bluuebunny/crossref_metadata_2025 huggingface dataset:
Implantation-induced disordering of CuPtB-ordered GaInP
Authors: Burkard M. , Englert A. , Geng C. , Mühe A. , Scholz F. , Schweizer H. , Phillipp F.
Published: July 2002

In this article, a comprehensive treatment of the implantation-induced disordering (IID) of CuPtB-ordered GaInP with regard to microstructuring is presented. Ion implantation reduces the thermal stability of the crystal, so that disordering occurs at temperatures smaller than 800 °C for which the ordered phase normally is stable. It is shown that IID is mediated by implantation defects and can be described quantitatively by a model based on defect annealing. From the temperature dependence of the disordering process an activation energy of 2.15 eV has been evaluated, which is supposed to be the migration enthalpy of group III vacancies. Lateral order/disorder structures were achieved by masked implantation using high resolution electron beam lithography for the definition of wire and dot implantation masks down to 35 nm width. These structures were examined using photoluminescence and transmission electron microscopy. Both methods show that the spatial resolution is determined by implantation straggling, whereas defect diffusion can be neglected. This is also confirmed by extracting the defect diffusion length from the disordering model.
Oxygen-related defects in low phosphorous content GaAs1−yPy grown by metal organic vapor phase epitaxy
Authors: Cederberg J. G. , Bray K. L. , Kuech T. F.
Published: July 2002

The mixed Group V ternary alloy GaAs1−yPy (y<0.17) has been grown by metal organic vapor phase epitaxy and doped with oxygen using the oxygen precursor, diethylaluminum ethoxide [C2H5OAl(C2H5)2]. Controlled oxygen doping was accomplished over the range of 0<y<0.17. Deep level transient spectroscopy measurements reveal the presence of several oxygen-related deep levels. These levels, previously found in GaAs:O, vary with alloy composition over the investigated range. An additional deep level, most probably associated with the presence of misfit-related defects, has been identified. Photoluminescence performed on the oxygen-doped samples indicates that band edge emission is reduced and lower energy emission features are introduced over the wavelength range of 1000–1200 nm as a result of oxygen incorporation.
The effect of substrate orientation on the properties of low temperature molecular beam epitaxial GaAs
Authors: O’Hagan S. , Missous M.
Published: July 2002

GaAs and related materials, grown by molecular beam epitaxy at 200–300 °C under normal, group V-rich conditions are highly nonstoichiometric, with excess group V concentrations of up to 1021 cm−3, and the material properties are defect controlled. Here we report on comparative studies of low temperature growth of GaAs on (100) substrates and on (111)A, (111)B, (311)A, and (311)B surfaces. We show that material grown on both the (111) faces, under As-rich conditions, becomes polycrystalline almost immediately after commencement of growth. We attribute this to faceting of the surface creating regions of excessive As incorporation and either directly nucleating misorientated growth or producing areas of extreme localized strain resulting in the breakdown of crystallinity. Layers grown on (311)A and (311)B surfaces are of good crystalline quality but are highly nonstoichiometric in both cases, contrary to the anticipation that the (311)A surface, having fewer available forward Ga bonds than the (100) or (311)B surfaces might have a lower affinity for As incorporation. We also describe the reduction of excess As incorporation into layers grown on both (311) surfaces in the presence of Si or Be doping concentrations of 1×1019 cm−3 or greater, as we previously reported for layers grown on (100) substrates. This is evidenced by the reduced lattice parameter and lower absorption in the near-band-edge infrared region due to As antisite defects, compared to undoped material. Also, little or no hopping conduction is seen in Be doped layers although they are totally compensated. A proportion of donors are active in the Si doped layers and the conduction mechanism seems to be normal band conductivity. Electrical activation of the Si donors and Be acceptors can be increased, however, only after high temperature annealing.
Stimulated etching of Si(100) by Cl2 molecular beams with hyperthermal translational energies
Authors: Teraoka Yuden , Nishiyama Iwao
Published: July 2002

Etching reaction of Si(100) is investigated by using Cl2 molecular beams with a hyperthermal translational energy up to 3.0 eV. The reaction rate is clearly enhanced by translational energy, and the threshold energy is 2.1 eV. The translational-energy-induced reaction rates are measured as a function of substrate temperature; the results closely fit Arrhenius plots as the sum of two components having activation energies of 2.7 and 1.2 eV. The higher energy, derived from the fit of the high-temperature region, agrees well with the pure thermal reaction, i.e., low-translational-energy reaction. The lower energy, which is observed here, is determined as the activation energy of the translational-energy-induced reaction. This energy is not affected by the translational energy from the threshold to 3.0 eV, whereas the etch rate increases with translational energy. These findings suggest that the translational energy contributes to the formation of a new chlorinated Si surface from which silicon chloride desorbs thermally with lower activation energy than from a chlorinated Si surface formed by a pure-thermal reaction.
Photoluminescence characterization of biaxial tensile strained GaAs
Authors: Kim Ki Soo , Yang Gye Mo , Shim Hyun Wook , Lim Kee Young , Suh Eun-Kyung , Lee Hyung Jae
Published: July 2002

Heteroepitaxial GaAs layers were grown on Si (001) substrates by metalorganic chemical vapor deposition. The tetragonal distortion induced by the lattice and the thermal expansion coefficient mismatches gives substantial effects on the acceptor energy level as well as the valence band structure. The biaxial tensile strain in GaAs layers is investigated using low-temperature photoluminescence. The origins of intrinsic exciton lines and carbon-related extrinsic lines observed in the photoluminescence spectra are identified by the two-band model. It is also found that the binding energy of the carbon acceptor is reduced as biaxial tensile strain increases.
Photoluminescence and electron subband population in modulation doped AlGaAs/GaAs/AlGaAs heterostructures
Authors: Požela J. , Jucienė V. , Namajūnas A. , Požela K. , Mokerov V. G. , Fedorov Yu. V. , Kaminskii V. E. , Hook A. V.
Published: July 2002

The anomalies in photoluminescence spectra of modulation doped AlGaAs/GaAs/AlGaAs double barrier heterostructures with various GaAs quantum well (QW) widths are observed. The intensity of the photoluminescence peak excited by Ar laser and associated with electron optical transitions from the second QW subband to the valence band becomes higher than for the case of the first (lowest) subband in the GaAs QW width range L=22.5–30 nm. The calculations of electron-polar optical phonon scattering rate show that the well-width dependence of the intersubband electron scattering rate due to emission of confined and interface optical phonons is responsible for the photoexcited electron subband population redistribution and anomalies in the photoluminescence spectra. The transition rate for the second QW subband electron binding to the exciton state is evaluated as 25×1010 s−1 at L<22.5 nm and 4×1010 s−1 at 22.5<L⩽30 nm.
Microstructure and magnetic properties of rapidly quenched Sm2(Fe,Al,Zr)17C1.5 ribbons with Zr additions
Authors: Tang Wei , Jin Zhi-Qiang , Zhang Jian-Ron , Lu Li-Ya , Zhang Shi-Yuan , Du You-Wei
Published: July 2002

The effect of Zr additions on the microstructure and magnetic properties of melt-spun Sm2Fe(15.5−x)ZrxAl1.5C1.5 alloys has been systematically studied. The additions of Zr to Sm2(Fe,Al,Zr)17C1.5 alloys result in the precipitation of a ZrC compound within intergranular regions. It is found that the Zr addition is helpful in eliminating free iron dendrites, and effective on inhibiting grain growth during annealing. The x-ray diffraction analysis and transmission electron microscopy observation show that annealed ribbons with the Zr addition consist of a multiphase of the 2:17-type carbide, α-Fe and ZrC compound with an average grain size of about 25–35 nm. A remanence of up to 55 emu/g along with a coercivity of up to 11.6 kOe is obtained for Sm2Fe15.3Zr0.2Al1.5C1.5 ribbons annealed at 750 °C for 30 min. It is suggested that the high coercivity and remanence may be attributed to the Zr addition.
Excimer laser irradiation of Si3N4 films deposited on Si
Authors: Ohmukai Masato , Takigawa Yasuo , Kurosawa Kou
Published: July 2002

We have investigated the effect of argon excimer laser irradiation of silicon nitride films deposited on silicon substrates. When the film thickness is equal to or greater than 40 nm, the irradiation induces amorphous silicon precipitation near the thin surface layer of the film, which has been clarified by means of x-ray photoelectron and Raman spectroscopy. The formation mechanism of amorphous silicon with the irradiation is discussed. The phase of silicon precipitation depends on the thermal properties of the substrate material.
Electron cyclotron resonance assisted chemical vapor deposition of carbon nitride films on diamond
Authors: Wu Kehui , Wang E. G. , Qing Jian , Xu Guichang
Published: July 2002

Carbon nitride films were prepared using an electron cyclotron resonance enhanced chemical vapor deposition apparatus. A two-step mode was adopted in which a diamond layer was first deposited onto the substrate (Si or Mo), and then the carbon nitride films were grown. Detailed x-ray photoelectron analyses show that the carbon and nitrogen atoms have formed a nonpolar covalent bond. The nitrogen concentrations in the films remain unchanged when the substrate temperatures vary from 100 to 700 °C, which suggests that a stable phase has formed.
Random telegraph signal: An atomic probe of the local current in field-effect transistors
Authors: Mueller H. H. , Schulz M.
Published: July 2002

The switching amplitude of random telegraph signals (RTSs) caused by trapping a single electron at an individual interface defect is studied in sub-μm-sized metal oxide field-effect transistors (MOSFETs). The amplitudes of RTSs depend on the channel nonuniformity and, in particular, on the current distribution in the immediate vicinity of the trap. We find that to a good approximation the RTS amplitude is proportional to the square of the local current density. This mathematical relation is tested and verified with the help of a computer simulation. RTS amplitudes may thus be used as atomic probes of the local current density. By the evaluation of 187 RTS amplitudes in different MOSFETs of the same type, we deduce for the first time a histogram showing the magnitude distribution of the local current density in such devices.
Photoconductive generation mechanism and gain in internal photoemission infrared detectors
Authors: Shen W. Z. , Perera A. G. U.
Published: July 2002

The photoconductivity generation mechanism and photoconductive gain in internal photoemission infrared detectors have been studied. A simple model is proposed and it shows that the photoconductive gain of internal photoemission detectors is less than but close to unity and independent of the number of the emitter layers, while the current responsivity is proportional to the number of emitter layers. The results, in good agreement with the experiments, are contrary to those of quantum-well photodetectors due to the different photocarrier generation mechanisms between the quantum-well photodetectors and internal photoemission detectors.
Electrically detected magnetic resonance signal from iron contaminated Czochralski silicon crystal
Authors: Mchedlidze T. , Matsumoto K.
Published: July 2002

The electrical detection of magnetic resonance (EDMR) measurement, a detection method for the spin-dependent recombination, was applied to characterize iron contaminated silicon samples grown by the Czochralski method. The observed signal was different than previously reported electron paramagnetic resonance signals from defects in silicon. In addition, as the signal was not detected from similarly contaminated samples prepared from floating zone grown silicon crystal, we propose that the signal originates from defects containing iron and oxygen, namely, from iron decorated oxide precipitates. The dependency of EDMR signal on different experimental conditions (microwave power, illumination intensity, and temperature) were studied.
Simulation of mid-infrared HgTe/CdTe quantum-well vertical-cavity surface-emitting lasers
Authors: Vurgaftman I. , Meyer J. R. , Dell J. M. , Fisher T. A. , Faraone L.
Published: July 2002

We theoretically show the feasibility of optically and electrically pumped Hg-based vertical-cavity surface-emitting lasers (VCSELs) that emit at midwave-infrared wavelengths up to thermoelectric cooler temperatures. The maximum operating temperature is significantly enhanced by employing a multiple quantum-well active region with very thin (20–30 Å) HgTe wells engineered to yield a strong suppression of both Auger recombination and intervalence free-carrier absorption. Hg0.65Cd0.35Te/Hg0.1Cd0.9Te distributed Bragg reflectors are employed for one or both of the mirrors defining the optical cavity. Detailed numerical simulations of VCSELs emitting at λ≈4.3 μm predict that for optical pumping at 1.06 μm, a maximum operating temperature of 220 K should be achievable for pulsed operation and 160 K in cw mode, with a cw power output of up to 2.6 mW per array element at 100 K. Injection VCSELs are predicted to operate up to 200 K for pulsed operation and 105 K for quasi-cw with a 10% duty cycle.
Oxidized nanocrystalline Fe–Cu pseudoalloy subjected to high pressure and electrodischarge pulses: Mössbauer and x-ray investigations
Authors: Gavriliuk A. G. , Voitkovsky V. S. , Sidorov V. A. , Filonenko V. P. , Tsiok O. B. , Khvostantsev L. G.
Published: July 2002

Nanocrystalline Fe15Cu85 pseudoalloy has been subjected to pulsed heating up to 1500 K at high pressure (8 GPa). Two regimes were studied: the direct heating using electrodischarge through the sample and indirect heating with the use of cylindrical heater around the sample. The temperature and time conditions in both types of experiments were adjusted to be equivalent. The discharge parameters (stored energy, discharge time, and magnitude of current pulse) were sufficient to move defects by conduction electrons, but insufficient to melt the sample. The properties of treated samples were studied using Mössbauer absorption spectra and x-ray diffraction for three types of samples: (a) primary powder treated by high pressure up to 8 GPa, (b) powder subjected to indirect pulsed heating at 8 GPa, (c) powder treated by electrical pulses at 8 GPa. The x-ray diffraction pattern of primary powder exhibits peaks of copper, iron, and copper oxide (CuO). The Mössbauer spectrum of primary powder exhibits six peaks of alpha iron and some peaks near zero velocity due to the small iron clusters in the copper matrix and ultrafine clusters of paramagnetic phase x-Fe2O3. The transformation of CuO to Cu2O takes place in the course of indirect heating, the Mössbauer spectrum being almost unchanged. The direct electrodischarge heating causes the appearance of new magnetic phase with the magnetic field on iron nucleus 505 kOe, which corresponds to α-Fe2O3. The formation of α-Fe2O3 was confirmed by x-ray diffraction. At the same time the transformation of CuO to Cu2O is incomplete. These experiments demonstrate that high density current pulses, causing the electron wind, can be a useful tool to influence the structure of nanocrystalline powder.
Lateral and cross-well transport of highly and moderately excited carriers in Si1−xGex/Si superlattices
Authors: Galeckas A. , Juodkazis S. , Vanagas E. , Netiksis V. , Petrauskas M. , Bitz A. , Staehli J. L. , Willander M.
Published: July 2002

Picosecond transient reflectometry and dynamic grating techniques have been applied to investigate the perpendicular and parallel transport of optically excited carriers in strained-layer Si0.83Ge0.17/Si superlattices. We present results of the carrier ambipolar diffusivity and effective lifetime measurements in the layered structure and substrate within the 1017–1020 cm−3 density range. The combined experimental data are discussed in terms of parallel and perpendicular diffusion of carriers, interface recombination, and lattice heating. The estimated lateral and cross-well diffusion constants are 10 and 0.16 cm2/s at room temperature, respectively.
Role of c-axis field components in making the transport E(J) characteristics of (Bi,Pb)2Sr2Ca2Cu3Ox tapes insensitive to the direction of field within the tape plane
Authors: Edelman H. S. , Larbalestier D. C.
Published: July 2002

We report extended (over four decades) electric field versus current density characteristics of (Bi,Pb)2Sr2Ca2Cu3O10 tapes in magnetic fields applied along each of the three principal tape axes at 77 K, and along the two principal axes in the tape plane at 4.2 K. The characteristics are almost independent of field direction in the tape plane at both temperatures. Similar insensitivity of the critical current to magnetic field direction observed in other polycrystalline high temperature superconductors has been ascribed to the percolative nature of current flow, while the insensitivity in (Bi,Pb)2Sr2Ca2Cu3O10(2223) films and Bi2Sr2CaCu2O8 single crystals has been attributed to c-axis conduction across Josephson junctions. We show that the insensitivity of the critical current density to the field direction of these tapes follows naturally from the fact that the dissipation depends only on the field component lying along the c axis of individual 2223 grains. Lorenz-force driven motion of the pancake vortices that form within each 2223 grain provides a dissipation mechanism that involves only the c-axis components of the field. The angular dispersion of the grains in the tape ensures a c-axis field component in most grains, regardless of the applied field direction. This produces the remarkable insensitivity of E(J) over some four orders of magnitude of the electric field.
Low-temperature photoluminescence study on Er-doped GaP grown by organometallic vapor phase epitaxy
Authors: Fujiwara Yasufumi , Curtis Anthony P. , Stillman Gregory E. , Matsubara Naoteru , Takeda Yoshikazu
Published: July 2002

Low-temperature photoluminescence spectra of Er-doped GaP grown by low-pressure organometallic vapor phase epitaxy was investigated under both above-band gap and below-band gap excitation conditions. Under all experimental conditions, similar Er-related spectra dominated by numerous sharp emission lines were obtained for the first time. The intensity dependence of the photoluminescence exhibits complicated behavior and is modeled using a unique energy-transfer process. The results obtained indicate that some Er-related luminescence centers are efficiently activated by direct excitation of electrons to some traps related to the luminescence centers.
Photoluminescence and free carrier interactions in erbium-doped GaAs
Authors: Culp T. D. , Cederberg J. G. , Bieg B. , Kuech T. F. , Bray K. L. , Pfeiffer D. , Winter C. H.
Published: July 2002

The photoluminescence properties of GaAs:Er doped with a new pyrazole and pyridine-based Er source, tris(3,5-di-tert-butylpyrazolato)bis(4-tert-butylpyridine)erbium(III), were investigated. These samples showed significantly stronger and sharper 1.54 μm Er3+ luminescence than in GaAs:Er samples doped with cyclopentadienyl-based Er sources. The efficient luminescence was associated with the Er–2O center, formed with unintentional oxygen impurities. The Er3+ emission was greatly reduced in n-type samples, whereas the emission remained strong in p-type samples. This trend suggests that either the free hole concentration is very important to the Er3+ excitation efficiency, and/or there is a strong Auger quenching mechanism which involves free electrons. A model based on the results of a two-beam experiment indicates the presence of strong Auger energy transfer from the Er-bound exciton to a free electron. Auger energy transfer from the excited Er3+ ion to a free electron was found to be much less important. The temperature dependence of the Er3+ emission was also examined. A decrease in intensity was observed at the lowest temperatures. This effect was attributed to the freeze-out of carriers onto a relatively shallow trap which could be related to either Er or shallow acceptors.
Cathodoluminescence study of InGaAs/GaAs quantum dot structures formed on the tetrahedral-shaped recesses on GaAs (111)B substrates
Authors: Sekiguchi Takashi , Sakuma Yoshiki , Awano Yuji , Yokoyama Naoki
Published: July 2002

InGaAs/GaAs quantum dot (QD) structures grown in tetrahedral-shaped recesses (TSR) etched into GaAs (111)B substrates are studied by means of cathodoluminescence (CL). Two main features are observed in the CL spectra. The lower energetic peak can be assigned to QDs in the bottom of the TSRs, while the higher energetic one corresponds to the quantum wells (QWs) formed at the sides of the TSRs. This can be confirmed by spatially resolved CL imaging. The behavior of QD and QW luminescence is studied by temperature and electron beam intensity dependence of CL. We will show that the formation of the QDs is strongly dependent on the size and the spacing of the TSRs.
Liquid-crystal alignment on polytetrafluoroethylene and high-density polyethylene thin films studied by optical second-harmonic generation
Authors: Dennis John R. , Vogel Viola
Published: July 2002

We have used optical second-harmonic generation to study surface molecular order in a liquid-crystal (4′-n-octyl-4-cyano-biphenyl, or 8CB) on shear-deposited polymer films. The films are highly oriented layers of polytetrafluoroethylene (PTFE) or high-density polyethylene (HDPE), with a surface topology of uniaxially aligned nanoscale ridges and grooves, which are used as versatile substrates for oriented growth and alignment of other materials. In nematic 8CB cells made with either polymer, the surface monolayers of 8CB were aligned along the polymer orientation axis, and showed C2ν symmetry. In the isotropic phase, the surface monolayer alignment in these cells was lost. Monolayers of 8CB evaporated onto either polymer showed little or no alignment. These data indicate that the PTFE and HDPE films do not produce the strong epitaxylike alignment seen on some cloth-rubbed polymer surfaces. Instead, alignment appears to be primarily caused by surface ridges through an elastic, bulk-mediated mechanism.
On the equilibrium charge density at tilt grain boundaries
Authors: Srikant V. , Clarke D. R.
Published: July 2002

The equilibrium charge density and free energy of tilt grain boundaries as a function of their misorientation is computed using a Monte Carlo simulation that takes into account both the electrostatic and configurational energies associated with charges at the grain boundary. The computed equilibrium charge density increases with the grain-boundary angle and approaches a saturation value. The equilibrium charge density at large-angle grain boundaries compares well with experimental values for large-angle tilt boundaries in GaAs. The computed grain-boundary electrostatic energy is in agreement with the analytical solution to a one-dimensional Poisson equation at high donor densities but indicates that the analytical solution overestimates the electrostatic energy at lower donor densities.
Nitridation processes on GaAs(001) surfaces: Optical, structural, and chemical analysis
Authors: Jung H. D. , Kumagai N. , Hanada T. , Zhu Z. , Yao T. , Yasuda T. , Kimura K.
Published: July 2002

Nitridation processes on GaAs(001) surfaces exposed to plasma-activated N2 were investigated by employing in situ reflectance-difference spectroscopy, reflection high-energy electron diffraction, and in-line Auger electron spectroscopy. We have found that a stable GaN layer is formed only when the As background pressure in the growth chamber is sufficiently low. Nitridation is significantly suppressed under a high background pressure of As. A possible mechanism of this effect and its implication to cubic-GaN growth on GaAs surfaces are discussed. Surface roughening induced by extensive nitridation is also discussed.
Electric field singularity at an electrode tip in a nonlinear electrical conductor
Authors: Vojta Agnes , Clarke David R.
Published: July 2002

By analogy with the well-established analysis for the elastic field singularity around a crack tip in a nonlinear elastic material, it is shown that the electric field, E, at the tip of a flat conducting plate in a nonlinear conducting material obeying jαEN has a field singularity of the form r−1/(N+1) where r is the distance from the plate. The corresponding current density scales with distance from the tip as jαr−N/(N+1). Electrical network simulations of a highly conducting electrode in a nonlinear conducting medium support these scaling relationships.
Thermal stresses in metal-coated optical fibers
Authors: Shiue Sham-Tsong , Lin Yi-Shyang
Published: July 2002

The thermal stresses in metal-coated optical fibers are analyzed. Several thermal stresses are important in a metal-coated optical fiber. First, the interfacial radial stress would produce microbending loss. Secondly, the thermally induced axial force in the glass fiber would induce buckling of the fiber and also results in an increase of bending loss. Thirdly, when the interfacial shear stress is larger than its shear strength, the metal coating would be delaminated from the glass fiber. Finally, when the normal stress in the metal coating is larger than its tensile strength, the metal coating would be broken. These thermal stresses could be minimized by appropriately selecting physical properties of the metal coating and its thickness. To minimize these thermal stresses, the Young’s modulus, thermal expansion coefficient, and Poisson’s ratio of the metal coating should be decreased. On the other hand, the thickness of the metal coating has an optimal value. Based on the strength consideration, an optimal design of the metal-coated optical fiber also is considered.
A pump with flat-ring-shaped magnets
Authors: Saotome H. , Hagiwara T. , Sato Y.
Published: July 2002

Previously, authors developed a novel linear actuator with flat-ring-shaped magnets and applied it to a pump. In this article, the output power of the actuator and the efficiency of the pump are obtained by experiments. A pair of flat-ring-shaped Nd–Fe–B magnets having N and S poles on the surface are put on the same axis and face each other. Rotating one of the ring magnets in the direction of its circumference, an oscillating force occurs on the other magnet. When this magnet is restricted to linear motion along the axis, the mechanical power driving the rotating magnet is transferred to the linear motion power. A pump was made with this mechanism. The efficiency of the pump is experimentally obtained, where water pressure difference between the intake and the outlet has been changed. As a result, the efficiency of the pump attains to about 45%. The efficiency of the actuator itself is very high, because the actuator has no copper and iron losses, and a negligibly small eddy current loss.
Monte Carlo investigation of the eight-state Potts model on quasiperiodic tilings
Authors: Ledue D. , Landau D. P. , Teillet J.
Published: July 2002

A Monte Carlo investigation of the eight-state Potts model on the two-dimensional (2D) quasiperiodic octagonal tiling with free boundary conditions is performed in order to determine the nature of a temperature-driven transition. It is shown that numerical data suffer from drastic free boundary effects that strongly disturb the probability distributions of the internal energy and, consequently, the scaling behavior of the specific heat. An alternative way consisting in analysing the core of the tilings is applied to pass over free boundary effects. This analysis combined with the Lee–Kosterlitz method allows one to evidence that the system undergoes a first-order transition as in 2D periodic lattices. The first-order type of scaling is observed for the maximum in the susceptibility of the core of the tilings but not for the maximum in the specific heat.
Bridge function and cavity correlation function for the Lennard-Jones fluid from simulation
Authors: Llano-Restrepo Mario , Chapman Walter G.
Published: July 2002

In this paper we present the results of a Monte Carlo simulation study of the cavity correlation function y(r) for the Lennard-Jones (LJ) fluid over a range of liquidlike densities and temperatures from the triple point to supercritical conditions. We also calculate the direct correlation function c(r) from simulations of the total correlation function h(r) and the Ornstein–Zernike relationship. The bridge function B(r) is calculated by difference. We show that these new results are useful in testing integral equation theories by comparing them with Rosenfeld and Blum’s prescription for the bridge function.
Laser investigation of the competition between rotational autoionization and predissociation in superexcited np Rydberg states of NO
Authors: Fujii Asuka , Morita Norio
Published: July 2002

Rotational superexcited np Rydberg states (n≥23, v=0) of NO have been state-selectively produced with a two-color double resonance excitation method, and not only the NO+ ions generated by autoionization but also nitrogen atoms of the predissociation fragment have been directly detected to investigate the competition between rotational autoionization and predissociation. As a result, it has been found that the predissociation efficiency of the np Rydberg states hardly depends on whether the rotational autoionization channels are open or not. This fact clearly demonstrates that the decay dynamics of these states is governed by predissociation, not by rotational autoionization.
Transition from static to kinetic friction in a model lubricated system
Authors: Demirel A. Levent , Granick Steve
Published: July 2002

Molecularly thin confined fluids were deformed in shear faster than structural relaxations in response to shear could be accomplished, such that with increasing deformation the systems passed from the rest state to sliding. The response of these systems—two atomically smooth mica sheets separated by a fluid comprised of globularly shaped molecules [octamethylcyclotetrasiloxane]—was studied as a function of film thickness of the fluid (from 80 to 10 Å, i.e, from ∼8 to ∼1 molecular dimensions), as a function of normal pressure, and as a function of deformation rate, using a modified surface forces apparatus. Whereas the linear response was always liquid-like provided that the deformation rate was sufficiently slow, a “stick-slip” transition from the rest state to sliding was observed when the deformation rate was large, provided that the oscillatory frequency sufficiently exceeded the inverse intrinsic relaxation time of the confined fluid. This transition was monotonic and reversible without hysteresis for relatively thick films but for thinner films was discontinuous with hysteresis. For films thicker than 3 molecular layers (ML), two length scales in deformation were observed; the films showed nonlinear force-deformation response beginning at a deformation amplitude of 3 Å but in general showed stick slip only when the deformation was larger than this. The critical deformation at the point of stick slip decreased from 9 to 3 Å with increasing normal pressure, indicating diminished plasticity of the confined structures with increasing normal pressure. The critical film thickness of 3 ML correlates with the possibility of one rather than more slip planes. The thinnest films under the highest compressive pressures showed moderate increase of the viscous shear force with increasing effective sliding velocity, but in general the viscous force reached a plateau in which force showed almost no dependence on sliding rate. In interpreting the results in the context of friction, static friction was identified with the elastic stress at rupture and kinetic friction was identified with the limiting maximum observed level of viscous force. After normalizing friction and normal forces by the contact area, the static friction coefficient was found to be 0.44 and the kinetic friction coefficient to be 0.14. In other words, as the normal pressure increased, the elastic force needed to rupture the system increased more rapidly than the limiting shear stress. The magnitude of the limiting shear stress increased exponentially with decreasing film thickness with a decay length of 1 molecular dimension. This decay length correlates well with the known exponential decay of oscillations in the static force–distance profile. The critical shear amplitude of 3 Å, relative to the molecular dimension of ≈9 Å, is reminiscent of early estimates by Frenkel of the point of instability when planes of atoms slide over one another.
Low-temperature experimental studies in molecular biophysics: a review
Authors: Blagoi Yu. P. , Sheina G. G. , Ivanov A. Yu. , Radchenko E. D. , Kosevich M. V. , Shelkovsky V. S. , Boryak O. A. , Rubin Yu. V.
Published: July 2002

The enormous contribution of Academician Boris I. Verkin in laying the foundation of the biophysics research school in Kharkov are recalled in the Jubilee year commemorating his 80th birthday. This review describes the development and realization of his ideas during the last two decades at Molecular Biophysics Department of the Institute for Low Temperature Physics and Engineering (ILTPE) in Kharkov. Main results of the studies of physical and chemical properties of biopolymer fragments and biologically active compounds using methods of low-temperature electron–vibrational spectroscopy, low-temperature secondary-emission mass spectrometry, and low-temperature luminescence spectroscopy are presented.
An experimental investigation on the stability of viscous drops translating through a quiescent fluid
Authors: Koh C. J. , Leal L. G.
Published: July 2002

The evolution of the shape of an initially nonspherical drop translating at low Reynolds number through a quiescent fluid is investigated experimentally. It is found that the drop reverts to a spherical shape when the degree of initial deformation is small enough. However, drops that are highly deformed initially are shown to deform continuously. Specifically, a prolate drop breaks up into multiple droplets as it rises, while an oblate drop deforms into a double-emulsion drop as it translates. The experimental results agree well with results obtained earlier from numerical simulations [Phys. Fluids A 1, 1309 (1989)].
Three-dimensional mode structure of the drift cyclotron loss-cone instability in a mirror trap
Authors: Koepke M. , McCarrick M. J. , Majeski R. P. , Ellis R. F.
Published: July 2002

The three-dimensional mode structure of the drift cyclotron loss-cone (DCLC) instability is characterized in an axisymmetric mirror machine. It is shown, for this experiment, that (1) the azimuthal mode structure does not assume the geometry associated with an azimuthal normal mode φ̃=f (r,z)exp(imθ) with a single integral value of m, (2) the presence of a cool-ion plasma stream does not, in general, stabilize the mode globally, and (3) the axial mode characteristics have finite structure.
Regular and chaotic bubble oscillations in periodically driven pressure fields
Authors: Smereka P. , Birnir B. , Banerjee S.
Published: July 2002

The motion of a single bubble in a periodically driven pressure field is examined from a geometric point of view using Poincaré maps. It is shown that the equations of motion can be transformed to a perturbation of a Hamiltonian system. The conditions determining nonlinear resonance are found; these correspond to subharmonic bifurcations. Further it is illustrated how the resonant response interacts with the nonresonant one to produce jump bifurcations. Results are also presented indicating that the periodic response undergoes a complex bifurcation sequence and a strange attractor forms. Finally it is demonstrated how the strange attractor disappears creating horseshoe maps that are associated with transient chaos. This gives some indication of the bifurcations that form the superstructure for single bubble oscillations.
Generation of Tollmien–Schlichting waves by harmonic excitation
Authors: Sengupta T. K. , Ballav Manish , Nijhawan Sandeep
Published: July 2002

The creation of wavy disturbances (Tollmien–Schlichting waves) by a localized two-dimensional disturbance source vibrating periodically at a fixed frequency in a Blasius boundary layer is discussed. The initial-boundary value problem mathematically models the vibrating ribbon experiment of Schubauer and Skramstad. By considering the ribbon vibration to start at t=0, in the model one gets the transient and the eventual periodic part of the solution. The results of the full simulation are compared with the time-asymptotic solution.
Nonexistence of magnetohydrodynamic equilibria with poloidally closed field lines in the case of violated axisymmetry
Authors: Salat A.
Published: July 2002

The existence of nonaxisymmetric toroidal magnetohydrodynamic (MHD) equilibria, whose magnetic field lines are closed after one poloidal turn around the magnetic axis C is investigated analytically. Up–down symmetry of the configuration with respect to the equatorial plane that contains the axis is assumed. In principle, nonaxisymmetry is manifested in the form of a noncircular axis or a variation of the geometry and/or magnetic field along a circular axis. It is proved that no equilibrium with a noncircular axis exists. For a circular axis, nonexistence is proved if the ellipticity of the cross section varies along C. Nor is variation of the triangularity, etc., up to the seventh Fourier mode with respect to the poloidal angle, allowed. For variations with still higher mode numbers, nonexistence is made plausible. For the magnetic field the situation is analogous. Nonaxisymmetric poloidal equilibria with equatorial mirror symmetry are thus practically ruled out. The method of investigation is an expansion in the distance from the magnetic axis, supported by an algebraic computer language. With growing order the number of constraints on the configuration increases until the quoted results are obtained in the sixth and higher orders.
Atomic structure of the GaAs/Si interface
Authors: Hull R. , Rosner S. J. , Koch S. M. , Harris J. S.
Published: July 2002

The atomic structure of the GaAs/Si(100) interface is studied using high resolution transmission electron microscopy. The samples studied were grown by molecular beam epitaxy using a conventional two-step growth process of a relatively low-temperature (405 °C) GaAs buffer layer followed by a higher temperature (575 °C) device layer. Following this growth procedure, the interface is found to be atomically rough with nonuniform distributions of steps, and to contain regions of disrupted crystallinity. These regions are found to be more prevalent in material which has undergone only low-temperature (buffer layer) GaAs deposition. By using the atomic resolution microscope at the National Center for Electron Microscopy, Lawrence Berkeley Laboratories, we are also able to atomically image the same area of interface along orthogonal 〈011〉 directions, observing qualitatively similar structures.
Radiation-induced space charge in polymer film capacitors
Authors: Kurtz S. R. , Anderson R. A.
Published: July 2002

A space-charge mapping technique was used to observe the formation of x-ray-induced space charge in poly(ethylene terephthalate) and polystyrene capacitor structures. Electronic transport processes, responsible for radiation-induced conductivity in these insulators, produced trapped space charge adjacent to the electrodes. These results were consistent with the conclusions of earlier photoconductivity measurements and revealed injection processes that limit the buildup of space charge.
Instabilities in external cavity injection lasers due to resonant self-pulsing
Authors: Phelan P. , O’Gorman J. , McInerney J. , Heffernan D.
Published: July 2002

Instabilities in self-pulsing semiconductor diode lasers in a long external ring cavity are reported. The frequency of the self-pulsations increases in discrete steps with the injection current and the light-current characteristic displays kinks at these steps. The observed frequencies are multiples of the external cavity resonance and the laser output is unstable when biased at currents corresponding to these kinks. Similar observations have been made in various cavities and other laser diodes.
Static atomic displacements in a CdTe epitaxial layer on a GaAs substrate
Authors: Horning R. D. , Staudenmann J.-L.
Published: July 2002

A (001)CdTe epitaxial layer on a (001)GaAs substrate was studied by x-ray diffraction between 10 and 360 K. The CdTe growth took place at 380 °C in a vertical gas flow metalorganic chemical vapor deposition reactor. Lattice parameters and integrated intensities of both the substrate and the epitaxial layer using the (00l) and (hhh) Bragg reflections reveal three important features. Firstly, the GaAs substrate does not exhibit severe strain after deposition and it is as perfect as a bulk GaAs. Secondly, the CdTe unit cell distorts tetragonally with a⊥>a∥ below 300 K. The decay of the (00l) reflection intensities as a function of the temperature yields a Debye temperature of 142 K, the same value as for bulk CdTe. Thirdly, a temperature-dependent isotropic static displacement of the Cd and the Te atoms is introduced to account for the anomalous behavior of the (hhh) intensities.
Metastable thermal donor states in silicon
Authors: Chantre Alain
Published: July 2002

Deep level transient spectroscopy has been applied to the study of defects introduced in silicon by short heat treatments at 450 °C. The behavior of an electron trap at Ec−0.15 eV is shown to provide strong evidence for the bistable nature of small thermal donor clusters.
Structure and magnetic properties of the dilute magnetic semiconductor Mn:CuInTe2
Authors: Lin L.-J. , Wernick J. H. , Tabatabaie N. , Hull G. W. , Meagher B.
Published: July 2002

The properties of bulk polycrystalline p-type Mn:CuInTe2 (with up to 12% Mn substitution) have been examined using polarized light microscopy, x-ray energy dispersive analysis, optical spectrometry, van der Pauw technique, and superconducting quantum interference device magnetometer measurements. The compound (Cu1−xMnx)InTe2 is a tetragonal chalcopyrite with lattice constants nearly equal to that of CuInTe2. On the other hand, attempted substitution of Mn in In sites produces a second phase composed of Cu and Te. The band gap of Mn:CuInTe2 is larger than 0.92 eV. Most samples show phonon-limited mobility, with the highest mobilities occurring at ≊75 K. Samples with 3% Mn substitution in Cu sites are paramagnetic. At higher Mn concentration (9%<x<12%), the (Cu1−xMnx)InTe2 samples show antiferromagnetic coupling and a measured effective magnetic moment up to ≊5.4μB/Mn atom. Samples with attempted Mn substitution in In sites (from 3% to 9%) show anomalous magnetic behavior at low temperatures.
Resonant Raman scattering and exciton-optical phonon coupling in CdTe/(Cd,Mn)Te quantum wells
Authors: Chang S.-K. , Nakata H. , Nurmikko A. V. , Gunshor R. L. , Kolodziejski L. A.
Published: July 2002

Resonant Raman scattering at the E0 gap from longitudinal optical phonons in CdTe/(Cd,Mn)Te multiple quantum wells has been studied near the strain split n=1 exciton ground state. A large resonance enhancement is observed with distinct incoming and outgoing channels, in agreement with luminescence excitation spectra. Phonons are observed from CdTe wells and (Cd,Mn)Te barriers, indicative of small valence-band offsets for this superlattice system. At extreme resonance, striking linewidth broadening and damping effects for the alloy phonon modes are witnessed.
Whole-sample and localized induced-absorption optical bistability in GaAlAs waveguides
Authors: Aitchison J. S. , Valera J. D. , Walker A. C. , Ritchie S. , Rodgers P. M. , McIlroy P. , Stegeman G. I.
Published: July 2002

Induced-absorption optical bistability has been studied in GaAlAs heterostructure rib and slab waveguides. Both whole-sample and localized switching have been observed within the same device. The localized switch, which occurs on the time scale of a few microseconds and at an input power of 10–20 mW, has been induced using a laser-diode source.
Electric field dependent exciton energy and photoluminescence quenching in GaInAs/InP quantum wells
Authors: Shorthose M. G. , Maciel A. C. , Ryan J. F. , Scott M. D. , Moseley A. , Davies J. I. , Riffat J. R.
Published: July 2002

We report the observation of electric field induced exciton energy shifts and photoluminescence quenching in GaInAs/InP multiple quantum wells. We have measured both the photocurrent and photoluminescence spectra from 100 Å wells contained with p+- and n+-InP layers in a conventional p-i-n structure; reverse bias voltages of up to 12 V were applied. The exciton peaks in the photocurrent spectrum are seen to broaden and shift to lower energy; the photoluminescence peak, which is due to n=1 excitonic and free-carrier recombination, also shifts to lower energy and is completely quenched at high voltages. These results are similar to those reported previously for GaAs quantum wells and ascribed to the quantum-confined Stark effect.
Electron microscopy and microanalysis of a YBa2Cu3Ox superconducting oxide
Authors: Dou S. X. , Bourdillon A. J. , Sorrell C. C. , Ringer S. P. , Easterling K. E. , Savvides N. , Dunlop J. B. , Roberts R. B.
Published: July 2002

The onset of superconductivity at 140 K has been observed in the high Tc Y-Ba-Cu-O system in samples consisting of 90% by volume of a single-phase oxide of average cation ratio Y:Ba:Cu≂1:2:3. A sharp superconducting transition in the resistivity has been measured, where Tc0=140 K, Tc=93.2 K, ΔTc=0.5 K, and ‘‘zero resistance’’ was observed at 92.0 K. Transmission electron microscopy and energy dispersive spectroscopy measurements revealed that the superconducting phase consisted of a large number of microcrystallites of ≂0.5 μm size and that these contained a very high defect microtwinned/faulted structure.
Epitaxial growth quality optimization by supercomputer
Authors: Clarke Shaun , Vvedensky Dimitri D.
Published: July 2002

Supercomputer simulations of molecular beam epitaxial growth are applied to modeling processing profiles. Illustration is provided by investigating the relative advantages of high incident beam flux growth, interrupted periodically to allow the growth front to relax, versus continual growth at a relatively low deposition rate.
GaAlAs gain-guided semiconductor lasers with a curved facet
Authors: Yamashita S. , Nakatsuka S. , Tanaka T. , Ono Y. , Chinone N. , Kajimura T.
Published: July 2002

GaAlAs gain-guided semiconductor lasers having a curved facet are fabricated by employing reactive ion beam etching. The use of the curved facet permits stabilization of transverse mode in the direction parallel to the junction plane, reduction of astigmatism, and multilongitudinal mode oscillation.
Nonlinear effects in coplanar GaAs/InGaAs strained-layer superlattice directional couplers
Authors: Das Utpal , Chen Yi , Bhattacharya Pallab
Published: July 2002

We report on the performance characteristics of InGaAs/GaAs strained-layer superlattice coplanar ridge-type directional couplers realized by molecular beam epitaxy. The measured power transfer characteristics with 1.15 μm incident photoexcitation demonstrate nonlinear coupling due to absorption associated with the tails of the excitonic resonances in the quantum wells. From a theoretical fit of the measured data, the nonlinear refractive index coefficient, n2, of the multiquantum well is found to be 2.25×10−7 cm2/W. This agrees very well with a value of n2=1.9×10−7 cm2/W obtained independently on the same material from interferometric measurements.