bluuebunny/crossref_metadata_2025 huggingface dataset:
Soldering reaction between eutectic SnPb and plated Pd/Ni thin films on Cu leadframe
Authors: Kim P. G. , Tu K. N. , Abbott D. C.
Published: July 2002

In the soldering reaction at 200 °C between eutectic SnPb and plated Pd/Ni on Cu leadframes, two thicknesses of Pd of 760 and 2500 Å were used. Even very thin Pd can lead to continuous spreading of the solder on the leadframe; no stable wetting angle can be observed. Anisotropic spreading and spike formation were found due to the mechanical effect of rolling of the leadframe, and they are more pronounced in the thinner Pd samples. In the interfacial reaction, we have observed the formation of a ternary Pd–Ni–Sn compound and Ni3Sn4. While spalling of the ternary compound occurs, no spalling of the Ni3Sn4 was found. The latter forms a rather uniform layer consisting of small scallop-type grains. We found that the growth rate of Ni3Sn4 is much slower, by a factor of about 10, than that of Cu6Sn5 in the reaction between Cu and eutectic SnPb.
Amino acid synthesis from an amorphous substance composed of carbon, nitrogen, and oxygen
Authors: Miyakawa Shin , Tamura Hideki , Sawaoka Akira B. , Kobayashi Kensei
Published: July 2002

Hydrogen cyanide, formaldehyde, and ammonia are considered important intermediates in amino acid synthesis by electric discharge. In this study, however, amino acid precursors were synthesized from a CO–N2 mixture free of hydrogen atoms. An amorphous film composed of carbon, nitrogen, and oxygen was given from a highly activated plasma. When exposed to atmospheric moisture, this film incorporated hydrogen atoms to yield amino acid. This is a mechanism for amino acid synthesis without involving hydrogen cyanide, formaldehyde, and ammonia.
Optical poling and its application in optical storage of a polyimide film with high glass transition temperature
Authors: Si Jinhai , Mitsuyu Tsuneo , Ye Peixian , Shen Yequan , Hirao Kazuyuki
Published: July 2002

The temperature dependence of optical poling of a side-chain azobenzene–polyimide film with high glass transition temperature was experimentally investigated. With the increase of the sample temperature the light-induced second-harmonic generation increased first, and then decreased as the temperature was near its glass transition temperature. The light-induced second-order nonlinear optical coefficient d33 for the optimum temperature was determined to be 6.9 pm/V, which was about 2.7 times that obtained at room temperature. Compared with optical poling at room temperature, this thermal-assisted optical poling not only enhanced d33 of the polyimide film, but also improved the polar order stability of molecules. Optical storage based on optical poling was also demonstrated experimentally.
Strain variations in InGaAsP/InGaP superlattices studied by scanning probe microscopy
Authors: Chen Huajie , Feenstra R. M. , Goldman R. S. , Silfvenius C. , Landgren G.
Published: July 2002

Strain-compensated InGaAsP/InGaP superlattices are studied in cross section by atomic force microscopy and scanning tunneling microscopy. Undulations in the morphology of the {110} cross-sectional faces are observed, and are attributed to elastic relaxation of this surface due to underlying strain arising from thickness and compositional variations of the superlattice layers. Finite element computations are used to extract a quantitative measure of the strain variation.
A photoconductive, miniature terahertz source
Authors: Lai Richard K. , Hwang Jiunn-Ren , Norris Theodore B. , Whitaker John F.
Published: July 2002

We discuss the performance of a micromachined, photoconductive terahertz emitter that is fabricated on low-temperature-grown GaAs. The device is mounted on a pair of single-mode optical fibers that allows the source to be freely positionable. A strong radiation burst is emitted due to the large magnetic moment created by the electrode. The emitter’s small feature size of 300 μm by 300 μm with a photoconductive switch area of 30 μm by 30 μm suggests its application for terahertz, time-domain, near-field spectroscopy and imaging.
Imaging the depletion zone in a Si lateral pn junction with scanning tunneling microscopy
Authors: Hildner M. L. , Phaneuf R. J. , Williams E. D.
Published: July 2002

Scanning tunneling microscopy (STM) and spectroscopy are used to characterize a lateral pn junction fabricated on a silicon (100) surface. STM images show both an electronic feature and a structural trench at the edges of the ion-implanted p-type regions. The electronic feature widens with applied reverse bias indicating that it is associated with the depletion zone. The electronic feature and its voltage dependence are described qualitatively by modeling the tip-junction system as a series of nonequilibrium metal–insulator–semiconductor diodes formed with a semiconductor of spatially variable carrier density.
Fatigue characteristics of SrBi2Ta2O9 thin films prepared by metalorganic decomposition
Authors: Zhang Z. G. , Liu J. S. , Wang Y. N. , Zhu J. S. , Yan F. , Chen X. B. , Shen H. M.
Published: July 2002

Polycrystalline SrBi2Ta2O9 (SBT) ferroelectric thin films were synthesized on Pt/Ti/SiO2/Si substrates by metalorganic decomposition. Electric measurements demonstrate that fatigue increases with decreasing switching voltage and frequency, and the suppressed polarization caused at a lower switching voltage can be recovered by switching at a higher voltage. This suggests that the domain walls of SBT thin films are weakly pinned and easily depinned by a higher external field. The polarization of SBT thin films annealed in air shows more degradation than that annealed in oxygen, which indicates that the oxygen vacancy also plays an important role in fatigue behavior of SBT thin films.
InGaN/GaN/AlGaN-based laser diodes with cleaved facets grown on GaN substrates
Authors: Nakamura Shuji , Senoh Masayuki , Nagahama Shin-ichi , Iwasa Naruhito , Yamada Takao , Matsushita Toshio , Kiyoku Hiroyuki , Sugimoto Yasunobu , Kozaki Tokuya , Umemoto Hitoshi , Sano Masahiko , Chocho Kazuyuki
Published: July 2002

After obtaining an epitaxially laterally overgrown GaN on sapphire by the metalorganic chemical vapor deposition method, GaN growth was continued up to a thickness of 200 μm by a hydride vapor phase epitaxy method. The InGaN multi-quantum-well-structure laser diode (LD) was grown on a free-standing GaN substrate, which was obtained by removing the sapphire substrate. The LDs with cleaved mirror facets showed an output power as high as 160 mW under room-temperature continuous-wave (CW) operation. The fundamental transverse mode was observed up to an output power of 80 mW. The lifetime of the LDs at a constant output power of 5 mW was about 180 h under CW operation at an ambient temperature of 50 °C, due to a high threshold current density of 14 kA/cm2.
Thermally stable rhenium Schottky contacts to n-GaN
Authors: Venugopalan H. S. , Mohney S. E.
Published: July 2002

The barrier heights of Re Schottky contacts to n-GaN were investigated by current–voltage (I–V) and capacitance–voltage (C–V) measurements. Both techniques indicate that the barrier height increases upon annealing at 500 °C for 10 min. After this anneal, a barrier height of 0.82 eV and ideality factor of 1.1 are obtained by I–V measurements performed at 150 °C. The C–V measurements performed at room temperature reveal a barrier height of 1.06 eV. These barrier heights are stable upon further short term annealing at temperatures as high as 700 °C. The Re Schottky contacts were also stable upon prolonged annealing for 24 h at 300 °C. The Re/n-GaN Schottky diode was chosen for study because of its anticipated thermodynamic stability against metallurgical reactions.
Nanometer-scale Si selective epitaxial growth on Si surface windows in ultrathin oxide films fabricated using scanning tunneling microscopy
Authors: Shibata Motoshi , Nitta Yoshiki , Fujita Ken , Ichikawa Masakazu
Published: July 2002

Using scanning tunneling microscopy (STM), nanometer-scale Si(111) and Si(001) windows in ultrathin SiO2 films are fabricated by electron-beam-induced thermal decomposition. At 450–630 °C, the oxidized Si surfaces are irradiated with a field emission electron beam from a STM tip with an energy of 70–150 eV and a current of 10–50 nA. The smallest window size is about 40 nm. The shape of the Si crystals selectively grown on the Si(001) windows is that of a frustum of a quadrangular pyramid, while that on the Si(111) windows is an (111) oriented two-dimensional island. We discuss the influence of the field emission electrons on the fabrication and the selective growth.
Highly packed InGaAs quantum dots on GaAs(311)B
Authors: Akahane Kouichi , Kawamura Takahiro , Okino Kenji , Koyama Hiromichi , Lan Shen , Okada Yoshitaka , Kawabe Mitsuo , Tosa Masahiro
Published: July 2002

We have fabricated highly packed and ordered In0.4Ga0.6As quantum dots (QDs) array on GaAs(311)B substrate without coalescence of QDs. Reflection high-energy electron diffraction and Auger spectra suggest the inhomogeneous distribution of In and Ga in QD. In concentration near the surface of QD is larger than that of the inside, and the inhomogeneous distribution of In and Ga in QDs prevents QDs from merging.
Piezoelectric effects on the optical properties of GaN/AlxGa1−xN multiple quantum wells
Authors: Kim H. S. , Lin J. Y. , Jiang H. X. , Chow W. W. , Botchkarev A. , Morkoç H.
Published: July 2002

Piezoelectric effects on the optical properties of GaN/AlGaN multiple quantum wells (MQWs) have been investigated by picosecond time-resolved photoluminescence (PL) measurements. For MQWs with well thicknesses 30 and 40 Å, the excitonic transition peak positions at 10 K in continuous wave (cw) spectra are redshifted with respect to the GaN epilayer by 13 and 45 meV, respectively. The time-resolved PL spectra of the 30 and 40 Å well MQWs reveal that the excitonic transition is in fact blueshifted at early delay times due to quantum confinement of carriers. The spectral peak position shifts toward lower energies as the delay time increases and becomes redshifted at longer delay times. We have demonstrated that the results described above are due to the presence of the piezoelectric field in the GaN wells of GaN/AlGaN MQWs subject to elastic strain together with screening of the photoexcited carriers. By comparing experimental and calculation results, we conclude that the piezoelectric field strength in GaN/Al0.15Ga0.85N MQWs has a lower limit value of about 560 kV/cm. The electron and hole wave function distributions have also been obtained. The implication of our findings on the practical applications of GaN based optoelectronic devices is also discussed.
Effect of detuning on the angular emission pattern of high-efficiency microcavity light-emitting diodes
Authors: Dill C. , Stanley R. P. , Oesterle U. , Ochoa D. , Ilegems M.
Published: July 2002

Results on molecular beam epitaxy-grown microcavity light-emitting diodes with InGaAs/GaAs quantum wells and a hybrid top mirror are presented. An external quantum efficiency of 14.8% is achieved for a 400 μm diam light-emitting diode. The strong influence of the spectral overlap between the spontaneous emission spectrum and the cavity resonance mode on the radiation pattern is shown. The angular emission profile is compared with model predictions for different detunings, and a very good agreement is obtained when the asymmetric spectral broadening of the intrinsic spontaneous emission is taken into account.
Electronic excitations and longitudinal optical phonon modes of self-assembled CdSe quantum dots revealed by microprobe studies
Authors: Lowisch M. , Rabe M. , Kreller F. , Henneberger F.
Published: July 2002

The electronic states of self-assembled CdSe quantum dots and the longitudinal optical phonon mediated energy relaxation among these states are investigated. Measurements with microprobe resolution demonstrate that each quantum dot develops specific phonon modes. There is no evidence for a well-defined phonon bottleneck in the intra-quantum-dot relaxation. We observe single quantum-dot bi-exciton emission and bi-exciton binding energies exceeding 20 meV.
Piezoresistive torque magnetometry below 1 K
Authors: Lupien Christian , Ellman Brett , Grütter Peter , Taillefer Louis
Published: July 2002

We have investigated the performance of piezoresistive cantilevers as magnetometers in the temperature range below 1 K. The de Haas-van Alphen effect was used to study the temperature dependence of a sample of κ-(BEDT-TTF)2Cu(NCS)2, fixed on the end of a cantilever, as a function of the excitation current through the piezoresistive device. We found that by using a small thermalizing wire connected directly to the sample, large excitations were not incompatible with sample temperatures remaining low, thereby establishing the use of these devices as sensitive magnetometers well below 1 K. A large hysteretic behavior observed at low fields (below 0.01 T) and low temperature (below ∼2 K) precludes their use in that regime.
Nucleation of chemical vapor deposited silicon nitride on silicon dioxide
Authors: Copel M. , Varekamp P. R. , Kisker D. W. , McFeely F. R. , Litz K. E. , Banaszak Holl M. M.
Published: July 2002

We have studied the early stages of silicon nitride chemical vapor deposition (CVD) on silicon dioxide using medium energy ion scattering. The growth mode consists of island nucleation followed by coalescence. Similar behavior is observed for films grown using different precursors and reactor environments, indicating that the growth mode is caused by the fundamental nonwetting nature of the nitride/oxide interface under the conditions used for CVD.
Exchange-biased magnetic tunnel junctions: Dependence of offset field on junction width
Authors: Moon K.-S. , Fontana R. E. , Parkin S. S. P.
Published: July 2002

Magnetic tunnel junction structures comprising a ferromagnetic layer pinned by exchange biasing and a “free” ferromagnetic layer were prepared using dc-magnetron sputtering and patterned with conventional optical lithography. Structures were prepared, square and rectangular in shape, with various widths and lengths. It was observed that the magnetic properties of the free layer vary systematically with the size of the junction. In particular, the offset field of the free-layer magnetic hysteresis loop, as measured resistively, is controlled by a combination of magnetostatic and ferromagnetic coupling between the pinned and free layers.
Laser-induced fluorescence and Langmuir probe determination of Cl2+ and Cl+ absolute densities in transformer-coupled chlorine plasmas
Authors: Malyshev M. V. , Fuller N. C. M. , Bogart K. H. A. , Donnelly V. M. , Herman Irving P.
Published: July 2002

Laser induced fluorescence detection of Cl2+ has been used to track their relative concentration in a high-density inductively (transformer) coupled (TCP) 10 mTorr chlorine plasma as a function of the 13.56 MHz radio frequency (rf) power. This relative Cl2+ number density was compared to the total absolute positive ion density (ni+=nCl++nCl2+) obtained with a Langmuir probe. Both nCl2+ and ni+ doubled with increasing rf power from 8 to ∼55 W in the capacitively coupled (dim) mode. Above the transition from a capacitively to inductively coupled (bright-mode) plasma at ∼85±35 W, nCl2+ decayed with rf power while ni+ continued to increase. Consequently, Cl2+ is the dominant ion in dim-mode operation and Cl+ is the dominant ion in bright-mode operation, at ni+⩾6×1010 cm−3. With the plasma operating in the reaction ion etch (RIE) mode (the stage powered at 14.56 MHz, and no TCP power) nCl2+ tracked ni+ over the entire range of powers (2–150 W). Thus, Cl2+ is the dominant ion during capacitively coupled RIE operation.
Influence of p-type doping on the degradation of ZnSe laser diodes
Authors: Albert D. , Nürnberger J. , Hock V. , Ehinger M. , Faschinger W. , Landwehr G.
Published: July 2002

We present results of optical degradation experiments on blue-green ZnSe-based diode laser structures. A micro-focused wavelength selectable laser is used for degradation and the luminescence from the degrading region is simultaneously recorded. The degradation speed at a given optical power density depends strongly on the doping of the structure and on the photon energy: undoped structures do not degrade, while the threshold photon energy to induce degradation depends on the degree of compensation in the p-doped layers. However, at high photon energies, an improvement in compensation does not slow down the degradation process, leading to the suggestion that the nitrogen acceptor itself is unstable under these conditions.
Sputter deposition-induced electron traps in epitaxially grown n-GaN
Authors: Auret F. D. , Goodman S. A. , Koschnick F. K. , Spaeth J.-M. , Beaumont B. , Gibart P.
Published: July 2002

We have used deep level transient spectroscopy to study the electrical properties of defects introduced in epitaxial n-GaN during sputter deposition of Au Schottky contacts. Four defects, located 0.22±0.02, 0.30±0.01, 0.40±0.01, and 0.45±0.10 eV below the conduction band, were characterized. The first of these defects has similar electronic properties as a radiation induced defect in GaN, while the second appears to be the same as a defect in the as-grown material. The latter two defects have not previously been observed in as-grown or processed epitaxial GaN.
Determination of the mobility gap of microcrystalline silicon and of the band discontinuities at the amorphous/microcrystalline silicon interface using in situ Kelvin probe technique
Authors: Hamma S. , i Cabarrocas P. Roca
Published: July 2002

A method to determine the mobility gap of thin films and the band discontinuities in heterojunctions is presented. It combines in situ contact potential measurements with dark conductivity activation energy measurements. The method is applied to determine the mobility gap of microcrystalline silicon (μc-Si:H) and the band discontinuities at the μc-Si:H/amorphous silicon (a-Si:H) interface. The mobility gap of μc-Si:H depends on its crystalline volume fraction and varies between 1.48 and 1.55 eV. The main band discontinuity occurs at the valence band side. The consequences of the band discontinuities on a-Si:H based solar cells using μc-Si:H doped layers are discussed.
Direct writing of low Tc superconductor-normal metal-superconductor junctions using a focused ion beam
Authors: Moseley R. W. , Booij W. E. , Tarte E. J. , Blamire M. G.
Published: July 2002

Using a focused ion beam, we have produced superconductor-normal metal-superconductor junctions by controllably removing a portion of the top layer of a patterned superconductor-normal metal thin film. The high-quality junctions showed Josephson coupling which scaled qualitatively with barrier properties and temperature as expected. The largest product of a junction’s critical current and the normal state resistance measured is 98 μV at 4.2 K. The method has good reproducibility and could be exploited in a number of superconducting electronics applications.
Elasticity and mechanical instabilities of diamond at megabar stresses: Implications for diamond-anvil-cell research
Authors: Zhao Ji-Jun , Scandolo S. , Kohanoff J. , Chiarotti G. L. , Tosatti E.
Published: July 2002

We present first-principles calculations of the stress–strain curves for diamond under nonhydrostatic tetragonal compression, in the regime of operation of diamond-anvil cells (DACs). We provide a parametrization of the stress components as a function of strain, which is potentially useful for the controlled design of DACs. A stability analysis shows that a tetragonal-shear mechanical instability sets in in diamond when σz−σr=200 GPa, where σz and σr are the axial and radial components of the stress, respectively.
Optimization of erbium-doped light-emitting diodes by p-type counterdoping
Authors: Neufeld E. , Markmann M. , Vörckel A. , Brunner K. , Abstreiter G.
Published: July 2002

In this letter, we report on the influence of the space charge region width in erbium- and oxygen-doped silicon light-emitting diodes on the electroluminescence (EL) power at 1.54 μm under reverse bias conditions. The space charge region width was varied by codoping the Si:Er:O layer with boron, thereby compensating the Er–O donors. We observe a strong enhancement of the EL power with increasing width. The data indicate the existence of a dark region of approximately 45 nm in the pn junction, in which no light is generated due to a lack of hot carriers which are necessary for impact excitation of Er3+ ions.
Dielectric property and microstructure of a porous polymer material with ultralow dielectric constant
Authors: Xu Yuhuan , Tsai Yi-pin , Tu K. N. , Zhao Bin , Liu Q.-Z. , Brongo Maureen , Sheng George T. T. , Tung C. H.
Published: July 2002

This letter reports the synthesis and dielectric properties of a porous poly(arylethers) material with an ultralow dielectric constant for interlayer dielectric applications in microelectronics. The porous polymer films were fabricated by a method of organic phase separation and evaporation. A dielectric constant of 1.8 was achieved for a porous film with an estimated porosity of 40%. The characterization of microstucture for the porous film showed numerous nanopores with an average size of 3 nm distributed uniformly throughout the film.
Segregation and strain relaxation in Au/Ni multilayers: An in situ experiment
Authors: Labat S. , Gergaud P. , Thomas O. , Gilles B. , Marty A.
Published: July 2002

We report on in situ real-time measurements of both stress and strain during growth of ultrathin layers, with submonolayer sensitivity. The in-plane lattice parameter is measured by reflection high energy electron diffraction and the stress is determined via the curvature measurements. The system studied is Au/Ni (i.e., Au on Ni and Ni on Au). We have evidenced a large asymmetry in the two different growths: Au (on Ni) shows a progressive elastic strain relaxation, whereas Ni (on Au) exhibits a strong interplay between the stress and the interfacial mixing.
Electromechanical response of cellular electret films
Authors: Sessler G. M. , Hillenbrand J.
Published: July 2002

Charged polypropylene films with a cellular structure show pseudopiezoelectric properties. Measurements of the direct and inverse electromechanical transducer constants of such films, relating to the operation as sensors and as actuators, respectively, yield values of ≈200 pC/N. These values can be explained with a theoretical model by assuming reasonable charge distributions and charge densities. The experimental and the theoretical results show the reciprocity of the transducer constants.
Bound exciton dynamics in GaN grown by hydride vapor-phase epitaxy
Authors: Pozina G. , Bergman J. P. , Paskova T. , Monemar B.
Published: July 2002

Temperature-dependent time-resolved photoluminescence measurements were performed on thick GaN layers grown by hydride vapor-phase epitaxy on Al2O3 substrates. Radiative lifetimes were determined for the neutral-donor–bound exciton with position at 3.478 eV and for two neutral-acceptor–bound excitons at 3.473 and 3.461 eV. We report a value of 3600 ps for the radiative lifetime of the acceptor–bound exciton transition at 3.461 eV. The dominant mechanism responsible for the nonradiative recombination of the bound excitons is shown to be connected with dissociation of the bound excitons into free excitons.
Sequential ion-induced stress relaxation and growth: A way to prepare stress-relieved thick films of cubic boron nitride
Authors: Boyen H.-G. , Widmayer P. , Schwertberger D. , Deyneka N. , Ziemann P.
Published: July 2002

It is shown that the bombardment of high quality cubic (c-) BN films with 300 keV Ar+ ions leads to a strong relaxation of their compressive stresses without destroying the cubic phase if the total ion fluence is kept below an upper limit. In addition, it was found that on top of such a stress-relieved film a further pure c-BN layer can be grown, but it builds up compressive stress again. Based on both results, a procedure is developed to grow thick (>1 μm) c-BN films (>80% c-BN) exhibiting low residual stress and long term stability under ambient conditions.
Role of 90° domains in lead zirconate titanate thin films
Authors: Ganpule C. S. , Nagarajan V. , Li H. , Ogale A. S. , Steinhauer D. E. , Aggarwal S. , Williams E. , Ramesh R. , De Wolf P.
Published: July 2002

We report observations of the ferroelectric domain structure in epitaxial lead zirconate titanate (PbZr0.2Ti0.8O3) ferroelectric thin films using piezoresponse microscopy. By manipulating the film thickness, a uniform two-dimensional grid of 90° domains (a domains, i.e., c axis in the plane of the film) has been induced. Our studies show that the out of plane polarization direction in the film is preferentially oriented. 90° domains have been observed as regions of low piezoresponse, as compared to the fully c axis oriented regions. We have studied the influence of these 90° domains and the domain walls on the nucleation of polarization reversal. We observe that the nucleation occurs preferentially at 90° domain interfaces. Polarization reversal is seen to occur through the nucleation and subsequent growth of “semicircular/elliptical” reverse domains, which eventually consume the entire region as a function of reversal time.
Influence of sample oxidation on the nature of optical luminescence from porous silicon
Authors: Coulthard I. , Antel W. J. , Freeland J. W. , Sham T. K. , Naftel S. J. , Zhang P.
Published: July 2002

Site-selective luminescence experiments were performed upon porous-silicon samples exposed to varying degrees of oxidation. The source of different luminescence bands was determined to be due to either quantum confinement in nanocrystalline silicon or defective silicon oxide. Of particular interest is the defective silicon-oxide luminescence band found at 2.1 eV, which was found to frequently overlap with a luminescence band from nanocrystalline silicon. Some of the historical confusion and debate with regards to the source of luminescence from porous silicon can be attributed to this overlap.
A new topological aspect of the arbitrary dimensional topological defects
Authors: Jiang Ying , Duan Yishi
Published: July 2002

We present a new generalized topological current in terms of the order parameter field φ to describe the arbitrary dimensional topological defects. By virtue of the φ-mapping method, we show that the topological defects are generated from the zero points of the order parameter field φ, and the topological charges of these topological defects are topological quantized in terms of the Brouwer degrees of φ-mapping under the condition that the Jacobian J(φ/v)≠0. When J(φ/v)=0, it is shown that there exist the crucial case of branch process. Based on the implicit function theorem and the Taylor expansion, we detail the bifurcation of generalized topological current and find different directions of the bifurcation. The arbitrary dimensional topological defects are found splitting or merging at the degenerate point of field function φ but the total charge of the topological defects is still unchanged.
Infrared quantum counter studies in europium doped lanthanum trifluoride
Authors: Tian Wenyan , Pandher Ranjit S. , Rami Reddy B.
Published: July 2002

Violet upconversion signals in the wavelength region 340–420 nm were observed from LaF3:Eu3+ under 583 nm dye laser excitation. The intensities of some upconversion signals enhanced ∼8× under double resonance excitation by a 583 nm dye laser and a 790.6 nm Ti: sapphire laser.
Laser mass-resolved spectroscopy and theoretical study of methyl-p-aminobenzoate(H2O)n (n=2,3,4) complexes
Authors: Fernández José A. , Longarte Asier , Unamuno Iñigo , Castaño Fernando
Published: July 2002

A combined computational and experimental study of the methyl-p-aminobenzoate(H2O)n, (n=2,3,4) complexes [MAB(H2O)n] is reported. Complexes potential energy surfaces were explored by ab initio density functional theory (DFT) methods, at the B3LYP/6-31G level, and the stable isomer structures and vibrational modes further computed at the B3LYP/6-31+G* level. A set of self-contained experimental techniques, including laser induced fluorescence (LIF), resonance enhanced multiphoton ionization mass-resolved spectroscopy (REMPI), two-color resonance enhanced multiphoton ionization mass-resolved spectroscopy (R2PI), “hole burning” spectroscopy (HB), and two-color ionization thresholds were used to study the spectra and other physical features of the complexes. Of the three title complexes only MAB(H2O)4 has been observed with our experimental methods, while the MAB(H2O)3 was formed by evaporation and MAB(H2O)2 was not detected at all. It has been shown that the observed MAB(H2O)4 complex has only one isomer with a hydrogen bonded water ring structure attached to the amino hydrogens and its low vibrational modes (up to 165 cm−1) have been assigned. A discussion of the results, including structures of stable isomers, isomer energies, ionization thresholds, and the difficulties in observing some solvated complexes is presented.
Heat generation approximation in modulation-doped field-effect transistors by the energy relaxation between carriers and phonons
Authors: Houng Mau-Phon , Wang Yeong-Her , Chong Kwok-Keung , Chu Chang-Hsing , Hung Chen-I , Miaw Jiunn-Way
Published: July 2002

A combination of the nonlinear charge-control model, rate balance equations, and electron-phonon scattering is used to simulate the hot-electron transport and heat generation problem of modulation-doped field-effect transistors. Based on the present model, the trend of high-field transport can be well demonstrated. In addition, the heat generation inside the channel can also be estimated by the energy flux balance equation with the energy relaxation rate formulated by polar-optical phonon scattering mechanism. Scaling down the gate length to submicron range, more heat power is generated inside the channel. However, the heat generation ratio reveals that hot electrons carry over half of the energy density gained from the applied electric field directly into drain region. For 0.1 μm gate length, more than 90% energy density is achieved. By reducing the gate length into the deep submicron range, the thermal impact on the drain side becomes more serious and complicated than the one found inside the channel.
Molecular weight dependent electroluminescence of silicon polymer near-ultraviolet light-emitting diodes
Authors: Hoshino Satoshi , Furukawa Kazuaki , Ebata Keisuke , Yuan Chien-Hua , Suzuki Hiroyuki
Published: July 2002

We investigated the electroluminescence (EL) of single-layer near-ultraviolet (NUV) light-emitting diodes (LEDs) made from poly[bis(p-n-butylphenyl)silane] (PBPS) with three different molecular weights (MWs). Although the NUV EL spectra of the three LEDs exhibited no noticeable differences, we observed a marked MW dependence on such aspects of the operating performance as the EL external quantum efficiency, EL threshold current density and electric field, which were improved as the MW of PBPS decreased. The MW dependence of the hole transport behavior suggested that the MW decrease promoted positive space charge formation in the PBPS layer during LED operation. We attributed the origin of the MW dependence of the LED performance to this positive space charge formation, which played an important role in improving the electron–hole supply balance from the external electrodes of the LED.
Resonant detection and frequency multiplication of terahertz radiation utilizing plasma waves in resonant-tunneling transistors
Authors: Ryzhii V. , Khmyrova I. , Shur M.
Published: July 2002

We propose the use of a resonant-tunneling transistor with high mobility of electrons in its quantum-well channel as a detector and a frequency multiplier of terahertz radiation and analyze its characteristics. The combination of strong nonlinearity of the resonant-tunneling current, negative differential conductivity, and excitation of plasma waves in the channel can provide higher performance of detectors and frequency multipliers based on the resonant-tunneling transistor compared to those considered previously.
Forward–backward semiclassical dynamics in the interaction representation
Authors: Shao Jiushu , Makri Nancy
Published: July 2002

The forward–backward semiclassical dynamics methodology [J. Phys. Chem. 103, 7753, 9479 (1999)] is reformulated in the interaction representation. The new version of the method allows for a fully quantum mechanical description of a low-dimensional subsystem of interest, along with a semiclassical forward–backward treatment of the solvent coordinates and their coupling to the reference subsystem. Application to the long-time tunneling dynamics in a symmetric double-well system coupled to a harmonic bath shows that the interaction FBSD is capable of capturing quantitatively the tunneling and decoherence effects induced by weakly dissipative environments.
Complex formation, rearrangement, and reaction in PhOH++ND3: Vibrational mode effects, recoil velocities, and ab initio studies
Authors: Green Richard J. , Kim Ho-Tae , Qian Jun , Anderson Scott L.
Published: July 2002

Vibrationally mode-selected phenol cations (C6H5OH+ and C6D5OH+) were reacted with ND3 in a guided-ion-beam instrument. Integral cross sections and recoil velocity distributions are reported as a function of collision energy and vibrational state. Three reactions are observed. A small signal is found for the [PhOH:ND3]+ adduct at low total energies, indicating the formation of a very long-lived complex. The major reaction is H/D exchange, generating PhOD++ND2H. Exchange is ∼40% efficient at low energies, strongly inhibited by collision energy, and strongly enhanced by excitation of PhOH+ vibrations. Recoil velocity distributions suggest that H/D exchange proceeds through a statistical complex at all energies. A precursor complex is invoked to explain the energy and vibrational state dependence. The endoergic proton transfer reaction is a minor channel at all energies, with dynamics intermediate between the direct and complex limits. Quantum chemistry and RRKM calculations are reported, providing an additional mechanistic insight.
Low-field magnetoresistive property of partially crystallized La0.5Sr0.5MnO3 thin films by pulsed laser deposition
Authors: Liu J.-M. , Huang Q. , Li J. , You L. P. , Xu S. Y. , Ong C. K. , Liu Z. G. , Du Y. W.
Published: July 2002

La 0.5 Sr 0.5 MnO 3 thin films deposited on (001) SrTiO3 substrates at different temperatures are prepared using pulsed laser deposition, and their electro- and magnetotransport properties are experimentally evaluated. The structure analysis reveals that the thin films show amorphous, mixed amorphous/nanocrystalline as well as epitaxial microstructures, respectively, depending on the deposition temperature. While the amorphous thin film exhibits variable range hopping conduction, the epitaxial sample is metal like and ferromagnetic. Enhanced low-field magnetoresistance at low temperature for the microstructure in which the nanocrystalline phase and amorphous phase coexist is demonstrated. It is argued that the amorphous layer separating the neighboring nanocrystals behaves as barrier for the spin-dependent tunneling, resulting in enhanced magnetoresistance at low magnetic field. The modified two-channel model where the insulating conduction channel and the spin-ordered and metallic conduction channel coexist in parallel is employed to explain the magnetotransport phenomena.
X-ray scattering studies of surfactant mediated epitaxial growth of Si/Ge/Si(001) heterostructures
Authors: Rodrigues W. , Sakata O. , Lee T.-L. , Walko D. A. , Marasco D. L. , Bedzyk M. J.
Published: July 2002

The strain and morphology of Si/Ge films grown by surfactant mediated molecular beam epitaxy on Si(001) with Bi as the surfactant were studied with grazing-incidence x-ray diffraction, x-ray reflectivity, low-energy electron diffraction, and Auger electron spectroscopy. Bi is observed to prevent the intermixing of Ge and Si layers by inhibiting Ge segregation in Si. Without a surfactant the critical thickness of Ge/Si(001) is 3 monolayers (ML). Using Bi, two-dimensional growth of Ge is observed for films up to 10 ML in thickness, with the onset of strain relaxation occurring at 7 ML of Ge growth. At 10 ML, the top Ge atomic layers are only partially relaxed. This is achieved by introducing roughness at the interface of the Ge and Si layers.
On second quantization of quantum groups
Authors: Grosse Harald , Schlesinger Karl-Georg
Published: July 2002

We construct a deformation of the function algebra on the quantum group SLq(2) into a trialgebra in the sense of Crane and Frenkel. We show that this naturally acts on the trialgebraic deformation of the Manin plane, previously introduced by the authors. Alternatively, one can view it as acting on the trialgebraic deformation of the fermionic Manin plane. We prove that the trialgebraic deformation of SLq(2) defines a 2−C*-category, a structure as needed for the superselection structure of massive two-dimensional quantum field theories. Besides this, we investigate another approach to trialgebra deformations of a bialgebra as a deformation of a Fock space construction over the bialgebra.
Optimal ensemble length of mixed separable states
Authors: Lockhart Robert
Published: July 2002

The optimal (pure state) ensemble length of a separable state A is the minimum number of (pure) states needed in convex combination to construct A. We study the set of all separable states with optimal (pure state) ensemble length equal to k or fewer. Lower bounds on k are found below which these sets have measure 0 in the set of separable states. In the bipartite case and the multiparticle case where one of the particles has significantly more quantum numbers than the rest the lower bounds are sharp. A consequence of our results is that for all two-particle systems, except possibly those with one qubit or those with a nine-dimensional Hilbert space, and for all systems with more than two particles the optimal pure state ensemble length for a randomly picked separable state is with probability 1 greater than the state’s rank. In bipartite systems with probabilty 1 it is greater than 1/4 the rank raised to the 3/2 power and in a system of p qubits with probability 1 it is greater than 22p/(1+2p), i.e., almost the square of the rank.
Dynamics of capillary evaporation. II. Free energy barriers
Authors: Leung Kevin , Luzar Alenka
Published: July 2002

We investigate the free energy barrier of vapor tube formation in a metastable liquid confined between hydrophobic walls. The model we use is a lattice gas model with nearest neighbor interactions whose evaporation dynamics has been reported in the preceding paper (paper I). We apply transition state theory and a constrained umbrella sampling technique, taking as our transition state a vapor pocket in the middle of the liquid layer. The calculated transmission coefficients show that the size of a vapor pocket is indeed a reasonable order parameter to describe capillary evaporation. The umbrella sampling method gives estimates of free energy barrier for vapor tube formation that are within an order of magnitude agreement with direct Monte Carlo simulation runs. In all the cases studied, the estimated free energy barriers are much smaller than those predicted by a previous mean-field approach.
Unrestricted Hartree–Fock band structure calculations for polymers: Application to a cross-talk system
Authors: Poulsen Tina D. , Mikkelsen Kurt V. , Fripiat Joseph G. , Champagne Benoı̂t
Published: July 2002

Unrestricted Hartree–Fock calculations for a one-dimensional infinite periodic system have been employed to characterize a cross-talk system between trans-1,4-polybutadiene and a small molecule, O2. The total energy, the energy band structure, and the longitudinal linear polarizability have been investigated. The presence of O2 has been found to influence in a quantitatively as well as a qualitative way the energy band structure of polybutadiene.
Layer selective determination of magnetization vector configurations in an epitaxial double spin valve structure: Si(001)/Cu/Co/Cu/FeNi/Cu/Co/Cu
Authors: Choi B. C. , Samad A. , Vaz C. A. F. , Bland J. A. C. , Langridge S. , Penfold J.
Published: July 2002

The magnetization vector configurations in an epitaxial Si(001)/Cu/Co/Cu/Fe20Ni80/Cu/Co/Cu double spin valve structure have been investigated at room temperature using polarized neutron reflectivity measurements. The layer-averaged magnetic moment per FeNi atom was found to be 0.99±0.06 μB at saturation, while the top (bottom) Co layer moment was 1.69±0.09 μB (1.67±0.08 μB), which are identical to the bulk values within experimental error. Furthermore, the Co magnetization vectors are found to lie in-plane but canted with respect to the applied field direction for an applied field strength smaller than the coercive fields (∼80 and 130 Oe) of the Co layers. This result indicates that a complete antiparallel alignment of the layer magnetizations is not reached causing a corresponding reduction in the giant magnetoresistance (GMR). These observations emphasize the importance of the detailed spin configuration in determining the GMR amplitude at low fields.
The frequency-dependent depolarization ratio of the low-frequency Raman scattering of two inorganic systems in their glassy, supercooled, and molten states
Authors: Yannopoulos S. N.
Published: July 2002

A temperature-dependent, low-frequency Raman study for a strong and a fragile glass-forming liquid is reported in order to elucidate the frequency and temperature sensitivity of the depolarization ratio spectrum, ρ(ω,T). Changes observed in ρ(ω,T) are directly reflected on the spectral features of the Raman coupling coefficient, Cαβ(ω). Our data provide evidence for polarization dependence of this coefficient, which has been completely overlooked in studies concerning the experimental determination of Cαβ(ω) through a comparison of neutron and reduced Raman spectra. The current status considering the frequency dependence of the coupling coefficient is briefly reviewed from the theoretical, experimental, and computer simulation points of view. The experimental data suggest that a reconsideration of the approaches employed should be undertaken.
Applying diatomic in molecules in excited electronic state calculations for flexible polyatomic molecules. The I3 molecule
Authors: Margulis C. J. , Coker D. F.
Published: July 2002

In this paper we calculate the 64 potential energy surfaces of the manifold of states of I3 that dissociate to three ground state iodine radicals as functions of bend and stretching coordinates. We describe the set of quantum rotations necessary to generate the full Hamiltonian matrix using only I2 experimental energies and describe the formal connection between this approach and the usual diatomic in molecules Hamiltonian formulation. The I3 excited state surfaces calculated in this way are used to compute various excited state features in the photoelectron spectrum of I3− and we use these results to interpret the recently measured spectrum.
Mesoscopic bead-and-spring model of hard spherical particles in a rubber matrix. I. Hydrodynamic reinforcement
Authors: Raos Guido , Allegra Giuseppe
Published: July 2002

Exploiting an electrostatic analogy, we show that the elastic forces between a set of rigid particles embedded in a phantom polymer network can be represented by a simple bead-and-spring model. The beads represent the particles and the springs the rubber matrix. The model is validated by Monte Carlo simulation of rubbers filled with hard spherical particles, at volume fractions between 0.1 and 0.3. We derive both the moduli and the full stress–strain curves, under uniaxial elongation. The model reproduces and extends previous theoretical results on the so-called hydrodynamic reinforcement effect.
Optical activity effects in second harmonic generation from anisotropic chiral thin films
Authors: Sioncke Sonja , Van Elshocht Sven , Verbiest Thierry , Persoons André , Kauranen Martti , Phillips Karen E. S. , Katz Thomas J.
Published: July 2002

Circular-difference effects in second-harmonic generation have been used to study chiral, anisotropic thin films of a helicene derivative. For such samples, these effects arise both from the chirality of the film and from its anisotropy. We show theoretically and experimentally that there is a fundamental difference between a circular-difference effect originating from chirality and anisotropy. A method is described that distinguishes the two contributions.
Self-diffusion process in water: Spatial picture of single-particle density fluctuations
Authors: Svishchev Igor M. , Zassetsky Alexander Yu.
Published: July 2002

A computer simulation methodology with which to study the single-particle dynamics in complex molecular liquids is presented. Molecular dynamics simulations of liquid water are performed in the temperature range of 238–473 K using the polarizable point charge (PPC) potential. The self part of the van Hove density–density correlation function is calculated. Using the Gaussian approximation of the van Hove function the mean self-diffusion coefficient for the PPC potential is calculated. The singularity temperature for supercooled PPC water, Ts=218 K, estimated from the self-diffusion data appears to agree well with most estimates for real water. In order to elucidate the spatial picture of the single-particle molecular density in this complex liquid and its time evolution, we explicitly resolve the self van Hove function in the local frame of water molecules. The self-diffusion tensor is introduced and numerically evaluated from this spatial (separation and direction dependent) self van Hove function. The fluctuations of the single-particle molecular density in liquid water appear to be spatially anisotropic (nonspherical). At low temperatures these dynamical heterogeneities in liquid water tend to increase.
Motions of diaquahydrogen ions and phase transitions in dodecatungstophosphoric acid hexahydrate studied by H1 nuclear magnetic resonance technique
Authors: Sato Setsuko , Ido Akiko , Ishida Hiroyuki
Published: July 2002

The motions of the planar diaquahydrogen ion H5O2+ in H3PW12O40⋅6H2O are studied by H1 nuclear magnetic resonance second moment (M2) and spin-lattice relaxation time (T1) measurements. The phase transitions were observed at 144 and 313 K by differential thermal analysis (DTA) and differential scanning calorimetry (DSC). Comparison between the experimental and calculated M2 values suggests that the pseudo-C4 reorientation around the central H+ occurs at temperatures higher than 140 K and that H3O+–H2O behavior appears at temperatures higher than 315 K. Since the longitudinal magnetization relaxed nonexponentially, T1 was also estimated theoretically by applying the hindered rotation treatment to magnetic relaxation for a five-interacting proton system. The enthalpy change (ΔH) of 260±30 Jmol−1 and the difference between the experimental and calculated T1 minimum values show that at the order–disorder phase transition point of 144 K the potential energy well for the H5O2+ ion changes from a single potential well to unequal potential wells whose energy difference is 4–5 kJ mol−1. The room temperature phase is dynamically disordered with the pseudo-C4 reorientation of the H5O2+ ion.