bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical scan-correction system applied to atomic force microscopy
Authors: Barrett R. C. , Quate C. F.
Published: July 2002

Nonlinearities inherent in the piezoelectric actuators used in high-resolution scanning probe microscopies limit the usefulness of the instruments for precision dimensional measurements of submicrometer to micrometer scale structures. These nonlinearities can result in images where the scale changes by over 40% from one region of the image to another. This paper describes a simple two-axis optical beam displacement sensor that is used to accurately measure the (x,y) position of a piezoelectric tube scanner used in an atomic force microscope. This sensor has a noise level of 6.1 Årms over a frequency range of 0.5 Hz–1 kHz and a stability of about 200 Å over a 30-min period. Two different methods were used to correct the scans: postimaging software image correction and real-time feedback scan correction. The software method allows fast imaging and does not alter the control of the microscope, but requires postimaging image processing. It also loses some image information because of interpolation errors and the necessity of cropping the image to recover a rectangular image area. Feedback correction uses the sensor and a control system to accurately position the scanner for each data point of the image. This method eliminates the postprocessing of the images as well as interpolation errors, but limits the achievable scan speed and also adds the noise of the sensor and control system to the scan itself. Finally, both scan-correction systems reduce imaging errors dramatically over conventional open-loop piezoelectric scanners. Both linear and quadratic scan distortion are reduced to about 1%–2% of the image size.
Radio-frequency discharge source for beams of atomic nitrogen and oxygen
Authors: Pollard J. E.
Published: July 2002

Supersonic beams of atomic nitrogen and oxygen are produced by an electrodeless rf discharge within a ceramic nozzle. Mixtures of N2 or O2 seeded in He are heated by 100 W at 38 MHz with a stagnation pressure of 200–250 Torr. The flux of reactive atoms is 1–3×1018 sr−1 s−1 with stream velocities of 3.2–3.6 km/s and a velocity spread of 20% FWHM. The source is relatively simple to construct, because it requires only passive cooling for operation in vacuum.
Determination of flux ionization fraction using a quartz crystal microbalance and a gridded energy analyzer in an ionized magnetron sputtering system
Authors: Green K. M. , Hayden D. B. , Juliano D. R. , Ruzic D. N.
Published: July 2002

A diagnostic which combines a quartz crystal microbalance (QCM) and a gridded energy analyzer has been developed to measure the metal flux ionization fraction in a modified commercial dc magnetron sputtering device. The sensor is mounted on a linear motion feedthrough and embedded in a slot in the substrate plane to allow for measuring the uniformity in deposition and ionization throughout the plane of the wafer. Radio-frequency (rf) power is introduced through a coil to ionize the Al atoms. The metal flux ionization fraction at the QCM is determined by comparing the total deposition rate with and without a bias that screens out the ions, but that leaves the plasma undisturbed. By varying the voltage applied to the grids, the plasma potential is determined. At a pressure of 35 mTorr, a magnetron power of 2 kW, and a net rf power of 310±5 W, 78±5% ionization was found.
Phase measurement of light absorption and scatter in human tissue
Authors: Chance B. , Cope M. , Gratton E. , Ramanujam N. , Tromberg B.
Published: July 2002

Analog and digital technologies are presented for precise measurement of propagation delay of photons from source and detector placed on portions of the human body. The goal of the apparatus design is to quantify absorption (μa) and scattering (μs′) induced by biological pigments and biological structures, respectively. Body tissues are highly scattering with a mean distance between scatterers of less than a mm (at 700–850 nm). Significant absorption is mainly due to 5%–10% of the tissue volume occupied by blood. Measurement of μa and μs′ is done by both time and frequency domain equipment. This article focuses upon frequency domain equipment because of its simplicity, reduced noise bandwidth, versatility, and the strong analogy to very high frequency/ultrahigh frequency communication devices, particularly those using phase modulation. Comparisons are made of homodyne and heterodyne systems together with evaluation of single and multiple side band systems, with particular emphasis on methods for multiplexed optical and radio frequencies by frequency encoding or time-sharing technologies. The applications of these phase modulation systems to quantitative brain and muscle blood oximetry, functional activity of the forebrain, and other important problems of medical science, are presented.
Infrared-to-visible CW frequency upconversion in Er3+-doped fluoroindate glasses
Authors: de Araújo Cid B. , Menezes L. S. , Maciel G. S. , Acioli L. H. , Gomes A. S. L. , Messaddeq Y. , Florez A. , Aegerter M. A.
Published: July 2002

We report on efficient frequency upconversion in Er3+-doped fluoroindate glasses. The process is observed under 1.48 μm laser diode excitation and results in the generation of strong blue (∼407 nm), green (∼550 nm), and red (∼670 nm) radiation. The main mechanism that allows for upconversion appears to be the energy transfer among Er3+ ions in excited states. The results illustrate the large potential of this new class of glasses for photonic applications.
Inversion domains in GaN grown on sapphire
Authors: Romano L. T. , Northrup J. E. , O’Keefe M. A.
Published: July 2002

Planar defects observed in GaN films grown on (0001) sapphire have been identified as inversion domain boundaries (IDBs) by a combination of high resolution transmission electron microscopy, multiple dark field imaging, and convergent beam electron diffraction techniques. Films grown by molecular beam epitaxy (MBE), metalorganic vapor deposition (MOCVD), and hydride vapor phase epitaxy (HVPE) were investigated and all were found to contain IDBs. The IDBs in the MBE and HVPE films extended from the interface to the film surface and formed columnar domains that ranged in width from 3 to 20 nm in the MBE films and up to 100 nm in the HVPE films. For the films investigated, the MBE films had the highest density, and the MOCVD films had the lowest density of IDBs. The nucleation of inversion domains (IDs) may result from step-related inhomogeneities of the GaN/sapphire interface.
Determining magnetic anisotropies from hysteresis loops
Authors: Weber W. , Allenspach R. , Bischof A.
Published: July 2002

A novel method for determining magnetic anisotropies from hysteresis loops is presented. While sweeping the loop, a magnetic field is applied perpendicularly to the sweep-field axis. This causes the magnetization to rotate reversibly in a wide field range and still reach saturation at finite fields. An example is given whereby surface and volume anisotropies are determined from magneto-optical Kerr effect loops in Co films grown on stepped Cu(001).
Bowing parameter of the band-gap energy of GaNxAs1−x
Authors: Bi W. G. , Tu C. W.
Published: July 2002

We report a study of nitrogen incorporation in GaAs using a N rf plasma source. The N composition can be increased by lowering the growth temperature. X-ray diffraction shows no phase separation. Optical absorption measurements indicate that GaNxAs1−x is a direct band-gap material in the N composition range studied (x⩽14.8%), rather than a semimetal, contrary to theoretical predictions based on Van Vechten’s model. Analyzing the N composition dependence of the band-gap energy of the alloy indicates a composition-dependent bowing parameter, consistent with the first-principles supercell calculations [L. Bellaiche, S. H. Wei, and A. Zunger, Phys. Rev. B 54, 17 568 (1996)].
Phonon confinement effects in the Raman scattering by TiO2 nanocrystals
Authors: Bersani D. , Lottici P. P. , Ding Xing-Zhao
Published: July 2002

Nanocrystalline TiO2 has been obtained by a sol-gel process by controlling the crystal size through the water/alkoxide ratio. Raman spectra of anatase nanocrystals with average sizes of 9.5–13.4 nm are reported and the correlation between the Raman band shape (peak position and linewidth) of the main feature at 144 cm−1 and the crystals dimension is discussed. While in this system a minor role is played by nonstoichiometry and pressure effects, a model based on the phonon confinement, which takes into account the size distribution as determined by the transmission electron microscopy images, correctly reproduces the Raman band shape change.
Metal–semiconductor–metal near-infrared light detector based on epitaxial Ge/Si
Authors: Colace L. , Masini G. , Galluzzi F. , Assanto G. , Capellini G. , Di Gaspare L. , Palange E. , Evangelisti F.
Published: July 2002

In this letter we report on a metal–semiconductor–metal photodetector based on thick relaxed Ge layers, epitaxially grown on silicon after insertion of a low-temperature-grown Ge buffer layer. The detector shows a good responsivity at normal incidence at both 1.3 and 1.55 μm, with a maximum responsivity of 0.24 A/W at 1.3 μm under a 1 V bias. A response time of about 2 ns has been measured.
Diffusion lengths of excited carriers in CdxZn1−xSe quantum wells
Authors: Chao L.-L. , Cargill G. S. , Snoeks E. , Marshall T. , Petruzzello J. , Pashley M.
Published: July 2002

Diffusion lengths of excited carriers in a CdxZn1−xSe multiple quantum well structure were determined for temperatures between room temperature and 8 K from cathodoluminescence measurements. The diffusion length was found to depend upon temperature and Cd concentration of the quantum well. For the highest Cd concentration (x=0.43), the diffusion length increased with temperature up to 225 K and then dropped at higher temperatures. Diffusion lengths were 0.21 μm at 8 K, 0.38 μm at 225 K, and 0.24 μm at room temperature. For the well with least Cd concentration (x=0.24), longer diffusion lengths were obtained. The nature of the diffusing carriers is also discussed.
Crystal structures and ferroelectric properties of SrBi2Ta2O9 and Sr0.8Bi2.2Ta2O9
Authors: Shimakawa Y. , Kubo Y. , Nakagawa Y. , Kamiyama T. , Asano H. , Izumi F.
Published: July 2002

Crystal structures of ferroelectric materials of stoichiometric SrBi2Ta2O9 (TC=300 °C) and Sr-deficient-and-Bi-excess Sr0.8Bi2.2Ta2O9 (TC=400 °C) were refined by neutron powder diffraction. Bi2O2 layer and TaO6 octahedra are considerably distorted and atomic displacements along the a axis cause ferroelectric spontaneous polarization. In Sr0.8Bi2.2Ta2O9, both Bi substitution and cation vacancies at the Sr site were revealed and a chemical composition of (Sr0.82Bi0.12)Bi2Ta2O9.0 was obtained. The calculated polarization of Sr0.8Bi2.2Ta2O9 is larger than that of the stoichiometric sample, which is consistent with observations of remanent polarization in thin-film capacitors. The Bi substitution and the cation vacancies at the Sr site enhance structural distortion in the TaO6 octahedra and lead to the larger ferroelectric spontaneous polarization and the higher Curie temperature.
Measurements of the THz absorption and dispersion of ZnTe and their relevance to the electro-optic detection of THz radiation
Authors: Gallot G. , Zhang Jiangquan , McGowan R. W. , Jeon Tae-In , Grischkowsky D.
Published: July 2002

Via THz time-domain spectroscopy, we have measured the absorption and index of refraction of single-crystal 〈110〉 ZnTe from 0.3 to 4.5 THz. We find that the absorption is dominated by two lower-frequency phonon lines at 1.6 and 3.7 THz and not by the transverse-optical (TO) -phonon line at 5.3 THz as previously assumed. However, the index of refraction is determined mainly by the TO-phonon line. Using these data, we discuss a frequency-domain picture of electro-optic detection of THz radiation below the TO-phonon resonance and compare with the photoconductive THz receiver over the same frequency range.
Inhomogeneous transport in heteroepitaxial La0.7Ca0.3MnO3/SrTiO3 multilayers
Authors: Jo Moon-Ho , Mathur Neil D. , Evetts Jan E. , Blamire Mark G. , Bibes Manuel , Fontcuberta Josep
Published: July 2002

We have investigated the electrical transport properties of heteroepitaxial La0.7Ca0.3MnO3(LCMO)/SrTiO3 multilayers; as the LCMO layer thickness was varied from 25 to 2.5 nm, the metallic transition was suppressed and enhanced magnetoresistance extended over low temperatures. The results of transport and magnetic measurements imply a vertically inhomogeneous magnetic structure in the LCMO layers, with magnetically disordered interfaces. Although strain is clearly implicated in this surface disorder, we show that intrinsic magnetic disorder must also be associated with the interfaces.
Voltage-controlled yellow or orange emission from GaN codoped with Er and Eu
Authors: Lee D. S. , Heikenfeld J. , Birkhahn R. , Garter M. , Lee B. K. , Steckl A. J.
Published: July 2002

Orange and yellow-colored light emission has been achieved at room temperature in the same elecroluminescent device (ELD) made on GaN thin films codoped with Er and Eu. The GaN film was grown by molecular-beam epitaxy on Si (111) substrates using solid sources for Ga, Er and Eu and a plasma source for N2. Simple Schottky devices were fabricated on the GaN films using indium–tin oxide (ITO) transparent electrodes. ELD spectra show that the yellow and orange colors result from the combination of green emission from Er (537, 558 nm) and red emission from Eu (621 nm). A color change was observed with applied bias, producing yellow at higher bias (−100 V) and orange at lower bias (−70 V). We have fabricated both relatively small (∼250 μm) and large (1.45 mm) ELDs. Parameters for the chromaticity diagram were calculated to be x=0.382, y=0.605 for the yellow emission and x=0.467, y=0.523 for the orange emission. This work shows the possibility of achieving any intermediate color in the spectrum from green to red by adjusting the concentration of Er and Eu in GaN.
Study of quantum well intermixing caused by grown-in defects
Authors: Haysom J. E. , Aers G. C. , Raymond S. , Poole P. J.
Published: July 2002

We show that the degree of thermally induced quantum well intermixing is dependent on the growth quality of epitaxial layer structures. Two different undoped quantum well/barrier structures are studied: an InGaAs/InGaAsP 1.5 μm laser-like structure, and an InGaAs/InP structure. In both cases, the temperature of growth of one or more layers is altered and results compared with a control wafer. Wafers with material grown at the lower temperature display blueshifts of up to 80 meV in the QW emission energy during the early stages of rapid thermal annealing. Wafers grown at temperatures that are more standard exhibit excellent stability. We examine shifts of both the heavy hole and light hole transitions as a function of intermixing via transmission measurements, and conclude that interdiffusion occurs primarily on the group V sublattice.
Cylindrical Langmuir probes beyond the orbital-motion-limited regime
Authors: Estes R. D. , Sanmartı́n J. R.
Published: July 2002

The current I to a cylindrical probe at rest in an unmagnetized plasma, with probe bias highly positive, is determined. The way I lags behind the orbital-motion-limited (OML) current, IOML∝R, as the radius R exceeds the maximum radius for the OML regime to hold, is of interest for space-tether applications. The ratio I/IOML is roughly a decreasing function of R/λDe−Rmax/λDe, which is independent of bias, with λDe the electron Debye length and Rmax/λDe roughly an increasing function of the temperature ratio, Ti/Te. The dependence of current on ion energy is used to discuss the effect of probe motion through the plasma, a case applying to tethers in low orbit.
Raman scattering on the B-site order controlled by A-site substitution in relaxor Perovskite ferroelectrics
Authors: Jiang Fuming , Kojima Seiji , Zhao Changlei , Feng Chude
Published: July 2002

The control of the B-site order of Pb(Mg1/3Nb2/3)O3 was studied by the partial substitution at the A site by both donor La3+/Bi3+ and acceptor K+1. Different A-site cations and their concentrations were chosen to modify the chemical order at the B site. The Raman scattering results over a wide temperature range (−190–300 °C) confirm that the first order scattering is dominant in phonon bands resulting from both ordered regions and disordered matrix. The degree of the B-site order is estimated by the full width at half maximum of the phonon bands associated with the ordered regions, specifically, the highest frequency breath-type A1g mode of the BO6. It is concluded that the order is enhanced with the donor La3+/Bi3+ doping, while with the acceptor K+1 doping, the order is suppressed. The effect of A-site substitution on the B-site order has thus been elucidated.
Dielectric properties of liquid ethanol. A computer simulation study
Authors: Saiz Leonor , Guàrdia Elvira , Padró Joan-Àngel
Published: July 2002

Static and dynamic dielectric properties of liquid ethanol have been studied as a function of the wave-vector number by computer simulation. Molecular dynamics simulations at room temperature have been performed using the optimized potentials for liquid simulations (OPLS) potential model proposed by Jorgensen [J. Phys. Chem. 90, 1276 (1986)]. The time dependent correlation functions of the longitudinal and transverse components of the dipole density as well as the individual and total dipole moment autocorrelation functions have been calculated. The infrared spectra and the dielectric relaxation of the liquid have been also analyzed. Results have been compared with the available experimental data. Special attention has been dedicated to investigate the molecular origin of the different analyzed properties.
Temperature dependence of the thermo-optic coefficient of InP, GaAs, and SiC from room temperature to 600 K at the wavelength of 1.5 μm
Authors: Della Corte Francesco G. , Cocorullo Giuseppe , Iodice Mario , Rendina Ivo
Published: July 2002

The thermo-optic coefficient ∂n/∂T has been measured from room temperature to 600 K at the wavelength of 1523 nm in three important semiconductors for fiber-optic device fabrication, namely, InP, GaAs, and 6H–SiC. The adopted technique is very simple and is based on the observation of the periodicity of the signal transmitted, at the desired wavelength, by an étalon made of the material under test, when it experiences a temperature variation. The values of ∂n/∂T measured in InP and GaAs at room temperature are in agreement with previously reported ones, but increase with temperature with a weak quadratic dependence. SiC conversely shows a lower thermo-optic coefficient (2.77×10−5 K−1) at 300 K, which, however, doubles for a 300 K temperature increase.
Coupled-cluster calculations on ferrocene and its protonated derivatives: Towards the final word on the mechanism of protonation of ferrocene?
Authors: Mayor-López María J. , Lüthi Hans P. , Koch Henrik , Morgantini Pierre Y. , Weber Jacques
Published: July 2002

The mechanism of the protonation of ferrocene, the simplest model for the electrophilic attack on a metallocene, has been studied extensively. However, neither experiment nor computation have reached agreement on the details of the mechanism. The different model calculations applied [Hartree–Fock, Möller–Plesset, and density functional theory (HF, MP2, and DFT) with different functionals] come to contradicting conclusions. As a complement to our previous work, we report the results obtained for neutral and protonated ferrocene using the coupled-cluster method [CCSD(T)] with polarized double- and triple-zeta basis sets. These calculations show that the metal-protonated and the agostic forms represent minima on the potential energy surface, whereas the ring-protonated form is higher in energy with no minimum structure identified. With regard to the reaction, these results indicate an exo reaction path. The CCSD(T) results are in good agreement with the predictions made by the DFT calculations, whereas the results obtained from the Hartree–Fock and MP2 computations appear to be incorrect.
Initiation of an early-stage plasma during picosecond laser ablation of solids
Authors: Mao Samuel S. , Mao Xianglei , Greif Ralph , Russo Richard E.
Published: July 2002

Picosecond time-resolved images of plasma initiation were recorded during pulsed-laser ablation of metal targets in an air atmosphere. An early-stage plasma was observed to form before the release of a material vapor plume. Close to the target surface, interferometry measurements indicate that the early-stage plasma has an electron number density on the order of 1020 cm−3. The longitudinal expansion of the ionization front for this plasma has a velocity 109 cm/s, during the laser pulse. In contrast, a material–vapor plume forms approximately 200 ps after the laser pulse, and it moves away from the target at 106 cm/s. The experimental observations of the early-stage plasma were simulated by using a theoretical model based on a two-fluids description of laser plasmas. The results indicate that the initiation of the plasma is due to air breakdown assisted by electron emission from the target.
Electron-impact ionization in classical nonideal plasmas
Authors: Kim Chang-Geun , Jung Young-Dae
Published: July 2002

Electron-impact ionization of hydrogenic ions is investigated in classical nonideal plasmas. The projectile–target interaction potential in classical nonideal plasmas is represented by the pseudopotential model. Semiclassical trajectory method is applied to the path of the projectile electron in order to visualize the ionization probability as a function of the impact parameter. The semiclassical ionization probability decreases with increasing the collective effect. It is also found that the collective effect increases with increasing the projectile energy.
Dielectrophoretic liquid actuation and nanodroplet formation
Authors: Jones T. B. , Gunji M. , Washizu M. , Feldman M. J.
Published: July 2002

Water, like any polarizable medium, responds to a nonuniform electric field by collecting preferentially in regions of maximum field intensity. This manifestation of dielectrophoresis (DEP) makes possible a variety of microelectromechanical liquid actuation schemes. In particular, we demonstrate a new class of high-speed DEP actuators, including “wall-less” flow structures, siphons, and nanodroplet dispensers that operate with water. Liquid in these microfluidic devices rests on a thin, insulating, polyimide layer that covers the coplanar electrodes. Microliter volumes of water, deposited on these substrates from a micropipette, are manipulated, transported, and subdivided into droplets as small as ∼7 nl by sequences of voltage application and appropriate changes of electrode connections. The finite conductivity of the water and the capacitance of the dielectric layer covering the electrodes necessitate use of rf voltage above ∼60 kHz. A simple RC circuit model explains this frequency-dependent behavior. DEP actuation of small water volumes is very fast. We observe droplet formation in less than 0.1 s and transient, voltage-driven movement of water fingers at speeds exceeding 5 cm/s. Such speed suggests that actuation can be accomplished using preprogrammed, short applications of the rf voltage to minimize Joule heating.
Optical shortpass filters based on macroporous silicon
Authors: Lehmann Volker , Stengl Reinhard , Reisinger Hans , Detemple Ralf , Theiss Wolfgang
Published: July 2002

A promising class of optical filters is introduced, based on diffraction at small apertures. The filters consist of straight pores with diameters in the micrometer regime and a length of up to one millimeter through a silicon wafer. In contrast to Bragg, Woods, or glass filters, the light is not transmitted in matter but in the medium inside the pores. The filters therefore show a true shortpass characteristic. Due to constructive interference between the high number of pores in an array, macropore filters are of high optical quality and may replace conventional filters in imaging systems.
Signal propagation in collisional plasma with negative ions
Authors: Kaganovich I. , Berezhnoi S. V. , Shin C. B.
Published: July 2002

The transport of charged species in collisional currentless plasmas is traditionally thought of as a diffusion-like process. In this paper, it is demonstrated that, in contrast to two-component plasma, containing electrons and positive ions, the transport of additional ions in multispecies plasmas is not governed by diffusion, rather described by nonlinear convection. As a particular example, plasmas with the presence of negative ions have been studied. The velocity of a small perturbation of negative ions was found analytically and validated by numerical simulation. As a result of nonlinear convection, initially smooth ion density profiles break and form strongly inhomogeneous shock-like fronts. These fronts are different from collisionless shocks and shocks in fully ionized plasma. The structure of the fronts has been found analytically and numerically.
A mixed quantum-classical molecular dynamics study of vibrational relaxation of a molecule in solution
Authors: Terashima Takeshi , Shiga Motoyuki , Okazaki Susumu
Published: July 2002

An application of mixed quantum-classical molecular dynamics to vibrational relaxation of the solute molecule in the solution has been investigated. In the present paper, we demonstrate that mean field approximation, rather than surface hopping approximation, works satisfactorily well for the vibrational relaxation of, at least, the CN− ion in the aqueous solution, where the potential surface of the solvent water depends little on the vibrational quantum state of the solute. The calculated relaxation time is in good correspondence to those obtained from other two different methods based upon the same potential model, i.e., Fermi’s Golden Rule with classical force autocorrelation function and path integral influence functional theory in its classical bath limit. The present method gives some interesting findings for the relaxation. For example, the energy relaxation time may be longer than the population relaxation time. With respect to the coupling to the solvent, water molecules in the first hydration shell make a leading contribution to the relaxation of the solute.
Orbits in the H2O molecule
Authors: Efstathiou K. , Contopoulos G.
Published: July 2002

We study the forms of the orbits in a symmetric configuration of a realistic model of the H2O molecule with particular emphasis on the periodic orbits. We use an appropriate Poincaré surface of section (PSS) and study the distribution of the orbits on this PSS for various energies. We find both ordered and chaotic orbits. The proportion of ordered orbits is almost 100% for small energies, but decreases abruptly beyond a critical energy. When the energy exceeds the escape energy there are still nonescaping orbits around stable periodic orbits. We study in detail the forms of the various periodic orbits, and their connections, by providing appropriate stability and bifurcation diagrams.
Local structure of Ge nanoislands on Si(111) surfaces with a SiO2 coverage
Authors: Kolobov Alexander V. , Shklyaev Alexander A. , Oyanagi Hiroyuki , Fons Paul , Yamasaki Satoshi , Ichikawa Masakazu
Published: July 2002

We have investigated the local structure and photoluminescence properties of ultrasmall Ge islands grown on Si(111) covered with SiO2. Scanning electron microscopy and transmission electron microscopy measurements show that the islands have a hemispherical shape, and depending on the growth temperature, can be either epitaxial or nonepitaxial. X-ray absorption near-edge structure measurements demonstrate that the nonepitaxial islands have the local structure of bulk diamond Ge and are very stable towards oxidation. The epitaxial islands are found to be partly oxidized, but no alloying with the Si substrate is observed. The nonepitaxial islands exhibit a photoluminescence peaked at 2.3 eV, which is typical of Ge nanocrystals embedded in SiO2. Possible mechanisms for the stability of the nonepitaxial Ge islands towards oxidation are discussed.
Far-infrared surface-plasmon quantum-cascade lasers at 21.5 μm and 24 μm wavelengths
Authors: Colombelli Raffaele , Capasso Federico , Gmachl Claire , Hutchinson Albert L. , Sivco Deborah L. , Tredicucci Alessandro , Wanke Michael C. , Sergent A. Michael , Cho Alfred Y.
Published: July 2002

Quantum-cascade lasers operating above 20 μm (at λ∼21.5 μm and λ∼24 μm) wavelength are reported. Pulsed operation was obtained up to 140 K and with a peak power of a few milliwatts at cryogenic temperatures. Laser action originates from interminiband transitions in “chirped” superlattice active regions. The waveguides are based on surface-plasmon modes confined at a metal–semiconductor interface. The wavelengths were chosen in order to avoid major phonon absorption bands, which are particularly strong at energies just above the reststrahlen band. We also report on a 21.5-μm-wavelength laser based on a two-sided interface-plasmon waveguide.
High concentration crossovers of polyelectrolyte solutions
Authors: Nishida K. , Kaji K. , Kanaya T.
Published: July 2002

In a few decades, several characteristic regimes and subregimes have been theoretically presented as functions of concentration C and contour length for flexible polyelectrolyte solutions. However, experimentally only the dilute and semidilute regimes have been well acknowledged. In this study, solutions of sodium poly(styrene sulfonate) are studied at various concentrations ranging from the semidilute regime to the bulk through the concentrated regime using small-angle and intermediate-angle x-ray scattering techniques. We have observed a single scattering peak being characteristic of polyelectrolyte solutions until C=4.1 mol/l. Above C=4.9 mol/l, such a characteristic peak disappears and instead, another new peak appears at higher q’s. In the plot of the maximum position qmax vs C, two crossover concentrations are found at C**=1.2 mol/l and C***=4.5 mol/l. The former is a continuous crossover from qmax∼C1/2 to qmax∼C1/4 which can be assigned to the crossover of the semidilute-to-concentrated regime, while the latter is a discontinuous crossover which may be assigned to that of the concentrated-to-swollen regime. In the swollen regime the characteristics of polyelectrolyte solutions are lost. The discontinuous crossover concentration C*** is about half the bulk concentration Cbulk=8.4 mol/l.
Excision of singularities by stringy domain walls
Authors: Kallosh Renata , Mohaupt Thomas , Shmakova Marina
Published: July 2002

We study supersymmetric domain walls on S1/Z2 orbifolds. The supergravity solutions in the bulk are given by the attractor equation associated with Calabi–Yau (CY) spaces and have a naked space–time singularity at some |ys|. We are looking for possibilities to cut off this singularity with the second wall by a stringy mechanism. We use the collapse of the CY cycle at |yc| which happens before and at a finite distance from the space–time singularity. In our example with three Kähler moduli the second wall is at the end of the moduli space at |yc| where also the enhancement of SU(2) gauge symmetry takes place so that |ye|=|yc|<|ys|. The physics of the excision of a naked singularity via the enhançon in the context of domain wall has an interpretation on the heterotic side related to R→1/R duality. The position of the enhançon is given by the equation R(|ye|)=1.
X-ray studies of phase transitions in solid oxygen
Authors: Prokhvatilov A. I. , Galtsov N. N. , Raenko A. V.
Published: July 2002

X-ray studies of the phase composition, lattice parameters, and molar volumes are carried out at the phase transitions in solid oxygen. In the region of the low-temperature transition a two-phase state is observed in a region having a width of 2 K on cooling of the samples and less than 0.8 K on heating. The width of the temperature hysteresis and the jump in volume at the transition are determined. The mean value of the volume jump is ∼0.5±0.2%. The structural characteristics found are convincing evidence that the low-temperature magnetic transformation in solid oxygen, like the high-temperature orientational transition, is a first-order transition. It is shown that at relatively high rates of passage through the β–γ transition, owing to the significant jump in volume and the high stress level, it is possible to supercool the γ phase down to helium temperature and to superheat the intermediate β phase almost to the melting temperature.
Formation of nanocrystallites governed by the initial stress in the ultrathin hydrogenated amorphous silicon films
Authors: Hazra Sukti , Sakata Isao , Yamanaka Mitsuyuki , Suzuki Eiichi
Published: July 2002

Spectroscopic ellipsometry identified E1 transitions at a lower energy than that for c-Si (3.38 eV). These transitions are generated from the Si paracrystallites or disordered crystallites in the ultrathin silicon films (2–10 nm) deposited by thermal chemical vapor deposition. During the growth of the film, paracrystallites expand gradually; disorder in the paracrystallites increases. Finally, a completely disordered Si network, i.e., the amorphous network, is generated. The presence of disorder crystallites in the ultrathin Si films acts as a constraint for the crystallization of the ultrathin films by rapid thermal annealing.
Monolithically integrated, flexible display of polymer-dispersed liquid crystal driven by rubber-stamped organic thin-film transistors
Authors: Mach P. , Rodriguez S. J. , Nortrup R. , Wiltzius P. , Rogers J. A.
Published: July 2002

This letter describes the monolithic integration of rubber-stamped thin-film organic transistors with polymer-dispersed liquid crystals (PDLCs) to create a multipixel, flexible display with plastic substrates. We report the electro-optic switching behavior of the PDLCs as driven by the organic transistors, and we show that our displays operate robustly under flexing and have a contrast comparable to that of newsprint.
Noise-induced escape on time scales preceding quasistationarity: New developments in the Kramers problem
Authors: Soskin S. M. , Sheka V. I. , Linnik T. L. , Arrayás M. , Kaufman I. Kh. , Luchinsky D. G. , McClintock P. V. E. , Mannella R.
Published: July 2002

Noise-induced escape from the metastable part of a potential is considered on time scales preceding the formation of quasiequilibrium within that part of the potential. It is shown that, counterintuitively, the escape flux may then depend exponentially strongly, and in a complicated manner, on time and friction.
Frequency behavior and the Mott–Schottky analysis in poly(3-hexyl thiophene) metal–insulator–semiconductor diodes
Authors: Meijer E. J. , Mangnus A. V. G. , Hart C. M. , de Leeuw D. M. , Klapwijk T. M.
Published: July 2002

Metal–insulator–semiconductor diodes with poly(3-hexyl thiophene) as the semiconductor were characterized with impedance spectroscopy as a function of bias, frequency, and temperature. We show that the standard Mott–Schottky analysis gives unrealistic values for the dopant density in the semiconductor. From modeling of the data, we find that this is caused by the relaxation time of the semiconductor, which increases rapidly with decreasing temperature due to the thermally activated conductivity of the poly(3-hexyl thiophene).
Manipulating the scaling factor of projective synchronization in three-dimensional chaotic systems
Authors: Xu Daolin , Li Zhigang , Bishop Steven R.
Published: July 2002

Scaling factor characterizes the synchronized dynamics of projective synchronization in partially linear chaotic systems but it is difficult to be estimated. To manipulate projective synchronization of chaotic systems in a favored way, a control algorithm is introduced to direct the scaling factor onto a desired value. The control approach is derived from the Lyapunov stability theory. It allows us to arbitrarily amplify or reduce the scale of the response of the slave system via a feedback control on the master system. In numerical experiments, we illustrate the application to the Lorenz system.
Dielectric relaxation in Ba-based layered perovskites
Authors: Kholkin A. L. , Avdeev M. , Costa M. E. V. , Baptista J. L. , Dorogovtsev S. N.
Published: July 2002

Ferroelectric materials with Bi-layered structure such as SrBi2Ta2O9 and SrBi2Nb2O9 are now intensively investigated in view of their applications in nonvolatile computer memories and high-temperature piezoelectric transducers. When Sr2+ is substituted with Ba2+, a significant disorder is induced and the material exhibits broadening of the phase transition. Such broadening is essential for applications since it allows achieving smooth temperature characteristics while maintaining high dielectric and piezoelectric properties. In this work, stoichiometric dense BaBi2Nb2O9 (BBN) ceramics are sintered using a mixed oxide route. Dielectric and ferroelectric properties are investigated in a broad range of temperatures and frequencies. Strong dispersion of the complex relative dielectric permittivity is observed including typical relaxor features such as shift of the permittivity maximum with frequency and broadening of the relaxation time spectrum with decreasing temperature. The dielectric relaxation obeys the Vögel–Fulcher relationship with anomalously low freezing temperature (Tf≈100 K), which is much lower than the permittivity maximum in the radio-frequency range. Polarization hysteresis loops testify linear properties of BBN at all temperatures above Tf. The properties of BBN ceramics are compared to conventional relaxor systems such as Pb(Mg, Nb)O3 and (Pb, La)(Zr, Ti)O3.
Barkhausen noise and size effects in magnetic microstructures
Authors: Callegaro Luca , Puppin Ezio , Ricci Simona
Published: July 2002

The properties of the Barkhausen noise in a series of permalloy squares have been measured with a magneto-optical hysteresigraph. The magnetic structures have been litographically defined in a permalloy film (thickness 160 nm) as squares having a size from 20 to 320 μm. The statistical distributions of the most significant parameters of the Barkhausen jumps have been extracted from the original data. At variance with respect to bulk and thin films, the jump amplitude distribution does not follow a well-defined power-law behavior. This observation is explained in terms of a transition between a discrete magnetization regime typical of small magnetic structures and the more usual regime observed in extended samples.
Epitaxial NaCl structure δ-TaNx(001): Electronic transport properties, elastic modulus, and hardness versus N/Ta ratio
Authors: Shin C.-S. , Gall D. , Kim Y.-W. , Desjardins P. , Petrov I. , Greene J. E. , Odén M. , Hultman L.
Published: July 2002

While metastable B1-NaCl-structure δ-TaNx is presently used in a variety of hard coating, wear-resistant, and diffusion barrier applications, it is a complex material exhibiting a wide single-phase field, x≃0.94–1.37, and little is known about its fundamental properties. Here, we report physical properties of epitaxial δ-TaNx layers grown as a function of x on MgO(001) by ultrahigh vacuum reactive magnetron sputter deposition. The room-temperature resistivity (ρ=225 μΩ cm), hardness (H=30.9 GPa), and elastic modulus (E=455 GPa) of δ-TaNx(001) are independent of x over the range 0.94–1.22. However, changes in the electronic structure associated with increasing x>1.22 lead to an increase in ρ with a decrease in H and E. All δ-TaNx(001) layers exhibit negative temperature coefficients of resistivity between 20 and 400 K due to weak carrier localization. δ-TaNx is superconducting with the highest critical temperature, 8.45 K, obtained for layers with the lowest N/Ta ratio, x=0.94. Based upon the above results, combined with the fact that the relaxed lattice constant a0 shows only a very weak dependence on x, we propose that the wide phase field in δ-TaNx is due primarily to antisite substitutions of Ta on N (and N on Ta) sites, rather than to cation and anion vacancies. To first order, antisite substitutions in TaNx are isoelectronic and hence have little effect on charge carrier density. At sufficiently high N/Ta ratios, however, simple electron-counting arguments are no longer valid since large deviations from stoichiometry alter the character of the band structure itself.
Ab initio calculations of the mechanical properties of Ti3SiC2
Authors: Holm Bengt , Ahuja Rajeev , Johansson Börje
Published: July 2002

We present results of first-principles calculations of the elastic constants and other mechanical properties of Ti3SiC2. This knowledge is important from a technological point of view, since the material shows promising characteristics, such as a combined metallic and ceramic appearance. We also confirm a recent experimental observation that the structure of Ti3SiC2 is stable at elevated pressures, and investigate some consequences of this fact.
Tabletop terahertz-wave parametric generator using a compact, diode-pumped Nd:YAG laser
Authors: Sato Atsushi , Kawase Kodo , Minamide Hiroaki , Wada Satoshi , Ito Hiromasa
Published: July 2002

We specifically developed a diode-pumped, Q-switched Nd:YAG laser to act as a pump source for a terahertz-wave parametric generator. A single Q-switched laser pulse (pulse width 12 ns, pulse repetition frequency 40 Hz) produced an output energy of 12 mJ. Utilizing this pump source, we were able to construct a compact (650×350 mm) terahertz-wave parametric generator using LiNbO3 crystals. We achieved a broad terahertz-wave spectrum, ranging from 1.2 to 2.4 THz, with a pump energy of 12 mJ.
Two-dimensional Fourier transform electronic spectroscopy
Authors: Hybl John D. , Albrecht Ferro Allison , Jonas David M.
Published: July 2002

Two-dimensional Fourier transform electronic spectra of the cyanine dye IR144 in methanol are used to explore new aspects of optical 2D spectroscopy on a femtosecond timescale. The experiments reported here are pulse sequence and coherence pathway analogs of the two-dimensional magnetic resonance techniques known as COSY (correlated spectroscopy) and NOESY (nuclear Overhauser effect spectroscopy). Noncollinear three pulse scattering allows selection of electronic coherence pathways by choice of phase matching geometry, temporal pulse order, and Fourier transform variables. Signal fields and delays between excitation pulses are measured by spectral interferometry. Separate real (absorptive) and imaginary (dispersive) 2D spectra are generated by measuring the signal field at the sample exit, performing a 2D scan that equally weights rephasing and nonrephasing coherence pathways, and phasing the 2D spectra against spectrally resolved pump–probe signals. A 3D signal propagation function is used to correct the 2D spectra for excitation pulse propagation and signal pulse generation inside the sample. At relaxation times greater than all solvent and vibrational relaxation timescales, the experimental 2D electronic spectra can be predicted from linear spectroscopic measurements without any adjustable parameters. The 2D correlation spectra verify recent computational predictions of a negative region above the diagonal, a displacement of the 2D peak off the diagonal, and a narrowing of the 2D cross-width below the vibrational linewidth. The negative region arises from 4-level four-wave mixing processes with negative transition dipole products, the displacement off the diagonal arises from a dynamic Stokes shift during signal radiation, and the narrow 2D cross-width indicates femtosecond freezing of vibrational motion.
Line broadening caused by Coulomb carrier–carrier correlations and dynamics of carrier capture and emission in quantum dots
Authors: Uskov A. V. , Magnusdottir I. , Tromborg B. , Mo/rk J. , Lang R.
Published: July 2002

Mechanisms of pure dephasing in quantum dots due to Coulomb correlations and the dynamics of carrier capture and emission are suggested, and a phenomenological model for the dephasing is developed. It is shown that, if the rates of these capture and emission processes are sufficiently high, significant homogeneous line broadening of the order of several meV can result.
Oxygen vacancy ordering in epitaxial La0.5Sr0.5CoO3−∂ thin films on (001) LaAlO3
Authors: Stemmer Susanne , Jacobson Allan J. , Chen X. , Ignatiev A.
Published: July 2002

High-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), and electron diffraction were used to study the microstructure of epitaxial La0.5Sr0.5CoO3−∂ thin films grown on (001) oriented LaAlO3 substrates. Films were characterized before and after annealing treatments under different oxygen partial pressures. EELS shows that annealing reduces the valence state of cobalt due to loss of oxygen. HRTEM image simulations show that the superstructure contrast observed in HRTEM can be explained by shifts of cations in planes containing ordered oxygen vacancies. The as-deposited film showed weak, short-range ordering of oxygen vacancies within nanometer-sized domains. The annealed film showed long-range order and a strong anisotropy in ordering, with the oxygen-deficient planes aligned parallel to the film/substrate interface. We propose that the anisotropy in ordering is a mechanism of stress relief in these films. Implications of the observed microstructure on the oxygen transport and surface oxygen exchange properties are discussed.
Interfacial phenomena related to the fabrication of thin Al oxide tunnel barriers and their thermal evolution
Authors: Dimopoulos T. , Da Costa V. , Tiusan C. , Ounadjela K. , van den Berg H. A. M.
Published: July 2002

In this work we study phenomena related to the plasma oxidation of thin metallic Al layers, to be used as insulating barriers in magnetic tunnel junctions. We investigate by barrier impedance scanning microscopy how the barrier’s over oxidation influences the local transport characteristics of the oxide layers and the results are correlated with the magnetotransport properties of patterned microsized as-deposited and annealed junctions. Interestingly, the oxygen reservoir existing at the ferromagnetic metal, degenerating the tunnel device, can be utilized for the improvement of the junction’s magnetotransport properties by means of thermal annealing processing.
Doping effect in layer structured SrBi2Nb2O9 ferroelectrics
Authors: Wu Yun , Forbess Mike J. , Seraji Seana , Limmer Steven J. , Chou Tammy P. , Nguyen Carolyn , Cao Guozhong
Published: July 2002

This article reports a systematic study of doping effects on the crystal structure, microstructure, dielectric, and electrical properties of layer-structured strontium bismuth niobate, SrBi2Nb2O9 (SBN), ferroelectrics. Substitution in both the A site (Sr2+ by Ca2+ and Ba2+) and B site (Nb5+ by V5+) up to 30 at % were studied. It was found that crystal lattice constant, dielectric, and electrical properties of SBN ferroelectrics varied appreciably with the type and amount of dopants. The relationships among the ionic radii, structural constraint imposed by [Bi2O2]2+ interlayers, and properties were discussed.
Photoluminescence study of excitons in homoepitaxial GaN
Authors: Martı́nez-Criado G. , Miskys C. R. , Cros A. , Ambacher O. , Cantarero A. , Stutzmann M.
Published: July 2002

High-resolution photoluminescence spectra have been measured in high-quality homoepitaxial GaN grown on a free-standing GaN substrate with lower residual strain than in previous work. Unusually strong and well-resolved excitonic lines were observed. Based on free- and bound exciton transitions some important GaN parameters are derived. The Arrhenius plot of the free A exciton recombination yields a binding energy of 24.7 meV. Based on this datum, an accurate value for the band-gap energy, EG(4.3 K) = 3.506 eV, can be given. From the donor bound excitons and their “two-electron” satellites, the exciton localization energy and donor ionization energy are deduced. Finally, estimates of the electron and hole masses have been obtained within the effective mass approximation.