bluuebunny/crossref_metadata_2025 huggingface dataset:
Measuring surface deformations in magnetic fluid by radioscopy
Authors: Richter Reinhard , Bläsing Jürgen
Published: July 2002

We present a quantitative method, capable of measuring the three-dimensional shape of a fluid surface. Utilizing the attenuation of x-rays in magnetic liquids, we are able to map amplitude and relief of large wave crests, a regime which is not accessible by the common optical methods. The technique is applied for measuring the static surface deformations of the Rosensweig instability. It will also be useful for measuring the surface profile of the Faraday instability in magnetic liquids, in standard fluids, in liquid metals, or even in granular material.
Molecular simulation of xenon adsorption on single-walled carbon nanotubes
Authors: Simonyan Vahan V. , Johnson J. Karl , Kuznetsova Anya , Yates John T.
Published: July 2002

Adsorption of xenon on single-walled (10,10) carbon nanotubes at a temperature of 95 K has been studied by molecular simulation and the results have been compared with recent experiments [A. Kuznetsova, J. T. Yates, Jr., J. Liu, and R. E. Smalley, J. Chem. Phys. 112, 9590 (2000)]. Simulations indicate that adsorption takes place primarily on the inside of the nanotubes at the experimental conditions. Interstitial and external adsorption were found to be negligible in comparison with adsorption inside the nanotubes. The coverage computed from simulation of 0.06 Xe–C is in good agreement with the experimentally measured value of 0.042 Xe–C. The isosteric heat of adsorption from simulation ranges from about 3000 to 4500 K as a function of coverage, which is consistent with the experimental desorption activation energy of 3220 K. Adsorption on the external surfaces of the nanotubes is observed to take place at Xe pressures that are larger than those probed in the experiments. The good agreement between simulations and experiments for the coverage and heat of adsorption indicate that the curvature of the nanotube does not substantially perturb the adsorption potential from that of a graphene sheet.
Performance characterization of twisted undulator
Authors: Smirnov A. V.
Published: July 2002

A special two-period insertion device is considered in terms of useful properties of spontaneous emission. General idea of the geometry can be interpreted as helical transformation applied to an original linear undulator structure to provide superimposed fast and slow transverse oscillations of the electron beam. Periodic sextupolar focusing effect of the device is calculated analytically. Three-dimensional hybrid magnetic design is treated numerically on the base of traditional and specially modified feasible structures. Main properties of the spontaneous emission of single electron were studied using a derived analytical form and numerical computations. The following features of the radiation are outlined: (i) reduction of relative content of higher odd harmonics as the “slow” imposed helical field increases; (ii) radiation enhancement as the number of helical turns increases; (iii) spectral line splitting and intensity adjustment; and (iv) opposite directions of circular polarization for adjacent spectral lines. In particular, unlike most insertion devices this structure does not require any magnetic readjustments or electromagnetic switching to provide opposite polarization. Potential technical advantages in application for single pass superradiant free electron lasers are also discussed.
Glass slits for collimating particle beams
Authors: Ballard A. , Bonin K.
Published: July 2002

We report here on the construction and use of a pair of matched glass collimating slits which are suitable for molecular beam work. These glass slits transmit nearly all (>85%) of an incident laser beam. Additionally, these slits are matched to better than 5–10 μm at a gap size of 200 μm.
Dependence of the transport properties on the long range order of β-phase Co0.50Ti0.50 alloy films
Authors: Lee Y. P. , Kim K. W. , Rhee J. Y. , Kudryavtsev Y. V. , Nemoshkalenko V. V. , Prokhorov V. G.
Published: July 2002

The influence of the structural disorder on the transport properties of Co0.50Ti0.50 alloy films in a temperature range of 4.2–300 K has been investigated without and with a magnetic field of 0.5 T. The absence of translational invariance in a disordered state leads to an increase in the resistivity and causes a change from the positive to negative temperature coefficient of resistance. This fact is explained by the partial localization of electronic states near the Fermi level. It was established that a partial structural disordering enhances the role of the electron-phonon-vibrating impurity scattering in the transport properties and also makes the spin-diffusive scattering rather noticeable. The appearance of a low-temperature resistivity minimum for the disordered Co0.50Ti0.50 alloy film arises from the quantum corrections to the electron–electron interactions in the presence of weak localization.
He
+
–ion irradiation effect on intersubband transitions in GaAs/AlGaAs multiple quantum wells
Authors: Berhane Y. , Manasreh M. O. , Weaver B. D.
Published: July 2002

Intersubband transitions in 3 MeV He+–ion irradiated GaAs–AlGaAs multiple quantum wells were studied using an optical absorption technique. The intersubband transitions were completely depleted in samples irradiated with doses as low as 1×1014 cm−2. Thermal annealing recovery of intersubband transitions was observed in samples irradiated with lower doses. On the other hand, intersubband transitions in heavily irradiated (doses>3×1014 cm−2) samples do not show thermal annealing recovery, which indicates that irradiation-induced defects are so severe that thermal annealing does not repair the damage. The total integrated areas of the intersubband transitions in irradiated samples and in a control sample were monitored as a function of annealing temperature.
T-shaped ballistic spin filter
Authors: Kiselev A. A. , Kim K. W.
Published: July 2002

We propose a semiconductor structure that can filter electron spin without using external magnetic fields. The structure consists of a T-shaped quasi-one-dimensional channel and an electrode placed near the channel intersection to control the spin-orbit interaction for electrons. Our calculations demonstrate that the proposed device can redirect electrons with opposite spins from an unpolarized source to left and right output leads, respectively, and, thus, serve as a spin filter. When the incident electron energy is in resonance with the quasilocalized zero-dimensional states at the intersection, polarization of the transmitted fluxes approaches 100%.
Shock timing technique for the National Ignition Facility
Authors: Munro David H. , Celliers Peter M. , Collins Gilbert W. , Gold David M. , Da Silva Luiz B. , Haan Steven W. , Cauble Robert C. , Hammel Bruce A. , Hsing Warren W.
Published: July 2002

Among the final shots at the Nova laser [Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)] was a series testing the VISAR (velocity interferometry system for any reflector) technique that will be the primary diagnostic for timing the shocks in a NIF (National Ignition Facility) ignition capsule. At Nova, the VISAR technique worked over the range of shock strengths and with the precision required for the NIF shock timing job—shock velocities in liquid D2 from 12 to 65 μm/ns with better than 2% accuracy. VISAR images showed stronger shocks overtaking weaker ones, which is the basis of the plan for setting the pulse shape for the NIF ignition campaign. The technique is so precise that VISAR measurements may also play a role in certifying beam-to-beam and shot-to-shot repeatability of NIF laser pulses.
Calibration of a helical resonator for microwave dielectric and conductivity measurements of metals
Authors: Song K. J. , Castner T. G.
Published: July 2002

The helical resonator (HR) is a useful resonant structure for the measurement of the microwave conductivity and dielectric response, but must be calibrated to obtain absolute values of these quantities. This has been accomplished by the measurement of frequency shifts and Q changes of thin disk samples of the metals Cu, Al, and Au and high purity Si and the use of the formula for (f−f0)/f due to Bethe and Schwinger. The measurements were made at 293, 77, and 50 K for the HR modes n=1–7(124 MHz–1.44 GHz). The results demonstrate the largest uncertain in the calculated values of Δf/f result from the z-axis variation of the E and H fields because of the helix short leading to large uncertainties in the stored energy U. The large dielectric response of the metals with their large values of dielectric “constant” −(ωpτeff)2 larger than 104 allows neglect of the Ez component contribution to Δf/f. However, uncertainties in the value of the T-independent τeff in the skin depth regime require the absolute calibration with high purity Si with ε∼11.7 at T=77 K. The calibration is accurate to ±15%.
Midinfrared photoluminescence from IV–VI semiconductors grown on silicon
Authors: McAlister D. W. , McCann P. J. , Namjou K. , Fang X. M. , Alkhouli O. , Wu H. Z.
Published: July 2002

Above room temperature continuous wave midinfrared photoluminescence has been observed from PbSe/Pb1−xSrxSe multiple quantum well structures grown by molecular beam epitaxy on (111) silicon. Emission energy from a sample with 10-nm-thick quantum wells varied from 336.1 to 343.7 meV as sample temperature was increased from 15 to 35 °C. At a heat sink temperature of 25 °C the emission energy varied from 336.8 to 339.9 meV as the current in the near-infrared diode pump laser was increased from 300 to 800 mA indicating an additional 8.2 °C of epilayer heating due to increased photon flux from the pump laser.
Quantitative photon tunneling and shear-force microscopy of planar waveguide splitters and mixers
Authors: Balistreri M. L. M. , Korterik J. P. , Veldhuis G. J. , Kuipers L. , van Hulst N. F.
Published: July 2002

A combined photon scanning tunneling and shear-force microscope has been developed to investigate the optical field distribution in a planar waveguide splitter and a multibranch mode mixer. The optical intensity distribution just above the surface of a planar waveguide is mapped with subwavelength resolution by a tapered optical fiber that probes the evanescent field. Simultaneously, the topography of the waveguide is recorded with subnanometer accuracy using a constant-distance feedback system based on shear-force detection with a tuning fork sensor. The experimental field patterns are quantitatively compared with field patterns simulated with the two-dimensional finite difference beam propagation method and a−mode solver. Good quantitative agreement between experiment and simulation is obtained. Moreover, the experiment reveals several details in the field distribution that results from incoupling conditions, mask imperfections, waveguide edges, and surface roughness. The surface effects in the optical field distribution are introduced by the use of the constant-distance feedback system.
Sound velocity in a model elastic superlattice composite with two dimensional periodic modulations
Authors: Mon K. K.
Published: July 2002

In this article, I study, by means of numerical solution to the wave equation, the long-wavelength sound velocities for a model of two component elastic superlattice composite. The composite is defined by a periodic array of parallel infinite rods of one component embedded in another elastic medium. This is a superlattice with periodic modulations in two directions. The wave equation is solved numerically with a finite plane wave basis and is used to calculate the long-wavelength sound velocities for the lowest and second lowest energy modes. I study the convergence in the number of Fourier components used and extrapolate with a power law. The deviations of the sound velocity squares from the uniform limits are observed to be proportional to the squares of the elastic constant modulation amplitude for the small amplitude regime. I observe similar results for a range of parameters and composite geometries.
Light induced structures in liquid crystalline side-chain polymers with azobenzene functional groups
Authors: Yaroschuk O. , Sergan T. , Lindau J. , Lee S. N. , Kelly J. , Chien L.-C.
Published: July 2002

We applied ellipsometry to study the distribution of azobenzene fragments in the films of two types of comblike polymers with azobenzene moieties in the side chains before and after ultraviolet (UV) light irradiation. The polymer with an alkyl chain at the end of the azobenzene fragment forms structures with preferred homeotropic alignment of the fragments. Irradiation of this polymer with nonpolarized UV light at normal incidence induces a homeotropic alignment of azobenzene fragments. Oblique irradiation induces tilted structures. Polarized UV light irradiation at normal beam incidence induces biaxial structures with a fanlike distribution of azobenzene fragments and preferably out-of-plane alignment. The polymer with polar nitro group at the end of the azobenzene moiety shows preferential in-plane orientation. The degenerate in-plane alignment is retained for normal irradiation with nonpolarized UV light. Excitation with polarized light provides highly ordered in-plane alignment of the azobenzene fragments perpendicular to the UV light polarization. Re-orientation of the fragments under a second UV exposure with orthogonal polarization involves out-of-plane rotation of the fragments.
Measurement of far-infrared waveguide loss using a multisection single-pass technique
Authors: Rochat Michel , Beck Mattias , Faist Jérôme , Oesterle Ursula
Published: July 2002

Waveguide loss measurements based on a multisection single-pass technique have been performed for both mid-infrared and far-infrared quantum cascade structures. The far-infrared quantum cascade structures are based on a vertical transition active region emitting at λ≈76 μm, embedded in a double-plasmon waveguide. The measured waveguide loss of 42±20 cm−1 agrees well with the calculated one based on free carrier absorption.
Matrix isolation spectroscopy of propylbenzene conformers and trapping sites
Authors: Dvir Shlomit , Haas Yehuda
Published: July 2002

Ultraviolet absorption and emission spectra of propylbenzene in an argon matrix are presented. The spectra are found to consist of several band series that are assigned to different trapping sites of the anti and gauche conformers of the molecule. Comparison with gas phase spectra and ab initio calculations allows the assignment of the vibronic bands of the emission spectrum. Some re-assignments of the observed vibronic bands are proposed. The relatively small number of trapping sites found for this asymmetric molecule is compatible with molecular dynamic simulations that are found to produce a small number of preferred sites.
Band gap engineering of amorphous silicon quantum dots for light-emitting diodes
Authors: Park Nae-Man , Kim Tae-Soo , Park Seong-Ju
Published: July 2002

Amorphous silicon quantum dots (a-Si QDs), which show a quantum confinement effect were grown in a silicon nitride film by plasma-enhanced chemical vapor deposition. Red, green, blue, and white photoluminescence were observed from the a-Si QD structures by controlling the dot size. An orange light-emitting diode (LED) was fabricated using a-Si QDs with a mean size of 2.0 nm. The turn-on voltage was less than 5 V. An external quantum efficiency of 2×10−3% was also demonstrated. These results show that a LED using a-Si QDs embedded in the silicon nitride film is superior in terms of electrical and optical properties to other Si-based LEDs.
Solvent nuclear quantum effects in electron transfer reactions. III. Metal ions in water. Solute size and ligand effects
Authors: Ando Koji
Published: July 2002

Solvent nuclear quantum effects in redox electron transfer (ET) reactions between metal ions in aqueous solution are studied via a molecular dynamics simulation analysis. The impacts of the solute size and charge variations together with the solvent ligand effects are examined by comparing with our previous study on a moderate size donor–acceptor system that assumed typical organic fluorescer-quencher molecules [J. Chem. Phys. 106, 116 (1997)]. It is shown that the spectral density function of the solvent coupling to ET, and consequently the quantum ET rate and its energy gap law, are strongly dependent on these variations of the system parameters. Two kinds of decomposition analysis, one into spatial contributions from inner- and outer-sphere solvations, and the other into motional frequency contributions from solvent intramolecular vibrations and intermolecular collective modes, are presented.
Thermodynamics modeling for moving contact line in gas/liquid/solid system: Capillary rise problem revisited
Authors: Fan H. , Gao Y. X. , Huang X. Y.
Published: July 2002

The nonequilibrium thermodynamics framework is tailored in the present paper to formulate the gas/liquid/solid system. In this system, there are two important issues, namely, the contact line motion and contact angle change, and shear stress singularity, during the dynamic evolution. Traditionally, the fluid mechanics approach has been applied to model these two issues. In the present paper, we applied a thermodynamics formulation to re-examine the first issue, i.e., the dynamic motion of the contact line and angle. It provides a new angle to understand the fundamentals of this classical problem. In order to verify the reliability of the present thermodynamics formulation, the capillary rise problem is revisited by using the formulation. A numerical result obtained based on the thermodynamics formulation is then compared with experimental test data. Excellent agreement between our analytical and experimental results gives us confidence for the future works on this approach.
Electronic structure of oxidized Ni/Au contacts on p-GaN investigated by x-ray absorption spectroscopy
Authors: Jan J. C. , Asokan K. , Chiou J. W. , Pong W. F. , Tseng P. K. , Tsai M.-H. , Chang Y. K. , Chen Y. Y. , Lee J. F. , Wu J. S. , Lin H.-J. , Chen C. T. , Chen L. C. , Chen F. R. , Ho J.-K.
Published: July 2002

X-ray absorption spectroscopy has been used to investigate the electronic structure of as-deposited and oxidized Ni/Au contacts on p-GaN. The Ni K-, L2,3-, and O K-edges x-ray absorption spectra clearly show the formation of NiO in the annealed contacts. Annealing in air increases Ni-site hole concentration and slightly shortens the nearest-neighbor Ni–O bond length, which enhances p–d hybridization and charge transfer from Ni to O. The observed very low specific contact resistance in the oxidized contacts is found to be due to the enhanced hole concentration at the Ni site.
The stabilizing effects of axial stretching on turbulent vortex dynamics
Authors: Nolan David S.
Published: July 2002

The evolution and dynamics of vortices in inviscid, incompressible flow under the influence of a deformation field which causes stretching in the axial direction are studied. These vortices are simulated using three-dimensional vortex methods which offer the advantages of having no inherent numerical dissipation and great computational efficiency for short to intermediate times. Two cases in particular are studied: The evolution of a stable, but strongly perturbed vortex, and the evolution of an unstable vortex. In both types of vortices, it is found that the stretching and radial inflow associated with the surrounding deformation field can suppress the growth of disturbances and slow the development of turbulence. This stabilization is even greater than what one would expect from the purely kinematic effects of the deformation field alone, indicating there is a negative feedback on the nonlinear vortex dynamics. The physical mechanisms for stabilization by stretching are discussed, along with potential applications for understanding the stability of intense geophysical vortices maintained by convection.
Study of the electronic structure of the actinide tetrabromides ThBr4 and UBr4 using ultraviolet photoelectron spectroscopy and density functional calculations
Authors: Beeching L. J. , Dyke J. M. , Morris A. , Ogden J. S.
Published: July 2002

Ultraviolet photoelectron spectra of UBr4 and ThBr4 have been recorded in the gas-phase and interpreted using relativistic density functional calculations. For ThBr4, eight bands were observed which are interpreted as ionization from the five Br 4p symmetry orbitals of a Br4 unit in Td symmetry, with three of the bands [the (4t2)−1, (1t1)−1, and (3t2)−1 ionizations] each being split into two by spin–orbit interaction. The observed splittings are rationalized in terms of the Th 6p and Br 4p contributions to the orbitals and by comparison with the known ultraviolet photoelectron spectra of ThF4 and ThCl4. The first vertical ionization energy (VIE) of ThBr4 was measured as (10.92±0.03) eV. UBr4 shows a very similar photoelectron spectrum with an extra band at (9.65±0.02) eV VIE. This is associated with a (5t2)−1 (U 5f,6d) ionization. Supporting matrix isolation infrared experiments were also carried out under very similar vaporization conditions to those used in the photoelectron spectroscopy experiments to check the composition of the vapor beams used. In these experiments, the T2 stretching modes of ThBr4 and UBr4 have been measured as 230±2 and 239±2 cm−1, respectively. Both the photoelectron and infrared matrix isolation spectra are consistent with an effective tetrahedral geometry for UBr4 and ThBr4.
Ultrafast spectroscopy of wavelength-dependent coherent photoionization cross sections of Li2 wave packets in the E1Σg+ state: The role of Rydberg states
Authors: Uberna Radoslaw , Amitay Zohar , Qian Charles X. W. , Leone Stephen R.
Published: July 2002

The significance of Rydberg states in the probing (via ionization) of Li2 wave packets has been studied through quantitative measurements of the relative coherent ionization cross sections in a two-color pump–probe femtosecond experiment. Following the preparation of a single intermediate rovibronic state with a cw laser, a femtosecond pump pulse (around 800 nm) creates a single two-state rotational wave packet by coherent excitation of the E1Σg+ (ν=9; J=27 and 29) states. The wave packet is then probed through ionization using time-delayed, wavelength tunable pulses (in the region 508–690 nm) while the total energy of the system is kept below the dissociation limit of Li2+. The background-free coherent ionization yield (for each probe wavelength) is measured as the relative oscillation amplitude of the single quantum beat time-dependent signal. The experimental results closely follow a relatively simple theoretical model, which is based on the assumption that the coherent ionization predominantly takes place via the excitation of high-n bound singly excited Rydberg states in the ionization continuum converging to the X 2Σg+ ground electronic state of Li2+. The best interpretation is that the high-n Rydberg states (above n∼25) undergo collisional ionization or autoionization and contribute to the measured coherent ionization signal, while the low-n Rydberg states undergo predissociation and do not contribute to the measured signal. An implication of the results is that the final states of the Li2 system, accessed by the above probe pulses, can be better approximated by a corresponding set of isolated discrete levels rather than by a continuum. This conclusion is important to experimental, as well as theoretical, coherent control and wave packet dynamics studies, in particular, when phase- and amplitude-shaped pump and probe pulses are employed. This study is also the first to investigate ionization of lithium dimer slightly below the E 1Σg+ shelf region with visible light.
Self-passivated copper as a gate electrode in a poly-Si thin film transistor liquid crystal display
Authors: Chae G. S. , Soh H. S. , Lee W. H. , Lee J. G.
Published: July 2002

Self-passivated copper as a gate electrode in the form of TiO/Cu/TiO/TiN/SiO2 has been obtained by annealing Cu/Ti/TiN/SiO2. The thickness of Ti in Cu/TiTiN was optimized at 150 Å by forming an 80 Å continuous TiO film on the outer surface of the Cu. The multilayer of SiO2/TiO/Cu/TiO/TiN/SiO2 showed stable electrical passivating properties against Cu diffusion into the top or bottom SiO2. Consequently, self-passivated copper has secured the dielectric properties of plasma enhanced chemical vapor deposition SiO2 and can be utilized as a gate electrode in low temperature poly-Si thin film transistor liquid crystal displays without sacrificing the low resistivity of Cu.
Phase-field model of domain structures in ferroelectric thin films
Authors: Li Y. L. , Hu S. Y. , Liu Z. K. , Chen L. Q.
Published: July 2002

A phase-field model for predicting the coherent microstructure evolution in constrained thin films is developed. It employs an analytical elastic solution derived for a constrained film with arbitrary eigenstrain distributions. The domain structure evolution during a cubic→tetragonal proper ferroelectric phase transition is studied. It is shown that the model is able to simultaneously predict the effects of substrate constraint and temperature on the volume fractions of domain variants, domain-wall orientations, domain shapes, and their temporal evolution.
Pressure and helium mixing effects on plasma parameters in temperature control using a grid system
Authors: Bai K. H. , Hong J. I. , Chung C. W. , Kim S. S. , Chang H. Y.
Published: July 2002

Pressure and He mixing effects on plasma parameters in electron temperature control using a grid system are investigated. Electron temperature is higher in lower pressure, when the electron temperature is high and not controlled. Electron density can be increased by about three times by decreasing the source gas pressure from 20 to 1 mTorr, and by about two times by He mixing in the temperature controlled region (diffusion region), while the electron density is decreased in the source region. This electron density increase is mainly due to the increase of the high energy electron population, and the measured electron energy distribution functions clearly show this.
Epitaxial growth and electronic structure of a C60 derivative prepared by using a solution spray technique
Authors: Shimada T. , Nakatani H. , Ueno K. , Koma A. , Kuninobu Y. , Sawamura M. , Nakamura E.
Published: July 2002

We demonstrate and analyze the epitaxial film formation of a molecular material that cannot be evaporated in vacuum due to thermal decomposition. A solution of the material is sprayed onto single crystalline van der Waals surfaces using a pulse valve under controlled vapor pressure of the solvent. Monolayer epitaxial films are obtained and we propose that the growth is mediated by liquid ultrathin films formed on the surface. Molecular arrangement and electronic structure of C60(CH3)5H films are studied by reflection high energy electron diffraction and ultraviolet photoelectron spectroscopy, respectively. The present technique will be useful to study the electronic structure of recently synthesized molecular materials.
Growth of a SiC layer on Si(100) from adsorbed propene by laser melting
Authors: Dragnea Bogdan , Boulmer Jacques , Débarre Dominique , Bourguignon Bernard
Published: July 2002

Carbon is incorporated into Si(100) to form a thin polycrystalline layer of SiC by laser melting the Si surface after adsorption of propene in ultrahigh vacuum. The SiC layer of thickness up to 25 nm is polycrystalline. Crystallites of size ≈100 nm are oriented with respect to the Si substrate and exhibit a diffraction pattern in low energy electron diffraction (LEED). The evolution of the surface is monitored in real time by recording the Si transient reflectivity at 675 nm at each laser pulse, and after exposure to the laser by LEED, IR spectroscopy, and atomic force microscopy. The formation of the SiC layer is accompanied by very strong variations of both the static and transient reflectivities, by the growth and narrowing of the IR peak assigned to β SiC, and by the increase of the C incorporation rate. The SiC overlayer is very stable against photodesorption, while initially small amounts of C on Si are photodesorbed in a few laser pulses. Recording the transient reflectivity during processing allows one to evidence that the laser absorption increases drastically as the SiC layer grows, resulting in (undesired) larger melting depth and duration that favor incorporation of C in Si below the SiC layer. SiC layers of improved quality might be obtained by active control of the laser fluence by means of the reflectivity transient.
Thermal reduction of activation energy of tricyclohexylphosphine on a rhodium crystal surface
Authors: Sadreev Almas F. , Sukhinin Yurii V. , Uvdal Kajsa , Pohl Anna
Published: July 2002

The activation energy of desorption of molecules adsorbed on crystal surfaces is defined by the interaction potential. This potential is reduced for finite temperature because of phonon excitations of the crystal surface. Moreover, rotations of molecules relative to the crystal surface contribute to the thermal reduction of the activation energy. As an application we consider organic phosphines on a rhodium surface. We interpolate the interaction by the Morse potential with parameters fitted to data obtained by density functional theory. The theoretical results are compared to temperature-programmed desorption experiments of tricyclohexylphosphine adsorbates on the rhodium crystal surface. The activation energy of desorption of the monolayers are calculated from temperature programmed desorption studies using the Redhead equation.
Self-assembly of colloidal spheres on patterned substrates
Authors: Ye Y.-H. , Badilescu S. , Truong Vo-Van , Rochon P. , Natansohn A.
Published: July 2002

We report here on the self-assembly of polystyrene colloidal spheres on patterned substrates. The substrate was a grating with a periodic one-dimensional-height profile. The two-dimensional (2D) arrays of colloidal spheres were obtained by the convective self-assembly method. It is shown that these structures strongly depend on the ratio between the diameter of the sphere (d) and the period of the grating (p) (x=d/p). When 1<x<1.15, and 1.15<x<2, the 2D array showed a centered-rectangular symmetry. For x=1.15 and 2, the ordered 2D array had a hexagonal symmetry structure. The angle between the grating groove direction and the crystal lattice vector (θ) is given by θ=sin−1(d/p). It is suggested that the variety of the 2D structures are formed by the capillary forces and the self-shadowing effects during the drying process.
Mesoscopic multiterminal Josephson structures. I. Effects of nonlocal weak coupling
Authors: Amin M. H. S. , Omelyanchouk A. N. , Zagoskin A. M.
Published: July 2002

We investigate nonlocal coherent transport in ballistic four-terminal Josephson structures (in which bulk superconductors (terminals) are connected through a clean normal layer, e.g., a two-dimensional electron gas). Coherent anisotropic superposition of macroscopic wave functions of the superconductors in the normal region produces phase slip lines (2D analogs to phase-slip centers) and time-reversal symmetry breaking 2D vortex states in it, as well as such effects as phase dragging and magnetic flux transfer. The tunneling density of local Andreev states in the normal layer is shown to contain peaks at the positions controlled by the phase differences between the terminals. We obtain the general dependence of these effects on the controlling supercurrent/phase differences between the terminals of the ballistic mesoscopic four-terminal SQUID.
Minority-carrier diffusion length in a GaN-based light-emitting diode
Authors: Gonzalez J. C. , Bunker K. L. , Russell P. E.
Published: July 2002

Minority-carrier diffusion lengths of electrons and holes were measured in a GaN-based light-emitting diode using the electron-beam-induced current technique in the line-scan configuration. A theoretical model with an extended generation source and a nonzero surface recombination velocity was used to accurately extract the diffusion length of the p- and n-type layers. A minority-carrier diffusion length of Ln=(80±6) nm for electrons in the p-type GaN layer, Lp=(70±4) nm for holes in the n-type GaN:Si,Zn active layer, and Ln=(55±4) nm for electrons in the p-type Al0.1Ga0.9N layer were determined. The results from this model are compared with two simpler and widely used theoretical models.
Desorption of polyatomic molecules from the Pt(111) surface by femtosecond laser radiation
Authors: Cai Lei , Xiao Xudong , Loy M. M. T.
Published: July 2002

We have experimentally studied femtosecond laser induced desorption of polyatomic molecules (NH3, ND3) from Pt (111), as a function of laser fluence and laser pulse duration (FWHM) from 120 fs to 1.0 ps. We found that the desorption yield does not depend on pulse width, in contrast to the results of a previously studied system, CO on the same metal surface. In addition, no isotope effect between NH3 and ND3 was observed. All these data are consistent with a thermal desorption mechanism instead of a hot-electron mediated process.
The Poisson equation in density fitting for the Kohn-Sham Coulomb problem
Authors: Manby F. R. , Knowles P. J. , Lloyd A. W.
Published: July 2002

A new density fitting approach to the Coulomb problem in Kohn–Sham and Hartree–Fock theory is introduced. Almost all of the 2- and 3-index repulsion integrals become simple overlap-like integrals, without approximation. The method is tested on numerous benchmark problems, which reveal that accuracy equal to or better than standard density fitting can be achieved with the evaluation of around a tenth of the number of Coulomb integrals. The scaling properties of the method are illustrated for polyalanine helices up to Ala16.
Cr:Er:Tm:Ho:yttrium aluminum garnet laser exhibiting dual wavelength lasing at 2.1 and 2.9 μm: Spectroscopy and laser performance
Authors: Walsh Brian M. , Murray Keith E. , Barnes Norman P.
Published: July 2002

Over 1.0 J of 2.1 μm laser energy and over 0.5 J of 2.9 μm laser energy have been demonstrated in a single flashlamp pumped solid state laser material, specifically Cr:Er:Tm:Ho:YAG. Flashlamp pumped laser operation of Ho:YAG at 2.1 μm and Er:YAG at 2.9 μm in various host materials is well known. We have developed an innovative laser system that operates at each of these wavelengths independently or simultaneously in a single solid state laser material with performance comparable to single wavelength systems Er:YAG and Cr:Tm:Ho:YAG. Variation of the flashlamp pump pulse length provides a method to discriminate between lasing at 2.1 and 2.9 μm. This effect results from Er→Tm→Ho energy transfer, the short lifetime of the upper lasing manifold in Er, the I11/24 manifold, and the relatively long upper laser level lifetime in Ho, the I75 manifold. This simple tuning method of achieving two widely separated wavelengths without the use of optical tuning elements has potential applications in remote sensing and medical lasers.
Effects of polarization on electroreflectance spectroscopy of surface-intrinsic n+-type doped GaAs
Authors: Sung Y. G. , Chiou S. J. , Wang D. P. , Lu Y. T. , Huang K. F. , Huang T. C.
Published: July 2002

The electroreflectance (ER) spectra of a surface-intrinsic n+-type doped (100) GaAs have been measured at various polarization angles of the probe beam. Several Franz–Keldysh oscillations were observed above the band-gap energy, thus enabling heavy- and light-hole transitions to be separated by the application of the fast Fourier transform to the ER spectra. From this, the ratios of the amplitudes of the light- to heavy-hole transitions versus angle of polarization were obtained. At a large incident angle (80°), the strength of the field of the probe beam in the normal direction of the sample (Fz) was varied from zero to a larger component. It was found that the ratios increased with increasing Fz which is consistent with the theory that the light-hole transition becomes more enhanced with z-polarized light.
In situ infrared-transmission study of vibrational and electronic properties during the formation of thin-film β-FeSi2
Authors: Fahsold Gerhard , Singer Kilian , Pucci Annemarie
Published: July 2002

Transition metal silicides can be formed by a solid phase reaction of metal films on silicon. During that thermally activated process, the system runs through chemically and crystallographically distinguishable phases with different properties concerning lattice vibrational modes and electronic transport. We demonstrate that in situ infrared transmission spectroscopy gives quantitative information on the development of both these properties during formation of the thin-film solid phases. We deposited a 12-nm-thick Fe film on Si(111) and then annealed this system at increasing temperatures. After each step of annealing we measured infrared spectra in normal transmission geometry. The changes in the broadband infrared transmission and the development of vibrational modes allow one to identify the subsequent formation of ε-FeSi and polycrystalline β-FeSi2 and to monitor the interplay of crystalline and electronic structure during formation of these phases.
Temperature dependence of 1/f noise in Pd/n-GaAs Schottky barrier diode
Authors: Singh R. , Kanjilal D.
Published: July 2002

Temperature dependence of 1/f noise in the Pd/n-GaAs Schottky barrier diode has been investigated. The noise measurements were performed under forward bias over a wide temperature range from 90 to 300 K and frequency range from 1 to 100 Hz. The noise spectra exhibited a frequency dependence proportional with 1/f′γ where γ varied between 0.8 and 1.2. It was observed that the spectral power density of current noise SI decreased with the increase in temperature up to 130 K. This behavior has been attributed to the spatial inhomogeneities of the Schottky barrier height at the metal–semiconductor interface. Above a temperature of 130 K, SI increased with the increase in temperature. This variation in SI has been attributed to the mobility and diffusivity fluctuations of the carriers within the space charge region of the Schottky barrier diode.
Electron and phonon temperature relaxation in semiconductors excited by thermal pulse
Authors: Gurevich Yu. G. , Logvinov G. N. , Carballo-Sanchez A. F. , Drogobitskiy Yu. V. , Salazar J. L.
Published: July 2002

Electron and phonon transient temperatures are analyzed in the case of nondegenerate semiconductors. An analytical solution is obtained for rectangular laser pulse absorption. It is shown that thermal diffusion is the main energy relaxation mechanism in the phonon subsystem. The mechanism depends on the correlation between the sample length l and the electron cooling length lε in an electron subsystem. Energy relaxation occurs by means of the electron thermal diffusion in thin samples (l≪lε), and by means of the electron–phonon energy interaction in thick samples (l≫lε). Characteristic relaxation times are obtained for all the cases, and analysis of these times is made. Electron and phonon temperature distributions in short and long samples are qualitatively and quantitatively analyzed for different correlations between the laser pulse duration and characteristic times.
Amplification of the signal in triode structures of ion detectors based on 6H-SIC epitaxial films
Authors: Lebedev A. A. , Strokan N. B. , Ivanov A. M. , Davydov D. V. , Savkina N. S. , Bogdanova E. V. , Kuznetsov A. N. , Yakimova R.
Published: July 2002

The possibility of about 50 times the inneramplification of signals in SiC-based detectors of short-range ions is shown. The detector has an n–p–n+-like structure, where the p-type base was grown epitaxially on a 6H n+-SiC substrate. To complete the structure a Schottky barrier was made on top. Detector parameters were investigated in a “floating base” regime. Alpha particles from 244Cm were used and the augmentation of signal (E) with increasing applied voltage (U) was investigated. A superlinear increase of E was observed with a significant (tens of times) amplification of the introduced by the alpha particle nonequilibrium charge. It was also found that the nonuniformity of the diffusion-drift carrier transport parameters in the films does not exceed 10%.
Simulation of higher harmonics generation in tapping-mode atomic force microscopy
Authors: Sahin O. , Atalar A.
Published: July 2002

In tapping-mode atomic force microscopy, nonlinear tip–sample interactions give rise to higher harmonics of the cantilever vibration. We present an electrical circuit to model the atomic force microscope cantilever with its first three flexural eigenmodes. An electrical circuit simulator is used to simulate the tapping-mode operation. Amplitude and phase responses of the third flexural eigenmode are obtained for different sample properties. It is found that amplitude and phase of higher harmonics depend highly on sample properties.
Nanoscale limited area growth of InAs islands on GaAs(001) by molecular beam epitaxy
Authors: Lee S. C. , Stintz A. , Brueck S. R. J.
Published: July 2002

Growth of InAs islands on a GaAs(001) substrate patterned with ∼50–200-nm diameter holes in an SiO2 mask overlayer providing selective GaAs nucleation areas is reported. The nanoscale pattern was generated in the SiO2 film by large-area interferometric lithography and dry etching. Two-dimensional, 285-nm period, arrays of InAs islands having heights of 10–15 nm with three different bottom diameters of 50–100, ∼150, and ∼200 nm were selectively grown on SiO2 patterned substrates by molecular beam epitaxy. Growth conditions were chosen to provide a very-low sticking coefficient of In atoms on the SiO2 surface suppressing volume contribution from migration of In atoms incident on the SiO2 mask region to nearby open GaAs surface areas. Formation of spherical-section InAs dots with diameters of about 50 nm relying on nanoscale-limited area growth is demonstrated. As the diameter of the hole increases beyond 150 nm, InAs islands deviate from a spherical section and self-assembled quantum dots confined within the open GaAs surface appear. A relation between dot formation and the nanoscale growth area is proposed, with a transition from single- to multiple-dot formation occurring at hole diameters of ∼100–150 nm.
Interstitial boron and oxygen related defects as the origin of the deep energy level in Czochralski-grown silicon
Authors: Ohshita Yoshio , Vu Tuong Khanh , Yamaguchi Masafumi
Published: July 2002

The structure of the B and O related defect complex in Czochralski (CZ)-grown Si crystal is theoretically studied by using ab initio calculations. When a B-doped CZ Si wafer is used as a solar cell material, light irradiation and/or minority carrier injection causes the solar cell conversion efficiency to be degraded. Because the interstitial B and O complex is considered to be the key defect causing the degradation, the stable position of interstitial B in O-doped Si crystal is determined by ab initio calculations using the Si29B1O1H36 cluster system. Although interstitial B cannot directly create a stable chemical bond with interstitial O, it is found to exist stably as an interstitial B–substitutional Si–interstitial O complex. The interstitial B and substitutional Si atoms in the defect are positively charged and generate a deep energy level, indicating that this complex efficiently attracts an electron due to its electrical force and that it might act as a recombination center. When this complex is generated by light irradiation and/or minority carrier injection, it shortens the lifetime of the electron resulting in the degradation of solar cell performance.
Extraction of trap states in laser-crystallized polycrystalline-silicon thin-film transistors and analysis of degradation by self-heating
Authors: Kimura Mutsumi , Inoue Satoshi , Shimoda Tatsuya , Tam Simon W.-B. , Lui O. K. Basil , Migliorato Piero , Nozawa Ryoichi
Published: July 2002

Trap states at the oxide-silicon interface and grain boundary in laser-crystallized polycrystalline-silicon thin-film transistors were extracted. The oxide-silicon interface traps and grain boundary traps can be extracted using the low-frequency capacitance–voltage characteristic and current–voltage characteristic, respectively. The traps above and below the midgap can be extracted using n-type and p-type transistors, respectively. The oxide-silicon interface traps consist of deep states and therefore seem to be caused by dangling bonds. The grain boundary traps consist of tail states and therefore seem to be caused by distortion of silicon-silicon bonds. Moreover, degradation by self-heating was analyzed. The oxide-silicon interface traps increase after the degradation. This means that silicon-hydrogen bonds are dissolved, and dangling bonds are generated. The grain boundary traps also increase a little.
Improved interfaces and magnetic properties in spin valves using Ni80Fe20 seed layer
Authors: Repetski Edward J. , Yang David X. , Chopra Harsh Deep , Chen P. J. , Egelhoff W. F.
Published: July 2002

The “giant” magnetoresistance (GMR) (Ni80Fe20)O–Co–Cu based “top” spin valves were studied with and without Ni80Fe20 as a seed layer. Microstructure examination shows that without the seed layer, the “free” and the “pinned” Co layers of the spin valves are highly irregular, discontinuous, and connected by pinholes across the Cu spacer layer, resulting in a large coupling >5.96 kA/m (>75 Oe) and a negligible GMR effect (<0.7%). The presence of Ni80Fe20 seed layer leads to continuous layers without pinholes and smooth interfaces in the (Ni80Fe20)O–Co–Cu, thereby essentially eliminating the coupling between the “free” and the “pinned” layers (0.23 kA/m or 2.9 Oe), a more than 25-fold reduction with respect to the seedless spin valves. Reduced detrimental coupling results in more than an order of magnitude increase in GMR (8.5%) in the NiFe seed layer spin valves. Domain studies confirm that the “pinned” and the “free” layers in seedless spin valves reverse their magnetization in an overlapping field range, and independently in spin valves deposited in the presence of a seed layer.
Comparison of current distributions in electroconvulsive therapy and transcranial magnetic stimulation
Authors: Sekino Masaki , Ueno Shoogo
Published: July 2002

We compared current density distributions in electroconvulsive therapy (ECT) and transcranial magnetic stimulation (TMS) by numerical calculations. The model consisted of an air region and three types of tissues with different conductivities representing the brain, the skull, and the scalp. In the ECT model, electric currents were applied through electrodes with a voltage of 100 V. In the TMS model, a figure-eight coil (6 cm diameter per coil) was placed on the vertex of the head model. An alternating current with a peak intensity of 3.0 kA and a frequency of 4.2 kHz was applied to the coil. The maximum current densities inside the brain in ECT (bilateral electrode position) and TMS were 234 and 322 A/m2, respectively. The results indicate that magnetic stimulators can generate comparable current densities to ECT. While the skull significantly affected current distributions in ECT, TMS efficiently induced eddy currents in the brain. In addition, TMS is more beneficial than ECT because the localized current distribution reduces the risk of adverse side effects.