bluuebunny/crossref_metadata_2025 huggingface dataset:
Room-temperature ferromagnetism in zincblende CrSb grown by molecular-beam epitaxy
Authors: Zhao J. H. , Matsukura F. , Takamura K. , Abe E. , Chiba D. , Ohno H.
Published: July 2002

Thin films of CrSb grown by solid-source molecular-beam epitaxy on GaAs, (Al, Ga)Sb, and GaSb are found to exhibit ferromagnetism. Reflection high-energy electron diffraction and high-resolution cross sectional transmission electron microscopy both indicate that the structure is zincblende. Temperature dependence of remanent magnetization shows that the ferromagnetic transition temperature is beyond 400 K.
On the continuum limit of fermionic topological charge in lattice gauge theory
Authors: Adams David H.
Published: July 2002

It is proved that the fermionic topological charge of SU(N) lattice gauge fields on the four-torus, given in terms of a spectral flow of the Hermitian Wilson–Dirac operator or, equivalently, as the index of the overlap Dirac operator, reduces to the continuum topological charge in the classical continuum limit when the parameter m0 is in the physical region 0<m0<2.
Time-resolved fluorescence and absorption spectroscopies of porphyrin J-aggregates
Authors: Kano Hideaki , Kobayashi Takayoshi
Published: July 2002

Dynamics of excited states in porphyrin J-aggregates has been investigated using femtosecond time-resolved fluorescence and absorption spectroscopies. An ultrafast relaxation process due to internal conversion (IC) from the S2-exciton state to the S1-exciton state is observed as an S2-fluorescence and a recovery from a bleaching of the S2-exciton state. The S2-fluorescence shows a sharp spectrum with almost no Stokes shift with a decay-time constant of 360±70 fs. In the transient absorption spectrum, the bleaching of the S2-exciton state disappears with a time constant of about 300 fs, which is in agreement with the result of the time-resolved fluorescence data. Relaxation dynamics of the S1-exciton following S2→S1IC is also studied and several relaxation processes such as an intra-aggregate vibrational energy redistribution, vibrational and phase-space coolings are investigated.
A new approximate analytical approach for dispersion relation of the nonlinear Klein–Gordon equation
Authors: Lim C. W. , Wu B. S. , He L. H.
Published: July 2002

A novel approach is presented for obtaining approximate analytical expressions for the dispersion relation of periodic wavetrains in the nonlinear Klein–Gordon equation with even potential function. By coupling linearization of the governing equation with the method of harmonic balance, we establish two general analytical approximate formulas for the dispersion relation, which depends on the amplitude of the periodic wavetrain. These formulas are valid for small as well as large amplitude of the wavetrain. They are also applicable to the large amplitude regime, which the conventional perturbation method fails to provide any solution, of the nonlinear system under study. Three examples are demonstrated to illustrate the excellent approximate solutions of the proposed formulas with respect to the exact solutions of the dispersion relation.
Development of microwave-optical double-resonance spectroscopy using a Fourier-transform microwave spectrometer and a pulsed laser
Authors: Nakajima Masakazu , Sumiyoshi Yoshihiro , Endo Yasuki
Published: July 2002

A new type of microwave-detected microwave-optical double-resonance (MODR) spectroscopy has been developed using a Fourier-transform microwave spectrometer and a tunable pulsed dye laser. In this method, a free-induction decay (FID) signal was detected instead of the microwave (MW) absorption. To demonstrate the performance, we measured the MODR spectra of the CCS and C4H radicals in supersonic jets generated by a pulsed-discharge nozzle. Since the pulsed sources are employed for both the optical and microwave radiations, it is possible to control the relative timing of irradiations of the MW and optical pulses. We were able to obtain two different types of spectra; one is the ordinary population labeling spectrum, and the other is a spectrum obtained by breaking the coherence of molecules. In the latter case, more than 50% of depletion of the FID signal was observed, which is unable to be attained when noncoherent phenomena are used to detect the double-resonance signal.
Measurement of complex optical constants of a highly doped Si wafer using terahertz ellipsometry
Authors: Nagashima Takeshi , Hangyo Masanori
Published: July 2002

We propose and demonstrate a terahertz (THz) time-domain spectroscopy combined with ellipsometry. The complex optical constants of a Si wafer with low resistivity are deduced from the measurements of the wave forms of reflected s- and p-polarized THz pulses without reference measurement. The obtained dispersion of refractive index above ∼0.2 THz shows good agreement with that predicted by the Drude theory. The complex optical constants deduced by the THz ellipsometry in the low-frequency region are strongly affected by the slight error of the ellipsometric angle originating mainly from the misalignment of the rotation angles of the polarizer and analyzer.
Tracking target and spiral waves
Authors: Jensen F. G. , Sporring J. , Nielsen M. , Sørensen P. G.
Published: July 2002

A new algorithm for analyzing the evolution of patterns of spiral and target waves in large aspect ratio chemical systems is introduced. The algorithm does not depend on finding the spiral tip but locates the center of the pattern by a new concept, called the spiral focus, which is defined by the evolutes of the actual spiral or target wave. With the use of Gaussian smoothing, a robust method is developed that permits the identification of targets and spirals foci independently of the wave profile. Examples of an analysis of long image sequences from experiments with the Belousov–Zhabotinsky reaction catalyzed by ruthenium-tris-bipyridyl are presented. Moving target and spiral foci are found, and the speed and direction of movement of single as well as double spiral foci are investigated. For the experiments analyzed in this paper it is found that the movement of a focus correlates with foci in the immediate neighborhood independently of how they were created.
Comparison of catalytically grown and arc-discharge carbon nanotube tips
Authors: Thiên-Nga Lê , Bonard Jean-Marc , Gáal Richard , Forró László , Hernadi Klara
Published: July 2002

We have performed a detailed transmission electron microscopy study of the tip of carbon nanotubes prepared by chemical vapor deposition (CVD) and by arc discharge. We found that a large proportion of the CVD-grown tubes have well-formed caps but that the graphitization of the walls is far from perfect. The tip shapes are also more rounded and the tip radius is higher when compared to arc-discharge tube tips. These features are discussed in relation with the growth mechanism and with the consequences they might have for applications in scanning probe microscopies.
Effects of viscous flow on residual stresses in film/substrate systems
Authors: Hsueh Chun-Hway , Lee Sanboh
Published: July 2002

An analytical model is developed to analyze the effects of viscous flow on residual stresses in film/substrate systems. This is achieved by utilizing the analogy between the governing field equation of elasticity and the Laplace transform with respect to time of the viscoelastic field equation. While viscous flow can occur in either the film or the substrate, analyses of how viscous flow in the substrate relaxes residual stresses in the film have not been performed. Also, the film thickness is often ignored in analyzing stress relaxation due to viscous flow in the film. The above two issues are studied in the present analysis. For the case of viscous flow in the film, the stress relaxation rate decreases with increasing film thickness. Conversely, for the case of viscous flow in the substrate, the stress relaxation rate increases with increasing film thickness. Compared to viscous flow in the film, viscous flow in the substrate results in slower stress relaxation in the system.
Absorption and emission in oligo-phenylene vinylene nanoaggregates: The role of disorder and structural defects
Authors: Spano Frank C.
Published: July 2002

The impact of exciton-phonon coupling and defect states on the photophysical properties of p-distyrylbenzene nanoaggregates is studied numerically. Molecular packing within aggregates is based on the known crystal structures of poly-p-phenylene vinylene (Type I) and the five phenyl group oligomer (Type II). Calculations of absorption and emission are conducted using a reduced basis set consisting of all one- and two-particle vibronic states. The calculated spectra are very similar for both aggregate types, the only substantial difference being the polarization directions for the J-band and 0-0 emission line. Under the noninteracting domains approximation the calculated nanoaggregate absorption spectrum is in excellent agreement with experiment, assuming an exciton coherence length of approximately 20 Å. In the calculated emission spectrum the 0-0 emission is uniquely polarized compared with the rest of the vibronic progression, also in agreement with experiment. The 0-0 emission intensity in defect-free Type I and II aggregates is linearly proportional to the total number of molecules, becoming superradiant beyond a certain size threshold. The 0-0 emission is highly sensitive to stacking faults and dislocations. These defects account for the measured Stokes shift, but quench the 0-0 emission (and superradiance) while only slightly affecting the rest of the vibronic progression. Adding orientational point defects to an aggregate with stacking faults and/or dislocations enhances the 0-0 oscillator strength, bringing the 0-0 emission intensity into good agreement with experiment.
Stimulated emission in blue-emitting Si+-implanted SiO2 films?
Authors: Luterová K. , Pelant I. , Mikulskas I. , Tomasiunas R. , Muller D. , Grob J.-J. , Rehspringer J.-L. , Hönerlage B.
Published: July 2002

We investigate the blue photoluminescence of Si+-implanted SiO2 films under picosecond UV excitation. The emission intensity exhibits a nonlinear increase with increasing excitation intensities, accompanied by pulse shortening. The photoluminescence decays nonmonoexponentially in time. However, the nonlinearities are not associated with significant spectral narrowing. To explain the results, we propose and numerically investigate a kinetic model based on competition between radiative (both spontaneous and stimulated) and nonradiative recombination in isolated luminescence centers in the SiO2 matrix. Good agreement between theoretical and experimental data seems to confirm the existence of stimulated emission in the films, however, under extremely high excitation densities only (approximately 100 MW/cm2).
Self-organized lateral ordering for vertically aligned PbSe/PbEuTe quantum-dot superlattices
Authors: Raab A. , Lechner R. T. , Springholz G.
Published: July 2002

The growth of vertically aligned self-assembled PbSe/Pb1−xEuxTe quantum-dot superlattices is found to result in a pronounced hexagonal lateral ordering tendency accompanied by a narrowing of the size distribution. In addition, the lateral dot separations and dot densities become tunable by changes in the spacer thickness with an almost twofold density increase for spacer thicknesses increasing from 100 to 275Å. Similar marked changes are also found for PbSe dot sizes and shapes. This could provide additional means for the tuning of the optical and electronic properties of the dots.
Theoretical investigation of iron carbide, FeC
Authors: Tzeli Demeter , Mavridis Aristides
Published: July 2002

Employing multireference variational methods (MRCI), we have constructed full potential-energy curves for the ground state (X 3Δ) and forty excited states of the diatomic carbide, FeC. For all states we report potential-energy curves, bond lengths, dissociation energies, dipole moments, and certain spectroscopic constants, trying at the same time to get some insight on the bonding mechanisms with the help of Mulliken populations and valence-bond–Lewis diagrams. For the X 3Δ state at the MRCI level of theory, we obtain a dissociation energy De=86.7 kcal/mol at a bond length re=1.581 Å. These values compare favorably to the corresponding experimental ones, De=91.2±7 (upper limit) kcal/mol and re=1.5924 Å. The first excited state (1Δ) is predicted to be 9.7 kcal/mol above the X-state as compared to an experimental value of 9.786 kcal/mol.
Effects of the matrix on self-organization of InAs quantum nanostructures grown on InP substrates
Authors: Li Hanxuan , Daniels-Race Theda , Hasan Mohamed-Ali
Published: July 2002

We have studied the influence of matrix materials on the self-organization of InAs nanostructures grown on InP substrates by molecular-beam epitaxy. Our results show that InAs quantum dots are formed on InAlGaAs, whereas quantum-wire-like structures are produced on InAlAs and InGaAs. Tuning from vertical anticorrelation in InAs/InAlAs superlattices to vertical correlation in InAs/InGaAs and InAs/InAlGaAs superlattices is observed, which is explained by the size effects in the nanostructure–nanostructure interaction.
Intrinsic noise of a micromechanical displacement detector based on the radio-frequency single-electron transistor
Authors: Zhang Yong , Blencowe Miles P.
Published: July 2002

We investigate the intrinsic noise of a micromechanical displacement detector based on the radio-frequency single-electron transistor (rf-SET). Using the noise analysis of a SET by Korotkov [Phy. Rev. B 49, 10381 (1994)] as our starting point, we determine the spectral density of the displacement noise due to the tunneling current shot noise. The resulting mechanical displacement noise decreases in inverse proportion to the increasing gate voltage. In contrast, the displacement noise due to the fluctuating SET island charge increases approximately linearly with increasing gate voltage. Taking into account both of these noise sources results in an optimum gate voltage value for the lowest displacement noise and hence best sensitivity. We show that a displacement sensitivity of about 10−4 Å and a force sensitivity of about 10−16 N are predicted for a micron-sized cantilever with a realizable resonant frequency 100 MHz and quality factor Q∼104. Such sensitivities would allow the detection of quantum squeezing in the mechanical motion of the micromechanical cantilever and the detection of single-spin magnetic resonance in magnetic resonance force microscopy.
Electron transport measurements of Schottky barrier inhomogeneities
Authors: Calvet L. E. , Wheeler R. G. , Reed M. A.
Published: July 2002

We report nonmonotonicities in the low-temperature current versus gate voltage characteristics of PtSi/Si Schottky Barrier metal–oxide–semiconductor field-effect transistors. Direct tunneling through the Schottky barrier is shown to limit the current and be superimposed with resonant peaks and oscillations. These structures are attributed to resonant tunneling through impurities located close to the interface and nonuniformities of the heterojunction. We thus demonstrate barrier height variations in electron transport through a relatively large metal/semiconductor contact area. The inhomogeneities result in different average Schottky barrier heights between devices, and cause height variations as a function of carrier concentration within a metal/semiconductor interface.
Mobility anisotropy in InSb/AlxIn1−xSb single quantum wells
Authors: Ball M. A. , Keay J. C. , Chung S. J. , Santos M. B. , Johnson M. B.
Published: July 2002

Three types of defects at the surface of InSb quantum well samples are identified: hillocks, square mounds, and oriented abrupt steps. The electron mobility in the quantum well correlates to the density of abrupt features, such that samples with a high density of anisotropic defects show anisotropy in the mobility. We propose that the dominant scattering mechanism associated with these abrupt features is a fluctuation in the quantum well morphology.
Excitation of remarkably nondispersive surface plasmons on a nondiffracting, dual-pitch metal grating
Authors: Hibbins Alastair P. , Sambles J. Roy , Lawrence Chris R.
Published: July 2002

A nondiffracting metallic lamellar grating formed from three equally spaced grooves per repeat period, with one being slightly shallower than the other two is examined at microwave frequencies. When filled with a slightly lossy dielectric, this structure supports a remarkably nondispersive surface plasmon polariton mode, which exhibits strong selective absorption of incident power. Measured reflectivities show excellent agreement with the results predicted by a rigorous coupled wave theory.
A class of cellular automata modeling winnerless competition
Authors: Afraimovich V. , Ordaz F. C. , Urı́as J.
Published: July 2002

Neural units introduced by Rabinovich et al. (“Sensory coding with dynamically competitive networks,” UCSD and CIT, February 1999) motivate a class of cellular automata (CA) where spatio-temporal encoding is feasible. The spatio-temporal information capacity of a CA is estimated by the information capacity of the attractor set, which happens to be finitely specified. Two-dimensional CA are studied in detail. An example is given for which the attractor is not a subshift.
Microgating carbon nanotube field emitters by in situ growth inside open aperture arrays
Authors: Hsu David S. Y.
Published: July 2002

Multiwalled carbon nanotubes were grown using chemical vapor deposition inside small apertures having a horizontal gate and a sidewall insulator spacer. Emission currents up to 140 nA per cell at 63 V have been obtained. These arrays have exhibited a gate current as low as 2.5% of the anode current throughout the entire gate voltage range, representing the lowest gate to anode current ratio of gated nanotube emitters reported to date. We attribute this feature to the emitter geometry and method of fabrication. The overall fabrication method required only a few and simple processing steps.
Vibration and two-photon absorption
Authors: Bishop David M. , Luis Josep M. , Kirtman Bernard
Published: July 2002

A treatment of nuclear vibrational motions in two-photon absorption is laid out in detail. Perturbation theory is used to develop working formulas for diatomic molecules and tested for the X 1Σ+–A 1Π transition of carbon monoxide. The results are compared with the exact numerical values. Certain vibrational contributions, not previously recognized, are found to make an important contribution to the two-photon absorption probability.
Transport properties of ferromagnet/insulator/semiconductor tunnel junctions
Authors: Manago T. , Suzuki Y. , Tamura E.
Published: July 2002

Spin-dependent transport of the photoexcited electrons in the semiconductor (p-GaAs)/insulator (Al2O3)/ferromagnetic metal (permalloy) junctions was investigated. As samples, homogeneous tunneling junctions were prepared on the flat and As-defect-free GaAs(111)B homoepitaxial surface by in situ oxidation of the Al layer and successive metallization by permalloy deposition. Spin-polarized electrons were excited in the GaAs by circularly polarized light and injected into the permalloy layer. Since the permalloy has almost zero magnetic circular dichroism at the vicinity of the band gap energy of GaAs, we can detect spin-dependent current exclusively. As a result, the energy dependence of the observed helicity asymmetry (1.44–3.05 eV) of the photoinduced current shows the absence of the spin-dependent tunneling in the sample. The importance of controlling the electron lifetime to obtain the spin-dependent tunneling was discussed.
Modeling of kink-shaped carbon-nanotube Schottky diode with gate bias modulation
Authors: Yamada Toshishige
Published: July 2002

A model is proposed for the recent gate voltage VG modulation experiment of a kink-shaped carbon nanotube (NT) Schottky diode [Z. Yao, H. Postma, L. Balents, and C. Dekker, Nature (London) 402, 273 (1999)]. Since larger VG increases both the forward and the reverse turn-on voltages of the diode, we show that: (1) the rectification must occur at the kink where the metallic and the semiconducting NTs meet, and not at the electrode contact, and (2) the semiconducting NT must be n type. The turn-on voltages are derived analytically as a function of VG considering the electrode contact contribution and a good agreement is obtained with the experimental data.
Calorimetric studies of the A1 to L1 transformation in FePt and CoPt thin films
Authors: Barmak K. , Kim J. , Shell S. , Svedberg E. B. , Howard J. K.
Published: July 2002

Using differential scanning calorimetry, the enthalpy (activation energy) for the A1 to L10 transformation in FePt and CoPt thin films have been measured as −10.2±2.1 kJ/g -atom (1.7±0.1 eV) and −3.1±0.2 kJ/g-atom (2.8±0.2 eV), respectively. FePt is further seen to transform at temperatures that are approximately 120 °C lower than those for CoPt. The enthalpy of grain growth that accompanies the transformation is shown to be negligibly small by comparison to the transformation enthalpy in both films. The impact of our findings on alloy development for ultrahigh density magnetic storage media is discussed.
Rate processes in nonlinear optical dynamics with many attractors
Authors: Arecchi F. T.
Published: July 2002

Kramers’ 1940 paper and its successive elaborations have extensively explored the transition rate between two stable situations, that is, in the language of system dynamics, the transition between the basins of attraction of two stable fixed point attractors. In a nonequilibrium system some of the above conditions may be violated, either because one of the two fixed points is unstable, as in the case of transient phenomena, or because both fixed points are unstable, as in the case of heteroclinic chaos, or because the attractors are more complex than fixed points, as in a chaotic dynamics where two or more strange attractors coexist. Furthermore, there is recent experimental evidence of space-time complexity consisting in the alternate or simultaneous oscillation of many modes, each one with its own (possibly chaotic) dynamics. In all the above cases, coexistence of many alternative paths implies a choice, either due to noise or self-triggered by the same interacting degrees of freedom. A review of the above phenomena in the case of nonequilibrium optical systems is here presented, with the aim of stimulating theoretical investigation on these novel rate processes.
Chaotic synchronization in Hamiltonian systems
Authors: Heagy J. F. , Carroll T. L.
Published: July 2002

It is shown that Hamiltonian systems can exhibit the phenomenon of chaotic synchronization. Specific attention is paid to the standard map. Analytic synchronization conditions are derived and numerically verified for the standard map. We report on experimental studies of an analog electronic circuit realization of a ‘‘piecewise linear standard map.’’ When coupled appropriately to a duplicate circuit, chaotic synchronization is observed. The relevance of this study to synchronization in other Hamiltonian systems is discussed.
Complex spatiotemporal convection patterns
Authors: Pesch W.
Published: July 2002

This paper reviews recent efforts to describe complex patterns in isotropic fluids (Rayleigh–Bénard convection) as well as in anisotropic liquid crystals (electro-hydrodynamic convection) when driven away from equilibrium. A numerical scheme for solving the full hydrodynamic equations is presented that allows surprisingly well for a detailed comparison with experiments. The approach can also be useful for a systematic construction of models (order parameter equations).
Convergence of Hamiltonian systems to billiards
Authors: Collas Peter , Klein David , Schwebler Hans-Peter
Published: July 2002

We examine in detail a physically natural and general scheme for gradually deforming a Hamiltonian to its corresponding billiard, as a certain parameter k varies from one to infinity. We apply this limiting process to a class of Hamiltonians with homogeneous potential-energy functions and further investigate the extent to which the limiting billiards inherit properties from the corresponding sequences of Hamiltonians. The results are mixed. Using theorems of Yoshida for the case of two degrees of freedom, we prove a general theorem establishing the “inheritability” of stability properties of certain orbits. This result follows naturally from the convergence of the traces of appropriate monodromy matrices to the billiard analog. However, in spite of the close analogy between the concepts of integrability for Hamiltonian systems and billiards, integrability properties of Hamiltonians in a sequence are not necessarily inherited by the limiting billiard, as we show by example. In addition to rigorous results, we include numerical examples of certain interesting cases, along with computer simulations.
Dielectrophoretic ratchets
Authors: Gorre-Talini L. , Spatz J. P. , Silberzan P.
Published: July 2002

We have experimentally applied some concepts of “force-free” motion to micron size particles (latex beads). The coupling of dissipation and local spatial asymmetry of the potential experienced by the beads can put them into motion. The potentials used in these experiments are of dielectrophoretic nature. To that end, electrodes of particular shapes were used in order to submit the considered suspensions to inhomogeneous ac electric fields. Two regimes were explored: i—the Brownian ratchet case in which a Brownian particle is successively trapped in a factory roof-like potential and left free to diffuse. ii—the shifted ratchets case in which two potentials exhibiting similar characteristics are applied successively, one of them being shifted by a fraction of their common period relatively to the other. In both cases, a good agreement with the theoretical predictions was observed. In particular, particles of different sizes were characterized by different macroscopic velocities leading to the prospect of promising separation techniques.
A spectroscopical measure for the exploration of phase space
Authors: Vega J. L. , Uzer T.
Published: July 2002

Many nonintegrable systems have eigenstates that typically require numerous basis states to represent them. We develop a criterion to judge the extent to which phase space is explored by the spectrum of such a Hamiltonian. Our criterion uses the eigenvalues rather than the eigenfunctions and is based on identifying a direct relation between the intensity of Shnirelman’s peak and the localization length. We illustrate our procedure by applying it to the spectrum of two prototypical nonintegrable systems, the kicked rotor and the kicked top.
Convection in horizontally vibrated granular material
Authors: Medved Milica , Dawson Damien , Jaeger Heinrich M. , Nagel Sidney R.
Published: July 2002

We report observations of convective motion in a container filled with granular material when it is vibrated in the horizontal direction. We find that the roughness of the boundaries and the container dimensions play an important role in determining the shape and number of the convection cells. When the container bottom and lateral walls are rough, the system typically exhibits four counter-rotating rolls stacked in two pairs on top of each other; for very low filling height, it is possible to observe a single row of rolls arranged laterally along the bottom of the container. With smooth walls, on the other hand, we find that the system typically forms only a single pair of counter-rotating convection rolls that originate in the two upper corners of the vibrated material; when the filling height is increased to a level that depends on the container width, we observe a transition to the four-roll state.
Deterministic nonlinearity in ventricular fibrillation
Authors: Small Michael , Yu Dejin , Harrison Robert G. , Robertson Colin , Clegg Gareth , Holzer Michael , Sterz Fritz
Published: July 2002

We provide numerical evidence that the electrocardiogram data collected from pigs during induced ventricular fibrillation cannot be described by a monotonic nonlinear transformation of linearly filtered noise. To establish this we use surrogate techniques and apply two test statistics: (1) the Takens’ maximum likelihood estimator of the Grassberger–Procaccia correlation dimension and (2) an improved correlation dimension estimation routine. The improved dimension estimates provide evidence that the correlation dimension of the underlying dynamics during the episode of VF in the first 30 s is slightly less than 6. This result is consistent and reproducible among subjects.
Magneto-optic recording technology (invited)
Authors: Kryder Mark H.
Published: July 2002

Magneto-optic recording has great potential as an erasable/rewritable storage technology. Bit densities of 108/cm2 are being achieved and improvements can be expected. This paper reviews the status and discusses the most critical issues of the technology. It is shown that the fundamental broadband signal-to-noise ratio in existing rare-earth transition-metal media is well above 20 dB and therefore adequate to maintain error rates less than 10−12. Substrate imperfections in optical media lead to error rates of 10−5–10−6, but even these relatively high raw bit error rates are corrected to 10−13 by error correction codes and hardware being used in write-once optical disk storage systems. The stability of magnetic domains recorded into magneto-optical materials may be explained by a model involving wall energy, demagnetizing fields, applied fields and wall coercivity, and there exist materials which fulfill the requirements necessary to have recorded data remain stable against fields up to 100 Oe and temperatures from 0 to 100 °C. Of more concern is the stability of the amorphous thin films against long-term annealing. Passivation with optical dielectrics is essential, and preliminary data on some rare-earth transition-metal alloys indicate that recorded data would be stable for five years when stored at 50 °C and that the media could withstand in excess of 1010 write/erase cycles with less that 10% change in minimum stable domain size. It is concluded that magneto-optic recording technology is a promising high-density recording technology with a place in the digital storage hierarchies of the future.
Study of forward I-V plot for Schottky diodes with high series resistance
Authors: Sato K. , Yasumura Y.
Published: July 2002

The current-voltage characteristics for Schottky barrier diodes with series resistance are discussed. It is shown that by using Norde’s function F(V)=V/2−(kT/q)ln(I/SAT2) at two different temperatures, barrier height, n-value or ideality factor, and series resistance can be determined even in the case 1<n<2.
Rapid thermal anneal and furnace anneal of beryllium-implanted Ga0.47In0.53As
Authors: Maier M. , Selders J.
Published: July 2002

The activation efficiency of conventional furnace annealing (550–750 °C, 30 min, capless, AsH3/H2 ambient) and rapid thermal annealing (800 °C,∼1 s, SiO2 cap, N2 ambient) has been compared in Be-doped Ga0.47In0.53As by investigating the carrier profiles and the atomic profiles. Nearly complete activation (100%) occurs during both 750 °C furnace annealing and rapid thermal annealing for an implanted fluence of 6×1013 Be ions/cm2. At a fluence of 6×1014 Be ions/cm2 a significantly higher activation (>50%) is found after rapid thermal annealing as compared with furnace annealing (30%) mainly due to out-diffusion of Be during furnace annealing. Low-temperature photoluminescence (2 K) revealed that comparable recrystallization is achieved in both methods. A high diffusivity of Be is apparent in the atomic depth profiles after furnace annealing. However, only insignificant redistribution is noticeable after rapid thermal annealing, probably due to gettering of the diffusing Be by defects.
The Slater–Pauling curve: First principles calculations of the moments of Fe1−cNic and V1−cFec
Authors: Johnson D. D. , Pinski F. J. , Staunton J. B.
Published: July 2002

We have performed calculations of the electronic structure of the random substitutional alloys Fe1−cNic and V1−cFec using the spin-polarized, self-consistent Korringa–Kohn–Rostoker coherent-potential approximation (KKR-CPA) method. This is a first principles method based on spin density functional theory and a local spin density approximation for the exchange and correlation functional. For fcc Fe1−cNic, a range of volumes were considered for 0.25<c<1.0, and it was found that the moments are sensitive to the volume in the Fe-rich alloys near the INVAR concentration. In bcc V1−cFec, we find the average moments to decrease linearly with vanadium concentration (in the Fe-rich alloys) and the vanadium moment to be antiparallel to the iron moment. The moments vanish when the vanadium concentration becomes greater than 0.7 which is in good agreement with experiment. In V1−cFec, in contrast to bcc Fe1−cNic, the bonding–antibonding ‘‘valley’’ of the minority density of states (in which the Fermi level is pinned) persists over a wide range of concentrations.
The effect of lanthanide substitution on the superconductivity in Ba2YCu3O7 and (La,Sr)2CuO4 (invited)
Authors: Takagi H. , Uchida S. , Eisaki H. , Tanaka S. , Kishio K. , Kitazawa K. , Fueki K.
Published: July 2002

The effect of the lanthanide substitution on the superconductivity in Ba2YCu3O7 and (La,Sr)2CuO4 is investigated. The substitution effect is completely different between the two Cu-oxide systems. In Ba2YCu3O7, the replacement of Y with lanthanides gives negligible change in the transition temperature, whereas a few percent of lanthanide substitution depresses Tc by about 10 K in the (La,Sr)2CuO4 system. The depression of Tc in the latter system depends on the atomic number of lanthanides. From the results of the magnetic susceptibility and the relationship between the lattice parameters and Tc, we conclude that the depression of Tc cannot be ascribed to the magnetic effect nor to the volume effect. Possible mechanisms for the depression of Tc are discussed.
Radially induced magnetic anisotropy in Fe-Cr-Co permanent magnets
Authors: Okada M. , Togashi R. , Sugimoto S. , Homma M.
Published: July 2002

Fe-Cr-Co magnets with radial anisotropy were developed by rotating samples in a magnetic aging furnace. The anisotropy is altered by the applied magnetic field strength. The Fe-25 wt. %Cr-12 wt. %Co-0.5 wt. %Ti alloy aged in a magnetic field of 100 Oe yields the magnetic properties of (BH)max=24 kJ m−3 (3 MGOe) and iHc=48 kA m−1 (600 Oe) parallel to radial direction, which are higher than those of (BH)max=7 kJ m−3 (0.9 MGOe), iHc=33 kA m−1, measured parallel to the circumferential direction. The increment of the applied magnetic field over 200 Oe alters the radial anisotropy into a circumferential one. Microstructural studies indicate that the iron-rich phase is elongated parallel to the direction of induced anisotropy. Computations of the magnetic flux distribution in disk samples confirm the experimental observations.
The effect of Al substitution on the coercivity of Nd-Fe-B magnets
Authors: Strzeszewski J. , Hadjipanayis G. C. , Kim A. S.
Published: July 2002

The effect of Al on the coercivity of Nd-Dy-Fe(Al)B magnets has been investigated with magnetic measurements, differential scanning calorimetry, and microstructure studies. The Al containing samples were found to have a much higher coercivity, a relatively steeper initial magnetization, a different Hc(Hef) curve, and a larger temperature coefficient of coercivity than the samples without Al. Morphology studies made with a scanning transmission electron microscope did not show any significant differences between the two samples. The only evident difference, found in x-ray and electron diffraction data, was in the c/a ratio of the Nd2Fe14B tetragonal phase which was higher for the sample with Al. Another difference was observed in the magnetization reversal mechanism. In the Al-containing samples the magnetization is reversed by changing the magnetization of entire grains, while for the samples without Al the magnetization is reversed by dividing into domains inside the grains.
Raman scattering in a Ga1−xInxP strained heterostructure
Authors: Abdelouhab R. M. , Braunstein R. , Bärner K. , Rao M. A. , Kroemer H.
Published: July 2002

Raman scattering was employed to study the optical phonon modes in Ga1−x Inx P heterostructures. The stress calculated from the pressure coefficient and the observed frequency shift for the GaP-like LO phonon is larger than the one obtained from the lattice mismatch given by the elastic theory; the possibility of other types of defects responsible for this difference is considered. The presence of internal stress in epitaxial layers could induce a switchover from one- to two-mode behavior. The modified random element isodisplacement model is used to explore this possibility.
Excitation mechanisms of the electron-beam-pumped atomic xenon (5d→6p) laser in Ar/Xe mixtures
Authors: Ohwa Mieko , Moratz Thomas J. , Kushner Mark J.
Published: July 2002

The atomic xenon laser operates on seven infrared transitions (1.73–3.51 μm) between the 5d and 6p manifolds. Intrinsic laser power efficiencies exceeding 5% have been previously obtained in Ar/Xe mixtures, principally at 1.73 μm (5d[3/2]1→6p[5/2]2). The kinetic mechanisms responsible for this performance, though, are not well understood. In this paper, we report on a computer model for the electron-beam-pumped xenon laser in Ar/Xe mixtures with which we have investigated some of these excitation mechanisms. Based on the results of a parametric study of power deposition (50 W cm−3 to 100 kW cm−3), gas pressure (0.5–6 atm), and xenon fraction, we suggest that the high efficiency obtained in Ar/Xe mixtures is due to rapid collisional cascade to the upper laser level of the 1.73-μm transition following dissociative recombination of ArXe+ and selective quenching of the lower laser level of the 1.73-μm transition by collisions with argon. The results of our model indicate that the decrease in laser performance at high Xe fractions results from electron-impact excitation of the lower laser levels (6s→6p) and quenching of the 5d manifold by collisions with atomic xenon. The degradation of laser performance at high specific power deposition is most likely due to electron-collision mixing of the 5d and 6p manifolds. As a result of the lower levels being cleared dominantly by atomic collisions, we predict that optimum performance is then obtained at higher gas pressures when increasing power deposition. The results of the model predict that optimum power deposition is obtained when the fractional ionization is ≊2–3×10−6.
Adhesion-induced deformations of polymeric substrates: Particle size dependence of the contact area
Authors: Rimai D. S. , DeMejo L. P. , Bowen R. C.
Published: July 2002

Adhesion-induced deformations of a polyurethane substrate in contact with cross-linked polystyrene spheres, having diameters ranging from less than 2 μm to approximately 12.5 μm were observed using scanning electron microscopy. The diameters of the contact areas were measured from the micrographs. It was found that the contact radius varied as the particle radius raised to the 0.75±0.05 power. Experimental results are compared to the predictions of various adhesion models. The results are also discussed in terms of the Dupré work of adhesion.
Temperature-compensated 2:17-type permanent magnets with improved magnetic properties
Authors: Liu S. , Ray A. E. , Mildrum H. F.
Published: July 2002

Computer programs have been developed to predict temperature coefficients of saturation magnetization for (Sm1−xGdx)2Tm17 magnets with higher Fe content from temperature-dependent magnetic properties of single rare-earth compositions. Very good agreement was obtained between the calculated and measured data. The composition Sm0.54Gd0.46 (Co0.63Fe0.29Cu0.06Zr0.02)7.69 exhibits a nearly zero temperature coefficient of magnetization from −60 to 150 °C. Its maximum energy product remains 16–17 MGOe in the temperature range of 20–200 °C. A room-temperature energy product of 19 MGOe was attained for the composition Sm0.55Gd0.45(Co0.608Fe0.316Zr0.02)7.72.
Cathodoluminescence and electroluminescence of undoped and boron-doped diamond formed by plasma chemical vapor deposition
Authors: Kawarada H. , Yokota Y. , Mori Y. , Nishimura K. , Hiraki A.
Published: July 2002

Visible luminescence between 2.0–3.5 eV of undoped and boron-doped diamond formed by plasma-assisted chemical vapor deposition has been investigated by cathodoluminescence. Electroluminescence from Schottky diode of boron-doped semiconducting diamond has been observed for the first time and found to be due to the same luminescent center as that of cathodoluminescence. In the particles or films where the content of nitrogen and boron was greatly reduced, the cathodoluminescence peaks occurred at 2.8–2.9 eV. The characteristics of these emission spectra are very similar to those obtained in type-IIa diamond where dislocations are luminescent. The doping of boron during the deposition form another luminescent center at 2.3–2.4 eV. From the monochromatic cathodoluminescence imaging, the luminescent regions differ in the two peaks. {100} sectors are much more luminescent than {111} sectors at the signal of 2.8 eV. This phenomenon has been discussed based on the difference in defect or impurity concentration of each sector.
On the Richardson constant for aluminum/gallium arsenide Schottky diodes
Authors: Missous M. , Rhoderick E. H.
Published: July 2002

Measurements have been made of the Richardson constant (A**) for Al/GaAs Schottky diodes in which the aluminum is deposited epitaxially by molecular beam epitaxy. These diodes are the nearest to ideal that have yet been reported. The value of (A**) for n-type GaAs, after allowing for the temperature variation of the barrier height and for the effect of tunnelling, was found to be (0.41±0.15)×104 A m −2 K−2. This is much lower than the previously accepted value, and confirms the low value reported by Srivastava, Arora, and Guha. Since there is no possibility of an interfacial layer in our diodes, we believe the low value of A** to be an intrinsic property of the Al/GaAs interface. The value of A** for Al/p-GaAs was found to be (7.0±1.5)×104 A m−2 K−2, which is lower than the theoretical value, though the discrepancy is not so large as for n-type GaAs. Because of the uncertainty in A**, values of barrier hights obtained from C−2 vs V plots are likely to be more reliable than those deduced from I/V characteristics.
Automatic modeling of physical phenomena: Application to ultrasonic data
Authors: Grabec Igor , Sachse Wolfgang
Published: July 2002

The physical modeling of natural phenomena is treated in this paper as an information transforming process which can be performed by a general modeler composed of an array of sensors, a central processing unit, a memory, and an array of actuators. The most characteristic property of such a system is its capability of predicting some properties of observed phenomena from partial perception and past experience. This property is realized if the central processing unit is acting as an estimator of a conditional average. As the condition, the partial set of signals from the sensor array is employed, while the complementary set is estimated from it. In the estimation the smoothed empirical probability distribution is employed. It is constructed from the empirical samples obtained by a set of prototype experiments and stored as a data base in the memory. The proposed modeler was simulated on a laboratory minicomputer which was part of a general data acquisition system. It was applied to the modeling of acoustic emission and ultrasonic scattering phenomena. It is shown experimentally that forward and inverse acoustic emission problems can be solved and a simple characterization of material inhomogeneities can be performed by the modeler. A mutual mapping of conditional data to estimated ones and the reverse is the basis of an iteration procedure which corresponds to a discrete dynamical process in the multidimensional data space. Numerical examples show that attractors of this process are the vectors of the data base. Therefore, the iteration can be efficiently applied for noise reduction. The operation of this dynamical system is comparable to the operation of neural networks in which the memory corresponds to the data base.
Gap solitons in nonlinear electrical superlattices
Authors: Bilbault J. M. , Remoissenet M.
Published: July 2002

We investigate the amplitude dependence of the gap transmission properties of nonlinear electrical superlattices of finite length, sandwiched between two linear electrical transmission lines. We show that once the amplitude of the incident sinusoidal wave, with a frequency which lies in the first linear gap, is greater than a certain threshold, the system switches to a transmitting state: the transmittance of the system which can become unity presents gap soliton-mediated bistability and hysteresis. The voltage-current characteristic also exhibits bistability. The transmittance curve calculated by using the characteristic matrix method is in good agreement with the results of our numerical simulations performed with conditions close to those of real experiments. The gap soliton envelope calculated analytically fits both the results obtained by the characteristic matrix method and those obtained by numerical simulations on the dynamics of the wave transmission inside the electrical superlattice, after subtracting out the harmonics effects.
Mechanism of diamond growth by chemical vapor deposition on diamond (100), (111), and (110) surfaces: Carbon-13 studies
Authors: Chu C. J. , D’Evelyn M. P. , Hauge R. H. , Margrave J. L.
Published: July 2002

The mechanism of diamond growth by hot-filament chemical vapor deposition (CVD) was investigated on the (100), (111), and (110) crystal faces of natural diamond by competition studies using carbon-13-labeled methane and carbon-12 acetylene. Homoepitaxial growth rates of 0.4, 0.5, and 1.3 μm/h were obtained for growth on the (100), (111), and (110) faces, respectively. The (100)- and (111)-oriented films were smooth initially, while films grown on (110) substrates quickly became rough. The (111) films had graphitic inclusions, as evidenced by the Raman spectrum, while the (100) and (110) films were graphite-free. The (111) films also exhibited substantial tensile stress, as indicated by a shift in the Raman peak and by spontaneous cracking in films grown thicker than 3.5 μm. The carbon-13 mole fraction of mixed 13C/12C diamond films grown on the diamond substrates was determined from the shift of the first-order Raman frequency, after correction for the shift due to stress. The carbon-13 mole fractions of methane, acetylene, and the methyl radical (derived indirectly from the mole fractions of H13C13CH and H12C13CH) in the gas phase were obtained by sampling the gas near the growth surface and subsequently determining its composition by matrix-isolation infrared spectroscopy. The carbon-13 mole fraction of the diamond film was equal to that of the methyl radical but differed significantly from that of acetylene for growth on all three crystal faces, indicating that the methyl radical is the dominant growth precursor under hot-filament CVD conditions regardless of the crystallographic orientation of the diamond substrate.
Experimental and theoretical studies of the performance of quantum-well infrared photodetectors
Authors: Andrews S. R. , Miller B. A.
Published: July 2002

We report on the performance of GaAs/AlGaAs quantum-well infrared detectors exhibiting intersubband absorption in the 8–14 μm band. The dark current characteristics have been investigated as a function of barrier width and electron density in the well and compared with a model which takes into account thermionic emission and thermally assisted tunneling in the Wentzel–Kramers–Brillouin approximation. The model gives a good description of the data and is extended to predict the variation of detectivity and background limited operating temperature with structural parameters.