bluuebunny/crossref_metadata_2025 huggingface dataset:
Temporal fluctuations: A fingerprint of surface chemical reactions
Authors: Sarfaty Moshe , Davidow Jed , Lymberopoulos Dimitris
Published: July 2002

A real-time fast Fourier transform of temporal fluctuations of optical emission and rf power signals captures surface chemical reactions. The observed fluctuations, previously considered as noise and traditionally time averaged, depend on process chemistry and surface materials. The coherent optical emission fluctuations are specific only to the etch by-products. Plausible explanations, such as plasma density or electron temperature modulations, plasma instabilities, rf coupling modulations, or external hardware effects are dismissed by the data. It has been confirmed that the observed frequency patterns are directly related to surface chemical reactions of the film exposed to the plasma. This finding can be used to diagnose, monitor, and control chemical surface reactions in real time.
Effect of interfacial silicon on the structural stability of C54–TiSi2 on SiO2
Authors: Suh Dongwoo , Kim Hong Seung , Kang Jin-Yeong
Published: July 2002

The formation of interconnect C54–TiSi2 on SiO2 substrates suffers severe structural instability upon rapid thermal anneal. To understand the structural instability from a mechanistic point of view, we investigated the interfacial features of the C54–TiSi2 on SiO2 using Auger electron spectroscopy and cross-sectional transmission electron microscopy. As a result, we noted that the surplus silicon layer retained at the interface between the C54–TiSi2 thin film and the SiO2 substrates is indispensable for the stabilization of the C54–TiSi2 on SiO2. We explained the affirmative role of the silicon layer retained at the interface in terms of the residual stress of the C54–TiSi2 thin film.
X-ray induced luminescence of solid argon at high pressures: A pressure probe
Authors: Baer Bruce J. , Yoo Choong-Shik , Cynn Hyunchae
Published: July 2002

Above 7 GPa, visible luminescence is observed when argon in a diamond anvil cell is irradiated by high energy x rays from a synchrotron source. The spectrum consists of two peaks which shift by −43.6±0.7 cm−1/GPa and −47.5±2.4 cm−1/GPa from ambient pressure emission energies of 17 900±40 and 17 300±140 cm−1, respectively. These shifts are linear up to 60 GPa and reversible on decompression. Extrapolation of the shifts to ambient pressure suggests that this is a 4p–5d atomic transition.
Direct measurement of the dependence of granular giant magnetoresistance on the relative orientation of magnetic granules
Authors: Dai Jianbiao , Tang Jinke
Published: July 2002

Experiments have been designed to vary the relative angle between the magnetic moments of different Co granules in Cu80Co20 granular system. The moments of granules are mostly aligned in the same direction by field cooling to low temperature in a high magnetic field. A small field applied at an angle relative to the cooling field rotates the moments of a portion of the granules that have small particle size and coercivity. It is found that the giant magnetoresistance (GMR) varies linearly with cos φ, where φ is the relative angle between the magnetic axes of granules. This behavior disappears if the sample is cooled in zero fields, or if the rotating field is too large or small, or if the measuring temperature is higher than the blocking temperature. Our results show that the GMR in granular structures has the same angular dependence as the layered films and confirm the existing theories and recent microscopic models of granular GMR suggesting a crucial role of the relative orientations of the magnetic granules in determining the spin dependent scattering.
Formation of amorphous nanostructured materials by liquid state spinodal decomposition
Authors: Leung C. C. , Guo W. H. , Kui H. W.
Published: July 2002

As molten Pd40.5Ni40.5P19 is undercooled to a temperature T that is well below its liquidus Tl, liquid state spinodal decomposition takes place. If T is far away from Tl, wavelength λ of the liquid spinodals can be less than 100 nm. At this point, if water quenching is applied, upon solidification the final morphology consists of multiple amorphous networks intertwined with each other.
Temperature characteristics of bipolar cascade lasers
Authors: Patterson S. G. , Lau E. K. , Pipe K. P. , Ram R. J.
Published: July 2002

The temperature characteristics of bipolar cascade lasers operating up to 80 °C with room temperature, continuous-wave differential slope efficiencies of 93%, are reported. The continuous-wave characteristic lasing threshold temperature, T0, of the device is found to be 103 K for heatsink temperatures below 40 °C, dropping to 56 K for heatsink temperatures in the range 50–80 °C. Two separate regions are found to exist for the characteristic temperature of the differential slope efficiency, T1. The temperature of the topside metal contact versus bias was measured directly using a microthermocouple probe. Finite element modeling showed good agreement with measured surface temperatures. The simulations indicate that, prior to the onset of lasing in the bottom active region, nonradiative recombination heating in the bottom active region significantly heats the (lasing) top active region, leading to a reduction in the differential slope efficiency of the bipolar cascade laser.
Second-harmonic spectroscopy of bulk boron-doped Si(001)
Authors: Lim D. , Downer M. C. , Ekerdt J. G.
Published: July 2002

The effect of bulk boron incorporation on the second-harmonic generation (SHG) spectrum of Si(001) films grown epitaxially by chemical vapor deposition is studied as a function of doping level and temperature. At room temperature, boron doping (NA∼1018 cm−3) strongly enhances and blueshifts the E1 resonance of the second-harmonic generation spectra to 3.4 eV. Surface hydrogen termination reverses this effect. The observed doping and temperature dependence are modeled as electric-field-induced SHG in the bulk depletion region. The results suggest applications of SHG as an in situ, noninvasive probe of electrically active dopants.
Photonic band gaps and flat band edges in periodically textured metallic microcavities
Authors: Salt M. G. , Tan W. C. , Barnes W. L.
Published: July 2002

The dispersion of optical modes in metallic microcavities is shown to be dramatically altered when wavelength-scale periodic texture is added to one of the cavity mirrors. We demonstrate that Bragg scattering of the cavity modes by such microstructure produces near-flat bands that remain significantly flat well away from the first Brillouin zone boundary. The electric fields associated with these modes are studied and are found to have field maxima that are enhanced over those pertaining to planar microcavities. The impact of this band structure upon emissive devices, especially in terms of the clear insensitivity to viewing angle, is discussed.
A technique to form a porous silicon layer with no backside contact by alternating current electrochemical process
Authors: El-Bahar A. , Nemirovsky Y.
Published: July 2002

We report here the formation of porous silicon under alternating current conditions. Instead of applying the usual direct current electrochemical process, an alternating current was applied with a given frequency and peak voltage. The porous silicon layer properties are equivalent to the properties that would be achieved by the standard direct current formation technique (i.e., same porosity level). The main advantages of this process are: (a) The alternating current formed porous silicon exhibits higher mechanical stability during the drying step than layers formed using the standard direct current technique. (b) The alternating current process can be performed without a deposited backside contact. These simplify the process and permit its integration with high temperature processing steps and clean furnaces of a modern very large sale-integrated technology.
Statistical analysis of near-field photoluminescence spectra of single ultrathin layers of CdSe/ZnSe
Authors: von Freymann G. , Kurtz E. , Klingshirn C. , Wegener M.
Published: July 2002

The statistical analysis of thousands of near-field photoluminescence spectra of single ultrathin CdSe layers at 20 K exhibits a strong positive correlation peak around 20 meV energy with a width of 5 meV. Our data are consistent with individual spectra which consist of sets of many pairs of lines. In each pair, the two lines must have comparable strength. We speculate about the origin of these pairs.
Luminescence spectroscopy of GaN in the high-temperature regime from room temperature to 900 °C
Authors: Young A. P. , Brillson L. J.
Published: July 2002

We have measured the optical luminescence spectrum of GaN from the near infrared to the near ultraviolet at elevated temperatures. Despite intense blackbody radiation above 600 °C, luminescence is observable at 3 eV or greater at temperatures as high as 900 °C, i.e., including near-band-edge emissions over the entire range of GaN growth temperatures. Elevated-temperature measurements also reveal thermally activated quenching of the midgap “yellow” defect luminescence. These results show that electron beams already used to monitor molecular-beam epitaxy growth can also provide temperature and defect information during the growth process.
GaN/W/W-oxide metal base transistor with very large current gain and power gain
Authors: Mochizuki K. , Uesugi K. , Asbeck P. M. , Gotoh J. , Mishima T. , Hirata K. , Oda H.
Published: July 2002

We demonstrate a GaN/W/W-oxide metal base transistor (MBT) whose collector is formed by oxidizing the intrinsic W base. The thickness of the nonoxidized intrinsic base of the fabricated collector-up MBT on a sapphire substrate was estimated to be 2–3 nm. Although the MBT showed large leakage, subtraction of the leakage from collector current revealed that the transistor had a very large small-signal direct current (dc) current gain of 87 dB and a dc power gain of 50 dB. This indicates that the GaN-based MBT is a possible candidate for microwave and millimeterwave amplifiers as well as for high-speed integrated circuits used in optical fiber communication system.
A simple algebraic derivation of the covariant anomaly and Schwinger term
Authors: Ekstrand C.
Published: July 2002

An expression for the curvature of the “covariant” determinant line bundle is given in even-dimensional space–time. The usefulness is guaranteed by its prediction of the covariant anomaly and Schwinger term. It allows a parallel derivation of the consistent anomaly and Schwinger term, and their covariant counterparts, which clarifies the similarities and differences between them.
Bit error probabilities in high temperature superconductor logic
Authors: Atkin I. L. , Satchell J. S. , Hirst P. J. , Humphreys R. G.
Published: July 2002

We present error rate simulations for a single flux quantum logic circuit which include statistical variations in the circuit parameters. We find that the fabrication yield, junction critical margins, and bit error probabilities of the nominal circuit can give a false impression of the performance of practical circuits. For our study circuit, the toggle flipflop, we find that, although the standard figures of merit may vary only marginally with design rules, the average error rate can differ by a factor of 104. Furthermore, for a state-of-the-art technology, the error rates of different realizations of the same design actually span 10 orders of magnitude. These results highlight the inadequacy of current figures of merit to gauge the performance of high temperature superconductor designs.
Statistical mechanics of rotationally inelastic molecule–surface scattering in the dynamical Lie algebraic method
Authors: Guan Daren , Yi Xizhang , Zheng Yujun , Ding Shiliang , Sun Jiazhong
Published: July 2002

The dynamical Lie algebraic method is used for the description of statistical mechanics of rotationally inelastic molecule–surface scattering. A main advantage of this method is that it can not only give the expression for evolution operator in terms of the group parameters, but also provide the expression for the density operator for a given system. The group parameters may then be determined by solving a set of coupled nonlinear differential equations. Thus, the expressions of the statistical average values of the translational-to-rotational energy transfer, the interaction potential, and their dependence on the main dynamic variables for the system are derived in terms of the density operator formalism in statistical mechanics. The method is applied to the scattering of NO molecules from a static, flat Ag(111) surface to illustrate its general procedure. The results demonstrate that the dynamical Lie algebraic method can be useful for describing statistical dynamics of gas–surface scattering.
A vacuum gauge using positive temperature coefficient thermistor as the sensor
Authors: Chatterjee Suman , Halder Ashim K. , Sengupta Kamalendu , Maiti Himadri S.
Published: July 2002

A vacuum gauge has been designed and fabricated using a new type of sensor element, the semiconducting barium titanate positive temperature coefficient (PTC) thermistor. It covers a much wider range and better resolution compared to conventional Pirani gauge and other thermistor vacuum gauges particularly in the low vacuum levels due to its extreme high temperature coefficient. The details of design criteria of the instrument and its performance characteristics have been discussed. Due to the unique autocurrent limiting and stabilizing capabilities of a PTC thermistor, the fabrication of the instrument appears to be much cheaper.
An extended-cavity diode laser with a circular output beam
Authors: Lancaster G. P. T. , Sibbett W. , Dholakia K.
Published: July 2002

We report a compact extended-cavity diode laser producing, directly, 30 mW of near diffraction limited circular output. A laser diode with an integrated microlens is mounted in the Littrow geometry giving 5 GHz of continuous and 5 nm discontinuous tuning around 780 nm. The beat note linewidth of two similar lasers is measured to be 300 kHz. Laser cooling and trapping of rubidium has been performed with this laser geometry.
Time-resolved two-band infrared radiometry of carbon tetrachloride under shock compression up to 10 GPa
Authors: Fat’yanov Oleg V. , Ogura Toshiyuki , Nicol Malcom F. , Nakamura Kazutaka G. , Kondo Ken-ichi
Published: July 2002

Time-resolved two-band infrared radiometers were constructed for measuring temperatures of carbon tetrachloride under shock compression. Radiance histories were observed with about 10 ns time resolution in two slightly overlapping bands, approximately 7–9.5 and 9–12 μm, for shock pressures from 3.3 to 9.4 GPa. The experimentally obtained temperatures (680–1370 K) were consistent with the results of theoretical calculations using equation-of-state data below 7 GPa and suggested the chemical reaction of CCl4 at 7.5 GPa.
Controllable method for fabricating single-wall carbon nanotube tips
Authors: Zhang Y. , Iijima S.
Published: July 2002

Single-wall carbon nanotubes (SWCNTs) were abruptly cut with a niobium substrate after a heating process in an ultrahigh vacuum transmission electron microscope. The cutting was attributed to a break of weakly bonded interface between carbon nanotubes and niobium carbide that formed as a product of a solid-phase reaction. This effect provided a controllable method for preparing well-defined SWCNT tips in future field-emission applications.
The Ideal Gas Thermodynamics of Diesel Fuel Ingredients. I. Naphthalene Derivatives and Their Radicals
Authors: Curran Henry , Wu Christine , Marinov Nick , Pitz William J. , Westbrook Charles. K. , Burcat Alexander
Published: July 2002

The molecular fundamentals of 21 naphthalene derivatives were investigated, calculated and evaluated, and their ideal gas thermodynamic properties were calculated, for the sake of simulating the combustion properties of diesel fuel. Ten of these species are stable molecules and 11 are radicals. The molecular fundamentals are calculated using Gaussian 94 ab initio and MOPAC 6 semiempirical programs. The results can be used to estimate the MOPAC performance with polyaromatic species.
Exchange coupling in NiFe/NiMn films studied by pseudo-Hall effect
Authors: Li Guohong , Yang Tao , Hu Qiang , Lai Wuyan
Published: July 2002

Pseudo-Hall effect in exchange biased NiFe/NiMn films was measured in magnetic fields rotated in the film plane. Besides accurate determination of the exchange biasing direction, coherent rotation and incoherent reversal of the magnetization of the NiFe layers can be distinguished from these measurements. Detailed magnetic anisotropy analysis shows that there is no additional uniaxial anisotropy induced by the interfacial coupling in this system, while the instability of antiferromagnetic grains may play an important role that is responsible for the difference between exchange biasing fields obtained by reversible and irreversible measurements.
Quantum stochastic models of two-level atoms and electromagnetic cross sections
Authors: Barchielli Alberto , Lupieri Giancarlo
Published: July 2002

Quantum stochastic differential equations have been used to describe the dynamics of an atom interacting with the electromagnetic field via absorption/emission processes. Here, by using the full quantum stochastic Schrödinger equation proposed by Hudson and Parthasarathy, we show that such models can be generalized to include other processes into the interaction. In the case of a two-level atom we construct a model on the basis of some physical requirements, the main being a balance equation on the fluxes of the ingoing and outgoing photons; in this model the atom-field interaction turns out to be due either to absorption/emission processes either to direct scattering processes, which simulate the interaction due to virtual transitions to the levels which have been eliminated from the description. To see the effects of the new terms, we consider both direct and heterodyne detection of the fluorescence light emitted by an atom stimulated by a monochromatic coherent laser and we deduce from these two detection schemes the expressions of the total, elastic and inelastic electromagnetic cross sections and the spectral distribution of the fluorescence light. The total cross section, as a function of the frequency of the stimulating laser, can present not only a Lorentzian shape, but the full variety of Fano profiles; intensity dependent widths and shifts are obtained. The fluorescence spectrum can present complicated shapes, according to the values of the various parameters; when the direct scattering is not important the usual symmetric triplet structure of the Mollow spectrum appears (for high intensity of the stimulating laser), while a strong contribution of the direct scattering process can distort such a triplet structure or can even make it to disappear.
Narrowing of high power diode laser arrays using reflection feedback from an etalon
Authors: Romalis M. V.
Published: July 2002

The spectrum of a high power multielement laser array is narrowed using reflection feedback from an 100 μm etalon placed in front of the laser. The full width at half maximum (FWHM) of the laser array is reduced by a factor of 2 with only 6% power loss. This reduction in FWHM is useful for optical pumping of alkali metals in the presence of high density buffer gas.
Micro-Raman study of high pressure induced graphite-diamond phase-structural transformation: The role of a nitrogen containing precursor
Authors: Klyui N. I. , Valakh M. Ya. , Pascual J. , Mestres N. , Petrusha I. A. , Zaika N. I.
Published: July 2002

CN x samples subjected to a high pressure-high temperature treatment were studied by micro-Raman spectroscopy, microhardness, and x-ray diffraction techniques. After treatment the CNx material leads to the formation of some highly ordered diamond crystals, showing an extraordinarily low broadening of the 1332 cm−1 Raman line (Δν=2.43 cm−1). We propose that the graphite-diamond phase-structural transformation takes place through the formation of rombohedral graphite inclusions in the treated sample and the corresponding effect of puckering of the graphite layers. The observation of the band at ∼1621 cm−1 in the Raman spectra of the matrix surrounding the diamond crystals may be considered as an evidence of the proposed model.
Formation of ultrafine nanostructure by crystallization of Zr52Al6Cu14Ni8Fe20 metallic glass
Authors: Mattern N. , Kühn U. , Neuefeind J. , Eckert J.
Published: July 2002

The short-range order and the crystallization behavior of amorphous Zr52Al6Cu14Ni8Fe20 alloys have been investigated by means of calorimetry and x-ray diffraction. The amorphous structure transforms upon annealing without formation of long-range order. An ultrafine microstructure consisting of about 60 vol % cubic NiZr2 crystal-like clusters with a size of 〈D〉≈2 nm embedded in a residual amorphous matrix phase forms as a first step of crystallization resulting from high nucleation rate combined with low growth velocity. In a second step growth of the clusters to crystals up to a mean diameter of 〈D〉≈4–5 nm takes place as a distinct process.
Tensor product of principal unitary representations of quantum Lorentz group and Askey–Wilson polynomials
Authors: Buffenoir E. , Roche Ph.
Published: July 2002

We study the tensor product of principal unitary representations of the quantum Lorentz group, prove a decomposition theorem, and compute the associated intertwiners. We show that these intertwiners can be expressed in terms of complex continuations of 6j symbols of Uq(su(2)). These intertwiners are expressed in terms of q-Racah polynomials and Askey–Wilson polynomials. The orthogonality of these intertwiners imply some relation mixing these two families of polynomials. The simplest of these relations is the orthogonality of Askey–Wilson polynomials.
Influence of adsorbates on the surface conductivity of chemical vapor deposition diamond
Authors: Szameitat M. , Jiang X. , Beyer W.
Published: July 2002

To investigate the mechanism of the diamond surface conductivity, temper experiments have been performed on chemical-vapor-deposited (CVD) diamond films under vacuum conditions. The surface conductivity of these films was measured as a function of temperature and in contact with different gas atmospheres. These results were compared with those obtained by gas evolution experiments performed on CVD diamond samples of the same kind, demonstrating the possible role of CO adsorption to the surface conductivity.
Osmotic second virial cross coefficients for star and linear polystyrenes
Authors: Striolo Alberto , Prausnitz John M.
Published: July 2002

Experimental osmotic second virial cross coefficients are reported for linear and 8-arm star polystyrenes in three solvents: toluene, cyclohexane, and methylcyclohexane. The osmotic second virial cross coefficient for 8-arm star and linear polystyrene is always positive and within the osmotic second virial coefficients measured for the single polymers. The positive cross coefficient indicates net repulsion between the two different polymers in dilute solution. The extent of repulsion is greatest in toluene and least in cyclohexane. To relate the macroscopic second virial coefficient to microscopic interactions, the potential of mean force between linear and 6-arm star polymers was computed by molecular simulation. The interaction between nonbonded polymer segments is given by a square-well potential. Well width was set equal to one half of the segment diameter. Different solvent conditions were investigated by using different well depths. Potentials of mean force were then used to compute the osmotic second virial cross coefficients.
Reactive removal of misfit dislocations from InGaAs on GaAs by lateral oxidation
Authors: Mathis S. K. , Chavarkar P. , Andrews A. M. , Mishra U. K. , Speck J. S.
Published: July 2002

Layers of hypercritical thickness InGaAs/AlAs/GaAs have been shown to strain relax when the underlying AlAs layer is laterally oxidized. InxGa1−xAs layers of composition 0.2⩽x⩽0.4 have been investigated at thicknesses in the range 5–20 times the Matthews–Blakeslee critical thickness hc. The amount of strain relieved does not depend on the InGaAs-layer thickness, the initial strain state, or the composition of the material, but it does strongly depend on the oxidation temperature. The structure of the strain-relaxed InGaAs layer has been investigated using plan-view transmission electron microscopy. It is shown that the misfit dislocation density in In0.3Ga0.7As grown to 15 times the critical thickness (660 Å) has been reduced by two orders of magnitude after lateral oxidation. It is proposed that this is due to interfacial oxidation, which consumes the misfit dislocation cores.
Lateral epitaxial overgrowth of GaSb on GaSb and GaAs substrates by metalorganic chemical vapor deposition
Authors: Yi S. S. , Hansen D. M. , Inoki C. K. , Harris D. L. , Kuan T. S. , Kuech T. F.
Published: July 2002

Lateral epitaxial overgrowth of GaSb on GaSb and GaAs substrates patterned with SiO2 or Si3N4 films by metalorganic chemical vapor deposition was accomplished using trimethylgallium and trimethylantimony. Transmission electron microscopy measurements show that coalesced films grown on GaSb substrates exhibit defect-free materials, while those on GaAs substrates show regular, small-angle crystal tilting originating from large lattice mismatch.
Second-order optical nonlinearity generated by doping the surface layer of silica with anions or cations
Authors: Qiu Mingxin , Vilaseca Ramon , Cojocaru Crina , Martorell Jordi , Mizunami Toru
Published: July 2002

A method for generating second-harmonic light from silica glass by doping its surface layer with ions is presented. When anions CO32−, OH− or Cl− are introduced from the cathodic surface during poling, the second-harmonic signal generated near that surface is enhanced by two orders of magnitude with respect to the case of poling without doping. The penetration depths of CO32−, OH− and Cl− have been measured by the etching technique. In contrast, by doping the anodic surface with cations Li+, Na+ or K+, the second-harmonic signal drops to one tenth of that corresponding to the case of poling without doping. This indicates that in the anodic surface the main charges are negative, and the positive charges neutralize them to decrease the strength of the electric field that is at the origin of the second-order susceptibility. Secondary ion mass spectroscopy detection shows the concentration distribution of the injected lithium in the surface layer. The second-harmonic signal from the anodic surface decreases monotonically with increasing poling-doping time up to 300 min. Doping from interfaces far from the electrodes is inefficient for influencing the second-harmonic generation.
Direct observation of lasing mode in a microdisk laser by a near-field-probing technique
Authors: Baba Toshihiko , Yamada Hiroshi , Sakai Atsushi
Published: July 2002

Evanescent field profiles of whispering gallery (WG) modes in a 1.6-μm-GaInAsP microdisk injection laser were investigated by scanning a nanoprobe near the periphery of a lasing microdisk. The increase in threshold current due to the light scattering by the probe and the corresponding decrease in laser light were observed experimentally. Two-dimensional images of the evanescent field, which agreed well with theoretical expectations, were obtained by the use of a Pt–Ir probe. The images indicate that the WG mode is strongly locked by the fourfold symmetry in the microdisk with center post claddings. This characteristic is favorable for the microdisk laser itself to be used as an active near-field probe. A lower contrast image obtained by the use of a tapered silica fiber probe suggests that the microdisk probe can be sensitive to both surface profiles and optical properties of the object.
Anomalous behavior of Co insertion to Al2O3 in CoFe/Al2O3/NiFe tunnel junctions
Authors: Tanoue S. , Yamasaki A.
Published: July 2002

We fabricated a tunnel junction 20 nm CoFe/0.75 nm Al2O3/t Co/0.75 nm Al2O3/20 nm NiFe (t=0–1 nm) to investigate the effect of Co layer insertion on Al2O3 layer, using direct sputtering Al2O3 target. At room temperature the magnetoresistance (MR) ratio decreased as the Co layer thickness increased and above 0.4 nm Co the MR ratio became zero. With decreasing temperature, the MR ratio of 0.3 nm Co sample rapidly increased and reached 14.6% at 50 K. On the other hand, the MR ratio of 0 nm Co sample gradually increased and reached 4.6% at 4.8 K. Temperature dependence of the MR ratio was interpreted by two-step tunneling. MR was enhanced by Co insertion, although the mechanism was not fully understood. Anomalous skirts were found in MR curves of 0.3 nm Co sample below 150 K and their behavior resembled granular tunneling magnetoresistance. This phenomenon may be interpreted if the 0.3 nm Co layer sandwiched between Al2O3 insulating layers has a paramagnetic state above 150 K and remains in a superparamagnetic state below 150 K, provided the Curie temperature of 0.3 nm Co layer is 150 K.
Thermally detected optical absorption, reflectance, and photoreflectance of In(As,P)/InP quantum wells grown by gas source molecular beam epitaxy
Authors: Disseix P. , Payen C. , Leymarie J. , Vasson A. , Mollot F.
Published: July 2002

This work reports an extensive optical study of a series of In(As,P)/InP strained quantum wells grown by gas source molecular beam epitaxy with various thicknesses and compositions. Thermally detected optical absorption, reflectance, and photoreflectance measurements have been performed in order to determine all the exciton energies. An envelope function model including band nonparabolicity, intervalence band coupling, and also possible group V element exchange at the interfaces, is used to interpret the experimental data. The fit of the optical transition energies leads to an accurate determination of the crucial structural and optical parameters. The arsenic concentration inside the wells is evaluated and the conduction band offset ratio (Qc=0.70) as well as the bowing parameter of In(As,P) (C=0.14 eV) are determined. These studies also provide useful information about the nanometer-scale P–As interface mixing.
Signature of GaN–AlN quantum dots by nonresonant Raman scattering
Authors: Gleize J. , Frandon J. , Demangeot F. , Renucci M. A. , Adelmann C. , Daudin B. , Feuillet G. , Damilano B. , Grandjean N. , Massies J.
Published: July 2002

Stackings of GaN quantum dots embedded in an AlN matrix, constituting periodic structures with a mean aluminum content in the 80%–90% range, have been investigated by Raman spectroscopy under excitation in the visible range, i.e., far from resonant conditions. For comparison, spectra of an alloy sample with approximately the same composition has been also recorded. The differences evidenced between these spectra give evidence for separate signatures of quantum dots and spacers of the multilayered structure. The mean (biaxial) strain in GaN dots and AlN spacers has been deduced from the measured phonon frequencies.
InGaN/GaN multiple quantum wells grown by metalorganic vapor phase epitaxy with mass transport
Authors: Pozina G. , Bergman J. P. , Monemar B. , Iwaya M. , Nitta S. , Amano H. , Akasaki I.
Published: July 2002

We report on studies of In0.12Ga0.88N/GaN heterostructures with three 35-Å-thick quantum wells (QWs) grown on sapphire substrates by metalorganic vapor phase epitaxy with employment of mass transport. The structure is demonstrated to show good structural and optical properties. The threading dislocation density is less than 107 cm−2 for the mass-transport regions. The photoluminescence (PL) spectrum is dominated by the rather narrow near-band gap emission at 2.97 eV with a linewidth of 40 meV. This emission has a typical PL decay time about 5 ns at 2 K within the PL contour. With increasing excitation intensity, an additional transition with longer decay time (about 200 ns) is enhanced at energy about 2.85 eV. The position of this line depends strongly on the excitation power. We explain the data in terms of a model, where the PL is a result of contribution from at least two nonequivalent QWs, which could be realized due to a potential gradient across the layers.
Flux trapping in a superconducting thin film as revealed by simulations and scanning superconducting quantum interference device microscope observations
Authors: Tanaka Keiichi , Morooka Toshimitsu , Odawara Akikazu , Chinone Kazuo , Mawatari Yasunori , Koyanagi Masao
Published: July 2002

Penetration of magnetic fluxes into a superconducting thin film that contained holes was studied for fast and slow cooling rates (0.18 and 0.009 K/s, respectively) under an ambient magnetic field of about 3 μT. By using a scanning superconducting quantum interference device microscope, we observed trapped magnetic fluxes both inside and outside of holes prepared in a superconducting film. The trapped fluxes outside the holes appeared in a regular arrangement when the superconducting film was cooled at the slow rate. Observed arrangements of the trapped fluxes were compared with a simulation that was based on a theoretical model that considered the surface barrier and the interaction among fluxes. The simulation explained well the observed arrangements of magnetic fluxes.
Diffraction efficiency and phase stability of poly(N-vinylcarbazole)-based photorefractive polymer composites as a function of azo-dye concentration
Authors: Smiley E. J. , McGee D. J. , Salter C. , Carlen C. R.
Published: July 2002

Diffraction efficiency and phase stability of photorefractive polymer composites doped with alkylether-substituted azobenzene dyes are studied as a function of dye concentration. Composites doped with 2,5-dimethyl-4-(4′-nitrophenylazo)phenyl benzyl ether (DMNPAPBE) are phase stable at all concentrations studied while composites doped with 2,5-dimethyl-4-(4′-nitrophenylazo)phenyl octyl ether (DMNPAPOE) exhibit a critical concentration of 47% above which the composite rapidly degrades due to dye crystallization. The glass transition temperatures Tg of DMNPAPBE- and DMNPAPOE-doped composites are approximately 67 and 57 °C, respectively. Diffraction efficiency measurements of DMNPAPOE-doped composites show a threefold increase at 100 V/μm over DMNPAPBE-doped composites at the same dye concentration and applied field.
The role of the molecular dynamics in the local intensity instabilities of large aperture dye lasers
Authors: Leyva I. , Guerra J. M.
Published: July 2002

We study experimentally the influence of the molecular dynamics in the local intensity fluctuations of large aperture dye lasers, and find dependencies on solvent viscosity and active molecular size. This is an example of the complexity of the still quite unknown nonlinear processes that underlie the pattern formation dynamics in large aperture optical systems, in which the diffraction has lost influence and the bulk dynamics dominate.
Flexible piezoelectric motor based on an acoustic fiber
Authors: Carotenuto Riccardo , Iula Antonio , Caliano Giosuè , Lamberti Nicola
Published: July 2002

A flexible, long (500 mm) and thin (diameter 0.8 mm) steel wire capable of transferring mechanical power from the stator of a piezoelectric motor to the rotor is presented. We experimentally demonstrate that the steel wire, acting as a waveguide and here called “acoustic fiber” by optical analogy, can transfer good mechanical power (maximum torque 1.4×10−5 N m, maximum rotational velocity 3200 rpm) with the unique property of high flexibility. The fiber is well suited to a variety of different applications in which the source of the mechanical power must be at a distance from the load.
Investigation of transport and magnetic properties of perovskite-type (La1−xSnx)yMnO3−δ epitaxial films
Authors: Guo Xiangxin , Chen Zhenghao , Dai Shouyu , Zhou Yueliang , Li Runwei , Zhang Hongwei , Shen Baogen , Cheng Huansheng
Published: July 2002

The transport and magnetic properties of perovskite-type epitaxial films of (La1−xSnx)yMnO3−δ have been studied. The results of the resistivity, magnetization, and Curie temperature as a function of the tin concentration show that the tin in the films has the same effect as a divalent ion. The spin-glass state has been found in the (La1−xSnx)yMnO3−δ films. The magnetic inhomogeneity and the strain in the films are responsible for this spin-glass behavior in the films. Additionally, the dependence of the spin-glass freezing temperature Tsg on the tin concentration also implies that the tin doping has a divalent effect.
Monte Carlo simulation of electron transport in simple orthorhombically strained silicon
Authors: Wang Xin , Kencke D. L. , Liu K. C. , Tasch A. F. , Register L. F. , Banerjee S. K.
Published: July 2002

We report on the electron transport properties of simple orthorhombically strained silicon studied by density-functional theory and Monte Carlo simulation. The six degenerate valleys near X points in bulk silicon break into three pairs with different energy minima due to the orthorhombic strain. The degeneracy lifting causes electron redistribution among these valleys at low and intermediate electric fields. Thus the drift velocity is enhanced under an electric field transverse to the long axis of the lowest valleys. Orthorhombically strained layers should be of interest in vertical SiGe-based heterostructure n-channel–metal–oxide–semiconductor field effect transistors. The simple orthorhombically strained Si grown on a Si0.6Ge0.4 sidewall has a low-field mobility almost twice that of bulk Si and an electron saturation velocity approximately 20% higher.
Characterizing mechanical resonators by means of a scanning acoustic force microscope
Authors: Sthal F. , Bourquin R.
Published: July 2002

A method of characterizing resonators is presented. This method is based on scanning acoustic force microscopy. Data on the topography and the vibration amplitude of the resonator are obtained simultaneously, by means of atomic force interaction. The normal component of the surface vibration of the resonator is evaluated. A tuning fork resonator and a 10 MHz SC-cut Boı̂tier à Vieillissement Amélioré quartz crystal resonator with adherent electrodes are studied. This analysis allows the cartography of the local characteristics of the resonator material to be made.
Application of photoluminescence phosphors to a phosphor-liquid crystal display
Authors: Do Y. R. , Bae J. W.
Published: July 2002

The photoluminescence properties and quantum efficiency of potential red, green, and blue (RGB) phosphors are measured under near-ultraviolet excitation (380–420 nm). The suitability of several phosphors is discussed for their application in phosphor-liquid crystal displays (LCDs). K5Eu2.5(WO4)6.25, SrGa2S4:Eu, and BaMgAl10O17:Eu phosphors are chosen as RGB phosphors for making a phosphor-LCD prototype. The color coordinates and optical properties of phosphor-LCDs are compared to those of conventional LCDs.
Kerr and Schwarzschild black-holes in three-dimensional relativity
Authors: Sen Dipak K.
Published: July 2002

We consider the Kerr and Schwarzschild black-hole space–times in the framework of a three-dimensional formulation of relativistic kinematics and field dynamics, in which local physical observers are represented by non-singular vector fields of bounded length in a three-dimensional pseudo-Riemannian space. A space–time is represented by a pair of 3-metric and a fundamental 3-vector field satisfying a set of basic equations, each solution of which determines uniquely a solution of the vacuum Einstein field equations. It is shown that the only spherically symmetric solution of our basic equations leads to the Schwarzschild space–time, thereby proving a version of Birkoff’s theorem in this formalism. The Schwarzschild horizon and the Kerr stationary limit are both related to the upper bound of the length of the corresponding physical 3-vector fields. An example of a solution of the vacuum Einstein field equations shows that nonstationary space–times can also be formulated in this three-dimensional relativity.
Performance and degradation of continuous-wave InGaN multiple-quantum-well laser diodes on epitaxially laterally overgrown GaN substrates
Authors: Kneissl Michael , Bour David P. , Romano Linda , Van de Walle Chris. G. , Northrup John E. , Wong William S. , Treat David W. , Teepe Mark , Schmidt Tanya , Johnson Noble M.
Published: July 2002

The performance and degradation characteristics of continuous-wave (cw) InGaN multiple-quantum-well laser diodes are reported. A cw threshold current as low as 62 mA was obtained for ridge-waveguide laser diodes on epitaxially laterally overgrown GaN on sapphire substrates grown by metalorganic chemical vapor deposition. Transmission electron microscopy reveals a defect density <5×107 cm−2 in the active region. The emission wavelength was near 400 nm with output powers greater than 20 mW per facet. Under cw conditions, laser oscillation was observed up to 70 °C. The room-temperature cw operation lifetimes, measured at a constant output power of 2 mW, exceeded 15 h. From the temperature dependence of the laser diode lifetimes, an activation energy of 0.50 eV±0.05 eV was determined.
Monte Carlo simulation of nonlinear Couette flow in a dilute gas
Authors: Montanero José Marı́a , Santos Andrés , Garzó Vicente
Published: July 2002

The direct simulation Monte Carlo method is applied to solve the Boltzmann equation in the steady planar Couette flow for Maxwell molecules and hard spheres. Nonequilibrium boundary conditions based on the solution of the Bhatnagar–Gross–Krook (BGK) model for the Couette flow are employed to diminish the influence of finite-size effects. Non-Newtonian properties are characterized by five independent generalized transport coefficients: a viscosity function, a thermal conductivity function, two viscometric functions, and a cross coefficient measuring the heat flux orthogonal to the thermal gradient. These coefficients depend nonlinearly on the shear rate. The simulation results are compared with theoretical predictions given by the Grad method and the BGK and the ellipsoidal statistical (ES) models. It is found that the kinetic models present a good agreement with the simulation, especially in the case of the ES model, while the Grad method is only qualitatively reliable for the momentum transport. In addition, the velocity distribution function is also measured and compared with the BGK and ES distributions.
Switching of the exchange bias in Fe/Cr(211) double-superlattice structures
Authors: te Velthuis S. G. E. , Jiang J. S. , Felcher G. P.
Published: July 2002

The reversal of the direction of the exchange bias in a “double-superlattice” system which consists of an Fe/Cr antiferromagnetic (AF) superlattice which is ferromagnetically coupled with an Fe/Cr ferromagnetic (F) superlattice through a Cr spacer layer, is observed. Magnetometry and polarized neutron reflectometry show that a switch in the bias direction occurs at a field (∼447 Oe) well below the field (14 kOe) necessary to saturate the AF superlattice and well below the field (2 kOe) where the AF superlattice initiates a spin–flop transition. The switching of the exchange bias cannot be explained in terms of a model of uniform rotation, but rather by breakdown into domains and reversal of the AF layers. The transparency of magnetic behavior of the double superlattice may be useful in understanding the behavior of traditional exchange bias systems.
Low-temperature Kerr spectroscopy on half-metallic Sr2FeMoO6
Authors: Rüdiger U. , Rabe M. , Güntherodt G. , Yin H. Q. , Dass R. I. , Goodenough J. B.
Published: July 2002

The polar Kerr rotation and ellipticity spectra of epitaxially grown (001)-oriented half-metallic Sr2FeMoO6 thin films have been determined in the photon energy range from 1.2 to 4.9 eV. The Kerr rotation spectrum shows three maxima at E=1.6, 4.0, and 4.65 eV. The maxima at 4.0 and 4.65 eV are consistent with spin-polarized band structure calculations for interband transitions from the O-2p to the minority-spin π*Mo/Fe and majority-spin Mo-t2g bands, respectively. The overall maximum intrinsic Kerr rotation is ΘK=−0.045° at a photon energy of 4.65 eV. The maximum of ΘK at E=1.6 eV coincides with a minimum in the reflectivity due to the plasma edge of Sr2FeMoO6 and, therefore, is not related to an interband transition.
Self-assembled Ge nanostructures as field emitters
Authors: Tondare V. N. , Birajdar B. I. , Pradeep N. , Joag D. S. , Lobo A. , Kulkarni S. K.
Published: July 2002

Self-assembled two-dimensional arrays of Ge islands on Si(111)7×7 were grown by depositing Ge on Si(111)7×7 substrates held at 650 K. It was observed that these islands were conical in shape as well as nearly uniform in size and shape. Consequently, the substrates of about 1 cm2 area were used as field-emitter arrays. It was found that the arrays exhibited a low onset voltage for field emission, large emission current, as well as high current stability.
Monte Carlo study of spin relaxation in AlGaAs/GaAs quantum wells
Authors: Bournel A. , Dollfus P. , Cassan E. , Hesto P.
Published: July 2002

An original approach is developed to investigate the electron spin dynamics in III–V quantum wells using a particle Monte Carlo transport model. We study the spin precession related to the D’yakonov–Perel’ mechanism, which is believed to be the predominant spin relaxation phenomenon in AlGaAs/GaAs quantum wells at room temperature. Using a Monte Carlo approach, the effect of electron/crystal scatterings on the D’yakonov–Perel’ mechanism can be both simply and accurately taken into account. Finally, including interface roughness in the calculation leads to spin relaxation times in good agreement with the experimental data.
Photoluminescence from pendant dye molecules mediated by exciton transport on helical polysilane chains
Authors: Tachibana Hiroaki , Kishida Hideo , Tokura Yoshinori
Published: July 2002

We have synthesized helical polysilanes on which the pendant groups are partly substituted by dye molecules. The introduction of a few percent of dye molecules into the substituents enhances the luminescent efficiency of the visible region, by behaving as luminescence centers. We have demonstrated that the excitation energy is transferred efficiently by free excitons on the Si backbones to reach the luminescence centers, where energy transfer occurs from the exciton to the dye molecule, and as a result sensitized luminescence occurs. The exciton transfer is found to occur over 50–100 monomer units on the polysilane backbone. This approach will open the way to fabricating a wavelength-tunable luminous device by utilizing the exciton transport on Si chains.
Multichannel data transmission system for rotating elements using a noncontacting slip ring
Authors: Li Jianfeng , Sutherland John W.
Published: July 2002

The collection or acquisition of dynamic data from elements that are rotating (e.g., gearing system, axle, engine propeller, and machine tool spindle) is a significant challenge in many engineering applications. Both contacting and remote telemetry-based methods have been employed to provide a data bridge between the rotating and stationary reference frames, however these methods suffer from several difficulties. A new method for the transmission of signals from the transducers on a rotating element is presented. Infrared light emitting diodes (LEDs) are placed on the circumference of the shaft or the end of it. Externally mounted photodiodes receive a frequency modulated light from the LEDs. Signal multiplexing enables multiple-channel signal transmission to be realized. The properties of the instrument are as follows: dynamic maximum frequency of 50 kHz, static sine distortion degree less than 1%, dynamic channel isolation of −40 dB, dynamic linear error less than 1%, and static/dynamic gains of 50–200. The proposed method has been validated through experiments performed on a prototype optical signal transmission system. Experimental results show excellent agreement with theoretical calculations.