bluuebunny/crossref_metadata_2025 huggingface dataset:
Capacitance–voltage relation for ceramics with positive temperature coefficient of resistance
Authors: Li Yong Gen , Cho Sung Gurl
Published: July 2002

A capacitance–voltage relation to determine the potential barrier height for ceramics with positive temperature coefficients of resistance (PTCR) was proposed. The suggested relation, (1/C2−1/2C02)2 versus V2, was derived based on the double Schottky barrier formation at the grain boundary and the linear distribution of bias along the depletion region. The linearity of the relation was evaluated for two types of PTCR ceramic specimens with different grain sizes. Also, the barrier heights determined from the plots were compared with those reported by other researchers. The potential barrier heights, determined at various temperatures for PTCR ceramics using the capacitance–voltage relation, were in the range of 0.36–0.80 V.
Room-temperature emission of GaAs/AlGaAs superlattice quantum-cascade lasers at 12.6 μm
Authors: Anders S. , Schrenk W. , Gornik E. , Strasser G.
Published: July 2002

We report on the lasing properties of quantum-cascade lasers that combine the advantage of strong electron confinement with the superior performance of superlattice structures. To achieve an optimum band offset between wells and barriers, we chose an Al content of the AlGaAs barriers of 45%. We fabricated both conventional Fabry–Pérot lasers, as well as single-mode-emitting distributed-feedback lasers. In pulsed mode, the lasers work well up to temperatures as high as 40 °C. The emission wavelength at this temperature is 12.6 μm. At 78 K, some lasers show a low threshold current density of 1.6 kA/cm2, while others show a high optical peak power of 240 mW.
Mini-conference on magnetic reconnection in space and astrophysical plasmas
Authors: Gary S. Peter
Published: July 2002

A mini-conference on magnetic reconnection in space and astrophysical plasmas was held as part of the American Physical Society Division of Plasma Physics Fall 2001 Meeting 30 October–2 November 2001. This paper provides a summary of some major physics issues discussed during this mini-conference.
Bi-quasi-Hamiltonian systems
Authors: Crampin M. , Sarlet W.
Published: July 2002

A general notion of bi-quasi-Hamiltonian systems is introduced and is related to previous work on various special cases of such systems.
Determining the relationship between local lattice strain and slip systems of dislocations around shallow trench isolation by convergent-beam electron diffraction
Authors: Toda Akio , Ikarashi Nobuyuki , Ono Haruhiko , Okonogi Kensuke
Published: July 2002

We clarified the generation of process-induced dislocations around a shallow trench isolation (STI) by using convergent-beam electron diffraction. Comparing the resolved shear strain (RSS) of 12 slip systems, we found that at the trench bottom corner the RSS on slip systems (1 −1 1)[0 1 1] and (1 −1 1)[1 0 −1] was largest in all slip systems. In fact, the dislocations of slip systems (1 −1 1)[0 1 1] and (1 −1 1)[1 0 −1] were observed around the trench bottom corner more often than those of any other slip systems. We also found that the large RSS at the trench bottom corner may be due to the corner shape or the intrinsic stress induced during oxidation. Therefore, to control dislocation around STI, the oxidation-induced stress at the trench bottom corner must be reduced, and the shape of the bottom corner must be controlled.
Algebraic dynamics in O*-algebras: A perturbative approach
Authors: Bagarello F. , Trapani C.
Published: July 2002

In this paper the problem of recovering an algebraic dynamics in a perturbative approach is discussed. The mathematical environment in which the physical problem is considered is that of algebras of unbounded operators endowed with the quasiuniform topology. After some remarks on the domain of the perturbation, conditions are given for the dynamics to exist as the limit of a net of regularized linear maps.
Plume emissions accompanying 248 nm laser ablation of graphite in vacuum: Effects of pulse duration
Authors: Claeyssens Frederik , Ashfold Micheal N. R. , Sofoulakis Emmanuel , Ristoscu Carmen G. , Anglos Demetrios , Fotakis Costas
Published: July 2002

We report a comparative study of the ultraviolet laser ablation of graphite, in vacuum, using nanosecond (34 ns), picosecond (5 ps), and femtosecond (450 fs) pulses of 248 nm radiation, focusing on the plume characteristics as revealed by wavelength, time- and spatially resolved optical emission spectroscopy. Nanosecond pulsed ablation gives a distinctively different optical emission spectrum from that observed with the two shorter pulse durations. Emissions attributable to electronically excited C*, C+* and C2* fragments are identified in the former, while the spectra obtained when using the shorter duration, higher intensity pulses contain additional lines attributable to C2+* species but none of the C* emission lines. As before [Claeyssens et al., J. Appl. Phys. 89, 697 (2001)], we consider that each atomic emission is a step in the radiative cascade that follows when an electron recombines with a Cn+ species (where n is one charge state higher than that of the observed emitter) formed in the original ablation process. Broadband visible radiation attributable to blackbody emission from larger particulates is also observed following ablation with any of the three laser pulse durations. Time gated imaging studies allow estimation of the velocity distributions of various of these emitting species within the plume, and their variation with incident laser fluence and/or intensity. The deduced multicomponent structure of the plume emission following excitation with short duration laser pulses is rationalized in terms of contributions from both nonthermal and thermal mechanisms for material ejection from the target. Use of longer duration (nanosecond) laser pulses offers the opportunity for additional laser-plume interactions, which we suggest are responsible for much of the observed emission in the nanosecond pulsed laser ablation of graphite.
Static Bloch sums for the square array
Authors: McPhedran R. C. , Nicorovici N. A.
Published: July 2002

We consider sums arising in doubly quasiperiodic Green’s functions for the Laplace equation, over the square array. The sums are represented as Fourier series, and it is shown that the coefficients in the series can be obtained as polynomials. We give expressions from which the first six array sums can be evaluated efficiently, and accurate to better than one part in 107, over most of the Brillouin zone.
Generic van der Waals equation of state and theory of diffusion coefficients: Binary mixtures of simple liquids
Authors: Rah Kyunil , Eu Byung Chan
Published: July 2002

A free volume theory of diffusion coefficients is formulated for binary mixtures of simple liquids. The free volume is defined by means of the generic van der Waals equation of state for mixtures, which is developed in this work, and computed in terms of the pair correlation function obtained by means of Monte Carlo simulations with a square-well potential model. The free volume thus computed is used to investigate the composition and temperature dependence of the binary diffusion coefficient of argon–krypton mixtures as well as the tracer diffusion coefficients of argon in liquid nitrogen and krypton in liquid argon. The present theoretical predictions compare very well with the experimental or simulation results available in the literature. The size and mass dependence of the ratio of the tracer diffusion to the solvent self-diffusion coefficients is also presented. This ratio is found to be almost independent of temperature and density. It therefore can be used to calculate the tracer diffusion coefficient from the self-diffusion coefficient and vice versa.
Dielectric properties of (100) textured thick Pb(ZrxTi1−x)O3 films with different Zr/Ti atom ratios
Authors: Cheng Jinrong , Zhu Wenyi , Li Nan , Cross L. Eric
Published: July 2002

Near 4-μm-thick Pb(ZrxTi1−x)O3 (PZT) films with Zr/Ti ratios of 60/40, 52/48, and 45/55 were coated onto platinized silicon substrates by using 2-methoxyethanol based sol-gel spin-on techniques with a special thermal treatment process. The scanning electron microscopy observations show the columnar growth of grains. The analysis of x-ray diffraction data indicates that all PZT films exhibit (100) texture. The dielectric constants and dissipation factors of the films were measured at elevated temperatures and frequencies. It is found that Curie points of 60/40, 52/48, and 45/55 films are at 350, 375, and 422 °C, respectively. All these films exhibit high dielectric constants and remnant polarizations. A permittivity of 1658 and remnant polarization of 35 μC/cm2 had been achieved for the 60/40 films. No enhancement of the dielectric constant was observed in films with a Zr/Ti ratio close to morphotropic phase boundary. The high dielectric constant observed in films with the higher Zr content was explained by the concept of domain engineering.
Piezoelectric effect on Al0.35−δInδGa0.65N/GaN heterostructures
Authors: Lo Ikai , Tsai J. K. , Tu Li-Wei , Hsieh K. Y. , Tsai M. H. , Liu C. S. , Huang J. H. , Elhamri S. , Mitchel W. C. , Sheu J. K.
Published: July 2002

Al 0.35−δ In δ Ga 0.65 N/GaN heterostructures have been studied by using transmission electron microscopy, photoluminescence, and Shubnikov–de Haas (SdH) measurements. In the sample of δ<0.01%, two SdH oscillations beat each other due to the population of the lowest two subbands. The carrier concentrations of these two subbands are 1.398 and 1.248×1013 cm−2 and the electric field at the interface is reduced to 2.19×104 V/cm, which is one order of magnitude smaller than that of Al0.35Ga0.65N/GaN heterostructure. We suggest that a small fraction of In atoms in the Al0.35−δInδGa0.65N can be used as a tuning parameter to control the strain and the piezoelectric field at the interface.
Geometric quantization of curvature energy in equipotential surfaces of ionic crystals
Authors: Gandy Paul J. F. , Klinowski Jacek
Published: July 2002

The curvature energies of triply periodic minimal surfaces (TPMS) and zero equipotential surfaces (ZEPS) of ionic crystals are both quantized with the Euler–Poincaré characteristic as the “quantum number,” and the curvature energy of the TPMS larger than that of the corresponding ZEPS. Quantization is imposed by the charge-defined metric.
Polarity inversion of GaN films by trimethyl–aluminum preflow in low-pressure metalorganic vapor phase epitaxy growth
Authors: Lim D. H. , Xu K. , Arima S. , Yoshikawa A. , Takahashi K.
Published: July 2002

The polarity of GaN films grown on nitrided (0001) sapphire substrates by low-pressure metalorganic vapor phase epitaxy was controlled by trymethyl–aluminum (TMAl) preflow prior to the growth of GaN buffer layer. The TMAl preflow served as forming a few monolayers of Al to modify the nitrided sapphire surface. The effects of the TMAl preflow on GaN epilayer polarities were investigated by coaxial impact collision ion scattering spectroscopy. It was shown that, by increasing the TMAl preflow time, the polarities of GaN epilayers were changed from a N polarity to a mixed polarity, and finally to a pure Ga polarity when the preflow time was over than 5 s. A schematic model of “two monolayers of Al” was proposed to understand the related mechanisms. The effects of the TMAl preflow on the epilayer quality were also evaluated by high-resolution x-ray diffraction.
Raman study of Zn1−xBexSe/GaAs systems with low Be content (x⩽0.20)
Authors: Pagès O. , Ajjoun M. , Bormann D. , Chauvet C. , Tournié E. , Faurie J. P. , Gorochov O.
Published: July 2002

We present an experimental and theoretical investigation of the Raman line shape of long-wavelength phonons with longitudinal optical (LO) symmetry from both sides of Zn1−xBexSe/GaAs (001) heterojunctions with special emphasis on samples with low Be content (x⩽0.20). First the built-in p-type LO phonon–plasmon (LO–P) coupled mode at the near-interfacial substrate is used as a sensitive probe to investigate the interfacial quality. The corresponding hole gas is reinforced when the ZnBeSe layers are nominally p doped by nitrogen. This provides clear evidence for effective hole transfer across the junction, and thereby indicates a minimized density of interfacial defects. In the nitrogen-doped layers hole densities as high as 1017 cm−3, in accordance with capacitance–voltage measurements, are directly inferred from clear weakening of the ZnSe-like LO mode due to LO–P coupling. Concerning the intrinsic properties of the alloys, we demonstrate that the asymmetric broadening of the ZnSe-like LO mode is determined by topological disorder only, and not by other possible mechanisms such as structural disorder, nonhomogeneity in the alloy composition, a distribution of tensile strain, or a Fano-type interference. This reveals a high structural quality that parallels the high interfacial quality. We also show that a spatial correlation model with Gaussian distribution applies to Zn1−xBexSe. Most of this study is supported by a quantitative treatment. We extend the phenomenological approach of Hon and Faust to equations of motion and polarization derived from the modified-random-element-isodisplacement model in order to achieve line shape analysis of the alloy-related LO and LO–P modes.
Goniometric instrument for light scattering measurement of biological tissues and phantoms
Authors: Bolt René A. , de Mul Frits F. M.
Published: July 2002

In this Note, we present a goniometric instrument for measuring the forward angular light scattering behavior of liquid or solid samples with high angular resolution and large dynamic range. The instrument is designed to work in the visible and near-infrared regions. Along with measuring the angular scattering behavior, the instrument functions as a refractometer to determine the refractive index of the sample.
Multiplexing of radio-frequency single-electron transistors
Authors: Stevenson T. R. , Pellerano F. A. , Stahle C. M. , Aidala K. , Schoelkopf R. J.
Published: July 2002

We present results on wavelength division multiplexing of radio-frequency single-electron transistors. We use a network of resonant impedance matching circuits to direct applied rf carrier waves to different transistors depending on carrier frequency. A two-channel demonstration of this concept using discrete components successfully reconstructed input signals with small levels of crosscoupling. A lithographic version of the rf circuits had measured parameters in agreement with electromagnetic modeling, with reduced crosscapacitance and inductance, and should allow 20–50 channels to be multiplexed.
Structure and piezoelectric properties of sol–gel-derived 0.5 Pb[Yb1/2Nb1/2]O3–0.5 PbTiO3 thin films
Authors: Zhang Q. Q. , Zhou Q. F. , Trolier-McKinstry S.
Published: July 2002

0.5 Pb[Yb1/2Nb1/2]O3–0.5 PbTiO3 thin films were deposited on (111) Pt/Ti/SiO2/Si substrates by sol–gel processing using a thin Pb(Zr0.52Ti0.48)O3 seed layer. X-ray diffraction analysis and scanning electron microscopy revealed that the films were strongly (111) oriented, mimicking the orientation of the (111) Pt/Ti/SiO2/Si substrate. No pyrochlore phase was observed by x-ray diffraction and the films had a uniform grain size of about 50–60 nm. The dielectric permittivity and loss factor varied only slightly with frequency in the range of 100–10 000 Hz. At 1 kHz, the dielectric permittivity was 1025 and dielectric loss was 0.028. The films exhibited good ferroelectric and piezoelectric properties. The remanent polarization (Pr) was 30 μC/cm2. The effective transverse piezoelectric e31,f coefficient (−4.8 C/m2) of the films was measured using a modified wafer flexure method.
Construction of diffusion algebras
Authors: Pyatov P. N. , Twarock R.
Published: July 2002

In Isaev et al. [J. Phys. A 34, 5815–5834 (2001)] diffusion algebras were introduced in the context of one-dimensional stochastic processes with exclusion in statistical mechanics. While Isaev et al. is based on the needs of the physicist reader and thus states results without proofs and focuses on the discussion of lower-dimensional examples, it is the purpose of this paper to present a construction formalism for diffusion algebras and to use the latter to prove the results in Isaev et al.
The reduction of a quantum system of three identical particles on a plane
Authors: Iwai Toshihiro , Hirose Toru
Published: July 2002

Quantum systems of three identical particles on a plane are analyzed from the viewpoint of symmetry. Upon reduction by rotation, such systems are described in the space of sections of a line bundle over a three-dimensional shape space whose origin represents triple collision. It is shown that if the total angular momentum is nonzero, then the wave section must vanish at the origin, while if it is zero, then the wave section can be finite at the origin. Since the particles are assumed to be identical, the quantum system admits the action of the symmetric group S3 as well, which stands for the group of particle exchanges and is commutative with rotation. Hence the reduced system still admits the S3 action, so that Bose and Fermi states can be discussed in the space of sections of the line bundle. A detailed analysis of a system of three free particles on a plane is presented in the latter part of the article.
Tris-(8-hydroxyquinoline)aluminum-based organic light-emitting devices with Al/CaF2 cathode: Performance enhancement and interface electronic structures
Authors: Lee J. , Park Y. , Lee S. K. , Cho E.-J. , Kim D. Y. , Chu H. Y. , Lee H. , Do L.-M. , Zyung T.
Published: July 2002

The device characteristics and the interface electronic structures of organic light-emitting devices based on tris-(8-hydroxyquinoline)aluminum were investigated with Al/CaF2, Al/LiF, and Al-only cathodes. Similar to the Al/LiF cathode, the Al/CaF2 cathode greatly improved the performance of the device over the Al-only cathode. However, a photoelectron spectroscopy study revealed that despite the performance improvement, the evolution of the new peaks during the Al/CaF2 cathode formation closely resembled those of the Al-only cathode rather than the Al/LiF cathode.
Stroboscopic single-shot detection of the flux state in a radio frequency superconducting quantum interference device
Authors: Cosmelli C. , Sciamanna F. , Castellano M. G. , Leoni R. , Torrioli G. , Carelli P. , Chiarello F.
Published: July 2002

The flux states of a rf superconducting quantum interference device (SQUID) can be used to investigate macroscopic quantum effects, the most challenging of which is the detection of quantum coherent oscillations, and are promising candidates for the implementation of quantum computing. The use of a proper readout system is of utmost importance for these purposes. In this letter we present experimental evidence of the possibility of using an underdamped, hysteretic dc SQUID to read out stroboscopically the flux states of a rf SQUID, allowing a single shot discrimination with an efficiency of 98%. The devices are all integrated on chip.
Charge-density wave as an electro-optical switch and memory
Authors: Ogawa N. , Miyano K.
Published: July 2002

Switching and memory effects based on the deformation of the charge-density wave (CDW) in K0.3MoO3 are demonstrated. The deformations, which greatly affect the CDW dynamic phase transition from creep phase to slide phase, can be set by an external electric field and be removed by photoexcitation. This property can be exploited to realize an electrically readable switch or memory device without incorporating any circuitry but two electrical leads.
Use of a pattern recognition algorithm to obtain a parametrization of low-mode and high-mode plasma states
Authors: Deranian R. D. , Groebner R. J. , Pham D. T.
Published: July 2002

A pattern recognition algorithm has been used to identify a small set of plasma parameters which provide good discrimination between the low-mode (L-mode) and high-mode (H-mode) states in the DIII-D tokamak [J.L. Luxon, R. Anderson, F. Batty et al., in Proceedings of the 11th Conference on Plasma Physics and Controlled Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159]. The parameters studied were the electron temperature Te, electron density ne, electron pressure Pe and their gradients, evaluated in the region where the H-mode transport barrier forms, and the plasma current Ip and toroidal magnetic field BT. The L-mode and H-mode states were found to separate well into a parameter space based on the temperature scale length Te/∇Te and the normalized temperature gradient ∇Te/Ip. These parameters provide a more reliable separation of the L- and H-states than do Te and ne. This parametrization provides constraints on theories and may provide insight into the physics of the L–H transition and of edge transport.
Study of photoluminescence at 3.310 and 3.368 eV in GaN/sapphire(0001) and GaN/GaAs(001) grown by liquid-target pulsed-laser deposition
Authors: Mah K. W. , Mosnier J.-P. , McGlynn E. , Henry M. O. , O’Mahony D. , Lunney J. G.
Published: July 2002

Epitaxial GaN films of thickness ∼1 μm have been grown on sapphire(0001) and GaAs(001) substrates using the liquid-target pulsed-laser-deposition technique in a 5 Torr nitrogen atmosphere. Detailed x-ray diffraction and photoluminescence studies were carried out for both types of samples. Significantly enhanced low-temperature photoluminescence emissions at 3.368 eV (I3) and 3.310 eV (I4) were observed for the material deposited on a GaAs(001) substrate at ∼800 °C. We propose a model to explain the emission mechanism for both lines in which the electrons and holes are confined in cubic inclusions within the hexagonal material, analogously to a type-I quantum well.
Stress in undoped and doped laser crystallized poly-Si
Authors: Lengsfeld P. , Nickel N. H. , Genzel Ch. , Fuhs W.
Published: July 2002

Raman measurements were performed on laser crystallized poly-Si on different substrates. Observed shifts of the Si LO–TO phonon peak are caused by stress originating from the film-substrate interface. The principal cause of the stress is the difference in the thermal expansion coefficients of substrate and film. Consequently, the amount of thermal stress critically depends on the choice of substrate. In the case of undoped samples on quartz, profiler and x-ray measurements confirmed the occurrence of tensile stress in the films. In the case of heavily doped samples, the change of the lattice parameter determined by x-rays is probably to a significant extend responsible for additional Raman shifts.
Temperature effects on the 1.0 and 1.3 μm emission lines of Nd3+ in a LaSc3(BO3)4 crystal host
Authors: Sardar Dhiraj K. , French Joe A. , Castano Francisco , Sayka Anthony , Salinas Felipe S.
Published: July 2002

A detailed investigation of the temperature effect on widths and shifts of the spectral lines of Nd3+ in LaSc3(BO3)4 has been performed. The spectral lines investigated correspond to the inter-Stark transitions R1→Y3 (1062 nm) and R1→Z4 (1344 nm) within the F3/24→4I11/2 and F3/24→4I13/2 intermanifold transitions, respectively. The widths of these lines and their shifts have been measured as a function of temperature. The linewidths are found to increase with increasing temperature for both transitions; the 1062 nm line shifts to the longer wavelengths, whereas the 1344 nm line shifts to the shorter wavelengths. The theory of phonon-ion interactions has been employed to explain the temperature dependencies of widths and shifts. These interactions give rise to the phonon relaxation processes that are responsible for both the broadening and shifts of the spectral lines.
HFCN open-shell isomers in solid argon. II. Excited-state tunneling isomerization HFC=N•→FC•=NH
Authors: Misochko Eugenii Ya. , Akimov Alexander V. , Goldschleger Ilya U. , Wight Charles A.
Published: July 2002

Photochemical reaction of the π-type fluoromethylene amidogen radical HFC=N• was studied by infrared, electroparamagnetic resonance, and laser-induced fluorescence spectroscopies in solid argon matrices. Laser excitation of the 0–0 band of the first excited state of HFC=N• at 488.1 nm leads to complete conversion to another structural isomer, the σ-type fluoroiminomethyl radical FC•=NH. The quantum yield of the photoisomerization reaction is ΦH≈0.5 at temperatures T⩽30 K. Experiments using pulsed laser excitation show that the rate constant for hydrogen atom transfer is kH=5×104 s−1 at T⩽30 K, whereas the rate constant of the analogous process in DFC=N• is only kD=30 s−1 under the same experimental conditions. The large kinetic isotope effect kH/kD≈1400 and absence of temperature dependence of kH show that the mechanism of photoisomerization is intramolecular tunneling transfer of the hydrogen atom in the first excited Ã 2A″ state. Simple one-dimensional consideration of the tunneling transfer of an H or D atom fails to describe the observed values of kH and kD. Therefore, multidimensional character of the hydrogen atom tunneling transfer should be taken into account, and a multidimensional potential energy surface of the excited state is required to describe correctly the chemical dynamics of the HFCN molecular system.
Theoretical study of a hot-electron device with a base consisting of a magnetic multilayer
Authors: Yamauchi Takashi , Mizushima Koichi
Published: July 2002

We studied the structure dependence of both the magnetoresistance (MR) ratio and the injection efficiency of a three-terminal device whose base consists of a magnetic multilayer. It was found that the injection efficiency of the device structure with a MR ratio over 200% does not exceed 0.08 at the voltage around 1 V, because electrons scattered elastically or inelastically in the ferromagnetic layer contribute to the increase of the MR ratio as well as that of the base current. The injection efficiency, however, increases with the voltage and is larger than 0.1 at 1.5 V with the MR ratio in excess of 200%.
Magnetostriction of the antiferromagnet NiCl2 in the homogeneous and multidomain states
Authors: Kalita V. M. , Lozenko A. F. , Trotsenko P. A.
Published: July 2002

Experimental data from measurements of the induced magnetostriction of the easy-plane two-sublattice layered antiferromagnet NiCl2 in the homogeneous and multidomain states at T=4.2 K are analyzed. It is shown that the intersublattice magnetoelastic interactions are dominant in NiCl2. The induced magnetostriction of the crystal in the single-domain state is due to canting of the spins of the sublattices toward the magnetic field direction. The induced magnetostriction in the multidomain state is due to a rearrangement of the domain structure and is independent of the small in-plane anisotropy. In both cases the values of the induced magnetostriction are directly proportional to the square of the value of the external magnetic field.
Multiple parallel conduction paths observed in depth-profiled n-GaN epilayers
Authors: Mavroidis C. , Harris J. J. , Jackman R. B. , Harrison I. , Ansell B. J. , Bougrioua Z. , Moerman I.
Published: July 2002

We have combined plasma etching with the Hall effect and resistivity measurements between 10 and 300 K to study the depth distribution of conduction in silicon (Si)-doped GaN epitaxial layers grown on sapphire substrates by two different metalorganic chemical vapor deposition processes. Reduction of the epitaxial layer thickness produces a linear decrease of the sheet carrier density with depth in the doped region, whilst in one sample, in the region less than ∼0.3 μm from the interface, the sheet carrier density tends to flatten out to a value of ∼3×1013 cm−2. The former is indicative of a uniform dopant distribution in the epitaxial material, and the latter reveals the existence of mobile charge near the interface. These experiments allow the properties of the doped material to be deconvoluted from those of the interface region, and the temperature dependence of these properties indicates the presence of two parallel conduction paths in the doped material: the conduction band and an impurity band. Thus a full analysis of GaN epitaxial layers is shown to require consideration of multiple parallel conduction processes, at the interface and in the bulk.
Molecular-beam epitaxy of (Cd,Mn)Se on InAs, a promising material system for spintronics
Authors: Grabs P. , Richter G. , Fiederling R. , Becker C. R. , Ossau W. , Schmidt G. , Molenkamp L. W. , Weigand W. , Umbach E. , Sedova I. V. , Ivanov S. V.
Published: July 2002

We describe the growth characteristics of a type of II–VI/III–V heterostructure, (Cd,Mn)Se/InAs, which shows promise for application in spintronics. We used a variety of approaches for growing the heterostructure, and found that a high epilayer quality could be obtained by incorporation of a thin ZnTe buffer layer between the two materials.
Mechanism of thermokinetical selection between carbon nanotube and fullerene-like nanoparticle formation
Authors: Louchev Oleg A. , Sato Yoichiro , Kanda Hisao
Published: July 2002

Open edge stability of carbon nanotube growth is suggested to depend on kinetically defined competition between pentagon and hexagon formation. Nanotube growth is possible when the time of hexagon formation, determined by the surface diffusion flux to the growth edge (proportional to carbon vapor pressure), is much lower than that of pentagon formation, which depends on the temperature. The competition of pentagon/hexagon formation at the growth edge together with thermal effects of condensation heat release and heat dissipation by radiation and collisions with inert gas (He), is shown to define selection between nanotube nucleus evolution into (i) continuous nanotube growth or (ii) fullerene- or cage-like nanoparticle formation. The involvement of catalyst nanoparticles enhances the formation of nanotubes by enhancing growth edge stability against pentagon formation under pressure-temperature conditions at which the open edge would otherwise become unstable closing nucleus into the fullerene-like nanoparticle.
Classification of supercritical gas adsorption isotherms based on fluid–fluid interaction
Authors: Murata K. , Yudasaka M. , Iijima S. , El-Merraoui M. , Kaneko K.
Published: July 2002

Based on an increase in fluid–fluid interaction, supercritical gas adsorption isotherms can be classified into three types, Henry-type isotherms, virial isotherms, and cooperative isotherms. The isotherms of the three types differ in terms of the compression factor and the adsorbed layer density. The goodness of this classification is supported by calculations based on the density functional theory. This classification is used for hydrogen adsorption isotherms on a single-wall carbon nanohorn obtained at different temperatures.
Continuous change of surface magnetization direction from perpendicular to planar in soft magnetic CoP multilayers
Authors: Favieres C. , Aroca C. , Sánchez M. C. , Madurga V.
Published: July 2002

Amorphous soft magnetic CoP electrolytic multilayers have been obtained with a controlled surface magnetic anisotropy. The surface magnetization easy direction can be continuously altered from an initial perpendicular position to a final arrangement within the longitudinal plane. This continuous change of magnetization direction is controlled during the sample production stage and, therefore, there is no need to subject the films to subsequent treatments such as, for example, thermal, stress, or magnetic field annealing. The magnetic domain configurations, as observed by means of the Bitter technique, have revealed a continuous variation from an initial stripe domain structure to a final situation in which the magnetization lies within the plane of the films and magnetic domain walls are observed. The magneto-optic behavior of these multilayers has also revealed this continuous evolution from an initial perpendicular easy direction for the magnetization to a configuration in which the surface magnetization has a planar longitudinal direction. The surface magnetization processes are hence modified from the initial rotation magnetization to final processes corresponding to magnetic wall displacements. Control of magneto-optic behavior can be of interest for technical applications, given there would be no need for sense windings. A phenomenological discussion about the evolution of the surface magnetization easy direction is also presented.
Comparison of colloidal effective charges from different experiments
Authors: Wette Patrick , Schöpe Hans Joachim , Palberg Thomas
Published: July 2002

We present measurements of effective charges in de-ionized aqueous suspensions of highly charged spherical latex colloids. For crystalline ordered samples the shear modulus G was measured using torsional resonance spectroscopy. It increases with increasing particle number density n. From fits of theoretical expressions based on a Debye–Hückel-type pair interaction potential, an effective charge ZG* was derived. On the other hand the effectively transported charge Zσ* was determined from the n dependence of the suspension conductivity. Both effective charges are independent of n within experimental error. For most species they scale with the ratio of radius to Bjerrum length. For all species, however, Zσ* is found to be systematically larger than ZG* by some 40%.
Percolative metal–insulator transition in layered manganites La1.4Sr1.6−yBayMn2O7 (y⩽0.50)
Authors: Zhu Hong , Tan Sun , Tong Wei , Zhang YuHeng
Published: July 2002

We have studied the magnetotransport properties of the La1.4Sr1.6−yBayMn2O7 (y⩽0.50) samples with a bilayered perovskite structure. For lightly doped samples, the metal–insulator transition accompanying a three-dimensional ferromagnetic transition can be understood commonly in terms of the double-exchange interaction. For heavier doped samples, the metal–insulator transition remains without a ferromagnetic transition and shifts to higher temperature with increasing Ba doping level. On account of the coexistence of short-range ferromagnetic ordering and paramagnetic state, the results can be explained by a recent percolation model based on phase separation. The percolative metal–insulator transition is consistent with the relative ferromagnetic metallic fraction change with temperature and doping level.
Laboratory measurement of water nucleation using a laminar flow tube reactor
Authors: Mikheev Vladimir B. , Irving Patricia M. , Laulainen Nels S. , Barlow Stephan E. , Pervukhin Viktor V.
Published: July 2002

A Laminar Flow Tube Reactor (LFTR) was used to study the nucleation of water vapor. Computational analysis was carried out to define the operating conditions of the LFTR suitable for water nucleation measurements. An interface between the LFTR and a mass spectrometer was developed to analyze the chemical content of the freshly nucleated water particles. Contaminants were detected in the initial configuration of the LFTR. As a result, improvements were made to the LFTR to achieve ultrahigh purity conditions in the system. The nucleation rate of water vapor as a function of supersaturation was measured over the temperature range 210–250 K. The first measurement of the nucleation rate of water at a temperature of 210 K was obtained. Reasonable agreement with the classical theory predictions is observed for temperatures in the range 230–250 K. However, below 220 K, classical theory begins to overestimate nucleation rates compared to experimental data and the disagreement grows with decreasing temperature. The experimental data obtained provide an excellent benchmark for further nucleation studies of binary, ternary, and more complex systems.
Diffusion study of the exchange-biased NiFe/MnIr/CoFe electrode in magnetic tunnel junctions
Authors: Yoon C. S. , Lee J. H. , Jeong H. D. , Kim C. K. , Yuh J. H. , Haasch Rick
Published: July 2002

The exchange-biased electrode of magnetic tunneling junctions with under- and overoxidized AlOx insulator layers were annealed at 300 °C to study the extent of the Mn diffusion. A significant amount of Mn diffusion into the AlOx layer was observed in the overoxidized junction sample from Auger electron spectroscopy and the extent of Mn diffusion increased with increasing oxidation time. X-ray photoelectron spectroscopy (XPS) proved that Mn found at the CoFe/AlOx interface in the overoxidized junction sample was in the form of Mn oxide. The XPS data suggest that the Mn diffusion was accelerated by the preferential oxidation of Mn at the CoFe/AlOx interface.
Dislocation behavior in InGaN/GaN multi-quantum-well structure grown by metalorganic chemical vapor deposition
Authors: Kim Y.-W. , Suh E.-K. , Lee H. J.
Published: July 2002

Propagation characteristics of dislocations were investigated in InGaN/GaN multi-quantum-well structures grown by metalorganic chemical vapor deposition. Threading dislocations with Burgers vector of b=〈1̄1̄20〉, emerged from the GaN buffer region, change their glide plane from normal to parallel to the growth plane when they meet InGaN wells. Dislocations gliding on the growth planes were pinned by quantum dots leaving two possible ways of propagation, changing their glide plane back to normal to the growth plane, {101̄0}, or extending loop shape pinned both ends by the quantum dots. Indium-rich quantum dots were formed on the InGaN quantum-well layers with size of 30±25 nm in diameter. It was estimated that the critical size of quantum dots to pin the dislocations is 30 nm.
Characterization of rapid-thermal-annealed InAs/In0.15Ga0.85As dots-in-well heterostructure using double crystal x-ray diffraction and photoluminescence
Authors: Krishna S. , Raghavan S. , Gray A. L. , Stintz A. , Malloy K. J.
Published: July 2002

The effect of rapid thermal annealing on a 10-layer InAs/In0.15Ga0.85As dots-in-a-well (DWELL) heterostructure was studied using double crystal x-ray diffraction (DCXRD) and photoluminescence (PL). From the x-ray rocking curves obtained for symmetric (004) and asymmetric (224) scans, the change in the in-plane and out-of-plane lattice constant and average composition in the DWELL structure were calculated. Thermally induced strain relaxation, which leads to an enhanced In/Ga interdiffusion preferentially along the growth direction, is believed to be the main mechanism for the changes in the structural and optical properties of the sample. Excellent correlation was observed between the PL and the DCXRD measurements.
Nanodomains of pyrochlore formed by Ti ion implantation in yttria-stabilized zirconia
Authors: Zhu S. , Zu X. T. , Wang L. M. , Ewing R. C.
Published: July 2002

The microstructural evolution of a single crystal of yttria-stabilized zirconia (YSZ) implanted with Ti has been studied by cross-sectional transmission electron microscopy (TEM). The implantation of 180 keV Ti ions to a dose of 1×1017 ions/cm2 was completed at room temperature. After annealing at 1100 °C in an Ar atmosphere for 2 h, a phase transition from the fluorite structure of ZrO2 to an isometric pyrochlore structure-type, A2B2O7, occurred due to cation ordering. High-resolution TEM revealed nanodomains of pyrochlore, Y2(TixZr1−x)2O7, with a≅10.24±0.05 Å. The nanodomains of the pyrochlore phase, embedded within the YSZ fluorite substrate, occurred in a depth range from 45 to 105 nm below the surface, which corresponds to Ti concentrations from ∼10 to ∼15 at. %. The nanoscale pyrochlore precipitates and the YSZ matrix have a completely coherent orientation.
Comparison of strain relaxation in InGaAsN and InGaAs thin films
Authors: Adamcyk M. , Schmid J. H. , Tiedje T. , Koveshnikov A. , Chahboun A. , Fink V. , Kavanagh K. L.
Published: July 2002

We compare the strain relaxation of In0.08Ga0.92As and In0.12Ga0.88As0.99N0.01 epitaxial thin films grown on GaAs (001) by elemental-source molecular-beam epitaxy. The epilayers we studied were essentially identical in their compressive lattice mismatch (0.62±0.02%), and thickness (600 nm). The strain state of the samples was determined by in situ substrate curvature monitoring, and by ex situ x-ray diffraction and plan-view transmission electron microscopy. We observe a slower rate of strain relaxation, and a 25% higher residual strain in the nitride. This is attributed to the presence of nitrogen interstitials in the InGaAsN epilayers and/or to the higher nitrogen bond strengths.
Nanometer-sized optical waveguides fabricated by anodic oxidation using a scanning near-field optical microscope
Authors: Onuki T. , Tokizaki T. , Watanabe Y. , Tsuchiya T. , Tani T.
Published: July 2002

We have fabricated an optical waveguide with a subwavelength cross section that propagates light. A metal–oxide core is partially embedded into a metal clad by anodic oxidation using the probe tip of a scanning near-field optical microscope (SNOM). Then, using the SNOM in transmission mode we have evidence of light propagating more than 5 μm in the waveguide whose core width and thickness are 300 and 70 nm, respectively.
Design criteria of a chemical reactor based on a chaotic flow
Authors: Tang X. Z. , Boozer A. H.
Published: July 2002

We consider the design criteria of a chemical mixing device based on a chaotic flow, with an emphasis on the steady-state devices. The merit of a reactor, defined as the Q-factor, is related to the physical dimension of the device and the molecular diffusivity of the reactants through the local Lyapunov exponents of the flow. The local Lyapunov exponent can be calculated for any given flow field and it can also be measured in experimental situations. Easy-to-compute formulae are provided to estimate the Q-factor given either the exact spatial dependence of the local Lyapunov exponent or its probability distribution function. The requirements for optimization are made precise in the context of local Lyapunov exponents.
Motion of grains down a bumpy surface
Authors: Samson L. , Ippolito I. , Bideau D. , Batrouni G. G.
Published: July 2002

We summarize in this article an extensive experimental and theoretical effort carried out to understand the behavior of a single ball when rolling down a bumpy surface. This may appear to be a simple problem but in fact is one that displays a rich variety of different behaviors which allow us to understand better dissipative systems such as granular media. Studies performed previously have shown that the motion of the single ball on the rough surface can be characterized by three different dynamic regimes according to the different values of the two control parameters, the inclination angle θ and the ratio Φ=R/r, where R is the radius of the rolling ball and r the radius of the glass beads which make up the rough surface. The three regimes are a decelerated regime A, a stationary regime B, characterized by a constant average velocity and a jumping regime C. This result was found to be independent of the composition of the rolling ball and the rough surface. It has been demonstrated that regime B is characterized by a viscous-like friction force that appears for specific parameter values. This friction force can be explained by a model whose central ingredient is the geometry of the surface. The trajectory of the ball in regime B can be pictured as a driven random walk motion where the fluctuations of the local velocities are due to collisions of the moving sphere and the surface grains. A detailed analysis of diffusive properties of the motion is discussed.
Luminescence of the rare-earth ion ytterbium in InP, GaP, and GaAs
Authors: Ennen H. , Pomrenke G. , Axmann A.
Published: July 2002

After implantation of ytterbium (Yb) in InP, GaP, and GaAs and subsequent annealing, we observed sharply structured photoluminescence bands at around 1.00 μm (1.24 eV). These emissions are assigned to the internal 4f-4f transitions 2F5/2→2F7/2 of Yb3+ (4f13). Isochronal annealing studies and variations of the implantation dosages for Yb in InP were performed to optimize the luminescence efficiency and to decide whether there are different Yb centers responsible for the luminescence bands. It is shown that the dominant luminescence band arises from a cubic Yb3+ center which resides substitutionally on a cation site (In or Ga). Zeeman measurements and photoluminescence excitation spectroscopy further support this interpretation. The influence of electron irradiation on the luminescence efficiency of InP:Yb has been investigated. It is found that the Yb3+ luminescence intensity is only weakly affected, while the near band-edge emission is strongly quenched.
Temperature dependence of semiconducting and structural properties of Cr-Si thin films
Authors: Nava F. , Tien T. , Tu K. N.
Published: July 2002

Electrical and structural properties of coevaporated Cr-Si thin alloy films and bilayer Cr/Si films as a function of annealing temperature from 10 to 1000 °K have been studied by in situ electrical resistivity and Hall measurements, and structural analysis including MeV 4He+ ion backscattering, x-ray diffraction, Auger electron spectroscopy combined with Ar sputtering, electron microprobe, and scanning and transmission electron microscopy. In the as-deposited state, the coevaporated alloy film was amorphous. Upon annealing, a sharp increase in resistivity occurred near 270 °C and the increase has been determined to be amorphous to crystalline CrSi2 phase transformation. The resistivity increased further with annealing up to 550 °C then a gradual decrease took place beyond 600 °C. In cooling, the resistivity increased monotonically with decreasing temperature. For the bilayer Cr/Si films, the annealing behavior is similar except the sharp increase in resistivity occurred around 450 °C due to the formation of CrSi2. The crystalline CrSi2 has been determined to be a semiconductor with an energy gap of 0.27 eV. It is p-type, having a hole concentration of 4×1019 cm−3 at room temperature and a hole mobility of 7.2×104×T (temp)−3/2 cm2/V sec in the acoustic scattering region. The kinetics of amorphous-to-crystalline transformation of Cr-Si alloy film in the temperature range of 225–25 °C has been determined to follow a t7 (time) dependence with an apparent activation energy of 1.13 eV.
Microscopic study of grain-boundary region in polycrystalline ferrites
Authors: Nakata A. , Chihara H. , Sasaki A.
Published: July 2002

The nature of grain boundary of manganese zinc ferrites is critical for magnetic properties. Many studies on grain-boundary structure and chemistry have been done, and those experimental results do not seem to fully describe the magnetic properties. In this study, close analysis of the grain-boundary region using high-resolution electron microscopy has made it clear that each grain consists of many small regions of 5–10 nm in size. The crystallographic orientation of each region is slightly different from each other in the order of 0.1–0.4°. And some defects are observed in lower permeability ferrites. Auger microprobe analyses clarify the segregation of Ca and Si and increase of Fe/O ratio at the grain boundary. The structural features will be discussed in relation to segregation and change of composition at the grain boundary.
Stress in silicon at Si3N4/SiO2 film edges and viscoelastic behavior of SiO2 films
Authors: Isomae Seiichi
Published: July 2002

Dislocation generation in silicon substrates at Si3N4/SiO2 film edges is investigated after thermal treatment at 900–1200 °C for 2 h in a N2 atmosphere. In order to explain the obtained experimental results, we propose a model to calculate stresses in silicon at the film edge. In the present model, it is assumed that the SiO2 film is a Maxwell viscoelastic solid, and stress values are obtained using the finite element method and analytical solutions under plain strain conditions. The calculated results show that the shape of the stress distribution at the Si3N4/SiO2 film edge is different from that at the Si3N4 film edge, and that stress in the former is small in comparison with that at the latter. As a result, it is proved that the present model gives a physical interpretation for the suppression of dislocation generation with Si3N4/SiO2 films.