bluuebunny/crossref_metadata_2025 huggingface dataset:
Linking rates of folding in lattice models of proteins with underlying thermodynamic characteristics
Authors: Klimov D. K. , Thirumalai D.
Published: July 2002

We investigate the sequence-dependent properties of proteins that determine the dual requirements of stability of the native state and its kinetic accessibility using simple cubic lattice models. Three interaction schemes are used to describe the potentials between nearest neighbor nonbonded beads. We show that, under the simulation conditions when the native basin of attraction (NBA) is the most stable, there is an excellent correlation between folding times τF and the dimensionless parameter σT=(Tθ−TF)/Tθ, where Tθ is the collapse temperature and TF is the folding transition temperature. There is also a significant correlation between τF and another dimensionless quantity Z=(EN−Ems)/δ, where EN is the energy of the native state, Ems is the average energy of the ensemble of misfolded structures, and δ is the dispersion in the contact energies. In contrast, there is no significant correlation between τF and the Z-score gap ΔZ=EN−Ems. An approximate relationship between σT and the Z-score is derived, which explains the superior correlation seen between τF and σT. For two state folders τF is linked to the free energy difference (not simply energy gap, however it is defined) between the unfolded states and the NBA.
Structure and magnetic properties of benzyl, anilino, and phenoxyl radicals by density functional computations
Authors: Adamo Carlo , Subra Robert , Di Matteo Andrea , Barone Vincenzo
Published: July 2002

A recently developed quantum mechanical approach devoted to the study of structural and magnetic properties of open-shell species was applied to the isoelectronic series formed by benzyl, anilino, and phenoxyl radicals. Hybrid Hartree–Fock/density functional models (here B3LYP) confirm their reliability, whereas, contrary to other cases, vibrational averaging and solvent effects play only a negligible role. The spin dependent properties are interpreted in terms of a new model including first and second order spin polarization effects.
Elastic properties of a diblock copolymer monolayer and their relevance to bicontinuous microemulsion
Authors: Matsen M. W.
Published: July 2002

A new self-consistent field theory (SCFT) method is introduced for evaluating the exact mean-field elastic properties of a diblock copolymer monolayer separating two immiscible homopolymer phases. The method is illustrated on ternary blends of A- and B-type homopolymers of equal size blended with AB diblock copolymers of symmetric composition. We make comparisons with weak- and strong-segregation calculations, as well as with an earlier less accurate SCFT-based calculation. These previous approaches are found to produce sizable degrees of inaccuracy, emphasizing the need for our improved method. To conclude, we address the significance of our calculation in the efforts to develop bicontinuous polymeric microemulsions. Our primary conclusion is that the molecular weight of the diblock copolymer must be small.
On the structure and thermodynamics of solvated monoatomic ions using a hybrid solvation model
Authors: Topol I. A. , Tawa G. J. , Burt S. K. , Rashin A. A.
Published: July 2002

The hydration free energies relative to that of the proton are calculated for a representative set of monatomic ions Z±. These include cationic forms of the alkali earth elements Li, Na, and K, and anionic forms of the halogens F, Cl, and Br. In the current model the relative ion hydration free energy is defined as Δ[ΔGhyd(Z±)]=G(Z±[H2O]n(aq))−G(H+[H2O]n(aq))−G(Z±(gas))−G(H+(gas)), where the solvated ions are represented by ion–water clusters coupled to a dielectric continuum using a self-consistent reaction field cycle. An investigation of the behavior of Δ[ΔGhyd(Z±)] as the number of explicit waters of hydration is increased reveals convergence by n=4. This convergence indicates that the free energy change for the addition of water to a solvated proton–water complex is the same as the free energy change associated with the addition of water to a solvated Z±–water complex. This is true as long as there are four explicitly solvating waters associated with the ion. This convergence is independent of the type of monatomic ion studied and it occurs before the first hydration shell of the ions (typically ⩾6) is satisfied. Structural analysis of the ion–water clusters reveals that the waters within the cluster are more likely to form hydrogen bonds with themselves when clustering around anions than when clustering around cations. This suggests that for small ion–water clusters, anions are more likely to be externally solvated than cations.
Deficiency indices and singular boundary conditions in quantum mechanics
Authors: Bulla W. , Gesztesy F.
Published: July 2002

We consider Schrödinger operators H in L2(Rn), n ∈ N, with countably infinitely many local singularities of the potential which are separated from each other by a positive distance. It is proved that due to locality each singularity yields a separate contribution to the deficiency index of H. In the special case where the singularities are pointlike and the potential exhibits certain symmetries near these points we give an explicit construction of self-adjoint boundary conditions.
When is a Hamiltonian system separable?
Authors: Marshall Ian , Wojciechowski Stefan
Published: July 2002

The Hamiltonian system given by H= 1/2  p2+V(q) with V∈C∞(Rn) is considered. A method for integrating such a system is that of separating the variables in the Hamilton–Jacobi equation. It is known that if such a separation is possible, then it can take place only when the equation is expressed in terms of generalized elliptic coordinates or in a degeneration of these. A criterion is proposed for deciding if separation is possible, and if it is, in which degeneration of elliptic coordinates it takes place.
Geometrical approach to inverse scattering for the Dirac equation
Authors: Jung Wolf
Published: July 2002

The high-energy-limit of the scattering operator for multidimensional relativistic dynamics, including a Dirac particle in an electromagnetic field, is investigated by using time-dependent, geometrical methods. This yields a reconstruction formula, by which the field can be obtained uniquely from scattering data.
Effective action of composite fields for general gauge theories in Batalin, Lavrov, and Tyutin covariant formalism
Authors: Lavrov P. M. , Odintsov S. D. , Reshetnyak A. A.
Published: July 2002

The gauge dependence of the effective action of composite fields for general gauge theories in the framework of the quantization method by Batalin, Lavrov and Tyutin is studied. The corresponding Ward identities are obtained. The variation of composite fields effective action is found in terms of new set of generators depending on composite field. The theorem of the on-shell gauge fixing independence for the effective action of composite fields in such formalism is proven. A brief discussion of gravitational-vector induced interaction for Maxwell theory with composite fields is given.
Superintegrable systems on the two-dimensional sphere S2 and the hyperbolic plane H2
Authors: Rañada Manuel F. , Santander Mariano
Published: July 2002

The existence of superintegrable systems with n=2 degrees of freedom possessing three independent globally defined constants of motion which are quadratic in the velocities is studied on the two-dimensional sphere S2 and on the hyperbolic plane H2. The approach used is based on enforcing the conditions for the existence of two independent integrals (further than the energy). This is done in a way which allows us to discuss at once the cases of the sphere S2 and the hyperbolical plane H2, by considering the curvature κ as a parameter. Different superintegrable potentials are obtained as the solutions of certain systems of two κ-dependent second order partial differential equations. The Euclidean results are directly recovered for κ=0, and the superintegrable potentials on either the standard unit sphere (radius R=1) or the unit Lobachewski plane (“radius” R=1) appear as the particular values of the κ-dependent superintegrable potentials for the values κ=1 and κ=−1. Some new superintegrable potentials are found, both on S2 and H2. The correspondence between superintegrable systems in spaces of zero and nonzero curvature is discussed.
Exotic solitons in magnets with strongly anisotropic exchange interaction
Authors: Kovalev A. S. , Gvozdikova M. V.
Published: July 2002

Exotic magnetic solitons (compactons and peakons) in magnets with extremely anisotropic exchange interaction are investigated on the basis of the classical Ising and XY-models.
Structural stability in two-dimensional model flows: Lagrangian and Eulerian turbulence
Authors: Danielson T. J. , Ottino J. M.
Published: July 2002

A two-dimensional approximation of the Navier–Stokes equations illustrates a specific instance of transition from Lagrangian to Eulerian turbulence. As the Reynolds number (Re) increases, the system describing the dynamics of the velocity field undergoes a transition from steady state to a limit cycle. At this point the flow displays chaotic advection—i.e., manifolds intersect transversely and Poincaré maps show the typical chaotic structure—but the velocity field itself is time-periodic and the power spectrum presents a fundamental frequency and its harmonics. As Re increases still further, the limit cycle bifurcates into a strange attractor producing a broadband power spectrum. The model suggests that Lagrangian turbulence (complex particle trajectories) might serve as a springboard for Eulerian turbulence (complex signal at a fixed point) and indicates a possible link between a kinematical view of flows and mixing and other viewpoints of turbulence based on strange attractors. The ideas can be generalized to three-dimensional flows; however, due to their simplicity, the flows generated are unable to mimic some key features of turbulence such as spatial uncorrelation of Eulerian signals.
The influence of velocity and density ratio on the dynamics of spatially developing mixing layers
Authors: Strykowski P. J. , Niccum D. L.
Published: July 2002

The dynamics of countercurrent mixing is examined in the shear layer of an axisymmetric jet. Experiments were designed to establish conditions of absolute instability in a spatially developing shear layer and to document how the instability influences the jet development. By applying suction around the jet periphery, shear-layer velocity ratios R greater than 1 could be studied. Here, R=(U1−U2)/(U1+U2), where U1 is the velocity of the forward jet stream and U2 is the velocity of the counterflowing stream created by suction. The density ratio S=ρ1/ρ2 of the mixing layer was also varied to determine the stability boundary in the S-R plane. The density of the forward stream ρ1 was increased by adding sulfur hexafluoride to the air jet, which provided density ratios between 1 and 5.1. Hot-wire anemometry and flow visualization revealed that a global transition occurs when conditions of absolute instability are established in the jet shear layers. One consequence of this transition is an abrupt decrease in the jet spread rate. The experimentally determined transition between globally stable and globally unstable flow regimes in the S-R plane agrees quite well with predictions of the convective/absolute instability boundary based on the linear stability theory [Pavithran and Redekopp, Phys. Fluids A 1, 1736 (1989)].
Physical condition for absolute instability in inviscid hydroelastic coupling
Authors: Triantafyllou George S.
Published: July 2002

In this paper the instability of infinitely long cylindrical structures submerged in a uniform inviscid flow is investigated. When the flow velocity is zero, two families of hydroelastic waves exist, with phase velocities that are equal in magnitude and have opposite signs. Positive values of the fluid velocity increase the magnitude of the phase velocity of the waves that propagate in the same direction as the flow, and decrease the magnitude of the phase velocity of the waves propagating opposite to the flow. It is shown by constructing an energy conservation law for the linearized equations of motion that both types of waves have initially positive energy, in the sense that they both require an energy source to be sustained. After a critical value of fluid velocity is exceeded the slow waves acquire negative energy, in the sense that they require an energy sink to be sustained. Coalescence between the ‘‘fast’’ (positive energy) and the ‘‘slow’’ (negative energy) waves creates waves with zero energy which do not require neither an energy source nor a sink to be sustained, and grow exponentially in time. It is further shown that the front end of any initially localized disturbance will propagate with a speed equal to the minimum group velocity of the fast waves and the back end will propagate with speed equal to the maximum group velocity of the slow waves. Consequently, if those two characteristic speeds have opposite signs, the instability is absolute, whereas if they have the same sign the instability is convective. It is shown that this simple physical criterion applies to a more general class of instabilities that involve the interaction of positive and negative energy waves.
Electron–ion hybrid mode due to transverse velocity shear
Authors: Ganguli G. , Lee Y. C. , Palmadesso P. J.
Published: July 2002

It has been established that electrostatic ion-cyclotron-like and Kelvin–Helmholtz modes can be sustained by a transverse velocity shear for L>ρi, where L is the velocity shear scale length and ρi is the ion gyroradius. Here it is shown that if ρe<L≪ρi, where ρe is the electron gyroradius, then a short wavelength electron–ion hybrid mode can be excited around the lower hybrid frequency. Like the Kelvin–Helmholtz instability, an explicit dependence on the second derivative of the dc electric field is essential for the growth of this mode.
Fluid motion of monomolecular films in a channel flow geometry
Authors: Stone H. A.
Published: July 2002

Surface pressure-driven flow of a monolayer in a channel flow geometry is studied and exact representations for the monolayer and subphase velocity fields are given in terms of solutions involving dual integral equations. The monolayer velocity profiles are examined as a function of the viscosity contrast between the monolayer and the subphase and the effects of a finite depth sublayer are investigated also. The calculated velocity profiles may prove useful for determining the effective surface viscosity of monolayer films from comparable experimental measurements.
Lattice gas automaton simulation of acoustic streaming in a two-dimensional pipe
Authors: Stansell Paul , Greated Clive A.
Published: July 2002

The lattice gas automaton fluid modelling technique is used to study acoustic streaming phenomena arising from the interaction of sound waves with no-slip boundaries in a two-dimensional pipe. It is demonstrated that this fluid simulation tool is able to reproduce the general form of the acoustic streaming flow field observed by Andrade [Proc. R. Soc. London 175, 1 (1883)] and predicted by Lord Rayleigh [Philos. Trans. R. Soc. London 175, 1 (1884)]. The differences that are evident may be due to the restrictions on the computing resource which prohibits the use of a lattice large enough to fulfill the conditions used by Rayleigh in his derivation of the acoustic streaming flow field.
Hydrodynamics of collisionless relativistic plasma with pressure anisotropy
Authors: Tsikarishvili E. G. , Lominadze J. G. , Javakhishvili J. I.
Published: July 2002

The closed set of relativistic hydrodynamical equations describing strongly magnetized collisionless plasma with an anisotropic pressure tensor is offered. This model is the generalization of Chew–Goldberger–Low [Proc. R. Soc. London Ser. A 236, 112 (1956)] theory for the relativistic case. It is shown that at high temperatures the equation of state differs significantly from the ones used earlier.
Static local field correction description of acoustic waves in strongly coupling dusty plasmas
Authors: Murillo M. S.
Published: July 2002

The kinetic equations for an interacting dust system with external time-dependent forces is considered from the Born–Bogolyubov–Green–Kirkwood–Yvon equations. A kinetic equation is obtained by writing the two-particle distribution function as a product of two one-particle distribution functions and the equilibrium radial distribution function. It is shown that a Vlasov-like equation is recovered with a collision term which is a functional of the pair correlation function. Wave behavior from the corresponding fluid equations is considered for a dusty plasma. The results are in qualitative agreement with previously obtained dispersion relations based on generalized hydrodynamics and the quasilocalized charge approximation.
Dust acoustic waves with dust charge fluctuations—revisited
Authors: Shukla P. K. , Resendes D.
Published: July 2002

The dust acoustic wave spectrum is rederived, taking into account dust charge fluctuations that are produced by orbital-motion limited and secondary electron currents in an unmagnetized dusty plasma containing negative or positive dust grains. It is found that the secondary electron current reduces the magnitude of fluctuating electron currents, which affect the propagation of the dust acoustic waves. The relevance of this investigation to enhanced scattering of electromagnetic waves off the dust-acoustic waves in the Earth’s polar mesosphere is discussed.
Spaced measurements and progress in understanding space plasma waves
Authors: Kintner P. M. , Schuck P. W. , Franz J. R.
Published: July 2002

Measuring plasma wave properties with spatially separated antennae observes wavelength, propagation speed, and direction as well as frequency. This approach has been pivotal to progress in understanding three phenomena in space plasmas. The first is lower hybrid solitary waves, which are waves with cylindrical symmetry propagating inside a density cavity. The second is electron phase-space holes, which are propagating vortices in phase space. The third is called broadband ELF (extra low frequency) electric fields and is an important but poorly understood wave process essential to transverse ion acceleration.
Low-frequency electrostatic dust-modes in a strongly coupled dusty plasma with dust charge fluctuations
Authors: Mamun A. A. , Shukla P. K. , Farid T.
Published: July 2002

Low-frequency electrostatic dust-modes are theoretically investigated accounting for dust grain charge fluctuation and for equilibrium grain charge inhomogeneity in a strongly coupled dusty plasma. A new stable extremely low-frequency mode, which is due to the inhomogeneity in the equilibrium dust grain charge, is found to exist in such a dusty plasma. It is also found that the dust-acoustic mode becomes unstable due to the effect of this equilibrium dust grain charge inhomogeneity. It is observed here that the influence of strong correlations in the dust fluid significantly modify the dispersion properties of this new mode as well as of the existing dust-acoustic mode. The implications of our results to recent experimental observations and to some space and astrophysical situations are briefly discussed.
Further evidence for two-component surface acoustic wave reflections from surface breaking slots
Authors: Dewhurst R. J. , McKie A. D. W. , Palmer S. B.
Published: July 2002

When laser-generated Rayleigh pulses interact with surface breaking slots in metals, two distinct reflections are present in the captured waveform. While the first component is accepted as arising from the direct reflection of a Rayleigh pulse from the top of the defect, the origin and features of the second component have been the subject of some discussion. This letter presents evidence to show that the second component also propagates at the Rayleigh velocity, lending support to the suggestion that it originates at the base of the slot and can be used to measure the slot depth.
Inversion layers on germanium with low-temperature-deposited aluminum-phosphorus oxide dielectric films
Authors: Chang R. P. H. , Fiory A. T.
Published: July 2002

Mixed aluminum-phosphorus oxide films deposited on Ge (100) under ultrahigh vacuum conditions at low temperatures (≊200 °C) are found to yield low interface state densities near the band edges which are sufficient for observing both p- and n-channel-type inversion layers. As a glass-promoting additive, it is speculated that phosphorus reduces bonding disorder at Ge/GeO2 interfaces.
Theoretical study of the electronic properties of semimagnetic superlattices
Authors: Wu G. Y. , Smith D. L. , Mailhiot C. , McGill T. C.
Published: July 2002

We present the first theoretical study of the electronic properties of superlattices formed from semimagnetic semiconductors. We explicitly consider the Cd0.8Mn0.2Te/Cd0.7Mn0.3Te superlattice system. We show magnetic field splittings both including (zero spin temperature) and neglecting (high spin temperature) the exchange interaction. We find that the exchange interaction dominates the magnetic effects. We present calculations of the derivative of the superlattice band gap with magnetic field as a function of the superlattice layer thickness. We show calculations of the change in superlattice band gap with magnetic field for several magnetic fields as a function of temperature.
Direct measurement of dispersive nonlinearities in GaAs
Authors: Lee Y. H. , Chavez-Pirson A. , Rhee B. K. , Gibbs H. M. , Gossard A. C. , Wiegmann W.
Published: July 2002

Nonlinear refractive index changes in 299 Å multiple quantum well GaAs were obtained directly by measuring Fabry–Perot transmission peak shifts. These changes crosscheck those obtained by Kramers–Kronig transformations of the nonlinear absorption under identical pumping conditions. High intensities saturate the exciton, so that nonlinear refractive index changes are dominated by nonresonant contributions. Thermal refractive index changes were also measured directly.
High-power quasi-cw monolithic laser diode linear arrays
Authors: Harnagel G. L. , Cross P. S. , Scifres D. R. , Welch D. F. , Lennon C. R. , Worland D. P. , Burnham R. D.
Published: July 2002

Optical power in excess of 25 W from one facet has been obtained from a 1-cm monolithic AlGaAs laser array for a pulse width of 150 μs (quasi-cw operation). The array consists of twenty 40-stripe lasers spaced along the bar, with etched grooves between each region to prevent transverse lasing and amplified spontaneous emission. About 80% of the facet width is utilized for active laser emission.
Bias-dependent photoresponse of p+i
n GaAs/AlAs/GaAs diodes
Authors: Melloch M. R. , McMahon C. P. , Lundstrom M. S. , Cooper J. A. , Qian Q-D. , Bandyopadhyay S.
Published: July 2002

We report photocollection efficiency measurements of p+in GaAs/AlAs/GaAs diodes fabricated on films grown by molecular beam epitaxy. Both the zero-bias and bias-dependent photocollection characteristics can be explained by assuming that the band discontinuity between AlAs and GaAs is mostly accommodated in the valence band.
New method of light-induced deposition of metal films on insulator-on-semiconductor substrates
Authors: Krawczyk S. K. , Kumar S. N.
Published: July 2002

We show in this work that the internal photoemission at the semiconductor-insulator interface in the semiconductor-insulator-electrolyte system can be used to initiate localized chemical reactions at the insulator-electrolyte interface. Using this concept, successful depositions of Cu and Ni films have been obtained on oxidized Si wafers. We have also found that the activation of the SiO2 surface with Pd atoms results in an efficient electron transfer at the SiO2-electrolyte interface and improves the adhesion of the deposited metal.
Resonant Raman effect in thin-layered AlAs-GaAs superlattices
Authors: Kobayashi Naoki , Toriyama Takeshi , Horikoshi Yoshiji
Published: July 2002

A new Raman line was found on ultrahigh AlAs-GaAs superlattices with an excitation energy near the band gap. The excitation profile revealed that the intensity of the new line was enhanced resonantly as well as those of the AlAs- and GaAs-like phonon lines as the excitation photon energy approached the photoluminescence peak energy. Polarization measurements showed that the enhanced lines were induced by the Fröhlich mechanism. The normal coordinate treatment of the lattice vibration indicates that these lines can probably be attributed to the longitudinal optical phonons of the E species.
Two- and three-dimensional magnetoluminescence measurements in a GaAs/AlGaAs multiple quantum well
Authors: Johnson G. R. , Cavenett B. C. , Kana’ah A. , Pakulis E. J. , Wang W. I.
Published: July 2002

We report a direct means of distinguishing between the two-dimensional and three-dimensional nature of luminescence bands in multilayered structures. Fixed energy interband excitation of a GaAs/AlGaAs multiple quantum well structure reveals oscillations of the luminescence intensity in a varying magnetic field resulting from resonant excitations between the electron and hole Landau levels. An angular dependence of the position of these Shubnikov–de Haas type oscillations shows directly the two-dimensional nature of the origin of a luminescence band.
Electroluminescence from the base of a GaAs/AlGaAs double heterojunction bipolar transistor
Authors: Levi A. F. J. , Hayes J. R. , Gossard A. C. , English J. H.
Published: July 2002

We have measured the electroluminescence spectrum of a double heterojunction bipolar transistor and found that a potential well formed at the base-collector junction acts as a preferential trap of low-energy electrons in the base. At high injection current densities the trap saturates. The subsequent buildup of carriers in the base changes the transistor turn-on characteristics.
18% efficient intrinsically passivated laser-processed silicon solar cells
Authors: Wood R. F. , Westbrook R. D. , Jellison G. E.
Published: July 2002

It is demonstrated that single-crystal silicon solar cells with efficiencies greater than 18% AM1.5 can be fabricated by glow discharge implantation and pulsed excimer laser annealing. A unique characteristic of these cells is that the surfaces are passivated intrinsically to give open circuit voltages as high as 640 mV without high-temperature oxidation. The simplicity of the processing is emphasized and the prospects for raising the efficiencies to even higher values are discussed.
Wavelength dependence of the excimer laser etching characteristics of a polymeric resist
Authors: Davis G. M. , Gower M. C.
Published: July 2002

The direct etching by excimer laser light of thin films of a polymeric resist, PMTM, has been studied at wavelengths of 157, 193, 222, and 248 nm. The energy absorbed per unit ablated volume near the etching threshold fluence indicates that a wavelength-dependent reaction mechanism causes ablation of the film. When laser wavelengths shorter than 222 nm are used at fluences close to threshold no evidence of heating of the polymer is observed. At 222 nm we observe thermal effects which cause melting and flow of the polymer while at 248 nm trapped pockets of volatile gas, which can subsequently explode, appear to be formed below the (molten) surface.
Influence of photodetachment on the switching characteristics of diffuse discharges containing oxygen
Authors: Schaefer G. , Hutcheson G. Z. , Schoenbach K. H. , Williams P. F.
Published: July 2002

Externally sustained discharges can be used as opening and closing switches in pulsed power systems. Admixtures of attachers allow rapid opening when the external sustaining source is terminated; however, they will increase losses in the closing phase. Photodetachment has been proposed as an additional control mechanism to overcome these losses. This letter presents measurements on photoionization sustained discharges in argon and nitrogen containing admixtures of oxygen under the influence of laser radiation. Strong changes of the voltage-current characteristics have been observed. The influence of parameters such as percentage of O2, laser power, laser pulse width, and circuit impedance is presented. Additionally, it is shown that photodetachment can enhance the stability of diffuse discharges.
Criteria for fluxon generation in long Josephson junctions by current pulses
Authors: Sakai S. , Samuelsen M. R.
Published: July 2002

In recent measurements in the time domain on the fluxon shape in long Josephson junctions the fluxons were generated by a current pulse injected into one end. We present here a perturbation treatment of the fluxon generation which we compare with numerical experiments. The agreement turns out to be excellent.
Abnormal-glow-discharge deposition of tungsten
Authors: Greenberg K. E.
Published: July 2002

α-tungsten films that adhere well to silicon dioxide were deposited using a dc abnormal-glow discharge through WF6, H2, and Ar. Film resistivities on the order of 30 μΩ cm and deposition rates as high as 200 Å/min were obtained without heating the substrate externally. X-ray diffraction, Auger electron spectroscopy, scanning electron microscopy, and transmission electron microscopy measurements indicate that electron scattering at the grain boundaries has limited the conductivity of the plasma-deposited films. Tungsten films having resistivities within a factor of two times that of bulk tungsten were produced with a two-step process utilizing plasma and conventional chemical vapor deposition.
Optically pumped mode-locked PbS1−xSex lasers
Authors: Vilhelmsson K. , Valk B. , Salour M. M. , Chu T. K.
Published: July 2002

We report the first optically pumped, mode-locked PbS1−xSex (x=0 and 0.3), semiconductor lasers with an external cavity. The materials lased at 4.1 and 4.8 μm, respectively. The output average power was estimated at about 1 mW when pumped with 250 mW average power from a mode-locked Nd-YAG laser. The demonstrated PbS1−xSex lasers were operated cw mode locked at a temperature of 8 K. Lasing was observed at temperatures up to 150 K. However, above 50 K it was necessary to chop the pump beam in order to maintain lasing. By varying the stoichiometric composition x, lasing throughout the wavelength span from 4 to 8 μm can be accomplished.
Surface emitting laser diode with bent waveguide
Authors: Ogura Mutsuo , Wu Ming-chiang , Hsin Wei , Whinnery John R. , Wang Shyh
Published: July 2002

A surface emitting laser diode (SELD) with a bent double heterostructure is fabricated on a grooved substrate. This SELD has a facet angle of 20 ° and lased at a threshold current of 120 mA. The external quantum efficiency is 33%. The far-field pattern has sharp peaks at 10 ° and 18 ° and wider emission bands between 25 ° and 45 °. Radiation loss by the bent waveguide is also estimated by the equivalent current source model.
Experimental determination of laser heated surface temperature distributions
Authors: Shaapur F. , Allen S. D.
Published: July 2002

A new scheme has been developed to measure laser-induced temperature distributions on solid surfaces, based on a first-order phase transition of an optically and thermally thin surface film where the transformed zone periphery marks the phase change isotherm of the film material. In order to test the technique, experimental measurements were made on a system for which analytical theoretical calculations and input parameters were available: steady-state 9.27-μm CO2 laser heating of semi-infinite SiC samples. The melting of polycrystalline films of NaCl (m.p. of 801 °C) and the eutectic mixture of NaCl-KCl (m.p. of 660 °C) spray deposited on the SiC surface served as the indicator. For a particular set of irradiation conditions, theoretical calculations were compared with experiment as a function of incident power.
Detection and measurement of local distortions in a semiconductor layered structure by convergent-beam electron diffraction
Authors: Maher D. M. , Fraser H. L. , Humphreys C. J. , Knoell R. V. , Bean J. C.
Published: July 2002

It is shown that local tetragonal distortion in a Si Ge0.05Si0.95 strained-layered structure can be detected and quantified from analyses of the high-order Laue zone lines which are present in the bright-field disks of [001] convergent-beam electron diffraction patterns. The detection of a tetragonal distortion is based on symmetry arguments, whereas the quantification requires a detailed analysis which is based on computer simulated patterns, assuming a kinematical approximation of the scattering process and including surface relaxation. The result of this quantitative analysis is in good agreement with the tetragonal distortion which is predicted by isotropic elasticity theory.
Quantum well surface-plasmon oscillator
Authors: Palmer A. Jay
Published: July 2002

We identify a new type of electromagnetic oscillator which utilizes the negative differential conductivity of a quantum well tunnel junction to drive surface-plasmon oscillations on the outside boundaries of the junction. We calculate the complex propagation constant as a function of frequency for the antisymmetric gap-mode surface plasmon on a state of the art GaAs/AlGaAs double-barrier tunnel junction. The surface-plasmon gain coefficient is found to be positive for frequencies up to 5 GHz. Single-mode oscillation of the surface plasmon at frequencies near 5 GHz is predicted on a tunnel junction with a lateral dimension equal to about 150 μm.
Microfabrication of molecular scale microstructures
Authors: Deckman H. W. , Abeles B. , Dunsmuir J. H. , Roxlo C. B.
Published: July 2002

Microfabrication techniques have been used to prepare a new class of molecular scale microporous materials. These materials are formed by chemically etching slots into alternate layers of a lithographically exposed amorphous superlattice cross section. The slot width is accurately controllable from 10 Å to more than 500 Å.
V-grooved inner-stripe laser diodes on a p-type substrate operating over 100 mW at 1.5 μm wavelength
Authors: Horikawa H. , Oshiba S. , Matoba A. , Kawai Y.
Published: July 2002

An output power of 110 mW cw has been achieved with a V-grooved inner-stripe laser diode on a p-type substrate emitting at 1.5 μm wavelength. Output powers over 100 mW could be obtained by determining the appropriate cavity length and the reflectivity of the front mirror facet, a parameter that depends heavily on output power. These lasers were fabricated without introducing anti-meltback layers. The meltback problem of the active layer was overcome using a low-temperature liquid phase epitaxy technique.
Negative differential resistance of double barrier diodes at zero bias
Authors: Liu H. C. , Coon D. D.
Published: July 2002

Zero-bias dc negative differential resistance of double barrier quantum well diodes in an ac field is explained. A physical picture is presented in terms of resonant tunneling phenomena. An adiabatic approximation is used limiting the present approach to applied ac frequencies lower than the ultimate device speed which is of the order of 1 THz for AlxGa1−x As/GaAs devices.
Supermode selection in diffraction-coupled semiconductor laser arrays
Authors: Wilcox J. Z. , Jansen M. , Yang J. J. , Ou S. S. , Sergant M. , Simmons W. W.
Published: July 2002

A modal gain analysis for diode laser diffraction-coupled arrays which takes into account evanescent coupling in the waveguide section of an array is presented. Theoretical predictions are supported by comparisons of patterns from typical double-heterostructure and large-optical-cavity lasers.
Model calculations of fields related to near-field optical scanning of a silver strip on a glass substrate
Authors: Marx Egon , Teague E. Clayton
Published: July 2002

The vertical energy flux density for the electromagnetic fields near the surface of a silver strip on a glass substrate is computed for an incident plane wave to aid in the measurement of the width of the strip. The dimensions of the strip cross section, e.g., 300 nm by 100 nm, are a fraction of the wavelength of the incident light , 632.8 nm. The flux 1 nm above the surface shows sharp spikes at the edges of the strip. The features of the fields near the surface could be used for accurate determination of the width of the strip by measurements up to about 30 nm above the strip. The effects of other variables are also shown in the figures.
Characteristics of annealed p/n junctions between GaAs and Si (100)
Authors: Unlu M. S. , Munns G. , Chen J. , Won T. , Unlu H. , Morkoç H. , Radhakrishnan G. , Katz J. , Verret D.
Published: July 2002

Electrical characteristics of GaAs( p)/Si(n) interface were determined from capacitance-voltage (C-V), current-voltage (I-V), and secondary ion mass spectroscopy (SIMS) measurements and compared to those on GaAs( p) epitaxial layes on GaAs(n) substrates. The comparison was made between the junctions as grown and after an anneal at 850 °C for 20 min in 10% forming gas under an As overpressure. For the GaAs/Si junction the ideality factor changed from 2 or larger to 1.5 and the apparent intercept voltage changed from 2.5 to 1.3 V after annealing. For the GaAs homojunction, the intercept voltage increased from 1.1 to 1.3 V. In addition, the excess current in the forward and reverse bias conditions dropped drastically in the heterojunction. No movement of the metallurgical junction was discernible to within the resolution capability of SIMS. The junction properties obtained by annealing suggest an atomic restructuring of the Si(100) interface during growth or annealing. These new results raise the possibility that the GaAs/Si interface can be made into an electrically viable junction and incorporated into active devices.
Evidence of interfacial melting during pulsed laser irradiation of Ni2Si on Si
Authors: Grimaldi M. G. , Priolo F. , Baeri P. , Rimini E. , Cullis A. G. , Chew N. G.
Published: July 2002

Interfacial melting of thin (∼65 nm) Ni2Si films thermally grown on 〈111〉 Si has been observed after pulsed laser irradiation. Low-energy implanted Bi was used as a marker to detect surface melting. Rutherford backscattering spectrometry has shown that the energy density threshold for interfacial mixing was lower than the one at which changes in the Bi profile occurred. Cross-section transmission electron microscopy performed on those samples has shown the existence of a reacted layer, ∼20 nm, at the silicide/silicon interface while no change in the structure of the outermost 50 nm was observed. The reacted layer formed sharp interfaces with both the underlying silicon and the silicide. Interfacial melting has been related to the presence in the phase diagram of an eutectic between the compound and the pure silicon.
Optical multistability and phase conjugation fidelity in a high-birefringence optical fiber
Authors: Trillo S. , Wabnitz S.
Published: July 2002

We derive an exact solution including pump depletion of collinear four-wave mixing in a birefringent Kerr medium. We find mirrorless optical multistability for a linearly polarized backscattered signal with orthogonal pumps at cw power levels using a few meters of polarization-maintaining silica fiber. An expression for the nonlinear phase distortion of ideal phase conjugation is obtained, showing a fundamental limitation to generating time-reversed wave fronts using Kerr nonlinearity.
Negative transconductance resonant tunneling field-effect transistor
Authors: Capasso Federico , Sen Susanta , Cho Alfred Y.
Published: July 2002

The operation of a new resonant tunneling transistor is reported. The field-effect transistorlike structure contains a double barrier in the gate. Resonant tunneling through the gate can be quenched by varying the drain or gate bias, leading respectively to negative conductance and negative transconductance in the drain current. Dramatic differences in the negative conductance and transconductance regions of the current-voltage characteristic are observed for opposite bias polarities. This different behavior directly demonstrates the role, in controlling resonant tunneling, of the electron accumulation and depletion layers in the channel adjacent to the double barrier.
Thulium barium copper oxide: A 90-K superconductor with a potential 1-MG upper critical field
Authors: Neumeier J. J. , Dalichaouch Y. , Ferreira J. M. , Hake R. R. , Lee B. W. , Maple M. B. , Torikachvili M. S. , Yang K. N. , Zhou H.
Published: July 2002

Upper critical magnetic fields Hc2(T) have been measured resistively for the high-temperature (Tc≳90 K) superconductors TmBa2Cu3O9−δ and YBa2Cu3O9−δ in applied magnetic fields H up to 9 T. If H at the resistive midpoint is defined as Hc2, then the upper critical field slopes (dHc2/dT)Tc at the superconducting transition temperatures Tc are respectively −2.8 and −1.1 T/K. The Tm-specimen initial slope is apparently the highest yet reported for 90 K superconductors. Extrapolations from this slope, based on standard, three-dimensional, dirty-limit Werthamer, Helfand, Hohenberg, and Maki theory, yield (1) zero-temperature upper critical fields Hc2(T=0)=99–175 T, where the range is determined by the degree of paramagnetic limitation; (2) Hc2(T=77 K)≊36 T, the upper critical field at the boiling point of liquid nitrogen.
Electron-hole recombination lifetimes in a quasi-zero-dimensional electron system in CdSxSe1−x
Authors: Shum Kai , Tang G. C. , Junnarkar Mahesh R. , Alfano R. R.
Published: July 2002

The recombination lifetimes for the radial and angular quantum number conserved 1S–1S and 1P–1P transitions from three-dimensionally confined electrons in CdSxSe1−x were measured by time-resolved photoluminescence (PL). The assignment of the observed transitions was supported by calculations of eigen energy levels and squared matrix element ratio for these transitions as well as well-resolved PL peaks arising from 1S–1S and 1P–1P transitions.
Field ion microscopy and imaging atom-probe mass spectroscopy of superconducting YBa2Cu3O7−x
Authors: Kellogg G. L. , Brenner S. S.
Published: July 2002

The structure and composition of the superconducting oxide YBa2Cu3O7−x have been examined in atomic detail by field ion microscopy and imaging atom-probe mass spectroscopy. The field ion samples were prepared from hot-pressed disks of the oxide powders. Atomic resolution images were obtained with either argon or hydrogen as the imaging gas. Individual layers of atoms were observed which could be field evaporated in a uniform, layer-by-layer manner. Imaging atom-probe analysis of the field ion tips indicated a metal composition which varied noticeably from sample to sample and an oxygen concentration which was consistently much too low.
Simultaneous delivery of neodymium:yttrium aluminum garnet and CO laser beams by a single As-S glass fiber
Authors: Arai Tsunenori , Kikuchi Makoto , Saito Mitsunori , Takizawa Masaya
Published: July 2002

The simultaneous energy delivery of neodymium:yttrium aluminum garnet (Nd:YAG) and CO laser beams by a single As-S glass fiber is described. The transmission loss in the fiber of a Nd:YAG laser beam was 2.6 dB/m under the energy delivery. A fiber output of 14 W was obtained at an approximately 1:1 power ratio from these lasers with a 400-μm core diameter fiber.
Electroabsorption in AlGaAs/GaAs multiple quantum well structures grown on a GaP transparent substrate
Authors: Lee H. C. , Dzurko K. M. , Dapkus P. D. , Garmire E.
Published: July 2002

The first measurements of electroabsorption in AlGaAs/GaAs multiple quantum wells grown on transparent GaP substrates are reported. High quality quantum well materials with sharp, well defined excitonic resonances are grown by employing a GaAsP intermediate layer between the quantum wells and the substrate. Good surface morphology and abrupt interfaces have been achieved with etch pit densities of 4×105 cm−2. A 7.5-meV shift of the absorption edge to lower energies is observed for field strengths of 8×104 V/cm. These structures are well suited for photonic switch array fabrication.
Photoresistivity and photo-Hall-effect topography on semi-insulating GaAs wafers
Authors: Look D. C. , Pimentel E.
Published: July 2002

By placing a semi-insulating GaAs wafer on a flat, rare-earth magnet, and irradiating the surface with two perpendicular slits of light to form a Greek cross configuration, it is possible to perform photoresistivity and photo-Hall-effect topography on the wafer. The technique is nondestructive in that the contacts are tiny, removable In dots which are placed only on the periphery. By varying the wavelength of the light, selective centers, such as EL2, can be mapped. We compare a 1.1-μm, photoexcited electron concentration map with a quantitative EL2 map on a 3-in. undoped, liquid-encapsulated Czochralski wafer.
Narrow two-dimensional electron gas channels in GaAs/AlGaAs sidewall interfaces by selective growth
Authors: Asai Hiromitsu , Yamada Syoji , Fukui Takashi
Published: July 2002

Narrow two-dimensional electron gas (2DEG) channels have been fabricated for the first time on GaAs/Al0.3Ga0.7As sidewall interfaces by selective growth using metalorganic chemical vapor deposition (MOCVD). The 4-μm-wide 2DEG channels are formed on the {111}A facets by controlling the facet formation in the selective growth layers only through MOCVD growth conditions. The angular dependence of Shubnikov–de Haas oscillations has confirmed the existence of 2DEG on the {111}A facets.
Low-temperature annealing characteristics of the lightly arsenic-doped polycrystalline silicon
Authors: Yang Chaosu
Published: July 2002

Low-temperature annealing characteristics of the lightly arsenic-doped polycrystalline silicon have been studied. It was found that, when the silicon dioxide layer covered on the polycrystalline silicon was moved after high-temperature processing and the polycrystalline silicon was annealed at low temperature, the conductance of the films would increase from one to four orders in magnitude. A possible mechanism for the interaction of the free oxygen in SiO2 with grain boundary is discussed.
Efficient 30 mW grating surface-emitting lasers
Authors: Evans G. A. , Carlson N. W. , Hammer J. M. , Lurie M. , Butler J. K. , Palfrey S. L. , Carr L. A. , Hawrylo F. Z. , James E. A. , Kaiser C. J. , Kirk J. B. , Reichert W. F.
Published: July 2002

A surface-emitting AlGaAs second-order distributed Bragg reflector laser using a graded index separate confinement heterostructure with a single quantum well has been fabricated. The surface emitted power is in excess of 30 mW with an external differential quantum efficiency of 20%. These values approach the performance of present commercially available edge-emitting diode lasers. Under severe current modulation conditions, the stable single longitudinal mode had nearly 30 dB wavelength sidemode rejection, and a near diffraction limited 0.51° full width half-power beam divergence of the single-lobe far-field pattern.
Optical bleaching in an epitaxial (Al,Ga)As Fabry–Perot resonator
Authors: Gourley P. L. , Drummond T. J.
Published: July 2002

We report studies of bleaching action in an epitaxial Fabry–Perot resonator grown by molecular beam epitaxy. These new structures hold exciting potential for surface-emitting lasers, optical switches, and optical modulators. The structure consisted of a multiple quantum well of AlGaAs and GaAs surrounded by two quarter-wave high reflectors with alternating layers of AlAs and AlGaAs. The spectral and temporal transmittances under intense optical pulses are reported here. The spectral data provide simultaneous information on nonlinear changes in both real and imaginary parts of the refractive index. The temporal data show that the transmittance can be switched up to three orders of magnitude in less than the experimental resolution of 300 ps, and provide evidence for time compression of optical pulses.