bluuebunny/crossref_metadata_2025 huggingface dataset:
Imaging local index variations in an optical waveguide using a tapping-mode near-field scanning optical microscope
Authors: Tsai Din Ping , Yang Chi Wen , Lo Shu-Zee , Jackson Howard E.
Published: July 2002

Imaging local index variations by using a form of modulated near-field scanning optical microscopy is suggested. To test these ideas, we have probed two different optical structures, one a well-characterized BK-7 glass prism in the total internal reflection configuration, and the other a side-polished optical fiber waveguide with a step index of refraction of 4.5×10−3. Using a recently developed tapping-mode tuning-fork near-field scanning optical microscope, we have obtained images showing distinct local index variations. This method may have applicability to the characterization of a wide variety of optical waveguide structures.
Enhanced electrical properties of Ba0.5Sr0.5TiO3 thin films grown by ultraviolet-assisted pulsed-laser deposition
Authors: Srivastava Anuranjan , Craciun Valentin , Howard Joshua M. , Singh Rajiv K.
Published: July 2002

In this letter, we report enhanced electrical performance of high-dielectric-constant barium strontium titanate (BST) thin films grown by an in situ ultraviolet (UV)-assisted pulsed-laser deposition (UVPLD) technique. In comparison with conventional pulsed-laser deposition (PLD) BST (i.e., films grown under similar conditions but without UV illumination) and films grown by other techniques, the UVPLD-grown films exhibited improved structural and electrical properties. The dielectric constant of 40-nm-thick films deposited at 650 °C by PLD and UVPLD were determined to be 172 and 281, respectively. The density of interface states at the flat-band voltage was found to be approximately 5.6×1011 eV−1 cm−2 for the UVPLD-grown BST films, which was almost an order of magnitude lower than that obtained for conventional PLD films. The leakage current density of the UVPLD-grown films was approximately 4×10−8 A/cm2 at 100 kV/cm, which was nearly 1.5 times lower than that obtained from the PLD deposited films. The equivalent silicon dioxide thickness of the best BST films grown was found to be around 10 Å.
Thermal-expansion effects in near-field optical microscopy fiber probes induced by laser light absorption
Authors: Gucciardi P. G. , Colocci M. , Labardi M. , Allegrini M.
Published: July 2002

We present a study on thermal-expansion effects induced by laser light coupling into metal-coated probes for near-field scanning optical microscopy. An expansion of the probe edge of 170 nm per mW of coupled power has been observed as an effect of the temperature raise due to light absorption in the metal coating. The phenomenon has been studied in both time and frequency domains by modulating the coupled laser power, in order to measure the typical time constants related to heat exchange processes, that turned out to be of the order of a few milliseconds. An analytical model, taking into account both the heat conduction through the coating and the convection losses, provides scales for the system parameters and fits the experimental data remarkably well.
Nanometer-scale conversion of Si3N4 to SiOx
Authors: Chien F. S.-S. , Chang J.-W. , Lin S.-W. , Chou Y.-C. , Chen T. T. , Gwo S. , Chao T.-S. , Hsieh W.-F.
Published: July 2002

It has been found that atomic force microscope (AFM) induced local oxidation is an effective way for converting thin (<5 nm) Si3N4 films to SiOx. The threshold voltage for the 4.2 nm film is as low as 5 V and the initial growth rate is on the order of 103 nm/s at 10 V. Micro-Auger analysis of the selectively oxidized region revealed the formation of SiOx. Due to the large chemical selectivity in various etchants and great thermal oxidation rate difference between Si3N4, SiO2, and Si, AFM patterning of Si3N4 films can be a promising method for fabricating nanoscale structures.
Comparison of nitrogen incorporation in SiO2/SiC and SiO2/Si structures
Authors: McDonald K. , Huang M. B. , Weller R. A. , Feldman L. C. , Williams J. R. , Stedile F. C. , Baumvol I. J. R. , Radtke C.
Published: July 2002

The nitrogen content of SiO2/SiC (4H) structures annealed in NO and N2O has been measured using nuclear reaction analysis. Samples were annealed in N15O18 or N215O at 1000 °C at a static pressure of 10 mbar for either 1 or 4 h. Annealing in N2O incorporates ∼1013 cm−2 of N and annealing in NO incorporates ∼1014 cm−2, both of which are an order of magnitude lower than in SiO2/Si. In the NO anneal, N is predominantly incorporated near the SiO2/SiC interface with an atomic concentration of ∼0.5%. As in the nitridation of SiO2/Si, two features are observed in SiO2/SiC after the NO anneal: a surface exchange of O in the oxide with the gas phase and NO diffusion and reaction at the interface. The surface exchange reaction in SiO2/SiC is similar to SiO2/Si, but there is a large difference in the incorporation of N at the interface.
Two-dimensional magnetic switching of micron-size films in magnetic tunnel junctions
Authors: Anguelouch A. , Schrag B. D. , Xiao Gang , Lu Yu , Trouilloud P. L. , Wanner R. A. , Gallagher W. J. , Parkin S. S. P.
Published: July 2002

The magnetic switching behavior of micron-size magnetic tunnel junctions has been studied in two-dimensional magnetic fields. By measuring junction resistance, we obtain information about the magnetization state of the free ferromagnetic layer. Magnetic properties of this layer are explored using the Stoner–Wohlfarth rotational model as a starting point. We use geometric parameters of the critical curves to obtain information about interlayer coupling and domain structure effects in the free layer.
Dielectric enhancement and ferroelectric anomaly of compositionally graded (Pb, Ca)TiO3 thin films derived by a modified sol–gel technique
Authors: Bao Dinghua , Yao Xi , Zhang Liangying
Published: July 2002

Compositionally graded (Pb, Ca)TiO3 thin films were prepared on platinum-coated silicon substrates by a monoethanolamine-modified sol–gel technique. The crystalline orientation and surface morphology of the graded films were related to the deposition sequence of the film layer. The dielectric constants, for up-graded and down-graded films annealed at 600 °C for 60 min were found to be 504 and 399, respectively. Both were larger than those reported for conventional (Pb, Ca)TiO3 thin films. The compositionally graded films had large polarization offsets in hysteresis loops when driven by an alternating electric field. The magnitude of polarization offsets displayed a power-law dependence on the electric field, and the direction of the offsets depended on the direction of the composition gradient with respect to the substrate. The offset, 120 μC/cm2 at 300 kV/cm driving electric field, was obtained. These results showed that compositionally graded (Pb, Ca)TiO3 thin films had enhanced dielectric properties and abnormal ferroelectric properties which can be used in various microelectronic devices.
Near-band-gap optical functions spectra and band-gap energies of GaNAs/GaAs superlattice heterostructures measured by spectroscopic ellipsometry
Authors: Šik J. , Schubert M. , Leibiger G. , Gottschalch V. , Kirpal G. , Humlı́ček J.
Published: July 2002

The optical properties of GaAs/GaNxAs1−x superlattice structures grown by metal–organic vapor-phase epitaxy are studied by variable angle-of-incidence spectroscopic ellipsometry for photon energies from 0.75 to 1.55 eV. We employ Adachi’s critical-point composite model, and we report the direct-band-gap energy E0 and the complex index of refraction of the GaNxAs1−x sublayers for x⩽3.3% from analysis of the ellipsometry data. We observe a strong redshift of E0 with increase in x, and a strong decrease of the E0 transition amplitude. The E0 values obtained for the superlattice structures are in good agreement with photoluminescence results, and also with previous reports from single epilayers. Structure, composition, layer thickness, and parallel and perpendicular lattice mismatch of the samples are studied by transmission electron microscopy and high-resolution x-ray diffraction investigations.
Correlation between strain and dielectric properties in ZrTiO4 thin films
Authors: Kim Taeseok , Oh Jeongmin , Park Byungwoo , Hong Kug Sun
Published: July 2002

Single-phase paraelectric ZrTiO4 thin films were synthesized at various temperatures using direct-current magnetron reactive sputtering. The dielectric constants (ε) and dielectric losses (tan δ) of as-deposited and annealed films were measured in the 100 kHz range using a Pt upper electrode and a phosphorous-doped Si (100) bottom electrode. Data showed that as the deposition temperature increased, the dielectric losses decreased, while the dielectric constants did not change much. Similar trends for dielectric losses were observed after annealing at 800 °C. These results of dielectric losses correlated well with strains in ZrTiO4 thin films, analyzed from x-ray diffraction peak widths at various scattering angles.
Photoluminescence studies and read/write process of a strong two-photon absorbing chromophore
Authors: Sivaraman R. , Clarson S. J. , Lee B. K. , Steckl A. J. , Reinhardt B. A.
Published: July 2002

The optical properties of a fluorene-based chromophore exhibiting a high two-photon absorption cross section have been investigated both in the pure state and as a guest–host system in poly(N-vinylcarbazole). Upon irradiation with a helium cadmium (He–Cd) laser at 325 nm, the guest–host sample exhibits a blueshift with a maximum emission at 459 nm. Information written onto the samples in the blue can also be read using an Ar+ laser. The chromophore undergoes a chemical change upon irradiation in air, and has been found to no longer exhibit upconversion after this transformation. Infrared analysis of the sample (the chromophore) before and after irradiation was carried out, and the spectra suggest the formation of a new conjugated species. Other applications we have developed using this phenomenon include the successful writing of optical device structures in polymer films where the AF-50 is a guest material.
Raman scattering and infrared reflectivity in [(InP)5(In0.49Ga0.51As)8]30 superlattices
Authors: Popović Z. V. , Cantarero A. , Camacho J. , Milutinovi A. , Latinovi O. , González L.
Published: July 2002

We have measured far-infrared and infrared reflectivity as well as Raman scattering in an [(InP)5(In0.49Ga0.51As)8]30 superlattice grown by molecular beam epitaxy. A numerical model for calculating the reflectivity coefficient for complex systems which includes superlattice, buffer layer, and substrate has been developed. The far-infrared reflectivity spectra consists of the superlattice confined and interface modes as well as the modes from the buffer layer (In0.49Ga0.51As) and the substrate (InP). In the infrared spectral range above 1000 cm−1 we observe only interference fringes from the buffer layer. A good agreement between calculated and experimental spectra is achieved. The folded longitudinal acoustic phonon doublet appears at about 39 cm−1 in the Raman scattering spectra. The frequency agrees well with a continuum model calculation. In the optical phonon spectral region we observe confined modes corresponding to both constituents. The modes representing vibrations of atoms at both interfaces: InP/InGaAs (230 cm−1 mode) and InGaAs/InP (240 and 260 cm−1 modes) have also been observed. The geometrical parameters of the sample, obtained from the fitting of the reflectivity data, agree well to the values of the layer thickness obtained by double crystal x-ray diffraction.
Proper construction of ab initio global potential surfaces with accurate long-range interactions
Authors: Ho Tak-San , Rabitz Herschel
Published: July 2002

An efficient procedure based on the reproducing kernel Hilbert space interpolation method is presented for constructing intermolecular potential energy surfaces (PES) using not only calculated ab initio data but also a priori information on long-range interactions. Explicitly, use of the reciprocal power reproducing kernel on the semiinfinite interval [0,∞) yields a set of exact linear relations between dispersion (multipolar) coefficients and PES data points at finite internuclear separations. Consequently, given a combined set of ab initio data and the values of dispersion (multipolar) coefficients, the potential interpolation problem subject to long-range interaction constraints can be solved to render globally smooth, asymptotically accurate ab initio potential energy surfaces. Very good results have been obtained for the one-dimensional He–He potential curve and the two-dimensional Ne–CO PES. The construction of the Ne–CO PES was facilitated by invoking a new reproducing kernel for the angular coordinate based on the optimally stable and shape-preserving Bernstein basis functions.
Dielectric cracking produced by electromigration in microelectronic interconnects
Authors: Chiras S. , Clarke D. R.
Published: July 2002

The development of stress during electromigration along Al lines, constrained within a dielectric in a coplanar test configuration, is measured. It is shown that, above a certain threshold current density, cracking of the dielectric is induced in the vicinity of the anode. Cracking of the dielectric leads to loss of mechanical constraint on the aluminum conductor which, in turn, leads to increases in electrical resistance with continued current flow. The electromigration-induced stresses are determined from the measured frequency shifts induced in a novel ruby strain sensor embedded immediately beneath the interconnect line on a sapphire substrate. The transparency of the sapphire substrate also facilitated the observation of a hitherto unreported form of dielectric cracking, namely cracking from the interconnect along internal interfaces. The observations of dielectric cracking are in agreement with a recent fracture mechanics model. Analysis of the stress data, together with the results of finite element calculations of the strain energy release rate for crack extension, gives a quantitative estimate of the effective valence Z*(=1.3±0.2) for aluminum.
Linear stability analysis for the multikink solution of the sine-Hilbert equation
Authors: Matsuno Yoshimasa
Published: July 2002

The linear stability analysis is performed for the multikink solution of the sine-Hilbert (sH) equation. We first solve the eigenvalue problem of the sH equation linearized about the multikink solution and then establish the completeness relation among the eigenfunctions. This relation enables us to solve the initial value problem of the linearized sH equation, proving the stability of the solution against infinitesimal perturbations. As an application of the completeness relation, we develop a direct multikink perturbation theory while employing the method of multiple time scales. We derive the evolution equations that describe the slow change of the kink parameters, as well as a nonsecular expression for the first-order correction to the multikink solution. A novelty of the present analysis is that the proof of the completeness relation is done by purely algebraic means with the help of the classical theory of algebraic equations.
Demonstration of optical gain at 1.06 μm in a neodymium-doped polyimide waveguide
Authors: Karve Gauri , Bihari Bipin , Chen Ray T.
Published: July 2002

Neodymium-doped polyimide waveguides were studied for optical amplifier applications. Photoluminescence at 890 nm, 1.06 μm, and 1.33 μm from neodymium ions in the polymer matrix was observed. Optical gain of about 8 dB was measured at 1.06 μm in a 5-cm-long multimode channel waveguide. Phase separation was observed in the doped polyimide. Large scattering losses in the waveguides due to the phase separation were reduced by planarizing the layer of doped polyimide with another layer of undoped polyimide. Such a waveguide amplifier has potential applications in lossless splitters, lossless fiber, and waveguide systems with large fanout.
Beamlet–beamlet interaction in a multi-aperture negative ion source
Authors: Fujiwara Yukio , Hanada Masaya , Okumura Yoshikazu , Watanabe Kazuhiro , Kawai Kenichi
Published: July 2002

Beamlet–beamlet interaction in an acceleration region of a multi-aperture hydrogen negative ion source was experimentally studied. Deflection angle of beamlets was measured in the case of both five-row beamlets and three-row beamlets at the beam energy ranging from 86 to 178 keV. The deflection angle of beamlets at the edge of a beam was about 3 mrad larger than that at the center of the beam. The deflection angle was independent of beam energy at the same perveance. Repulsive force due to the beamlet–beamlet interaction proved to be inversely proportional to the square of distance approximately. From a beam trajectory calculation, shaping of a grid was confirmed to be effective to compensate the beamlet–beamlet interaction.
Third-order nonlinear optical response and energy transfer in static disordered systems
Authors: Yang Mino , Fleming Graham R.
Published: July 2002

The signals generated by various third-order nonlinear spectroscopies (transient absorption, transient grating, and three-pulse photon echo peak shift) on energy transfer systems are investigated by carrying out model calculations. Focusing on the understanding of basic features, we employ a simple model system consisting of pairs of energy donors and acceptors. We assume that energy transfer occurs via the Förster mechanism (weak electronic coupling). Static disorder in the transition energies of the chromophores induces inhomogeneities in the rate of energy transfer and in the optical response. The interplay between these two processes is discussed. We show that the peak shift experiment has advantages over the other types of experiment in clarifying the nature of disorder which affects the mechanism and time scale of energy transfer. For example, the peak shift technique can reveal the extent to which disorder is correlated in individual chromophore assemblies, and the extent to which energy transfer can correlate the energies of donors and acceptors. Finally the ability of the peak shift method to distinguish and quantify diagonal and off-diagonal disorder in energy transfer systems is discussed.
Testing van der Waals interactions with quantum dynamics: Repulsive anisotropy and well depth in the LiH+He system
Authors: Bodo E. , Gianturco F. A. , Martinazzo R. , Paesani F. , Raimondi M.
Published: July 2002

Recent calculations on the rigid rotor surface for the LiH–He interaction [B. K. Taylor and R. J. Hinde, J. Chem. Phys. 111, 973 (1999)] surmised that the well depth and anisotropy of the new surface differ substantially from the latest valence bond calculations for the same system [F. A. Gianturco, S. Kumar, S. K. Pathak, M. Raimondi, M. Sironi, J. Gerratt, and D. L. Cooper, Chem. Phys. 215, 227 (1997)]. We examine in detail the performance of both these potential energy surfaces once employed to generate quantum observables which can be compared with experiments. Our computations clearly show that the two surfaces exhibit small differences in their short-range repulsive anisotropy and larger differences in well depths. The latter however cannot be assessed as yet from existing experiments. The different behavior of their wells in supporting van der Waals bound states is also examined and spectroscopic experiments are suggested for detecting possible transitions between such states.
Theory of electromagnetic field imaging and spectroscopy in scanning near-field optical microscopy
Authors: Porto J. A. , Carminati R. , Greffet J.-J.
Published: July 2002

We derive a general expression for the signal in scanning near-field optical microscopy. This expression, based on the reciprocity theorem of electromagnetism, is an overlapping integral between the local electric field and a function that characterizes the tip. In particular, it includes the dependence on wavelength, illumination conditions, and polarization. To illustrate the potentiality of this theory, we discuss the polarization behavior and the spectral response of the apertureless setup.
Electronic and vibronic contributions to two-photon absorption of molecules with multi-branched structures
Authors: Macak Peter , Luo Yi , Norman Patrick , Ågren Hans
Published: July 2002

The electronic and vibronic contributions to the two-photon absorption of a series molecules with multi-branched structures have been studied using ab initio response theory. The results indicate that the electronic coupling between different branches alone cannot explain the experimental finding of a strong enhancement of the two-photon absorption cross section over the single branch structure, whereas it is predicted that vibronic contributions can play an important role in this respect. It is shown that for multi-branched molecules the use of circularly polarized light can increase the two-photon absorption cross section by a factor of 1.5 over linearly polarized light excitation.
Strong modification of photoluminescence in erbium-doped porous silicon microcavities
Authors: Zhou Y. , Snow P. A. , Russell P. St. J.
Published: July 2002

A microcavity composed of porous silicon multilayer mirrors was electrochemically etched and doped with erbium. Measurements of the reflectivity and photoluminescence spectra are presented. Thermal processing under a nitrogen atmosphere optically activated the erbium ions. Photopumping yielded room temperature emission around 1.54 μm from the erbium-doped samples with the emitted light strongly modified by the microcavity structure. Emission spectra with a peak at 1.536 μm had a full width at half maximum of ∼6 nm.
Low-frequency electrical noise of high-speed, high-performance 1.3 μm strained multiquantum well gain-coupled distributed feedback lasers
Authors: Chen X. Y. , Deen M. J. , Peng C. X.
Published: July 2002

Measurements of low-frequency electrical noise (LFN) in an in-phase gain-coupled distributed feedback lasers with etched quantum-well active-layers emitting at 1.3 μm wavelength have been conducted. In particular, the injected current dependence of LFN is investigated over a wide range of injection current (from 10−2 μA to 60 mA). Pure 1/f noise spectra were observed in all measurements. The current dependence of the 1/f noise strongly correlates to the I–V characteristics. We find that noise from different mechanisms dominates when the lasers operate in different ranges of injection currents.
Coarsening kinetics of Ni3Sn4 scallops during interfacial reaction between liquid eutectic solders and Cu/Ni/Pd metallization
Authors: Ghosh G.
Published: July 2002

The thickening and radial growth kinetics of Ni3Sn4 scallops formed during interfacial reaction between liquid eutectic solders and electroplated Ni/Pd metallization scheme on Cu substrate is studied. Selective etching of solder revealed three-dimensional morphology, and the dynamical phenomena, such as faceting, competitive growth, and coalescence of Ni3Sn4 scallops during interfacial reaction. The growth kinetics of the Ni3Sn4 scallops in the submicron length scale was analyzed using an Arrhenius-type of equation. Both kinetics exhibited nonparabolic behavior with the time exponent greater than three. The thickening of the Ni3Sn4 layer during interfacial reaction was accompanied by the concomitant coarsening of the scallops. The coarsening kinetics during early stages of interfacial reaction was characterized by (i) a temporal law with the time exponent greater than three, (ii) a decrease in the average number of scallops per unit volume with reaction time, and (iii) an increase in the standard deviation of the normalized size distribution with reaction time. The temporal laws for growth kinetics are discussed in terms of the effects of characteristic microstructural length scale and the existing coarsening theories. Among the coarsening theories, both the temporal law and the characteristics of radial size distributions were found to be consistent with the predictions of a recent Monte Carlo simulation of liquid-phase sintering in a two-phase system where the volume fraction of the second phase was very high.
Low field electron mobility in 6H-SiC
Authors: Dhar Subhabrata , Ghosh Subhasis
Published: July 2002

Experimental and theoretical studies of temperature and doping dependencies of electron mobility in 6H-SiC are reported. Low field electron mobility has been calculated by iterative technique and compared with experimental data. The following scattering mechanisms, i.e., impurity, polar optical phonon, acoustic phonon, and intervalley optical phonon are included in the calculation. Ionized and neutral impurity scattering have been treated beyond the Born approximation using a phase-shift analysis. We have found that neutral impurity scattering is very important in 6H-SiC due to large donor activation energy. Acceptor concentration is used as a parameter for fitting experimental data. Comparisons with published as well as our experimental data confirm the present calculation over a wide range of temperatures and electron concentrations.
Suppression of intervalley scattering in Ga(As)Sb quantum wells
Authors: Hall K. C. , Leonard S. W. , van Driel H. M. , Kost A. R. , Selvig E.
Published: July 2002

Femtosecond time-resolved reflectivity was measured near the 1.55 μm absorption edge of several GaAsxSb1−x/AlSb quantum well samples. On the basis of differences in the reflectivity recovery kinetics and plateau values, we deduce that Γ–L intervalley scattering can be effectively suppressed for x⩾0.19. This is consistent with calculations incorporating confinement and strain effects which give L–Γ energy separations of 29 (x=0) and 109 meV (x=0.19). Suppression of intervalley scattering can lead to increased internal quantum efficiency and higher carrier mobility in 1.55 μm based devices.
Peripheral current analysis of silicon p–n junction and gated diodes
Authors: Czerwinski A. , Simoen E. , Poyai A. , Claeys C.
Published: July 2002

The peripheral reverse current Ip in silicon p–n junctions sets the leakage and standby power limits in modern integrated circuits. In order to study its origin more in depth, a detailed analysis of the reverse current through a gated diode is developed here. In particular, it is shown that the study of the reverse current component associated with the thick field oxide under depletion and inversion provides a sensitive tool. In addition, combining the gate bias dependence with the temperature variation of the reverse gated diode current allows us to identify its different components, namely, the diffusion JpDIF, the depletion region generation JpGENblk, and the surface generation current density JpGENsrf. Based on this analysis, it is demonstrated that the peripheral diffusion current shows a remarkable increase with gate bias VG, while for standard diodes an increase with the reverse voltage VR is revealed. This bias dependence has to be taken into account when studying the activation energy of the diffusion and generation parts of the peripheral current. It is finally demonstrated that the proposed gated-diode analysis of the peripheral diode current is markedly more sensitive than analysis of the standard p–n junctions with a large perimeter.
Magneto-optic Kerr effect investigation of cobalt and permalloy nanoscale dot arrays: Shape effects on magnetization reversal
Authors: Johnson J. A. , Grimsditch M. , Metlushko V. , Vavassori P. , Ilic B. , Neuzil P. , Kumar R.
Published: July 2002

Using the magneto-optic Kerr effect (MOKE) and magnetic force microscopy we have investigated the shape dependence of magnetization reversal in a series of cobalt and permalloy nanoscale dot arrays. The patterns were produced by e-beam lithography combined with e-beam deposition and lift-off techniques. To avoid pattern to pattern variations in growth-induced anisotropy and or thickness, elliptical elements of varying aspect ratio were deposited simultaneously on a single substrate. All arrays were 1.0×2.5 μm tetragonal lattices and were fabricated with 300 Å thickness of cobalt and with 200 and 300 Å thickness of permalloy. From MOKE data we extract the field at which the onset of switching occurs for each array. For each material (viz. Co or permalloy), and thickness (200 or 300 Å), the switching field versus aspect ratio can be interpreted by a single nucleation field. The dependence of the nucleation barrier on the material and its thickness is discussed.
New setup for angular distribution measurements of Auger electrons from fixed in space molecules
Authors: Guillemin R. , Shigemasa E. , Le Guen K. , Ceolin D. , Miron C. , Leclercq N. , Ueda K. , Morin P. , Simon M.
Published: July 2002

A new experimental setup for measurement of the angular distributions of energy selected Auger electrons emitted from fixed in space molecules is presented. The system is based on two identical ion detectors with a small angular acceptance placed at 0° and 90° relative to the polarization axis of the incident radiation, and a high luminosity double-toroidal electron analyzer combined with position sensitive detection. This setup allows selection of the molecular alignment for σ and π ionization channels simultaneously and provides an energy and ejection angle measurement of the outgoing electron. The performance in terms of energy and angular resolution, as well as the calibration procedure, are discussed. Sample results obtained on the carbon monoxide ionized above the C 1s threshold are presented.
The infrared spectrum of the O⋯H⋯O fragment of H5O2+: Ab initio classical molecular dynamics and quantum 4D model calculations
Authors: Vener Mikhail V. , Kühn Oliver , Sauer Joachim
Published: July 2002

The gas phase IR spectrum of the O⋯H⋯O fragment of H5O2+ and its deuterated analogue are calculated using ab initio classical molecular dynamics based on a MP2 potential energy surface. The assignment of the bands is made in terms of the quantum four-dimensional model calculations of anharmonic frequencies and intensities. Comparing low and high kinetic temperature simulations the importance of anharmonicities of the potential energy surface for understanding the vibrational band structure is highlighted. It is shown that any reasonable simulation of IR spectra of systems with very strong hydrogen bonds has to account for the dipole moment function beyond the linear approximation.
Infrared conductivity mapping for nanoelectronics
Authors: Knoll B. , Keilmann F.
Published: July 2002

With ever shrinking dimensions in microelectronics, the conductivity performance of charge carriers approaches physical limits and demands tighter control. We show that near-field microscopy carried out at sufficiently long infrared wavelengths—below the plasma frequency—selectively detects and characterizes subsurface mobile carriers with 30 nm resolution, timely for next generation chips as well as for fundamental research, e.g., on low-dimensional electron systems.
Computation of three-dimensional flows past circular cylinder of low aspect ratio
Authors: Mittal S.
Published: July 2002

Results are presented for finite element simulation of three-dimensional unsteady flows past cylinders of low aspect ratio. The end-conditions are specified to model the effect of a wall that may correspond to the flow in a wind tunnel, water channel or a tow-tank experiment with a cylinder having large end-plates. Results are computed for Reynolds number 100, 300, and 1000 for a cylinder of aspect ratio 16. The computations confirm that it is the end conditions for the finite cylinder that determine the mode of vortex shedding (parallel or oblique). Preliminary results for Re=106 flow past a cylinder of aspect ratio 8 are also presented. The Re=100 and 1000 flows exhibit oblique mode of vortex shedding. The flow for Re=100 is very organized, devoid of any vortex dislocations and is associated with only one cell along the cylinder span. The flow at Re=1000 is interspersed with vortex dislocations and the vortex shedding angle varies, both, temporally and spatially. The presence of vortex dislocations is responsible for the breakdown of spanwise coherence of vortex structures. Mode B pattern of vortex shedding is observed. Flow at Re=300 results in flow patterns that correspond to the wake transition regime. Mode A and Mode B patterns of vortex shedding in addition to vortex dislocations are observed at different time instants. The present results indicate that the wake transition regime, that is known to occur in the Re range 190–250 for large aspect-ratio cylinders, is either extended and/or delayed for a cylinder of small aspect ratio with “no-slip” walls.
Magnetic permeability imaging of metals with a scanning near-field microwave microscope
Authors: Lee Sheng-Chiang , Vlahacos C. P. , Feenstra B. J. , Schwartz Andrew , Steinhauer D. E. , Wellstood F. C. , Anlage Steven M.
Published: July 2002

We describe a scanning near-field microwave microscope which uses a loop probe to measure local magnetic properties of metallic samples on a length scale of 200 μm. We demonstrate imaging at 6 GHz through spatiallyresolved ferromagnetic resonance experiments on a single crystal of the colossal magneto-resistive material La0.8Sr0.2MnO3. We find the experimental results are qualitatively and quantitatively well described by a simple model of the system.
Calculated magnetic properties of two-dimensional arrays of nanoparticles at vanishing temperature
Authors: Russier V.
Published: July 2002

We calculate the magnetic properties of a monolayer of spherical and uniaxial single domain magnetic nanoparticles, with dipolar interaction, at vanishing temperature. The particles are located on the sites of a lattice of either square or hexagonal structure. We focus on both the magnetic properties (magnetization curve, coercive field) and the orientational structure of the magnetic moments in the layer. We find that the structure of the lattice plays an important role on the orientational structure of the moments, especially in the case of a strong dipolar coupling: an ordered state takes place at a length scale of a few interparticle distances, the nature of which (ferromagnetic or antiferromagnetic) changes with the symmetry of the lattice. However, the magnetic properties are nearly independent of the structure of the lattice.
The Rys quadrature revisited: A novel formulation for the efficient computation of electron repulsion integrals over Gaussian functions
Authors: Dupuis Michel , Marquez Antonio
Published: July 2002

A novel formulation of the Rys quadrature algorithm for the calculation of the electron repulsion integrals over Gaussian basis functions is presented. The new algorithm is specifically designed for high contractions. As for the original Rys quadrature algorithm, the new algorithm is very efficient for high angular momentum functions. In addition it is also equally efficient for low angular momentum functions. The new algorithm takes unique advantage of (1) the numerical Rys quadrature methodology in (2) dealing with charge distributions a la McMurchie–Davidson and in (3) scaling integral blocks as a means of transferring angular momentum a la Gill–Head–Gordon–Pople. An analysis of the algorithm suggests very favorable floating-point operation counts.
Short range interaction potentials between anions in crystals
Authors: Pyper N. C.
Published: July 2002

The ab initio computation of uncorrelated short range two-body anion–anion potentials Vs0(rAA) can yield two apparent anomalies. First, despite the common understanding that the repulsion between two closed shell species arises from the overlap of their wave functions, compression of the anion electron densities sometimes increases Vs0(rAA), even though the overlap is reduced. Second, attractive Vs0(rAA) are occasionally predicted at large ionic separations rAA. These apparent anomalies arise because Vs0(rAA) is the sum of a permutation term Vperm0(rAA) arising from interionic electron exchange plus a penetration term Vpen0(rAA), independent of such exchange, equal to the nonpoint Coulombic electrostatic interaction. This is attractive at realistic rAA and reduced in magnitude by ionic compression. Vperm0(rAA) is always repulsive and is decreased by ionic compression except occasionally at large rAA involving an attractive Vs0(rAA). The latter increases are explained by analyzing Vperm0(rAA) into two further terms: one involving Vpen0(rAA). Uniform electron gas density functional predictions of Vperm0(rAA) are oversensitive to the ion density, thereby missing compression-induced enhancements of Vs0(rAA). Ab initio predictions of Vpen0(rAA) and Vperm0(rAA) are presented both for “optimal” Vs0(rAA) computed using anion wave functions optimal for each crystal geometry and for “frozen” Vs0(rAA), where the entire potential is computed using the anion wave function optimal for a geometry very close to that of the crystal at equilibrium. This data plus the total “frozen” Vs(rAA) consisting of Vs0(rAA) plus an approximate electron correlation contribution were required to parametrize both previous compressible ion model studies and the refinements presented in the next paper.
Physical aging and nanostructure of poly(methyl methacrylate): Effect of methanol
Authors: Etienne S. , David L. , Surovtsev N. , Duval E.
Published: July 2002

The effect of physical aging on the macroscopic thermophysical properties of amorphous polymers is well documented. The problem of physical aging of amorphous polymers at a nanoscopic scale is addressed in this work. To achieve this goal, we take advantage of low-frequency Raman scattering spectroscopy (LFRS) that makes it possible to investigate the structure at such a scale and its evolution by observing the LFRS intensity in the spectral range of the boson peak. The physical aging is usually observed according a given thermal history (i.e., quenching and isothermal aging below the liquid glass transition temperature). A particular procedure is considered out in this paper. Namely, the specimen of poly(methyl methacrylate) experiences a physical aging at room temperature (i.e., far below its conventional Tg) during a drying process following immersion in methanol until saturation. It is shown that this procedure increases the physical aging rate but in an inhomogeneous way. This observation together with low-frequency mechanical spectroscopy measurements is in agreement with a nonhomogeneous structure model of amorphous matter.
Structural and magnetic properties of Co/AlN multilayers
Authors: Zanghi D. , Traverse A. , Petroff F. , Maurice J.-L. , Vaurès A. , Dallas J. P.
Published: July 2002

Granular Co/AlN multilayers, with Co thicknesses varying from 0.2 to 3 nm, were prepared by sputtering. The goal was to prepare Co clusters embedded in a ceramic matrix and to study their magnetic properties. The local surrounding of the Co atoms was characterized via x-ray absorption spectroscopy. The magnetic properties were found to depend markedly on the Co thickness. The findings can be explained in terms of strong chemical interaction between Co and AlN.
Effect of correlated random copolymers at a liquid–liquid interface: Is it comparable with that of diblock copolymers?
Authors: Denesyuk N. A.
Published: July 2002

In this paper we return to the problem of adsorption of correlated random copolymers at a liquid–liquid interface. We explain our particular interest in the exponentially decaying sequential correlations, which are obtained by a specially developed “coloring” procedure. In comparison with our previous work [N. A. Denesyuk and I. Y. Erukhimovich, J. Chem. Phys. 113, 3894 (2000)], we present an improved analytical analysis, based on the full Green functions of the chains under consideration. Calculating the interfacial tension enables us to introduce a new definition of the adsorption point and to compare it with that applied within the ground state dominance approximation. Bearing in mind the possible practical applications, we additionally consider the case of diblock copolymer. We show that, for equal selectivities and mean compositions, adsorption is always deeper for the chains of diblock copolymers. Nevertheless, for a wide range of identified parameters correlated random copolymers can still serve as effective compatibilizers, comparable with diblock copolymers and polyblock chains.
Determination of the emission zone in a single-layer polymer light-emitting diode through optical measurements
Authors: Granlund Thomas , Pettersson Leif A. A. , Inganäs Olle
Published: July 2002

We study the emission zone in a single-layer polymer light-emitting diode. The emission zone is found by studying the angular distribution of the electroluminescence. The emission is modeled by accounting for optical interference. We account for birefringence of the anode layer in our model. The active polymer was, however, found to be isotropic. The anode consists of a single-layer of the conducting polymer complex poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonate) (PEDOT-PSS), with enhanced conductivity. As a cathode we use plain aluminum. By using only PEDOT-PSS we avoid having a thin metal layer or indium-tin-oxide as the anode in the path of the escaping light. The active material is a substituted polythiophene with excellent film forming properties. A comparison between the experimental and calculated angular distribution of light emission from a single-layered polymer light-emitting diode was shown to be in good agreement for the spectral region studied. By assuming a distribution of the emission zone, we deduce the position as well as the width of the zone.
Charge-coupled device systems for recording two-dimensional multi-mega-ampere z-pinch data
Authors: Failor B. H. , Coleman P. L. , Levine J. S. , Song Y. , Sze H. , LePell P. D. , Coverdale C. A. , Deeney C. , Pressley L. , Schneider R.
Published: July 2002

Hardware and software have been developed for recording and displaying accurate image and spectral data produced by z-pinch plasma radiation sources at the Double-EAGLE facility at Maxwell Physics International. Desktop computers are used to acquire the data, analyze it, and display and print the results. Of the four charge-coupled device (CCD) image recording systems implemented, two record x rays directly and two record optical light emission from electron–excited phosphors. The CCD systems required careful shielding to allow them to operate in the harsh radio frequency noise environment. During a series of shots at the SATURN facility at Sandia National Laboratories, the quality of a keV x-ray spectrum recorded directly with a CCD compared well with an equivalent spectrum recorded with 2497 film.
Magnetic aftereffect and electrical conductivity in double perovskite Ba2FeMoO6
Authors: de Francisco C. , Muñoz J. M. , Zazo M. , Flores A. G. , Íñiguez J. , Torres L.
Published: July 2002

In this work a study of the magnetic aftereffect and electrical conductivity in Ba2FeMoO6 is presented. Aftereffect measurements have been performed by recording the time evolution of magnetic permeability just after the sample demagnetization, while the electrical conductivity has been obtained by means of the four-probe technique working under ac excitation. The isochronal relaxation spectrum was constructed by evaluating at each temperature the relative variation of permeability between the end of demagnetization and a set of indexed time windows according to the fastness of the magnetic relaxation. The magnetic aftereffect spectra show two well-defined peaks related to the magnetic phase transition and the orientational processes that can be fitted by activation energies close to 0.65 eV. Electrical conductivity presents metallic behavior in the ferromagnetic region and semiconducting response in the paramagnetic zone with activation energy of 0.03 eV.
A fluid–kinetic hybrid electron model for electromagnetic simulations
Authors: Lin Zhihong , Chen Liu
Published: July 2002

A fluid–kinetic hybrid electron model for electromagnetic simulations of finite-β plasmas is developed based on an expansion of the electron response using the electron–ion mass ratio as a small parameter. (Here β is the ratio of plasma pressure to magnetic pressure.) The model accurately recovers low frequency plasma dielectric responses and faithfully preserves nonlinear kinetic effects (e.g., phase space trapping). Maximum numerical efficiency is achieved by overcoming the electron Courant condition and suppressing high frequency modes. This method is most useful for nonlinear kinetic (particle-in-cell or Vlasov) simulations of electromagnetic microturbulence and Alfvénic instabilities in magnetized plasmas.
Tunnel magnetoresistance in magnetic tunnel junctions with ZnS barrier
Authors: Guth M. , Da Costa V. , Schmerber G. , Dinia A. , van den Berg H. A. M.
Published: July 2002

A first experimental evidence of a significant tunneling magnetoresistance signal of about 5% at 300 K for a magnetic tunnel junction consisting of hard and soft magnetic layers separated by a 2 nm ZnS semiconducting barrier is reported. The samples have been grown by sputtering on Si(111) substrate at room temperature and have the following structure: Fe6 nmCu30 nmCoFe1.8 nmRu0.8 nmCoFe3 nmZnSxCoFe1 nmFe4 nmCu10 nmRu3 nm. The hard magnetic bottom electrode consists of the artificial antiferromagnetic structure in which the rigidity is ensured by the antiferromagnetic exchange coupling between two FeCo layers through a Ru spacer layer. Barrier impedance scanning microscope (BISM) measurements reveal a good homogeneity of the barrier thickness. Electric transport measurements over square tunnel elements with lateral sizes between 3 and 100 μm, exhibit a typical tunnel current–voltage variations and tunnel resistance of 2–3 kΩ μm2 with small variations which never exceed a factor of 2, which is in good agreement with the BISM results. This good reproducibility of the junctions is very promising for MRAMs and transistors applications.
Effect of grain size on magnetoresistance in Ba2FeMoO6
Authors: Han H. , Han B. J. , Park J. S. , Lee B. W. , Kim S. J. , Kim C. S.
Published: July 2002

Effect of grain size on magnetoresistance (MR) and electrical and magnetic properties of double perovskite Ba2FeMoO6 (BFMO) have been investigated. BFMO samples with different grain size have been prepared by the conventional solid state reaction followed by sintering in a stream of 5% H2/Ar at 1100 °C with various sintering times. The size of grain increases with increasing of sintering time. Temperature dependent magnetization is the same for samples with different grain size. However, electrical resistivity ρ(T) and MR are affected by the grain size; the ρ(T) and the MR value decrease with an increase in grain size. In spite of the different grain size, the ρ(T) shows a metallic behavior below the ferromagnetic transition temperature. Low magnetic field applied to the sample considerably reduces the resistivity over the whole temperature. The magnitude of MR for the sample sintered for 5 h is as large as 31% and 7% with the magnetic field of 0.7 T at 12 and 300 K, respectively.
Charge states of divacancies in self-implanted doped Si
Authors: Szpala S. , Simpson P. J.
Published: July 2002

The charge states of divacancies induced by 5 MeV self-implantation of doped silicon were investigated by positron annihilation methods. For low doping concentrations, results were found to be in agreement with the predictions of Fermi statistics. For the case of heavily boron-doped silicon (nB=1×1019 cm−3) an anomalous single-negative divacancy charge state was detected. We attribute this to the introduction of new levels in the band gap, due to the capture of boron by divacancies, resulting in a boron-divacancy complex. Detailed analysis of positron annihilation spectra suggests that the boron does not reside on a nearest-neighbor site to the divacancy. Isothermal annealing experiments yield activation energy of 0.9±0.1 eV for migration of this defect.