the study of heterogeneous reactions is described. In particular, nitric oxide removal on carbonaceous materials at temperatures simulating pulverized coal firing is reported. Carbon black particulates were suspended and heated behind reflected shock waves in order to simulate flame-borne particulates. To meet the specific requirement of preshock conditions, a novel and simple powder aspirator was designed and fabricated for dispersing carbon black. Particle concentrations measured by a gravimetric method and by in-line light transmission were compared. Changes of nitric oxide concentration, reaction products and volatile species emitted from the particulates were analyzed with gas chromatographic techniques. Representative results of nitric oxide reduction by carbon black particulates at concentrations up to 50 g/m3 are provided, and indicate that single-pulse shock tubes coupled with a simple powder aspirator may be used to infer global rates for gas/solid reactions of potential importance in combustion.
ac susceptibility apparatus for measuring the transition temperature of high-T
c crystals, sintered samples, and films
Authors: Xenikos Dimitrios G. , Lemberger Thomas R.
Published: July 2002

We describe a simple ac magnetic susceptibility system which can be used to observe the superconducting transition temperature in the range 4.2–300 K. The apparatus is extremely flexible and has been tested using high-quality YBa2Cu3O7−x single crystals with volume as small as 2×10−5 cm3, much larger sintered polycrystalline samples of the same compound, and finally thin films deposited over a surface area of 1 cm2. The system measures resonant frequency variations of a parallel LC circuit with the superconductor close to the inductor. Typical frequencies and fields are in the ranges 0.2–10 MHz and 1–2 mG ac in the inductor.
Measurement of spectral and spatial intensity distributions of synchrotron radiation in soft x-ray region by means of helium gas scattering
Authors: Gotoh S. , Taguchi T. , Okamura S. , Hisatsugu T.
Published: July 2002

We measured the spectral and spatial intensity distributions of synchrotron radiation (SR) in the soft x-ray region using an energy-dispersive method with a Si(Li) detector at the Photon Factory. By measuring the intensity of x rays scattered by helium gas, we could reduce the high SR intensity to the sensitivity range of a Si(Li) detector and, by selecting the scattering angle, we used this method as a polarimeter. In the soft x-ray region above 2 keV, the measured spectral and spatial profiles of the direct beam transmitted through a Be window were almost the same as the calculated values, but absolute intensity was about 30% less than calculated. This is largely due to the geometric errors of the measuring system. We also measured the intensity distribution of the beam reflected from fused quartz mirrors. The spectrum profiles agreed with the calculated values, but absolute intensity was somewhat less than expected, and the spatial distribution had a complicated profile because of imperfections in the mirrors.
Current controlled variable resistors through superconductors
Authors: Lakshmi M. V. S. , Ramkumar K. , Satyam M.
Published: July 2002

This note attempts to describe a current controlled variable resistor conceived based on the property of critical current associated with a superconductor. This idea has been tested using high-temperature superconductor films.
Measurement of the Hall coefficient using van der Pauw method without magnetic field reversal
Authors: Levy Miguel , Sarachik M. P.
Published: July 2002

The van der Pauw geometry has been widely used for the measurement of resistivities and Hall coefficients. Although the measurement of a Hall coefficient requires a finite magnetic field, it should be noted that van der Pauw’s expression is valid only in the limit of zero field; in addition to the Hall contribution, measurements in a finite magnetic field generally include a term associated with field-induced changes in the longitudinal resistivity. Although a simple solution to this problem entails taking the difference between readings in opposite field directions, there are circumstances where this may be impractical. In this note we present a straightforward extension of the van der Pauw calculation which allows a determination of the Hall coefficient from quantities measured in one field direction only.
Delay-time modulation spectroscopy using a cw mode-locked Nd:YAG laser synchronized with the synchrotron radiation pulses (invited)
Authors: Mitani Tadaoki , Okamoto Hiroshi , Takagi Yoshihiro , Watanabe Makoto , Fukui Kazutoshi , Koshihara Shinya , Ito Chihiro
Published: July 2002

Picosecond light pulses synchronized with successive pulses of synchrotron radiation have been generated using a cw mode-locked Nd:YAG laser. By improvement of the time characteristics of the synchronized pulses, transient absorption measurements have been made in a short time domain of 0.05–11 ns using a delay-time modulation technique.
A new data acquisition system for time-resolved powder diffraction
Authors: Clark S. M. , Miller M. C.
Published: July 2002

A new data acquisition system tailored to the rapid collection of time-resolved powder diffraction data has been constructed. It is used for the study of structural phase transitions at the energy-dispersive powder diffraction facility of the UK synchrotron radiation source (SRS). The data acquisition system and some data collected during a study of the 196 °C phase transition in ammonium chloride are presented here.
Radio-frequency bandpass filter for NMR applications
Authors: Mustarelli Piercarlo , Savoldi Faustino , Scotti Sergio
Published: July 2002

A high-power, lumped-elements, interdigital bandpass filter is described which has been used to improve the isolation between decouple and observe channels of NMR spectrometers, and to ‘‘clean’’ the transmitter signals. With no calculations, and with the help of a sweeper or a spectrum analyzer, filters of this type can be easily assembled and tuned over the 90–400 MHz range, typical of the proton decoupling frequencies.
Parametric optimization of an easily constructed pulsed xenon-ion laser
Authors: Dai C. M. , Wu K. H. , Hsieh W. F. , Chuu D. S.
Published: July 2002

An easily fabricated xenon-IV ion laser is described. The various parameters considered are the gas pressures, the excitation voltages, and the repetition rates. Based on our results we believe that there exists an optimal excitation voltage and an optimal repetition rate for achieving largest lasing power and good reliability.
Development of a frequency-stabilized compact light source for an optically pumped Cs frequency standard
Authors: Ikegami Takeshi , Ohshima Shin-Ichi , Nakadan Yasuhiro , Koga Yasuki
Published: July 2002

A compact light source using a laser diode, whose frequency was stabilized to a saturated absorption spectrum of the Cs-D2 line (λ=852 nm), was developed for use of an optically pumped cesium frequency standard. The temperature of the laser diode was stabilized within 0.1 mK by two stages of feedback systems. Frequency stabilization to the saturated absorption spectrum was performed by modulating the injection current to the laser diode with 100 kHz. The frequency stability was measured by taking the beat of two light sources thus stabilized, and it was estimated to be 4.4×10−11(15 kHz) at an averaging time of 600 ms.
Detector distance selection for neutron time-of-flight temperature measurements
Authors: Lerche R. A. , Remington B. A.
Published: July 2002

Fuel ion temperatures for laser-driven, inertial confinement fusion targets are often determined by neutron time-of-flight (TOF) techniques. The error in the temperature measurement is a minimum at a target-to-detector distance that depends on both target and detector characteristics. The error is dominated by the detector response at shorter distances and by the number of detected neutrons at larger distances. We develop equations that relate the temperature error to the target ion temperature, the number of neutrons detected, target-to-detector distance, and the detector impulse response; and present sample calculations of error for D-D and D-T plasmas observed by typical Nova neutron TOF detectors. The detector placement is important for minimizing temperature error for target yield below 1010 neutrons.
Tangential imaging for fluctuation studies
Authors: von Goeler S. , Fredrickson E. , Bell R. , Bitter M. , Grek B. , Hill K. , Hsuan H. , Johnson D. , Long D. , McGuire K. , Parsells R. , Renda G.
Published: July 2002

Tangential imaging is being pursued on TFTR as a method to study instabilities with high poloidal mode number m. These instabilities cannot be seen with conventional perpendicular viewing soft x-ray diode arrays because the emission is integrated along a line of sight. Computer simulations of tangential imaging indicate that excellent spatial resolution can be obtained. Prerequisites are (a) that the line of sight is nearly parallel to a magnetic field line at the point of closest approach to the center of the plasma, and (b) that there exists a steep gradient of the plasma radiation that is used for the measurements. The TFTR plans for a tangential, two-dimensional imaging, soft x-ray diode array will be outlined.
The choice of optimum exposure time for a simple electron counting system
Authors: Hall B. D. , Reinhard D. , Monot R.
Published: July 2002

The question of an optimum exposure time for an event counting charge coupled device electron detection system is discussed [B. D. Hall, M. Flüeli, J. -P. Borel, and R. Monot, Rev. Sci. Instrum. (in press)]. The system is a null-event detector, providing a single piece of information per exposure: whether an event has not been detected. It is shown that when the number of exposure periods is fixed, optimum operation conditions require high exposures, in spite of the loss of information due to multiple events being detected as single ones.
A holographic relaxation spectrometer with phase-modulated detection
Authors: Odear Robert M. , Park Il Hyun , Xia Keqing , Johnson Charles S.
Published: July 2002

A holographic relaxation spectrometer is described which incorporates high-frequency phase modulation of the probe or reference beams by means of an electro-optic modulator. Heterodyne detection at the modulation frequency or two times the modulation frequency provides signals that are proportional to the electric field amplitude of the diffracted light. This system permits heterodyne detection of a transient diffraction signal in a single-shot experiment. The requirements for phase stability are somewhat relaxed and samples that deteriorate under laser irradiation can be studied. Diffraction amplitudes for the phase and absorbance components of laser-induced gratings can be recorded separately under certain conditions.