bluuebunny/crossref_metadata_2025 huggingface dataset:
Dislocation density reduction via lateral epitaxy in selectively grown GaN structures
Authors: Zheleva Tsvetanka S. , Nam Ok-Hyun , Bremser Michael D. , Davis Robert F.
Published: July 2002

The microstructure and the lateral epitaxy mechanism of formation of homoepitaxially and selectively grown GaN structures within windows in SiO2 masks have been investigated by transmission electron microscopy (TEM) and scanning electron microscopy. The structures were produced by organometallic vapor phase epitaxy for field emission studies. A GaN layer underlying the SiO2 mask provided the crystallographic template for the initial vertical growth of the GaN hexagonal pyramids or striped pattern. The SiO2 film provided an amorphous stage on which lateral growth of the GaN occurred and possibly very limited compliancy in terms of atomic arrangement during the lateral growth and in the accommodation of the mismatch in the coefficients of thermal expansion during cooling. Observations with TEM show a substantial reduction in the dislocation density in the areas of lateral growth of the GaN deposited on the SiO2 mask. In many of these areas no dislocations were observed.
Short wavelength (λ∼3.4 μm) quantum cascade laser based on strained compensated InGaAs/AlInAs
Authors: Faist Jérôme , Capasso Federico , Sivco Deborah L. , Hutchinson Albert L. , Chu Sung-Nee G. , Cho Alfred Y.
Published: July 2002

Growth of quantum cascade lasers based on strain-compensated InxGa1−xAs/InyAl1−yAs and operating at a wavelength shorter than 4 μm is reported. Pulsed mode operation of these lasers up to T=280 K is reported with a high T0. Continuous wave powers as high as 120 mW are reported at cryogenic temperatures (15 K).
Neutron diffraction study of electrostrictive coefficients of prototype cubic phase of relaxor ferroelectric PbMg1/3 Nb2/3 O3
Authors: Zhao J. , Glazounov A. E. , Zhang Q. M. , Toby Brian
Published: July 2002

The electrostrictive coefficients of prototype cubic phase of PbMg1/3Nb2/3O3 are studied using a high-resolution neutron powder diffraction experiment. The key idea of the approach is to “eliminate” from the material the inhomogeneity related to the presence of the double-phase mixture “polar regions/nonpolar matrix,” and to study two structurally homogeneous states: prototype cubic at temperatures above 600 K and rhombohedral ferroelectric at temperatures below 220 K. The obtained value of volumetric electrostrictive coefficient, Qh=(8.3±1.0)×10−2 ×(m4/C2), is consistent with those of other perovskite ferroelectrics with a prototype cubic phase.
Self-organized formation of hexagonal pore arrays in anodic alumina
Authors: Jessensky O. , Müller F. , Gösele U.
Published: July 2002

The conditions for the self-organized formation of ordered hexagonal structures in anodic alumina were investigated for both oxalic and sulfuric acid as an electrolyte. Highly ordered pore arrays were obtained for oxidation in both acids. The size of the ordered domains depends strongly on the anodizing voltage. This effect is correlated with a voltage dependence of the volume expansion of the aluminum during oxidation and the current efficiency for oxide formation. The resulting mechanical stress at the metal/oxide interface is proposed to cause repulsive forces between the neighboring pores which promote the formation of ordered hexagonal pore arrays.
Theoretical investigation of edge dislocations in AlN
Authors: Wright A. F. , Furthmüller J.
Published: July 2002

The structures and formation energies of neutral and charged edge dislocations in AlN are investigated via density-functional-theory calculations. Stoichiometric structures having full and open cores are considered as well as nonstoichiometric structures having aluminum or nitrogen vacancies along the dislocation core. Formation energies are found to depend strongly on the Fermi level, due to the presence of defect levels in the band gap, and on growth conditions for the case of the nonstoichiometric structures. A structure having aluminum vacancies along the dislocation core is predicted to be most stable in n-type material grown under nitrogen-rich conditions, whereas a nitrogen-vacancy structure is most stable in p-type material grown under aluminum-rich conditions. Estimates are also given for defect energy levels in the gap.
Spatially resolved exciton trapping in a voltage-controlled lateral superlattice
Authors: Zimmermann S. , Schedelbeck G. , Govorov A. O. , Wixforth A. , Kotthaus J. P. , Bichler M. , Wegscheider W. , Abstreiter G.
Published: July 2002

With spatially resolved photoluminescence experiments we demonstrate voltage-controlled trapping of excitons in a submicron scale lateral potential superlattice imposed on a semiconductor quantum well. The potential modulation is achieved by two interdigitated field-effect electrodes on top of the sample surface. Both parallel and vertical electric field components strongly modify the optical properties of the quantum well. We show that the lateral modulation of the strength of the quantum confined Stark effect results in an effective lateral exciton potential that can be probed by spatially resolved measurements of the excitonic luminescence. We demonstrate that excitons may be confined to the regions of strongest vertical fields, in which the effective exciton energy is lowest. Spatial resolution of the observed photoluminescence signal allows for a qualitative understanding of the exciton transport and trapping process.
Effect of cooling rate on the precipitation of quasicrystals from the Zr–Cu–Al–Ni–Ti amorphous alloy
Authors: Xing L. Q. , Eckert J. , Löser W. , Schultz L.
Published: July 2002

The Zr57Cu20Al10Ni8Ti5 alloy solidifies into an amorphous phase upon rapid quenching or casting at low cooling rates. However, the amorphous alloys formed at different cooling rates exhibit different crystallization behavior. The slowly cooled bulk amorphous alloys prepared by copper mold casting reveal a first crystallization peak at 715 K upon heating at 0.33 K/s and crystallize via precipitation of an icosahedral quasicrystalline phase in the first crystallization step. The rapidly quenched ribbons exhibit a first crystallization peak at 720 K and crystallize by simultaneous precipitation of the quasicrystalline phase together with Zr2Cu and Zr2Ni intermetallic phases in the first stage of crystallization. It is supposed that the undercooled Zr57Cu20Al10Ni8Ti5 melt has a tendency to develop an icosahedral short-range order, which is favored by low cooling rate. As a result, the bulk amorphous alloy has a short-range order close to a quasicrystalline phase. In contrast, the structure of the ribbon is more similar to that of the melt at high temperature and the atoms are oriented more randomly.
Flux pinning in ternary (Nd0.33Eu0.33Gd0.33)Ba2Cu3Oy melt-processed superconductors
Authors: Koblischka M. R. , Muralidhar M. , Murakami M.
Published: July 2002

The flux pinning characteristics of ternary melt-processed (Nd0.33Eu0.33Gd0.33)Ba2Cu3Oy (NEG) superconductors are studied in the temperature range 60⩽T⩽90 K. NEG samples exhibit a strongly developed peak effect in the dependence of the critical current densities on the external field, Ha. The scaling of the pinning forces versus the reduced field h=Ha/Hirr (where Hirr denotes the irreversibility field) yields a peak at h0=0.5 which is an indication of pinning provided by a spatial variation of the transition temperature. The presence of a weaker superconducting second phase is demonstrated by means of field cooling and warming experiments in fields up to 7 T. Furthermore, we discuss the possible effect of the magnetic moments of Gd and Nd on the flux pinning.
High-speed maskless laser patterning of indium tin oxide thin films
Authors: Yavas O. , Takai M.
Published: July 2002

Patterning characteristics of indium tin oxide thin films using different wavelengths of a diode-pumped Q-switched Nd:YLF and a flashlamp-pumped Nd:YAG laser have been studied. While a ripplelike structure in the etched line was formed due to incomplete material removal when the first harmonic of the Nd:YLF or Nd:YAG laser was used, a residue-free line could be obtained using the fourth harmonic of the Nd:YLF laser even at higher scan speeds. The observed differences in the morphology could be attributed to different absorption characteristics at the infrared and ultraviolet wavelengths. High process speeds in excess of 1 m/s could be achieved.
Ruthenium: A superior compensator of InP
Authors: Dadgar A. , Stenzel O. , Näser A. , Zafar Iqbal M. , Bimberg D. , Schumann H.
Published: July 2002

The 4d-transition metal ruthenium presents a new dopant to fabricate thermally stable semi-insulating InP layers for both electron and hole injection. The layers are grown by low-pressure metalorganic chemical vapor deposition using tertiarybutylphosphine and trimethylindium as source materials for InP growth. Using bis(η5-2,4-dimethyl-pentadienyl)ruthenium(II) as precursor Ru doping concentrations of the order of 4×1018 cm−3 are achieved, determined by means of secondary ion mass spectroscopy. The Ru diffusion coefficient in InP is determined to DRu(800 °C)⩽1×10−15 cm2/s which is four orders of magnitude smaller than for Fe. Resistivities obtained under electron and hole injection are above 6×107 Ω cm and 5×108 Ω cm, respectively. In deep level transient spectroscopy measurements under electron emission and hole emission, one deep level each with concentrations around 1016 cm−3, is observed.
Andreev reflection in Si-engineered Al/InGaAs hybrid junctions
Authors: De Franceschi Silvano , Giazotto Francesco , Beltram Fabio , Sorba Lucia , Lazzarino Marco , Franciosi Alfonso
Published: July 2002

Andreev-reflection dominated transport is demonstrated in Al/n-In0.38Ga0.62As superconductor–semiconductor junctions grown by molecular beam epitaxy on GaAs(001). High junction transparency was achieved in low-doped devices by exploiting Si interface bilayers to suppress the native Schottky barrier. It is argued that this technique is ideally suited for the fabrication of ballistic transport hybrid microstructures.
White light emission induced by confinement in organic multiheterostructures
Authors: Xie Z. Y. , Huang J. S. , Li C. N. , Liu S. Y. , Wang Y. , Li Y. Q. , Shen J. C.
Published: July 2002

A technique for inducing white-light emission from organic multiheterostructures is proposed. The configuration of organic multiheterostructure white-light emitting diodes is ITO/TPD(50 nm)/BePP2(5 nm)/TPD(4 nm)/BePP2:rubrene(5 nm)/TPD(4 nm)/Alq3(10 nm)/Al. Triphenyldiamine derivative (TPD) is used as a hole-transporting layer and the potential barrier layers. Blue fluorescent phenylpyridine beryllium (BePP2), orange fluorescent rubrene, and green fluorescent aluminum complex (Alq3) are used as three primary colors. BePP2 and BePP2 doped with rubrene act as the potential wells sandwiched between TPD barrier layers, in which excitons are confined. Alq3 is used as an electron-transporting green-color emitter. The white-light emission spectrum covers a wide range of the visible region, and the Commission Internationale de l’Eclairage coordinates of the emitted light are (0.32, 0.38) at 9 V. The maximum brightness and luminous efficiency of this device are 4000 cd/m2 (at 17 V) and 0.4 lm/W, respectively.
In–Ga intermixing in low-temperature grown GaAs delta doped with In
Authors: Bert N. A. , Chaldyshev V. V. , Musikhin Yu. G. , Suvorova A. A. , Preobrazhenskii V. V. , Putyato M. A. , Semyagin B. R. , Werner P.
Published: July 2002

Low-temperature grown GaAs films with indium delta layers are studied by transmission electron microscopy. The delta layers in the as-grown film are found to be as thick as four monolayers (ML) independently of a nominal In deposit of 0.5 or 1 ML, a thickness which reflects the film surface roughness during the low-temperature growth. A pronounced In–Ga intermixing is observed in the films subjected to 500–700 °C isochronal anneals. The In–Ga interdiffusion diffusivity is evaluated. The effective activation energy for In–Ga interdiffusion is found to be 1.1±0.3 eV which is significantly smaller than a value of 1.93 eV for a stoichiometric GaAs. The difference seems to result from a loss of the gallium vacancy supersaturation upon annealing, and is consistent with an annihilation enthalpy of 0.8 eV.
Enhanced precipitation of excess As on antimony delta layers in low-temperature-grown GaAs
Authors: Bert N. A. , Chaldyshev V. V. , Suvorova A. A. , Preobrazhenskii V. V. , Putyato M. A. , Semyagin B. R. , Werner P.
Published: July 2002

Delta doping with antimony isovalent impurity has been employed as a precursor for two-dimensional precipitation of excess arsenic in GaAs grown by molecular-beam epitaxy at low substrate temperature (LT-GaAs), and subsequently annealed. LT-GaAs films delta doped with indium isovalent impurity showed previously to provide two-dimensional As cluster sheets were studied for comparison. Small clusters observed by transmission electron microscopy at the Sb delta layers had an unusual lens shape and, probably, nonrhombohedral microstructure. These clusters induced strong local strains in the surrounding GaAs matrix. After annealing under the same conditions, the clusters at the Sb delta layers were found to be bigger than those at the In delta layers. Additionally, nucleation of the arsenic clusters at the Sb delta layers occurs at a relatively low annealing temperature. The observed precipitation features indicate that delta doping with Sb is more effective for two-dimensional precipitation of the excess As in LT-GaAs as compared with In delta doping.
Aluminum gallium nitride short-period superlattices doped with magnesium
Authors: Saxler A. , Mitchel W. C. , Kung P. , Razeghi M.
Published: July 2002

Short-period superlattices consisting of alternating layers of GaN:Mg and AlGaN:Mg were grown by low-pressure organometallic vapor phase epitaxy. The electrical properties of these superlattices were measured as a function of temperature and compared to conventional AlGaN:Mg layers. It is shown that the optical absorption edge can be shifted to shorter wavelengths while lowering the acceptor ionization energy by using short-period superlattice structures instead of bulk-like AlGaN:Mg.
Potential imaging of operating light-emitting devices using Kelvin force microscopy
Authors: Shikler R. , Meoded T. , Fried N. , Rosenwaks Y.
Published: July 2002

We report on the measurements of two-dimensional potential distribution with nanometer spatial resolution of operating light-emitting diodes. By measuring the contact potential difference between an atomic force microscope tip and the cleaved surface of the light emitting diode, we were able to measure the device potential distribution under different applied external bias. It is shown that the junction built-in voltage at the surface decreases with increasing applied forward bias up to flatband conditions, and then inverted. It is found that the potential distribution is governed by self-absorption of the sub-band-gap diode emission.
White-light-emitting organic electroluminescent devices based on interlayer sequential energy transfer
Authors: Deshpande R. S. , Bulović V. , Forrest S. R.
Published: July 2002

We demonstrate efficient, molecular organic white-light-emitting devices using vacuum-deposited thin films of red luminescent [2-methyl-6-[2-(2,3,6,7-tetrahydro-1H, 5H-benzo [ij] quinolizin-9-yl) ethenyl]-4H-pyran-4-ylidene] propane-dinitrile (DCM2), doped into blue-emitting 4, 4′ bis [N-1-napthyl-N-phenyl-amino]biphenyl (α-NPD), and green-emitting tris-(8-hydroxyquinolinato) aluminum(III) (AlQ3). The luminescent layers are separated by a hole-blocking layer of 2,9-dimethyl, 4,7-diphenyl, 1,10-phenanthroline (BCP), whose thickness is on the order of a typical Förster transfer radius of 30–40 Å. Excitons formed on α-NPD sequentially transfer their energy via a Förster mechanism to AlQ3 across the BCP layer, and from AlQ3 to DCM2. This interlayer sequential energy transfer results in partial excitation of all three molecular species, thereby producing white light emission. The thickness of the blocking layer and the concentration of DCM2 in α-NPD permit the tuning of the device spectrum to achieve a balanced white emission with Commission Internationale d’Eclairage chromaticity coordinates of (0.33, 0.33). The spectrum is largely insensitive to the drive current, and the devices have a maximum luminance of 13 500 cd/m2. At a luminance of 100 cd/m2, the quantum and power efficiencies are 0.5% and 0.35 lm/W, respectively.
Synthesis, structure, and optical properties of colloidal GaN quantum dots
Authors: Mićić O. I. , Ahrenkiel S. P. , Bertram D. , Nozik A. J.
Published: July 2002

Colloidal chemistry was used to synthesize GaN quantum dots. A GaN precursor, polymeric gallium imide, {Ga(NH)3/2}n, which was prepared by the reaction of dimeric amidogallium with ammonia at room temperature, was heated in trioctylamine at 360 °C for one day to produce GaN nanocrystals. The GaN particles were separated, purified, and partially dispersed in a nonpolar solvent to yield transparent colloidal solutions that consisted of individual GaN particles. The GaN nanocrystals have a spherical shape and mean diameter of about 30±12 Å. The spectroscopic behavior of colloidal transparent dispersion has been investigated and shows that the band gap of the GaN nanocrystals shifts to slightly higher energy due to quantum confinement. The photoluminescence spectrum at 10 K (excited at 310 nm) shows band edge emission with several emission peaks in the range between 3.2 and 3.8 eV, while the photoluminescence excitation spectrum shows two excited-state transitions at higher energies.
Analysis of the polar direction of GaN film growth by coaxial impact collision ion scattering spectroscopy
Authors: Sumiya M. , Tanaka M. , Ohtsuka K. , Fuke S. , Ohnishi T. , Ohkubo I. , Yoshimoto M. , Koinuma H. , Kawasaki M.
Published: July 2002

Nondestructive determination of the polarity of GaN has been achieved by the use of coaxial impact-collision ion scattering spectroscopy analysis. The polarity of a GaN film with a smooth surface on non-nitrided c-plane sapphire was identified (0001) (Ga face; +c). GaN films with a 20 nm buffer layer on nitrided sapphire had (0001̄) (N face; −c) polarity and a hexagonal faceted surface. The influence of both the buffer layer and of substrate nitridation on the polarity of wurtzite {0001} GaN films deposited by two-step metal organic chemical vapor deposition (MOCVD) has been investigated. The polarity of the buffer layer on a nitrided sapphire substrate was altered by varying its thickness or the annealing time. It was found that the polarity of the GaN film is determined by the polarity of the annealed buffer layer; MOCVD-GaN films on buffer layers with +c and −c polarity have either +c (smooth surface) or −c (hexagonal facet) polarity, respectively.
Determination of photoluminescence mechanism in InGaN quantum wells
Authors: Riblet Philippe , Hirayama Hideki , Kinoshita Atsuhiro , Hirata Akira , Sugano Takuo , Aoyagi Yoshinobu
Published: July 2002

We report on the unambiguous experimental determination of the photoluminescence mechanism in a set of In0.25Ga0.75N quantum wells. Instead of studying the photoluminescence for different In contents, we have investigated it as a function of the quantum well width in combination with a similar study performed on GaN quantum wells. In this way, we show that the photoluminescence is not coming from quantum dots or very localized states in the quantum well, but from the quantum well itself under the influence of a piezoelectric field induced by strain. The previously reported abnormal photoluminescence shifts and temperature dependencies can thus be explained.
Microwave synthesis of fullerenes from chloroform
Authors: Xie Su-yuan , Huang Rong-bin , Yu La-jia , Ding Jie , Zheng Lan-sun
Published: July 2002

Fullerenes C60 and C70 were synthesized continuously via microwave plasma from chloroform at low pressure argon atmosphere, and identified in toluene extracts of soot, using high-performance liquid chromatography coupled with atmospheric pressure chemical ionization mass spectrometry and diode-array spectrophotometric detection. Yield of C60 (0.3%–1.3%) and C70 (0.1%–0.3%) and their ratio depend on the temperature gradient and the collision probability. Besides fullerenes, a series of perchlorinated carbon clusters were found in the synthesis products. Since frameworks of the products are fragments of fullerenes, the further investigation of this process may lead to a better knowledge of the formation mechanisms of fullerenes.
Injection of a magnetic domain wall into a submicron magnetic wire
Authors: Shigeto K. , Shinjo T. , Ono T.
Published: July 2002

Two types of magnetic wires (150 nm width) with trilayer structure consisting of NiFe (20 nm)/Cu (20 nm)/Co (20 nm) were prepared. One was connected to a square pad (0.5×0.5 μm2) at one end, while the other has a symmetrical shape with two flat ends. Magnetization reversal was detected sensitively by magnetoresistance measurement. Switching field of the Co layer for the wire with a pad was much smaller than that for the wire without a pad. This indicates that a domain wall nucleates initially in the pad and is injected into the wire at the switching field. This model for the magnetization reversal process is supported by the angular dependence of the switching field.
Broken symmetry in low-temperature BaTiO3 phases: Strain effects probed by Raman scattering
Authors: Osada Minoru , Kakihana Masato , Wada Satoshi , Noma Tatsuo , Cho Woo-Seok
Published: July 2002

Micro-Raman spectroscopy is utilized to study the temperature dependence of domain structures in a high-quality BaTiO3 bulk crystal. For T=4–100 K, the spectra of the initially 90° domain wall region at room temperature exhibit the splitting of A1 mode at ∼244 cm−1 with the appearance of an additional peak at ∼263 cm−1, the behavior of which is different from that of a sharp peak observed at ∼244 cm−1 in the single-domain region. The results imply that the strain fields exist in the domain walls at low temperatures, thus altering the local crystal symmetry.
Strain-balanced GaAsP/InGaAs quantum well solar cells
Authors: Ekins-Daukes N. J. , Barnham K. W. J. , Connolly J. P. , Roberts J. S. , Clark J. C. , Hill G. , Mazzer M.
Published: July 2002

A strain-balance multiquantum well (MQW) approach to enhance the GaAs solar cell efficiency is reported. Using a p-i-n diode structure, the strain-balanced GaAsP/InGaAs MQW is grown on a GaAs substrate and equals a good GaAs cell in terms of power conversion efficiency. The cell design is presented together with measurements of the forward bias dark current density, quantum efficiency, and 3000 K light-IV response. Cell efficiencies under standard air mass (AM) 1.5 and AM 0 illumination are projected from experimental data and the suitability of this cell for enhancing GaInP/GaAs tandem cell efficiencies is discussed.
Nanoscale backswitched domain patterning in lithium niobate
Authors: Shur V. Ya. , Rumyantsev E. L. , Nikolaeva E. V. , Shishkin E. I. , Fursov D. V. , Batchko R. G. , Eyres L. A. , Fejer M. M. , Byer R. L.
Published: July 2002

We demonstrate a promising method of nanoscale domain engineering, which allows us to fabricate regular nanoscale domain patterns consisting of strictly oriented arrays of nanodomains (diameter down to 30 nm and density up to 100 μm−2) in lithium niobate. We produce submicron domain patterns through multiplication of the domain spatial frequency as compared with the electrode one. The fabrication techniques are based on controlled backswitched poling.
Short-range order in ultrathin film titanium dioxide studied by Raman spectroscopy
Authors: Yakovlev V. V. , Scarel G. , Aita C. R. , Mochizuki S.
Published: July 2002

It is demonstrated here that nonresonant Raman spectroscopy can be used for unequivocal determination of short-range order in ultrathin films, using different structures of titanium dioxide as the model system. Titania films as thin as 7 nm sputter deposited on 〈111〉 Si have been analyzed and their phase content determined.
Scanning tunneling microscopy study of Si(111) surface morphology after removal of SiO2 by synchrotron radiation illumination
Authors: Gao Y. , Mekaru H. , Miyamae T. , Urisu T.
Published: July 2002

The surface morphology of Si(111) was investigated using scanning tunneling microscopy after desorption of surface SiO2 by synchrotron radiation (SR) illumination. The surface shows large regions of atomically flat Si(111)-7×7 structure, and is characterized by the formation of single bilayer steps nicely registered to the underlying crystal structure. This is in sharp contrast to Si(111) surfaces after thermal desorption of SiO2 at temperatures 880 °C and above, where the surface steps are much more irregular. The registration of the surface steps to the underlying crystal structure indicates that the Si(111) surface reaches thermodynamic equilibrium under SR irradiation at temperatures much lower than that necessary for thermal desorption.
Near-infrared to visible up-conversion in a forward-biased Schottky diode with a p-doped channel
Authors: Sandhu J. S. , Heberle A. P. , Alphenaar B. W. , Cleaver J. R. A.
Published: July 2002

Near-infrared radiation of wavelength 1.5 μm is up-converted to a visible wavelength of 818 nm by internal photoemission in a Schottky diode with a modulation p-doped channel. The near-infrared light incident upon the metal–semiconductor interface excites electrons from the metal into the semiconductor. The electrons then drift into the quantum well where they recombine radiatively, producing luminescence at the shorter wavelength of 818 nm. The intensity of the luminescence is strongly dependent on bias, and turns on at a forward bias of −0.7 V.
Indium segregation in InGaN quantum-well structures
Authors: Duxbury N. , Bangert U. , Dawson P. , Thrush E. J. , Van der Stricht W. , Jacobs K. , Moerman I.
Published: July 2002

Direct evidence for In-segregation in InGaN/GaN quantum-well structures is given via highly spatially resolved energy dispersive x-ray analysis performed in a dedicated scanning transmission electron microscope. The In fluctuations become increasingly pronounced in the vicinity of dislocations. The latter assist In diffusion and cause severe Ga/In intermixing.
Perovskite rare-earth nickelates in the thin-film epitaxial state
Authors: Novojilov M. A. , Gorbenko O. Yu. , Graboy I. E. , Kaul A. R. , Zandbergen H. W. , Babushkina N. A. , Belova L. M.
Published: July 2002

We have succeeded in the preparation of thin films of rare-earth nickelates RNiO3 (R=Pr, Nd, Sm, and Gd) under reduced oxygen pressure <0.02 bar by metalorganic chemical-vapor deposition owing to their epitaxial stabilization on perovskite substrates. The film–substrate lattice mismatch is critical for the epitaxial stabilization of RNiO3 phases. Increase of the lattice mismatch or film thickness results in the deposition of rare-earth oxides and NiO instead of RNiO3. The epitaxial films of nickelates were strained and consisted of 90° domains with the orthorhombic Pnma structure. The transport properties of the strained films on LaAlO3 were similar to those of the bulk material of the same composition under applied pressure of 9 kbar but they were different from the properties of the bulk material under ambient pressure. The result implies that transport properties of RNiO3 films with sharp metal-to-insulator transition can be effectively tuned by the control of the lattice strain.
Electronic structure of Ti3SiC2
Authors: Ahuja R. , Eriksson O. , Wills J. M. , Johansson B.
Published: July 2002

We have investigated the electronic structure of Ti3SiC2 by means of first-principles self-consistent total-energy calculations within the local-density approximation using the full-potential linear-muffin-tin-orbital method. Our calculated volume is in good agreement (within 4%) with experiment. Also, the calculated values for c/a and positional parameters are in excellent agreement with the experimental data. We have compared the electronic structure of Ti3SiC2 with TiC. Our calculations show that Ti3SiC2 has a chemical bonding similar to the one of TiC, which is predominantly covalent.
Grain-boundary effects on the electrical resistivity and the ferromagnetic transition temperature of La0.8Ca0.2MnO3
Authors: Fu Yonglai
Published: July 2002

In this letter, grain-boundary effects on the electrical transport and the ferromagnetic transition temperature were investigated in La0.8Ca0.2MnO3. The different ferromagnetic transition temperature (45 K) and the different metal–semiconductor transition temperature (62 K) were observed in the samples with different grain boundaries. Wide grain boundaries can induce large grain-boundary effects on grain, and small grain size also induces large grain-boundary effects on grain due to the increase of grain boundaries. The possible mechanism of grain-boundary effect is explained as the strain effect of grain induced by the distortion at grain boundaries. The strain of grains should enhance the ferromagnetic double exchange interaction.
High-efficiency organic dye-doped polymer optical fiber lasers
Authors: Kuriki Ken , Kobayashi Takeyuki , Imai Nana , Tamura Toshihiko , Nishihara Susumu , Nishizawa Yukihisa , Tagaya Akihiro , Koike Yasuhiro , Okamoto Yoshi
Published: July 2002

A graded-index polymer optical fiber containing rhodamine 6G in poly(methyl methacrylate-co-2-hydroxyethyl methacrylate) has been fabricated. Lasing behavior of the fiber was observed by pumping with a frequency-doubled Q-switched Nd: yttrium–aluminum–garnet laser. A slope efficiency of 43% and a lifetime of 110 000 shots at a repetition rate of 10 Hz have been observed. With a 1.5 mJ pump energy, an output energy of 640 μJ was produced.
Second-type self-similar solutions to the ultrarelativistic strong explosion problem
Authors: Best Paul , Sari Re’em
Published: July 2002

The well-known Blandford–McKee solution describes the ultrarelativistic flow in a spherical blast wave enclosed by a strong shock. It is valid when the density of the external medium into which the shock propagates varies with the distance r from the origin as r−k, for k<4. These are first-type self-similar solutions in which the shock Lorentz factor Γ varies as Γ2∝t−m, where m=3−k to ensure energy conservation. New second-type self-similar solutions, valid for k>5−3/4≈4.13, are presented. In these solutions Γ varies as Γ2∝t−m with m=(3−23)k−4(5−33) so that the shock accelerates and the fraction of the flow energy contained in the vicinity of the shock decreases with time. The new solutions are shown to be in excellent agreement with numerical simulations of the flow equations. It is proved that no second-type self-similar solutions exist for k<5−3/4≈4.13.
Determination of the size, shape, and composition of indium-flushed self-assembled quantum dots by transmission electron microscopy
Authors: McCaffrey J. P. , Robertson M. D. , Fafard S. , Wasilewski Z. R. , Griswold E. M. , Madsen L. D.
Published: July 2002

Single and multiple layers of self-assembled InAs quantum dots (QDs) produced by the indium-flush technique have been studied by transmission electron microscopy (TEM) in an effort to develop techniques to reproducibly grow QDs of uniform size and shape. To monitor the changes in QD dimensions, plan-view samples of capped single layers were studied as well as cross-sectional samples of QDs in multiple layers and stacks. The changes in the observed round- and square-shaped QD images under various plan-view TEM imaging conditions, as well as the contrast reversal in the center of QD images viewed in cross-section are modeled using the many-beam Bloch-wave approach, including strain. The sizes and shapes of the QDs are determined through the interpretation of the observed (primarily strain) contrast in plan-view and the observed (primarily atomic number) contrast in cross-sectional TEM.
Oxygen incorporation in thin films of In2O3:Sn prepared by radio frequency sputtering
Authors: Mergel D. , Stass W. , Ehl G. , Barthel D.
Published: July 2002

Polycrystalline In2O3:Sn (ITO) films have been prepared by reactive radio frequency diode sputtering of an oxidic target using various oxygen/argon mixtures. They mostly contain more oxygen than the ideal crystal. When deposited at low pressure, the crystals have an expanded lattice (up to 3.5%) and, for small oxygen addition to the sputter gas, are also denser than the ideal crystal. This is explained by an incorporation of additional oxygen in the bixbyite structure, possibly into constitutional vacancies. Upon annealing, the lattice relaxes, however, the additional oxygen remains in the films. A model of oxygen segregation into grain boundaries is developed, that could also explain the grain–subgrain structure sometimes reported for ITO films.
Vacuum ultraviolet photochemistry of CH4 and isotopomers. II. Product channel fields and absorption spectra
Authors: Wang Jen-Han , Liu Kopin , Min Zhiyuan , Su Hongmei , Bersohn Richard , Preses Jack , Larese John Z.
Published: July 2002

In part I of this work the relative velocities and anisotropies of the atomic H and D fragments from methane photolysis at 10.2 eV were measured. In this paper the relative abundance of the methyl and methylene fragments are reported. A complete set of quantum yields for the different photodissociation channels of each isotopomer is obtained by combining the two sets of data. Previously it was found that H atoms are almost four times more likely than D atoms to be ejected; now it is found that hydrogen molecule photofragments are much richer in H atoms than in D. Overall, the heavier D atoms are more likely than the H atoms to remain attached to the carbon atom. An implication for astrophysics is discussed. The VUV absorption spectra of CH4 and CH3D are almost identical both at room temperature and 75 K. There is, as expected, no variation in the absorption spectrum with temperature. Evidence is given that all or almost all of the methylene is produced in the a 1A1 and not in the ground B13 state.
Theoretical simulations of the He79Br2 B, v=8←X, v″=0 excitation spectrum: Spectroscopic manifestation of a linear isomer?
Authors: Hernández Marta I. , González-Lezana Tomás , Delgado-Barrio Gerardo , Villarreal Pablo , Buchachenko Alexei A.
Published: July 2002

Possible manifestations of a linear isomer of a rare gas–halogen molecule van der Waals complex in its B←X excitation spectrum are analyzed using a continuous one-parametric family of X-state potential energy surfaces (PESs) with variable depths of minima in the T-shaped and linear configurations. For the HeBr2 complex as an example, the propensities in the frequencies and intensities of the representative transitions from T-shaped and linear isomers are analyzed and the variation of the whole spectrum with the topology of the X-state PES is established. Qualitatively good agreement with the experimental HeBr2 spectrum clearly suggests that the unassigned secondary band of the observed spectrum is likely formed by transitions from the linear isomer, whose energy is very close to that of the T-shaped one. Present results provide strong evidence for the possibility to detect a linear isomer of rare gas–halogen molecule complexes via conventional excitation spectroscopy. © 2000 American Institute of Physics.
Experimental determination of scanning probe microscope cantilever spring constants utilizing a nanoindentation apparatus
Authors: Holbery J. D. , Eden V. L. , Sarikaya M. , Fisher R. M.
Published: July 2002

A rapid, nondestructive, and accurate method for determining the normal spring constants of scanning probe microscopy cantilevers is presented. Spring constants are determined using a commercial combination atomic force microscope and nanoindentation apparatus configured with a W-indenter tip geometrically configured into either a scanning tunneling microscope pointed tip or chisel shape that may be placed onto the cantilever of interest with high accuracy. A load is applied to the cantilever tip and the corresponding displacement is measured. From the force–displacement curve, the spring constant is determined. For cantilevers with spring constants greater than 1 N/m, the derived spring constants are believed to be accurate to within ±10%, with better accuracy for stiffer levers. This method has been used to measure the stiffness of cantilevers from several manufacturers.
Excitation, propagation, and damping of electron Bernstein waves in tokamaks
Authors: Ram A. K. , Schultz S. D.
Published: July 2002

The conventional ordinary O-mode and the extraordinary X-mode in the electron cyclotron range of frequencies are not suitable for core heating in high-β spherical tokamak plasmas, like the National Spherical Torus Experiment [M. Ono, S. Kaye, M. Peng et al., in Proceedings of the 17th International Atomic Energy Agency Fusion Energy Conference (International Atomic Energy Agency, Vienna, 1999), Vol. 3, p. 1135], as they are weakly damped at high harmonics of the electron cyclotron frequency. However, electron Bernstein waves (EBW) can be effective for heating and driving currents in spherical tokamak plasmas. Power can be coupled to EBWs via mode conversion of either the X-mode or the O-mode. The two mode conversions are optimized in different regions of the parameter space spanned by the parallel wavelength and wave frequency. The conditions for optimized mode conversion to EBWs are evaluated analytically and numerically using a cold plasma model and an approximate kinetic model. From geometric optics ray tracing it is found that the EBWs damp strongly near the Doppler-broadened resonance at harmonics of the electron cyclotron frequency.
Geometry optimization of large biomolecules in redundant internal coordinates
Authors: Paizs Béla , Baker Jon , Suhai Sandor , Pulay Peter
Published: July 2002

We present an improved version of our recent algorithm [B. Paizs, G. Fogarasi, and P. Pulay, J. Chem. Phys. 109, 6571 (1998)] for optimizing the geometries of large molecules. The approximate Cholesky factorization technique has been generalized to the case of redundant coordinates, and an alternative approach involving use of the B†B matrix in the iterative coordinate back transformation is described. The generalized full Cholesky factors of B†B are very sparse and the corresponding force and geometry transformations are fast and numerically stable, permitting us to apply this technique for internal coordinate geometry optimization of molecules containing thousands of atoms. As an example we present optimization data on alpha-helical alanine polypeptides, and various globular proteins. Results for the alanine polypeptides indicates that internal coordinate optimization is clearly superior to the first-order Cartesian optimization techniques generally used in force field calculations. The largest system investigated is alpha-helical Ac-(Ala)999-NH2 containing 9999 atoms, which was successfully optimized using less than a megaword of memory. Optimization of various globular proteins shows that our procedure can easily deal with highly redundant (including full primitive) coordinate sets.
On the ultraviolet photofragmentation of hydrogen iodide
Authors: Alekseyev Aleksey B. , Liebermann Heinz-Peter , Kokh Daria B. , Buenker Robert J.
Published: July 2002

An ab initio configuration interaction (CI) study including spin-orbit coupling is carried out for the ground and low-lying excited states of the HI molecule by employing a relativistic effective core potential for the iodine atom. The computed spectroscopic constants for the X 1Σ+ ground and b 3ΠΩ Rydberg states are in good agreement with available experimental data, as are the vertical excitation energies for the repulsive a 3Π1, a 3Π0+, and A 1Π1 states of the A band. The a 3Π0+ state is found to possess a shallow minimum of 600 cm−1 depth outside the Franck–Condon region, at ≈5.1 a0. The electric-dipole moments have also been calculated for transitions from the ground to the A band states. Contrary to what is usually assumed, the a 3Π1, A 1Π1←X0+ transition moments are found to depend strongly on internuclear distance. Employing the computed potential energy and transition moment data, partial and total absorption spectra for the A band are calculated and the I* quantum yields, ΦI*(ν), are determined as a function of excitation energy. The maximal ΦI*(ν) values are calculated to be 0.55–0.59 and lie at 39 000–40 000 cm−1, which agrees well with experimental results. The influence of the t 3Σ1+ state and of the nonadiabatic effects on the ΦI*(ν) values is found to be negligible in the essential part of the A band. Finally, it is shown that significantly higher I* quantum yield values (up to 0.8–0.9) may be achieved when vibrationally hot HI molecules are excited in the appropriate spectral range.
Local probing of the giant magnetoresistance
Authors: Theeuwen S. J. C. H. , Caro J. , Radelaar S. , Canali L. , Kouwenhoven L. P. , Marrows C. H. , Hickey B. J.
Published: July 2002

We have contacted the tip of a scanning tunneling microscope (STM) to a Co/Cu magnetic multilayer to locally measure the giant magnetoresistance (GMR) of the multilayer. Apart from a point-contact GMR, the measured MR also reflects a magnetostriction effect in the STM. The resulting GMR ratios are typically 10%, with occasional ratios up to 60%. We attribute spot-to-spot variations of the ratio to differences in the local structure of the magnetic multilayer.
Inverse atomic densities and inequalities among density functionals
Authors: Angulo J. C. , Romera E. , Dehesa J. S.
Published: July 2002

Rigorous relationships among physically relevant quantities of atomic systems (e.g., kinetic, exchange, and electron–nucleus attraction energies, information entropy) are obtained and numerically analyzed. They are based on the properties of inverse functions associated to the one-particle density of the system. Some of the new inequalities are of great accuracy and/or improve similar ones previously known, and their validity extends to other many-fermion systems and to arbitrary dimensionality.
A direct-dynamics study of the zwitterion-to-neutral interconversion of glycine in aqueous solution
Authors: Fernández-Ramos Antonio , Smedarchina Zorka , Siebrand Willem , Zgierski Marek Z.
Published: July 2002

The mechanism of interconversion between the neutral and zwitterionic forms of glycine in aqueous solution is studied theoretically. It is argued that indirect transfer via a water bridge is a plausible alternative to the generally assumed direct transfer mechanism. The argument is based on model calculations in which the system glycine-water is represented by a 1:6 supermolecule embedded in a dielectric continuum. Optimized geometries and vibrational frequencies are obtained at the Hartree–Fock level with a 6-31G* basis set, and at the second-order Mo/ller–Plesset frozen-core level with the 6-31+G* basis set for the neutral and zwitterionic forms, and for their transition state. At both levels the energetics are corrected by single-point quadratic configuration interaction calculations, including single and double substitutions with frozen-core inner-shell orbitals. Both models reproduce the observed endothermicity of the transfer better than models that use only a limited number of discrete water molecules without a continuum and models solely based on the continuum approximation. In the optimized structures of this complex and of complexes with fewer water molecules, one of the water molecules always bridges the two functional groups. In the 1:6 complex, two of the other water molecules form hydrogen bonds with the amino hydrogens, two others with the carboxyl oxygens, and the sixth water molecule forms a bridge between the two water molecules attached to the amino group. The interaction of this supermolecule with the bulk solvent is treated by means of the Onsager model. The transition state calculated with the two models implies that the mechanism of interconversion is concerted transfer of two protons along the amino–water–carboxyl bridge. The dynamics calculations are performed with a multidimensional instanton model that includes solvent reorganization. For both models the calculated transfer rate constants are about an order of magnitude larger than the observed rate constants, indicating that the indirect mechanism can easily account for the observed dynamics. These results confirm the plausibility of the indirect mechanism of proton transfer via a water bridge in aqueous solutions of glycine.
Small angle neutron scattering measurements of nanoscale lithographic features
Authors: Wu Wen-li , Lin Eric K. , Lin Qinghuang , Angelopolous Marie
Published: July 2002

State-of-the-art lithographic techniques are able to fabricate structures for the semiconductor and other nanofabrication industries with dimensions below 150 nm. The relentless drive to further miniaturize semiconductor devices has placed increasingly stringent demands on current microscopy-based techniques for precisely measuring the size and the quality (line-edge roughness) of lithographically produced features. Using newly developed neutron optics, we demonstrate the first application of small-angle neutron scattering to nondestructively and quantitatively measure both the dimension and the quality of 150 nm lines fabricated on single crystal silicon wafers.
Design of multipolar plasmon excitations in silver nanoparticles
Authors: Krenn J. R. , Schider G. , Rechberger W. , Lamprecht B. , Leitner A. , Aussenegg F. R. , Weeber J. C.
Published: July 2002

We report on the experimental observation of multipolar plasmon excitations in lithographically designed elongated silver particles. In contrast to spheres, where the extinction bands of the respective multipolar plasmons overlap considerably to form a broad spectrum, spectrally well-separated extinction bands corresponding to plasmons of multipolar order up to n=6 are found. The results agree well with numerical simulations based on the Green’s Dyadic method.
Electric properties of urea and thiourea
Authors: Pluta Tadeusz , Sadlej Andrzej J.
Published: July 2002

The major linear and nonlinear electric properties of urea and thiourea have been calculated at different levels of approximation with respect to the treatment of the electron correlation contribution. The basis set saturation effects were studied by comparing the results computed in three different basis sets of increasing flexibility. The so-called (first-order) polarized (Pol) basis have been found to give excellent results for dipole moments and dipole polarizabilities. Their performance in calculations of the first hyperpolarizability is also quite good. However, calculations of the second hyperpolarizability require the use of recently developed second-order polarized (HyPol) basis sets. The frequency-dependent dynamic polarizabilities and first and second hyperpolarizabilities of urea and thiourea have been calculated in the time-dependent Hartree–Fock approximation and then scaled to account for the electron correlation contribution and the basis set extension effect. This method gives useful estimates of the magnitude of different nonlinear optical processes. In particular thiourea was found to have attractively high value of the third-order optical susceptibility. The replacement of the C=O group of urea by its sulfur counterpart brings in general quite a large increase of the efficiency of the third-order optical processes.
Effects of external electric field on the interfacial properties of weakly dipolar fluid
Authors: Warshavsky V. B. , Bykov T. V. , Zeng X. C.
Published: July 2002

In the framework of modified mean-field density-functional theory, effects of a uniform electric field on the interfacial properties of a model dipolar fluid [Teixeira and Telo da Gama, J. Phys.: Condens. Matter 3, 111 (1991)] are studied. Both density and orientational order-parameter profiles of the planar vapor-liquid interface are obtained as a function of the field strength. For the dipolar fluids with reduced dipole moment μ0*⩽1, we find that the field (under the condition μ0*E*/T*≪1) can shift the surface tension by few percent. We also find that the electric field actually reduces the thermodynamical surface tension but enhances the mechanical surface tension at the equimolar dividing surface. To detect these field effects on the surface tension we estimate the field strength which can be as high as 108 V/m.