bluuebunny/crossref_metadata_2025 huggingface dataset:
Semi-insulating C-doped GaN and high-mobility AlGaN/GaN heterostructures grown by ammonia molecular beam epitaxy
Authors: Webb J. B. , Tang H. , Rolfe S. , Bardwell J. A.
Published: July 2002

A method of growing semi-insulating GaN epilayers by ammonia molecular beam epitaxy through intentional doping with carbon is reported. Thick GaN layers of high resistivity are an important element in GaN-based heterostructure field-effect transistors. A methane ion source was used as the carbon dopant source. The cracking of the methane gas by the ion source was found to be the key to the effective incorporation of carbon. High-quality C-doped GaN layers with resistivities greater than 106 Ω cm have been grown with high reproducibility and reliability. AlGaN/GaN heterostructures grown on the C-doped semi-insulating GaN-based layers exhibited a high-mobility two-dimensional electron gas at the heterointerface, with room-temperature mobilities typically between 1000 and 1200 cm2/V s, and liquid-nitrogen-temperature mobilities up to 5660 cm2/V s. The carrier density was almost constant, with less than 3% change over the measured temperature range.
Observation of valence band electron emission from n-type silicon field emitter arrays
Authors: Ding Meng , Kim Han , Akinwande Akintunde I.
Published: July 2002

Electron emission from the valence band of n-type Si field emitter arrays is reported. High electrostatic field at the surface of Si was achieved by reducing the radius of the emitter tip. Using oxidation sharpening, 1 μm aperture polycrystalline Si gate, n-type Si field emitter arrays with small tip radius (∼10 nm) were fabricated. Three distinct emission regions were observed: conduction band emission at low gate voltages, saturated current emission from the conduction band at intermediate voltages, and valence band plus conduction band emission at high gate voltages. Emission currents at low and high voltages obey the Fowler–Nordheim theory. The ratio of the slopes of the corresponding Fowler–Nordheim fits for these two regions is 1.495 which is in close agreement with the theoretical value of 1.445.
Design and performance of a space based high temperature superconductivity experiment
Authors: Polturak E. , Koren G. , Ayalon M. , Flohr I. , Waller R. , Guelman M.
Published: July 2002

We describe an experiment aimed at testing the long term survivability of high temperature superconductors (HTSCs) in space. The experiment consists of a thin YBCO film integrated with a cryocooler. The experiment was launched into orbit aboard the TECHSAT II satellite in July 1998. Since then, the superconducting film was tested periodically, and has shown little or no degradation of its properties. Our experiment should provide the basic long term survivability data needed for advanced applications of HTSC in space, in the field of satellite communications.
Polaron dissociation in conducting polymers by high electric fields
Authors: Rakhmanova S. V. , Conwell E. M.
Published: July 2002

It has been accepted that addition of an electron or hole to a conducting polymer chain causes the chain to distort, creating a large polaron. We show, by numerical solution of the coupled equations for the electron wave functions and the lattice displacements on a polyacetylene chain with an extra electron, that the polaron does not form in electric fields ⩾6×104 V/cm. In fields ⩾106 V/cm, an already formed polaron will be dissociated. Implications for current versus voltage of a light-emitting diode at high fields will be discussed.
Room-temperature optically pumped CdHgTe vertical-cavity surface-emitting laser for the 1.5 μm range
Authors: Roux C. , Hadji E. , Pautrat J.-L.
Published: July 2002

The demonstration of a room-temperature CdHgTe surface-emitting laser is reported. A planar heterostructure with two high-index contrast dielectric mirrors deposited after growth and after etching off the substrate has been realized. The emission wavelength of 1.57 μm is nearly independent of temperature (dλ/dT=0.02 nm/K) and the multimode linewidth is 10 meV. The pulsed threshold power is as low as 21 mW for a 300 μm spot. Lasing is observed up to 300 K and the peak output power exceeds 700 mW
Diameter modification of silicon nanowires by ambient gas
Authors: Zhang Y. F. , Tang Y. H. , Peng H. Y. , Wang N. , Lee C. S. , Bello I. , Lee S. T.
Published: July 2002

Si nanowires (SINWs) with different diameters have been synthesized by laser ablation in different ambient gases. SINWs with the diameter distribution peaks at ∼13.2 and ∼9.5 nm have been obtained respectively in He and Ar (5% H2). SINWs produced in N2 had the smallest peak diameter at 6 nm, and are mixed in with some spherical particles with diameters ranging from ∼9 nm to several hundreds nm. Elements from the ambient gas were not detected in the SINWs. SINWs produced in Ar(5% H2) and N2 atmospheres exhibited photoluminescence and spectral blue-shift with diameter reduction, which are attributable to two-dimensional quantum confinement effects in crystalline nanowires.
Multiwalled carbon nanotubes growth in anodic alumina nanoholes
Authors: Iwasaki Tatsuya , Motoi Taiko , Den Tohru
Published: July 2002

Carbon nanotubes (CNTs), standing perpendicularly to a substrate with an electrode, were fabricated by thermal catalytic decomposition of ethylene from Co particles electrochemically embedded at the bottom of anodic alumina nanoholes. The thermal durability of the alumina nanoholes for the CNTs growth process was achieved by using Nb as an underlying electrode. The CNTs were electrically connected to the electrode through the conductive paths, which were formed at the bottom of alumina nanoholes by Nb ion migration from the underlying electrode during anodization.
Thermomagnetic writing on 29 Gbit/in.2 patterned magnetic media
Authors: Haginoya Chiseki , Koike Kazuyuki , Hirayama Yoshiyuki , Yamamoto Jiro , Ishibashi Masayoshi , Kitakami Osamu , Shimada Yutaka
Published: July 2002

We have demonstrated a thermomagnetic writing on a 29 Gbit/in.2 patterned medium with perpendicular magnetic anisotropy, by using Joule heat produced by a current flowing between a magnetic dot in the medium and a magnetic force microscope tip. The possible application of this thermomagnetic method to future patterned media is also discussed.
Third-order optical nonlinearity in ZnO microcrystallite thin films
Authors: Zhang Weili , Wang H. , Wong K. S. , Tang Z. K. , Wong G. K. L. , Jain Ravinder
Published: July 2002

We report the measurement of third-order optical nonlinearity (χ(3)) of ZnO microcrystallite thin films near the excitonic resonance at various temperatures using the femtosecond degenerate four-wave-mixing technique. The measured χ(3) response times are 270, 240, and 160 fs at 4.2 K, 77 K, and room temperature, respectively. The values of χ(3) range from 10−7 to 10−4 esu. The maximum values of χ(3) always correspond to the absorption peaks at different temperatures. Room-temperature excitonic enhancement of χ(3) is also observed.
Room-temperature electroluminescence from erbium-doped porous silicon
Authors: Lopez Herman A. , Fauchet Philippe M.
Published: July 2002

We demonstrate stable room-temperature electroluminescence (EL) at 1.54 μm from erbium-doped porous silicon devices under both forward- and reverse-bias conditions. Erbium was infiltrated in the pores (⩽1019 cm−3) by cathodic electrochemical migration of the ions followed by high-temperature annealing (950–1100 °C) in an oxygen and nitrogen environment. The devices exhibit an exponential EL dependence in both bias conditions as a function of input power. In reverse bias, the external quantum efficiency reaches 0.01%. The EL intensity decreases by a factor of 24 for reverse bias and 2.6 for forward bias when the temperature increases from 240 to 300 K. The different device characteristics in forward and reverse biases suggest that different excitation mechanisms are responsible for EL.
Is sulfur a donor in diamond?
Authors: Kalish R. , Reznik A. , Uzan-Saguy C. , Cytermann C.
Published: July 2002

Homoepitaxial diamond layers grown by chemical-vapor deposition in the presence of H2S, which were published to exhibit n-type conductivity, are carefully analyzed both electrically and structurally. Hall-effect measurements as a function of temperature clearly show the samples to exhibit p-type conduction, with an activation energy, carrier concentrations, and mobilities which very much resemble those of B-doped p-type diamond. Secondary-ion-mass spectroscopy confirms that indeed the samples, previously claimed to be n type due to a donor state attributed to sulfur, contain enough unintentional boron to explain the observed p-type features.
Structural characterization and strain relaxation in porous GaN layers
Authors: Mynbaeva M. , Titkov A. , Kryganovskii A. , Ratnikov V. , Mynbaev K. , Huhtinen H. , Laiho R. , Dmitriev V.
Published: July 2002

Surface and subsurface structures of porous GaN prepared by anodizing epitaxial GaN layers grown on SiC substrates are investigated by atomic-force microscopy. Comparison of the images of the porous GaN surfaces with those taken on planes cleft perpendicular to the surface shows that the pores are formed along the boundaries of columnar structures of the original GaN films. X-ray investigations show that the porous GaN has less residual stresses than the initial GaN epitaxial layers. Use of porous GaN as a buffer layer for growth of low-stress GaN is proposed.
Time-resolved electroabsorption measurement of the electron velocity-field characteristic in GaN
Authors: Wraback M. , Shen H. , Carrano J. C. , Li T. , Campbell J. C. , Schurman M. J. , Ferguson I. T.
Published: July 2002

A femtosecond optically detected time-of-flight technique that monitors the change in the electroabsorption associated with the transport of photogenerated carriers in a GaN p–i–n diode has been used to determine the room-temperature electron transit time and steady-state velocity as a function of electric field. The peak electron velocity of 1.9×107 cm/s, corresponding to a transit time of 2.5 ps, is attained at 225 kV/cm. The shape of the velocity-field characteristic is in qualitative agreement with theoretical predictions.
Heteroepitaxial growth of cubic GaN on Si(001) coated with thin flat SiC by plasma-assisted molecular-beam epitaxy
Authors: Wang D. , Hiroyama Y. , Tamura M. , Ichikawa M. , Yoshida S.
Published: July 2002

High-quality cubic GaN films were grown on Si(001) coated with flat ultrathin SiC under different Ga/N flux ratios. The 2.5-nm-thick cubic SiC film proved to be an effective buffer layer for cubic GaN growth on Si(001). Under a Ga-rich condition, films with local atomically flat surfaces were obtained, and the x-ray diffraction full-width at half maximum of (002) peak was 19 min for a 0.82-μm-thick film. The reduced SiC surface roughness decreased the defect density in the GaN epilayers. Under a N-rich condition, the GaN films showed statistical roughening of the surface and a characteristic columnar structure. Under the Ga-rich condition, the columns grew up and then laterally coalesced, so that an atomically flat surface with flat areas in size from 0.05 to 0.40 μm was formed.
Magnetoresistance in n- and p-type Ag2Te: Mechanisms and applications
Authors: Schnyders H. S. , Saboungi M.-L. , Rosenbaum T. F.
Published: July 2002

We compare the large magnetoresistive response of slightly nonstoichiometric Ag2±δTe for a wide range of hole (p⩽8×1017 cm−3) and electron (n⩽4×1018 cm−3) carrier densities. In the p-type material alone, a characteristic peak in the resistivity ρ(T,H) is dramatically enhanced and moves to higher temperature with increasing magnetic field, resulting in a high field (H∼5 T) magnetoresistance that is sizeable even at room temperature. By contrast, n-type specimens are geared for low-field (H<0.1 T) applications because of a striking linear field dependence of the magnetoresistance that appears to be restricted to the Ag-rich materials.
Operating lifetime of phosphorescent organic light emitting devices
Authors: Burrows P. E. , Forrest S. R. , Zhou T. X. , Michalski L.
Published: July 2002

We investigate the continuous operating lifetime of organic light emitting devices (OLEDs) using the phosphorescent dopant, 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum (II) as the light emitting molecule. We characterize devices based on two different electron transporting hosts: tris-(8-hydroxyquinoline) aluminum and 4,4′-N,N′-dicarbazolyl-biphenyl (CBP). The OLEDs lose ∼25% of their luminance in the first 50 h of operation, followed by extremely slow degradation with negligible growth of dark spots. The device lifetime of CBP-based phosphorescent OLEDs projected to 50% initial brightness is >107 h at a mean current density of 10 mA/cm2 under 50% duty cycle pulsed operation. These extremely long lifetimes are speculated to be an intrinsic property of electrophosphorescent OLEDs, where radiative phosphors significantly shorten the lifetime of potentially reactive triplet states in the conductive host material.
Influence of Si doping on optical characteristics of cubic GaN grown on (001) GaAs substrates
Authors: Li Z. Q. , Chen H. , Liu H. F. , Wan L. , Zhang M. H. , Huang Q. , Zhou J. M. , Yang N. , Tao K. , Han Y. J. , Luo Y.
Published: July 2002

The photoluminescence (PL) properties of Si-doped cubic GaN with different carrier concentrations were investigated at room temperature. The epilayers were grown on GaAs (001) by radio-frequency molecular-beam epitaxy. It was found that when the carrier concentration is increased from 5×1015 to 2×1018 cm−3, the PL peak shifted towards low energy, from 3.246 to 3.227 eV, and the PL linewidth increased from 77.1 to 121 meV. The PL peak shift is explained by the band gap narrowing effect due to the high doping concentration. The PL linewidth includes two parts: one is doping concentration independent, which is caused by the imperfection of samples and phonon scattering; the other is doping concentration dependent. We assign the second part to the broadening by the microscopic fluctuation of the doping concentration. The experimental measurements are in good agreement with the model.
New proof and generalizations of the Demidowitsch–Schneider criterion
Authors: Giraldo Luis , Gascón Francisco G.
Published: July 2002

A new proof of the Demidowitsch–Schneider criterion on the absence of closed trajectories of R3 is given. The new proof is generalized and applied in several directions: Rn vector fields (n>3),… .
Influence of the quantum-well thickness on the radiative recombination of InGaN/GaN quantum well structures
Authors: Bai J. , Wang T. , Sakai S.
Published: July 2002

Temperature-dependent photoluminescence (PL) measurements are performed on In0.23Ga0.77N/GaN single-quantum-well structures with different well thickness. Based on a band-tail model, the exciton localization effect is studied. The exciton localization effect is enhanced by increasing quantum-well thickness up to 2.5 nm. If the quantum-well thickness is further increased to above 2.5 nm, the exciton localization effect becomes weak. Finally, when the quantum-well thickness is increased to 5 nm, the exciton localization effect cannot be observed. In addition, the PL intensity decreases monotonically with increasing the quantum-well thickness. In connection with an excitation-power dependent PL measurement, the result of the quantum-well thickness dependent PL intensity can be attributed to quantum confined Stark effect, which becomes particularly strong in the wide quantum-well structure. Based on our optical investigation, the presented article indicates that the emission mechanism is dominated by the exciton localization effect in the thin quantum-well structures, while the quantum confined Stark effect dominates the radiative recombination in the wide quantum-well structures. Since understanding the emission mechanism is very important for further improving the performance of an InGaN/GaN-based optical device, the presented results in this article should be highly emphasized.
Phonon lifetimes in bulk AlN and their temperature dependence
Authors: Kuball M. , Hayes J. M. , Shi Ying , Edgar J. H.
Published: July 2002

We report on the Raman analysis of the phonon lifetimes of the A1(LO) (longitudinal optical) and E2(high) phonons in bulk AlN crystals and their temperature dependence from 10 to 1275 K. Our experimental results show that amongst the various possible decay channels, the A1(LO) phonons decay primarily into two phonons of equal energy (Klemens model), most likely longitudinal-acoustic phonons, whereas the E2(high) phonon decays asymmetrically into a high-energy and a low-energy phonon. Possible decay channels of the E2(high) phonon have been shown to include combinations of E2(low) and acoustic phonons. Phonon lifetimes of the A1(LO) phonon and the E2(high) phonon of 0.75 and 2.9 ps, respectively, were measured at 10 K.
On the performance of diagrammatic complete active space perturbation theory
Authors: Choe Yoong-Kee , Finley James P. , Nakano Haruyuki , Hirao Kimihiko
Published: July 2002

Excited states of C2, H2O, CO, and formamide are studied to illustrate the performance and accuracy of second-order diagrammatic complete active space perturbation theory (D-CASPT2) [J. Chem. Phys. 108, 1081 (1998)]. Comparisons are made with other ab initio methods and also full configuration interaction (FCI) calculations or experiment. Excitation energies computed by the D-CASPT2 method are quite accurate showing an average deviation of 0.1 eV from the FCI values for C2 and H2O. The CO and formamide excitation energies yield average deviations of 0.1 and 0.2 eV from experiment, respectively. The computational cost of this method is reduced to a great extent compared to the MRMP method.
A discrete-velocity, stationary Wigner equation
Authors: Arnold Anton , Lange Horst , Zweifel Paul F.
Published: July 2002

This paper is concerned with the one-dimensional stationary linear Wigner equation, a kinetic formulation of quantum mechanics. Specifically, we analyze the well-posedness of the boundary value problem on a slab of the phase space with given inflow data for a discrete-velocity model. We find that the problem is uniquely solvable if zero is not a discrete velocity. Otherwise one obtains a differential-algebraic equation of index 2 and, hence, the inflow data make the system overdetermined.
Size-dependent electrical behavior of spatially inhomogeneous barrier height regions on silicon
Authors: Rossi Robert C. , Tan Ming X. , Lewis Nathan S.
Published: July 2002

A series of ordered, periodic arrays of low barrier height n-Si/Ni nanometer-scale contacts interspersed among high barrier height n-Si/liquid contacts were prepared by evaporating Ni through bilayers of close-packed latex spheres deposited on n-Si. By varying the diameter of the spheres from 174 to 1530 nm, geometrically self-similar Si/Ni structures were produced having triangular Si/Ni features ranging from approximately 100 to 800 nm on a side. The resulting Si surfaces were used as electrodes in methanolic electrochemical cells containing LiClO4 and 1,1′-dimethylferrocene+/o. The dark current density–voltage properties of the resulting mixed barrier height contacts were strongly dependent on the size of the low barrier height contact regions even though the fraction of the Si surface covered by Ni remained constant.
Sources and control of instrumental drift in the surface forces apparatus
Authors: Heuberger M. , Zäch M. , Spencer N. D.
Published: July 2002

Instrumental drift in the surface forces apparatus (SFA) has been carefully scrutinized. A diversity of different contributions with different characteristic time constants could be distinguished. The face seal of the functional attachment was identified as a potential weak point in the mechanical loop of the instrument. We compared drift in three different design variants and found that the drift rate may vary over four orders of magnitude. We believe that the presented results are applicable to a number of different SFA types.
Concerted electron and proton transfer: Transition from nonadiabatic to adiabatic proton tunneling
Authors: Georgievskii Yuri , Stuchebrukhov Alexei A.
Published: July 2002

A concerted electron–proton transfer reaction is discussed, in which proton tunneling occurs simultaneously with electronic transition. It is assumed that the potential in which the proton moves is formed by two electronic states, which in the absence of their interaction would cross in the region between the two minima of the proton adiabatic potential. The proton tunneling between the two wells is, therefore, coupled to a switch between the two electronic states. The later occurs only when the proton is in the tunneling region under the barrier. A simple analytical expression for the tunneling matrix element TDA is derived, which is uniformly correct for small and large values of the electronic coupling. For small electronic coupling our expression coincides with that obtained in the nonadiabatic theory of proton-coupled electron transfer reactions. For large electronic coupling the expression is reduced to that obtained in the Born–Oppenheimer approximation. The transition from nonadiabatic to adiabatic tunneling is governed by the magnitude of the Landau–Zener parameter defined for the tunneling process. The obtained result is discussed in the context of the proton tunneling time.
Influence of viscoelasticity and interfacial slip on acoustic wave sensors
Authors: McHale G. , Lücklum R. , Newton M. I. , Cowen J. A.
Published: July 2002

Acoustic wave devices with shear horizontal displacements, such as quartz crystal microbalances (QCM) and shear horizontally polarized surface acoustic wave devices, provide sensitive probes of changes at solid–solid and solid–liquid interfaces. Increasingly the surfaces of acoustic wave devices are being chemically or physically modified to alter surface adhesion or coated with one or more layers to amplify their response to any change of mass or material properties. In this work, we describe a model that provides a unified view of the modification in the shear motion in acoustic wave systems by multiple finite thickness loadings of viscoelastic fluids. This model encompasses QCM and other classes of acoustic wave devices based on a shear motion of the substrate surface and is also valid whether the coating film has a liquid or solid character. As a specific example, the transition of a coating from liquid to solid is modeled using a single relaxation time Maxwell model. The correspondence between parameters from this physical model and parameters from alternative acoustic impedance models is explicitly given. The characteristic changes in QCM frequency and attenuation as a function of thickness are illustrated for a single layer device as the coating is varied from liquid like to that of an amorphous solid. Results for a double layer structure are explicitly given and the extension of the physical model to multiple layers is described. An advantage of this physical approach to modeling the response of acoustic wave devices to multilayer films is that it provides a basis for considering how interfacial slip boundary conditions might be incorporated into the acoustic impedance used within circuit models of acoustic wave devices. Explicit results are derived for interfacial slip occurring at the substrate–first layer interface using a single real slip parameter, s, which has inverse dimensions of impedance. In terms of acoustic impedance, such interfacial slip acts as a single-loop negative feedback. It is suggested that these results can also be viewed as arising from a double-layer model with an infinitesimally thin slip layer which gives rise to a modified acoustic load of the second layer. Finally, the difficulties with defining appropriate slip boundary conditions between any two successive layers in a multilayer device are outlined from a physical point of view.
Transport phenomena in rotating rarefied gases
Authors: Sharipov Felix , Gramani Cumin Liliana M. , Kremer Gilberto M.
Published: July 2002

The mass, heat, and momentum transfer through a rarefied gas confined between two coaxial rotating cylinders is studied. The evaporation and condensation of the gas on the cylinder surfaces is assumed. The state of equilibrium is perturbed by small deviations of pressure, temperature, and angular velocity on the inner cylinder. The S model of the Boltzmann equation has been solved numerically by the discrete velocity method. The fields of the pressure, temperature, velocity, heat flux vector, and shear stress are calculated in the wide range of the Knudsen number. Further, the Onsager reciprocity relations between the cross effects are verified.
Theory and simulation of ion noise in microwave tubes
Authors: Manheimer W. M. , Freund H. P. , Levush B. , Antonsen T. M.
Published: July 2002

Since there is always some ambient gas in electron beam devices, background ionization is ubiquitous. For long pulse times, the electrostatic potentials associated with this ionization can reach significant levels and give rise to such observed phenomena as phase noise in microwave tubes. This noise is usually associated with the motion of ions in the device; therefore, it is called ion noise. It often manifests itself as a slow phase fluctuation on the output signal. Observations of noise in microwave tubes such as coupled-cavity traveling wave tubes (CC-TWTs) and klystrons have been discussed in the literature. In this paper, a hybrid model is discussed in which the electron beam is described by the beam envelope equation, and the ions generated by beam ionization are treated as discrete particles using the one-dimensional equations of motion. The theoretical model provides good qualitative as well as reasonable quantitative insight into the origin of ion noise phenomena. The numerical results indicate that the model reproduces the salient features of the phase oscillations observed experimentally. That is, the scaling of the frequency of the phase oscillations with gas pressure in the device and the sensitive dependence of the phase oscillations on the focusing magnetic field. Two distinct time scales are observed in simulation. The fastest time scale oscillation is related to the bounce motion of ions in the axial potential wells formed by the scalloping of the electron beam. Slower sawtooth oscillations are observed to correlate with the well-to-well interactions induced by the ion coupling to the electron equilibrium. These oscillations are also correlated with ion dumping to the cathode or collector. As a practical matter, simulations indicate that the low frequency oscillations can be reduced significantly by using a well-matched electron beam propagating from the electron gun into the interaction circuit.
Impedance of a ferromagnetic sandwich strip
Authors: Sukstanskii A. , Korenivski V. , Gromov A.
Published: July 2002

A theoretical approach for calculating the impedance of a three-layer sandwich, consisting of two metallic ferromagnetic layers separated by a non-magnetic conductive layer, is presented. The Maxwell equations for the electromagnetic field coupled with the Landau–Lifshitz equations for the magnetization dynamics are solved, which enables one to describe the system over a wide frequency range, including the ferromagnetic resonance. Two sandwich strip structures are analyzed, both having thickness much less than the width, and the width much less than the length: a “closed” structure with the magnetic film closing at the edges along the width, and an “open” structure without flux closures where all the layers have the same width. The impedance for the two structures is calculated and analyzed as a function of the physical parameters of the device and frequency. The “closed” structure is more efficient magnetically and exhibits a highly inductive response to much higher frequencies than the “open” structure. The analytical results obtained are directly applicable to practical design of GHz inductive components.
Material properties of bulk InGaAs and InAlAs/InGaAs heterostructures grown on (111)B and (111)B misoriented by 1° towards 〈211〉 InP substrates
Authors: Yeo W. , Dimitrov R. , Schaff W. J. , Eastman L. F.
Published: July 2002

High-quality bulk InGaAs and InAlAs/InGaAs heterostructures have been grown on InP substrates with different orientation using molecular-beam epitaxy. It was found that the electrical and structural properties were strongly dependent on growth temperature and substrate misorientation. The electrical and structural properties of the film were investigated by high-resolution x-ray diffraction, Nomarski microscope, and Hall measurements. Full-width at half-maximum of 380 arcsec for bulk InGaAs on (111)B and 70 arcsec on (111)B misoriented by 1° towards 〈211〉 InP substrates were measured by x-ray diffraction. The room temperature electron Hall mobility for bulk InGaAs of 5100 cm2/V s, doped with Si concentration at the mid-1017/cm3, and two-dimensional electron gas mobility of 11 200 cm2/V s, and sheet density of 3.0×1012/cm2 for InAlAs/InGaAs heterostructures on (111)B misoriented by 1° towards 〈211〉 InP substrates were achieved.
Highly excited vibronic eigenfunctions in a multimode nonadiabatic system with Duschinsky rotation
Authors: Fujisaki Hiroshi , Takatsuka Kazuo
Published: July 2002

We study the characteristics of vibronic eigenfunctions of a multidimensional nonadiabatic system and their consequences in the quantum spectra. As an illustrative example, we investigate the properties of highly excited eigenfunctions of Heller’s multimode nonadiabatic system. The system consists of two diabatic states and two-dimensional (two-mode) harmonic potentials that are nonadiabatically coupled with the Condon approximation and with an appropriate magnitude of the Duschinsky angle. “Quantum chaos” thus produced has no classical counterpart. In addition to rather characterless chaotic eigenfunctions that are uniformly widespread in configuration space, we have found highly excited localized eigenfunctions of two extreme types which favor either the diabatic picture or adiabatic picture. As a result, the features of the associated quantum spectra are strongly affected by the initial preparation of a wave packet. This finding suggests that one can control the rate of nonadiabatic transitions such as that for electron transfer by using laser techniques or by choosing appropriate solvents.
Nucleation of a noncritical phase in a fluid near a critical point
Authors: Sear Richard P.
Published: July 2002

Phase diagrams of some globular proteins have a fluid–fluid transition as well as a fluid–crystal transition. Homogeneous nucleation of the crystal from the fluid phase near the critical point of the fluid–fluid transition is examined. As the fluid–fluid critical point is approached, the number of molecules in the critical nucleus, the nucleus at the top of the free energy barrier to nucleation, is found to diverge as the isothermal compressibility. This divergence is due to a layer of the fluid phase of width equal to the fluid’s correlation length which surrounds the core of the nucleus; the number of molecules in a crystalline environment in the nucleus does not diverge. The free energy barrier to nucleation remains finite but its derivative with respect to the chemical potential is equal to minus the number of molecules in the critical nucleus, and so diverges.
Structure of liquid ethylene glycol: A molecular dynamics simulation study with different force fields
Authors: Saiz L. , Padró J. A. , Guàrdia E.
Published: July 2002

The structure of liquid ethylene glycol at room temperature is examined by performing molecular dynamics (MD) simulation studies for several different liquid phase force fields. We compare the properties obtained and analyze the differences which arise from the use of these models. A thorough study of molecular conformation and intermolecular structure for the different potential models is carried out given that three of the studied force fields have the same intermolecular parameters and different intramolecular interactions. In addition, the effect of molecular shape on the intermolecular structure is discussed. Due to the important role played by the highly directional forces occurring in hydrogen bonded systems, in their intermolecular structure and in the macroscopic properties of the system, we pay special attention to the analysis of the features of the hydrogen bonding patterns present in the liquid. Revealing an overall agreement with the available structural experimental data, the results obtained show that, for the simulated models, the intermolecular structure is rather similar. The dynamics of the system is studied through the self-diffusion coefficients and, in contrast to the structural properties, the results obtained for the distinct models are quite different.
Ab initio molecular dynamics simulation of the Cu(110)–water interface
Authors: Izvekov Sergei , Mazzolo Alain , VanOpdorp Kirk , Voth Gregory A.
Published: July 2002

The results of a first principles simulation of the Cu(110)–water interface are presented. The calculations were carried out using ab initio Car–Parrinello molecular dynamics simulation within a pseudopotential formalism and the BLYP generalized gradient approximation to the exchange-correlation potential. An analysis of the structural properties of the interface shows that absorption of H2O molecules occurs on the metal top sites which is in agreement with experimental results. The electronic structure of the interface has also been explored. It was found that there is a strong coupling of the water overlayer with the metal crystal electronic states. However, the surface-states charge density is undisturbed by the presence of water. The empty surface states are seen to not be quenched by the presence of water, which is also in qualitative agreement with existing experiments.
Energy landscapes, global optimization and dynamics of the polyalanine Ac(ala)8NHMe
Authors: Mortenson Paul N. , Wales David J.
Published: July 2002

A parallel searching algorithm using eigenvector-following is used to generate databases of minima and transition states for an all-atom model of Ac(ala)8NHMe. The AMBER force field of Cornell et al. [J. Am. Chem. Soc. 117, 5179 (1995)] is employed both with and without a simple implicit solvent. We use a master equation approach to analyze the dynamics of both systems, and relate the results to the potential energy landscapes using disconnectivity graphs.
Determination of processing damage in thin polycrystalline Ir films using Bragg-peak fringe analysis
Authors: Saenger K. L. , Noyan I. C.
Published: July 2002

We present an analysis of Bragg–Brentano diffraction spectra from blanket thin (10–20 nm) films of polycrystalline Ir, sputter deposited on thermally oxidized Si wafers. We observed that postdeposition annealing sharpened the Ir diffraction peaks, produced enhanced texture, and resulted in the formation of well-defined thickness fringes that were symmetric around the 111 Ir reflection. Scanning electron microscopy confirmed that the fringes were caused by the increased coherence lengths of the annealed grains. Annealed samples subsequently subjected to reactive ion etching exhibited asymmetric fringe patterns, with lower intensity fringes on the high angle side of the 111 Ir peak. Reannealing these samples restored the symmetric fringes. Analysis of the fringe patterns using simple equations and modeling programs in the public domain yielded valuable structural information about the film and the changes caused by processing. These results were verified by x-ray reflectivity measurements and modeling. We conclude that thickness fringe analysis using standard x-ray systems and simple programs can provide cost-effective process diagnostics for high atomic number thin film structures.
Collective modes in a strongly coupled dusty plasma
Authors: Kaw P. K.
Published: July 2002

It is widely recognized that in a typical dusty plasma encountered in the laboratory or outer space, the dust component is in a strongly coupled state because the interaction energy of neighboring dust particles due to shielded Coulomb (“Yukawa”) forces is much larger than their thermal energy. Low frequency collective modes involving the motion of dust particles are therefore greatly influenced by the strong correlation effects in the dust component. In this paper a dispersion relation for low-frequency collective modes using a generalized hydrodynamics model for the dust component has been derived. Strong correlation effects are described in terms of viscoelastic transport coefficients and a finite relaxation time for the memory kernel. Novel collective effects such as new corrections to dispersion terms for longitudinal dust acoustic waves and the existence of transverse shear waves supported by strong correlations have been identified. New physical processes involving nonuniform charge number equilibria and delayed charging effects which could drive the shear wave instability have also been studied. A report on some new experiments where self-excited transverse shear modes are seen when the dust component of the plasma is in a strongly correlated fluid-like state is also presented.
Spectrum asymptotics of high-frequency acoustic emission generated by surface explosion
Authors: Berman A. , Likhterov L.
Published: July 2002

The problem of high-frequency acoustic emission generated by an underwater explosion at the free surface of inviscid liquid is considered. Flow is assumed to be irrotational that allows an approximate equation of motion for the boundary of expanding breakdown to be constructed in analogy to the Rayleigh equation for a spherical bubble. The surface of the cavity (having the “bowl”-like form) is replaced by the circular cylinder whose height is equal to its diameter. The simple solution is obtained if the adiabatic exponent is taken equal to 1.2. It is shown that, in the first approximation, the spectral density of an acoustic energy emitted by surface explosion varies as the minus 1.1 power of the frequency and decreases approximately by 3.3 decibels per octave. Such slow decrease of the spectral density versus the frequency is the consequence of accepted assumption about the existence of potential flow after an energy deposition.
Electronic structures and magnetism of LaNi5–xFex compounds
Authors: Yang J. B. , Tai C. Y. , Marasinghe G. K. , Waddill G. D. , Pringle O. A. , James W. J.
Published: July 2002

A systematic study of the magnetic moments and hyperfine interactions of the LaNi5–xFex compounds was performed using the spin-polarized tight-binding-linear-muffin-tin-orbital methods (TBLMTO). The calculated results were compared with neutron diffraction and Mössbauer experimental data and found to be in good agreement. Fe atoms exhibit more localized magnetic moments in LaNi5–xFex when the Fe content x is lower than 1.0. The calculated magnetic moments of the Fe atoms approach values as large as 2.5μB due to the dilution effect. The strong s–d hybridization between the Ni and La atoms and the neighboring Fe in the Fe-substituted samples results in a large transferred valence contribution to the total hyperfine field. It is found that the Fe and Ni magnetic moments decrease abruptly at a unit cell volume which is 5% smaller than the observed volume in LaNi4Fe due to the instability of the Fe magnetic moment. LaNi5 becomes ferromagnetic when the unit cell volume is larger than its original volume.
Model dielectric function spectra of GaAsN for far-infrared and near-infrared to ultraviolet wavelengths
Authors: Leibiger G. , Gottschalch V. , Rheinländer B. , Šik J. , Schubert M.
Published: July 2002

We study the optical properties of tensile strained GaAs1−yNy (0%⩽y⩽3.7%) single layers for photon energies from 0.75 to 4.5 eV and for wave numbers from 100 to 600 cm−1 using spectroscopic ellipsometry. The intentionally undoped GaAsN layers were grown pseudomorphically on top of undoped GaAs buffer layers deposited on Te-doped (001) GaAs substrates by metalorganic vapor phase epitaxy. We provide parametric model functions for the dielectric function spectra of GaAsN for both spectral ranges studied here. The model functions for photon energies from 0.75 to 4.5 eV excellently match dielectric function data obtained from a numerical wavelength-by-wavelength inversion of the experimental data (point-by-point fit). Critical-point analysis of the point-by-point fitted dielectric function is performed in the spectral regions of the fundamental band gap and the critical-point transitions E1 and E1+Δ1. The band-gap energy is redshifted whereas the E1 and E1+Δ1 transition energies are blueshifted with increasing y. For y⩽1.65% the observed blueshift of the E1 energy is well explained by the sum of the effects of biaxial (001) strain and alloying. The GaAsN layers show a two-mode phonon behavior in the spectral range from 100 to 600 cm−1. We detect the transverse GaAs- and GaN-sublattice phonon modes at wave numbers of about 267 and 470 cm−1, respectively. The oscillator strength of the GaN-sublattice resonance increases linearly due to alloying and tensile strain. We compare our results from the single layers with those obtained previously from tensile and compressively strained GaAsN/GaAs and GaAsN/InAs/GaAs superlattices.
Conduction band offset and electron effective mass in GaInNAs/GaAs quantum-well structures with low nitrogen concentration
Authors: Pan Z. , Li L. H. , Lin Y. W. , Sun B. Q. , Jiang D. S. , Ge W. K.
Published: July 2002

We have investigated the optical transitions in Ga1−yInyNxAs1−x/GaAs single and multiple quantum wells using photovoltaic measurements at room temperature. From a theoretical fit to the experimental data, the conduction band offset Qc, electron effective mass me*, and band gap energy Eg were estimated. It was found that the Qc is dependent on the indium concentration, but independent on the nitrogen concentration over the range x=(0–1)%. The me* of GaInNAs is much greater than that of InGaAs with the same concentration of indium, and increases as the nitrogen concentration increases up to 1%. Our experimental results for the me* and Eg of GaInNAs are quantitatively explained by the two-band model based on the strong interaction of the conduction band minimum with the localized N states.
Generalization of a two-dimensional micromagnetic model to nonuniform thickness
Authors: Porter D. G. , Donahue M. J.
Published: July 2002

A two-dimensional micromagnetic model is extended to support simulation of films with nonuniform thickness. Zeeman and crystalline anisotropy energies of each cell scale with the cell thickness, while the exchange energy of a pair of neighbor cells scales by a weight dependent on the thicknesses of both cells. The self-magnetostatic energy is computed by scaling the moment of each cell by its thickness, and adding a local correction to the out-of-plane field. The calculation of the magnetostatic field for a 10×10×1 oblate spheroid is shown to be more accurate by the nonuniform thickness model than by a uniform thickness model. With the extended model a 530×130×10 nm film in the shape of a truncated pyramid with tapering over the 15 nm nearest the edges is shown to have smaller switching field and different reversal mechanism compared with uniform thickness films of similar size and shape.
Mutual inductance of superconducting thin films
Authors: Coffey Mark W.
Published: July 2002

We consider the concentric, two-coil mutual inductance technique. Highly accurate approximations for the complex-valued mutual inductance of thin film superconductors in the Meissner state are derived. The ratio of the mutual inductance in the superconducting state M to the mutual inductance in the normal state M0 is derived for axisymmetric geometry and evaluated in limiting cases. This ratio is characterized not only by the two-dimensional screening length but by a combination of the macroscopic lengths of the coil geometry. The results have practical application for the electromagnetic response of superconducting thin films. Subsets of the results are relevant for inverting the mutual inductance to recover the penetration depth absolutely, and in studying the proximity effect.
Room-temperature excitons in wide-gap layered-oxysulfide semiconductor: LaCuOS
Authors: Ueda K. , Inoue S. , Hosono H. , Sarukura N. , Hirano M.
Published: July 2002

Sharp optical absorption and emission peaks near the band gap (Eg≈3.1 eV) were observed in LaCuOS polycrystalline thin films at room temperature. The absorption peak was able to be detected at temperatures as high as 490 K, and its intensity remarkably increased with decreasing temperature. The spectral position of the absorption peak and its temperature dependence almost agreed with those of the emission peak. It was concluded that the sharp absorption and emission peaks originate from excitons. On the basis of semiquantitative consideration about the excitons, it is suggested that the electronic-structure characteristic of the layered-crystal structure of LaCuOS is responsible for the stability of the excitons. The observation of the exciton absorption and emission at room temperature revealed that LaCuOS is a promising material for optoelectronic applications such as light-emitting devices.
Evaluation of an attenuation system for an infrared free-electron laser
Authors: Awazu Kunio , Fukami Yuko
Published: July 2002

This study concerns the attenuation of the incident power from a free-electron laser (FEL) in steps of up to two orders of magnitude, while maintaining optical beam geometry. We have developed and evaluated the properties of a FEL attenuation filter system, which can be attached and detached, with the position of the focusing lens and the target sample fixed. As a result, we have established that our FEL attenuation filter system enables two orders of magnitude of power control for infrared wavelengths between 5.5 and 6.5 μm. We conclude that the attenuation system is extremely useful for various FEL applications.
Interaction of benzene (Bz) with Pt and Pt2: A theoretical study on Bz–Pt2, Bz2–Pt, Bz2–Pt2, and Bz3–Pt2 clusters
Authors: Majumdar D. , Roszak S. , Balasubramanian K.
Published: July 2002

Extensive ab initio calculations have been carried out on benzene (Bz)–platinum complexes (Bz–Pt2, Bz2–Pt, Bz2–Pt2, and Bz3–Pt2) using a variety of computational techniques. Both physisorbed structures and energetically lower chemisorbed species were found. Complete active space multiconfiguration self-consistent field (CASMCSCF), multireference singles and doubles configuration-interaction (MRSDCI), density functional (DFT), and Møller–Plessett second order perturbation (MP2) calculations were employed to predict Bzm–Ptn structures. While the DFT and MP2 calculations also consistent with the MRSDCI techniques predict chemisorbed structures to be lower, the CASMCSCF method seems to favor physisorbed structures. The effect of spin-orbit coupling on the binding energies of complexes with the Pt atom and the Pt2 dimer were considered. The computed dissociation energies are consistent with the relative abundance of these clusters found in the time-of-flight mass spectra. The low-energy staircase structures of Bz2–Pt, Bz2–Pt2, and Bz3–Pt2 complexes found in this study could be electrically conducting.