bluuebunny/crossref_metadata_2025 huggingface dataset:
Amplification of ultrashort laser pulses in the photolytically driven XeF(C–A) active medium
Authors: Tcheremiskine V. I. , Sentis M. L. , Mikheev L. D.
Published: July 2002

Amplification of ultrashort laser pulses is demonstrated in a gas medium of the photolytical XeF(C–A) laser. A gas mixture of XeF2:N2:Ar was excited by vacuum-ultraviolet radiation produced by a high-current multichannel sliding discharge, which was initiated along one side of dielectric laser cavity. A small-signal gain of 1.6×10−3 cm−1 is observed for a seed pulse at 488 nm with ∼150 fs duration. The use of a second optical source together with the device optimization promise a multiple increase in the gain and the obtaining of high-contrast TW femtosecond pulses as a result of direct amplification, without application of a stretching/compression technique.
The persistence of eigenstates in open quantum dots
Authors: Akis R. , Bird J. P. , Ferry D. K.
Published: July 2002

We show that transport in open quantum dots can be mediated by single eigenstates, even when the dot leads support several propagating modes. The broadening of these few robust states, whose wave functions are generally localized within the interior of the dot, is found to be virtually independent of the lead width. Our results therefore indicate that a proper discussion of the specific nature of the individual eigenstates of the closed system is critical to determining their influence on transport through open dots.
Infrared spectroscopy to probe structure and growth dynamics in Fe+-(CO2)n clusters
Authors: Gregoire G. , Duncan M. A.
Published: July 2002

Fe + -( CO 2 ) n ion-molecule complexes are produced by laser vaporization in a pulsed-nozzle cluster source. These species are analyzed and mass-selected using a specially designed reflectron time-of-flight mass spectrometer. Infrared photodissociation of these complexes is investigated with an optical parametric oscillator/amplifier system using wavelengths near the CO2 asymmetric stretch vibration (2349 cm−1). Dissociation occurs by successive elimination of CO2 molecules. Tunable laser experiments obtain infrared resonance-enhanced photodissociation spectra for these complexes. Small complexes have CO2 asymmetric stretch resonances shifted to higher frequency than the free CO2 mode. The blueshift decreases initially with cluster size, but becomes nearly constant after the n=4 cluster. Argon-tagged complexes, e.g., Fe+-(CO2)n⋅Arm, photodissociate via the same CO2 resonances by elimination of argon. Except for the n=1 complex, bands for the tagged complexes occur at the same frequency as those for the corresponding CO2 complex without argon. Larger complexes exhibit additional resonances near the free CO2 asymmetric stretch indicating “surface” molecules not attached to the metal. Blueshifted resonances also persist in these complexes attributed to “core” ligands attached to the metal ion. In the largest clusters studied (n=9–14), additional resonances with an intermediate blueshift are measured associated with “caged” CO2 molecules not attached to the metal. These measurements demonstrate that infrared photodissociation spectroscopy has exciting potential to study clustering structures and dynamics around metal ions.
Piezo-, pyro-, ferroelectric, and dielectric properties of (Pb0.88Sm0.08)(Ti1−xMnx)O3/polyetherketoneketone 50/50 vol % ceramic/polymer composites
Authors: Barranco A. Peláiz , Martı́nez O. Pérez , Das-Gupta D. K.
Published: July 2002

( Pb 0.88 Sm 0.08 )( Ti 1−x Mn x ) O 3 / polyetherketoneketone 50/50 vol % ceramic/polymer composite films were studied with regard to their piezo-, pyro-, ferroelectric, and dielectric properties. The dielectric relaxation reflects the interaction between the constituent phases. Appropriate values of the dielectric permittivity, remanent polarization, and piezoelectric coefficients were obtained in the same way as those of other ceramic/polymer composite films. However, the coercive field, the pyroelectric coefficient, the pyroelectric figure of merit, and the electromechanical coupling factor were lower and the mechanical losses were higher. These results are discussed taking into account the incorporation of Sm3+ and Mn4+ into the perovskite structure in the ceramic phase.
Epitaxial growth of dielectric CaCu3Ti4O12 thin films on (001) LaAlO3 by pulsed laser deposition
Authors: Lin Y. , Chen Y. B. , Garret T. , Liu S. W. , Chen C. L. , Chen L. , Bontchev R. P. , Jacobson A. , Jiang J. C. , Meletis E. I. , Horwitz J. , Wu H.-D.
Published: July 2002

High dielectric CaCu3Ti4O12 (CCTO) thin films were epitaxially grown on (001) LaAlO3 (LAO) substrates by pulsed laser deposition. Microstructural studies by x-ray diffraction, pole figure measurements, and transmission electron microscopy show that the as-grown films are good single crystalline quality with an interface relationship of (001)CCTO//(001)LAO and [100]CCTO//[100]LAO. Dielectric property measurements show that the films have an extremely high dielectric constant with value of 10 000 at 1 MHz at room temperature. It is interesting to note that the twinned substrate results in the formation of twinning or dislocations inside the CCTO film.
Influence of the torsion stress on the circumferential magnetization curves of CoFeBSi amorphous wires
Authors: Betancourt I. , Valenzuela R.
Published: July 2002

Circumferential magnetization of as-cast amorphous CoFeBSi wires was calculated in terms of the real component of the complex inductance and the ac applied field. Experimental plots, measured at low frequency (f=1 kHz) and under torsion strain, showed a maximum in magnetization values for a wire clockwise rotation of ∼120°, in contrast with the decreasing trend for counterclockwise rotations. In addition, the wall propagation field also exhibited a clear minimum for ∼120°, in agreement with the magnetization maximum. These results are interpreted on the basis of an helical anisotropy induced during the preparation of the samples.
A Monte Carlo simulation for the stripping of the √3×√3 R30° alkanethiol lattice from Au(111) terraces and steps
Authors: Martin H. , Vericat C. , Andreasen G. , Vela M. E. , Salvarezza R. C.
Published: July 2002

Monte Carlo simulations of a model for the stripping of √3×√3 R30° alkanethiol lattices from terraces and steps of a (111) metal face in aqueous solutions are presented. In the model the stripping probability of an adsorbed alkanethiolate molecule depends on the applied potential, on intermolecular forces that stabilize the alkanethiol layer, and on the presence of substrate defects. Stabilizing intermolecular forces are also responsible for alkanethiolate aggregate formation during the stripping process. Snapshots and voltammograms derived from the model reproduce experimental STM images and electrochemical data for alkanethiol stripping from the Au(111) surface.
Adsorption behavior and current–voltage characteristics of CdSe nanocrystals on hydrogen-passivated silicon
Authors: Walzer K. , Quaade U. J. , Ginger D. S. , Greenham N. C. , Stokbro K.
Published: July 2002

Using scanning tunneling microscopy and spectroscopy we have studied both the geometric distribution and the conduction properties of organic shell capped CdSe nanocrystals adsorbed on hydrogen-passivated Si(100). At submonolayer concentrations, the nanocrystal distribution on the surface was found to be highly nonhomogeneous, with an aggregation of most of the nanocrystals into islands of monolayer thickness. I–V spectra collected on nanocrystals adsorbed on n- and p-type substrates showed a strong difference in the conduction behavior, caused by the substrate: CdSe nanocrystals on n-Si:H caused a widening of the surface band gap by 1 eV with respect to the gap of the substrate, while a significant narrowing of the gap was observed for nanocrystals on p-Si:H. This experimental result could be explained by modeling the system as a metal–insulator–semiconductor (MIS) diode. Using this model we have found that the current through the MIS junction is limited by the nanocrystals only in one bias direction, while in the other bias direction the current is limited by the semiconducting substrate. This property may be of relevance for the construction of hybrid electronic devices combining semiconductor electrodes with nanoscale elements such as nanocrystals or organic molecules.
Excitation of oscillations of the magnetic induction in a Nb–Ti slab as a result of a thermomagnetic flux avalanche
Authors: Chabanenko V. V. , Rusakov V. F. , Yampol’skiı̆ V. A. , Szymczak H. , Piechota S. , Vasiliev S. , Nabialek A.
Published: July 2002

Weakly damped oscillations of the magnetic induction with a frequency of ∼103 Hz are observed in a hard superconductor. The excitation of oscillations in the mixed state of a Nb–Ti slab is the result of giant magnetic flux avalanches arising in the development of a thermomagnetic instability. The existence time of the oscillatory phenomena is tens of times greater than the duration of the avalanche. These oscillations contain information about the vortex matter of a superconductor.
Features of the magnetization of an antiferromagnet with single-ion anisotropy of the easy-plane type and with ion spins S=1
Authors: Kalita V. M. , Ivanova I. M. , Loktev V. M.
Published: July 2002

A theory is proposed for the two-step phase transition from the singlet to a ferromagnetic state in systems of the ABX3 type. It is shown that the transition is due to a substantial rearrangement of the single-ion spectrum. The critical fields and magnetic susceptibility are calculated.
Optical properties of nitrogenated tetrahedral amorphous carbon films
Authors: Zhang X. W. , Cheung W. Y. , Ke N. , Wong S. P.
Published: July 2002

The chemical composition, structural, and optical properties of nitrogenated tetrahedral amorphous carbon (ta-C:N) films deposited by a pulsed filtered vacuum arc deposition system were characterized by non-Rutherford backscattering spectroscopy, Raman spectroscopy, and ultraviolet-visible spectroscopy. It was observed that the amount of nitrogen atoms incorporated into the ta-C:N films, as well as the sp2 fraction of the films, increased with increasing nitrogen pressure PN during deposition. As a result, the optical band gap of the ta-C:N films also decreased with increasing PN. At a fixed nitrogen partial pressure of 4×10−3 Pa, the nitrogen content was found to first increase with increasing substrate negative bias voltage (−Us), up to a maximum of about 14.5 at. % at −Us of 100 and 150 V, then decreases with further increase of −Us. The sp2 fraction however increased monotonically with increasing −Us. The optical band gap of the ta-C:N films initially increased with increasing −Us, up to a maximum at a certain −Us, and then decreased with further increase in −Us. The variation of the optical band gap with the negative substrate bias voltage was discussed in terms of the different sp2-bonded carbon configurations existing in the films and the graphitization of the ta-C:N films, as indicated by the Raman and density measurement results.
Dielectric response and structural properties of TiO2-doped Ba0.6Sr0.4TiO3 films
Authors: Jia Q. X. , Park B. H. , Gibbons B. J. , Huang J. Y. , Lu P.
Published: July 2002

Dielectric Ba0.6Sr0.4TiO3 films doped with different weight percentages of TiO2 were deposited by pulsed-laser deposition. The dielectric constant, dielectric loss, and the dielectric tunability of the films were found to be a strong function of the weight ratio of TiO2/Ba0.6Sr0.4TiO3. Compared to the pure Ba0.6Sr0.4TiO3, the TiO2-doped Ba0.6Sr0.4TiO3 exhibits lower dielectric loss while maintaining a significantly adjustable dielectric constant and desirable capacitance tunability. The change of the dielectric properties of TiO2-doped Ba0.6Sr0.4TiO3 films is closely related to the change in microstructure of the films as revealed by transmission electron microscopy.
Femtosecond dynamics of exciton bleaching in bulk GaN at room temperature
Authors: Huang Yin-Chieh , Chern Gia-Wei , Lin Kung-Hsuan , Liang Jian-Chin , Sun Chi-Kuang , Hsu Chia-Chen , Keller Stacia , DenBaars Steven P.
Published: July 2002

Femtosecond transient transmission pump–probe technique was used to investigate exciton dynamics in a nominally undoped GaN thin film at room temperature. An exciton ionization time of 100–250 femtoseconds was observed by the time-resolved pump–probe measurement. A comparison experiment with pre-excited free carriers also confirmed the observation of the exciton ionization process in bulk GaN.
Mechanism of the room-temperature persistent spectral hole burning in borate glasses doped with Eu3+
Authors: Chung Woon Jin , Heo Jong
Published: July 2002

Spectroscopic properties and room-temperature persistent spectral hole burning mechanisms of Eu3+-doped borate glasses were investigated. The depth of the burnt hole increased with the amount of carbon powders (i.e., degree of reducing atmosphere). This was attributed to the formation of defects which can donate free electrons for the photoreduction of Eu3+→Eu2+. Holes survived >104 s at 20 K and approximately 40% of hole areas were preserved after annealing at 280 K. The photoreduction of Eu3+ to Eu2+ is a primary mechanism of hole burning and this hypothesis was supported by spectral hole properties, relaxation properties as well as changes in the local structure of Eu3+ in glasses.
Dynamic tuning of optical waveguides with electrowetting pumps and recirculating fluid channels
Authors: Mach P. , Krupenkin T. , Yang S. , Rogers J. A.
Published: July 2002

We have developed an approach for using electrowetting actuation in recirculating fluidic channels to achieve dynamic tuning of optical fiber structures. The electrically controlled and fully reversible motion of the fluids and lubricants in these channels alters the refractive index profile experienced by the optical waveguide modes of the fiber. When combined with in-fiber gratings and etched fibers, this fluidic system yields dynamically adjustable narrow and broadband fiber filters, respectively. The nonmechanical operation of these systems, their ability to support switching speeds on the order of milliseconds, and their excellent optical characteristics indicate a promising potential for electrowetting-actuated fluidic tuning in optical fiber devices and other photonic components.
Low-temperature transverse dielectric and pyroelectric anomalies of uniaxial tungsten bronze crystals
Authors: Ko Jae-Hyeon , Kojima Seiji , Lushnikov S. G. , Katiyar R. S. , Kim Tae-Hoon , Ro Ji-Hyun
Published: July 2002

Low-temperature dielectric and pyroelectric properties were investigated for filled and unfilled tungsten bronze (TB) crystals, i.e., (Sr0.61Ba0.39)5Nb10O30, (K0.5Na0.5)1.0(Sr0.75Ba0.25)4.5Nb10O30, K5.80Li3.82Nb10.12O30, and K5.20Li2.34Nb10.88O30 with the spontaneous polarizations along the c axis at room temperature except for the last one. In all samples, transverse dielectric constant along the a axis showed a universal dielectric dispersion in the low-temperature range resembling a diffuse phase transition. Characteristic relaxation times obtained from the dielectric loss peaks followed the Arrhenius law, signifying thermal activation processes. The distribution of relaxation times seems to be caused by the distribution of activation energy, which has been successfully obtained by scaling the temperature- and frequency-dependent dielectric losses. This broadening may be attributed to the intrinsic randomness due to the charge disorder and quenched random fields caused by the unfilled structure and the off-stoichiometry of the TB compounds. Observed low-temperature dynamics were isotropic in the ab plane, and were insensitive to the poling conditions, degree of disorder, and high-temperature phase transitions. Structural phase transition accompanying a polarization tilt from the c axis toward the [110] axis, which was suggested to be the origin of the low-temperature anomalies, was not confirmed in our study. Some dynamics which are still active in the ferroelectric phase, like the concerted rotations of the oxygen octahedra, have been suggested as one of the possible origins of the observed phenomena.
Transmission properties of composite metamaterials in free space
Authors: Bayindir Mehmet , Aydin K. , Ozbay E. , Markoš P. , Soukoulis C. M.
Published: July 2002

We propose and demonstrate a type of composite metamaterial which is constructed by combining thin copper wires and split ring resonators (SRRs) on the same board. The transmission measurements performed in free space exhibit a passband within the stop bands of SRRs and thin wire structures. The experimental results are in good agreement with the predictions of the transfer matrix method simulations.
Correlation between interface traps and gate oxide leakage current in the direct tunneling regime
Authors: Loh Wei Yip , Cho Byung Jin , Li Ming Fu
Published: July 2002

Our experiment shows that when the gate oxide thickness is scaled to direct tunneling regime, the gate leakage current, and the number of interface traps increase in a discrete manner rather than in a gradual increment. A direct correlation between the increments of the gate leakage current and interface traps, irrespective of stressing polarity, is also observed. The discrete increase in gate current is due to degradation at localized spots rather than a uniform degradation over the entire gate area. The increment is also observed over a wide voltage range unlike interface-trap-assisted tunneling previously reported which occurs mainly near the flat-band voltage. A possible mechanism is proposed based on the observations.
Triode field emitter with a gated planar carbon-nanoparticle cathode
Authors: Park Kyung Ho , Seo Woo Jong , Lee Soonil , Koh Ken Ha
Published: July 2002

We fabricated a triode field emitter with a normal gate structure and a planar cathode of carbon nanoparticles (CNPs), which consisted of good quality graphitic sheets encapsulating metal (carbide) cores. For the quantitative analysis of the emission from the CNP triode emitter, we carried out a two-dimensional numerical calculation of electrostatic potential using the finite element method. As it turned out, a radial variation of electric field was very important to account for the emission from a planar emitting layer. By assuming the work function of 5 eV for CNPs, a set of consistent Fowler–Nordheim parameters, together with the radial position of emitting sites, were determined.
Shallow states at SiO2/4H-SiC interface on (112̄0) and (0001) faces
Authors: Yano Hiroshi , Kimoto Tsunenobu , Matsunami Hiroyuki
Published: July 2002

Shallow interface states at SiO2/4H-SiC were examined on (112̄0) and (0001) faces using metal–oxide–semiconductor (MOS) capacitors. The MOS capacitors were fabricated by wet oxidation on both faces to investigate the difference in the energy distribution of interface state density. The parallel conductance as a function of frequency was measured at room temperature, and high-frequency capacitance (C)–voltage (V) curves were measured both at room temperature and 100 K. By the conductance method, the interface state density on (112̄0) was revealed smaller than on (0001) at shallow energies, while at deeper energies the relation changes to opposite situation. High-frequency C–V curves at 100 K show a large positive flatband voltage shift and a large injection-type hysteresis on (0001) samples, while those were small on (112̄0), indicating another evidence of smaller interface state density near the conduction band edge on (112̄0).
Growth of single-wall carbon nanotubes within an ordered array of nanosize silica spheres
Authors: Lan A. , Iqbal Z. , Aitouchen A. , Libera M. , Grebel H.
Published: July 2002

Single-wall carbon nanotubes (SWCNTs), which were well-separated individuals or, small bundles of two to five tubes, were produced in a controllable manner by disproportionation of CO at 700 °C and 1 atm in the voids of an ordered array of silica spheres with embedded bimetallic Mo/Co catalytic particles. A combination of transmission electron microscopy, electron energy-loss spectroscopy, and Raman scattering excited with an Ar laser at λ=514.5 nm (2.41 eV), was used to characterize the nanotubes. The corresponding Raman spectra strongly indicate that these nanotubes are primarily chiral. This method offers a valid route for a high-quality single-step production of SWCNT composites for mechanical, electronic, and optoelectronic applications.
Controlled ice nucleation in microsized water droplet
Authors: Ning Du , Liu X. Y.
Published: July 2002

Controlled ice nucleation in a microsized water droplet was examined using a microdroplet suspension method. This method allows us to minimize the effect of the container wall and foreign particles. The influence of the water droplet volume on microdrop nucleation is examined quantitatively. It is found that at the microscale, the volume effect exerts a large influence on nucleation kinetics. This technique enabled us to quantify the effect of lysozyme on ice nucleation kinetics. It turns out that lysozyme promotes ice nucleation at relatively high supercoolings by decreasing the interfacial free energy between the nucleating phase and foreign particles. This quantitative measurement of the interfacial effect of additives (impurities) on ice nucleation kinetics will exert a significant impact on the research on the mechanism of antifreeze and nucleation promotion.
Phase-sensitive second-harmonic microscopy reveals polarity of topologically centrosymmetric molecular crystals
Authors: Kluge S. , Budde F. , Dohnke I. , Rechsteiner P. , Hulliger J.
Published: July 2002

Phase-sensitive second-harmonic microscopy is applied to reveal grow-in polarity in topologically centrosymmetric crystals of 4-chloro-4′-nitrostilben. As predicted by the Markow model of layer-by-layer polarity formation, growth along + and −b-direction in P21/c is producing optical nonlinearity in both sectors associated with the b axis. Present experiments show that formation of a pyroelectric symmetry class is a stochastic property of molecular crystals grown from dipolar compounds and near to thermodynamic equilibrium.
Structural characterization of nonpolar (112̄0) a-plane GaN thin films grown on (11̄02) r-plane sapphire
Authors: Craven M. D. , Lim S. H. , Wu F. , Speck J. S. , DenBaars S. P.
Published: July 2002

In this letter we describe the structural characteristics of nonpolar (112̄0) a-plane GaN thin films grown on (11̄02) r-plane sapphire substrates via metalorganic chemical vapor deposition. Planar growth surfaces have been achieved and the potential for device-quality layers realized by depositing a low temperature nucleation layer prior to high temperature epitaxial growth. The in-plane orientation of the GaN with respect to the r-plane sapphire substrate was confirmed to be [0001]GaN‖[1̄101]sapphire and [1̄100]GaN‖[112̄0]sapphire. This relationship is explicitly defined since the polarity of the a-GaN films was determined using convergent beam electron diffraction. Threading dislocations and stacking faults, observed in plan-view and cross-sectional transmission electron microscope images, dominated the a-GaN microstructure with densities of 2.6×1010 cm−2 and 3.8×105 cm−1, respectively. Submicron pits and crystallographic terraces were observed on the optically specular a-GaN surface with atomic force microscopy.
Observation of Coulomb staircases of both tunneling current and displacement current in nanomechanical double barrier tunneling structures
Authors: Nagano Kouhei , Okuda Atsushi , Majima Yutaka
Published: July 2002

Displacement current staircase due to Coulomb blockade is observed together with a tunneling current staircase in nanomechanical double barrier tunneling structures that consist of scanning vibrating probe/colloidal Au particles/vacuum/PtPd substrate. We discuss the motion of single electrons on and through the Au particles from both the tunneling and displacement current staircases, and demonstrate the electron shuttle phenomenon due to nanomechanical probe vibration.
Quantitative analysis of the polarization fields and absorption changes in InGaN/GaN quantum wells with electroabsorption spectroscopy
Authors: Renner F. , Kiesel P. , Döhler G. H. , Kneissl M. , Van de Walle C. G. , Johnson N. M.
Published: July 2002

Electroabsorption measurements are reported for wurtzite InGaN/GaN quantum wells. The electroabsorption technique allows exact quantitative analysis of absorption and absorption changes in InGaN quantum wells and barrier layers, with recorded field-induced absorption changes as large as 7000 cm−1 below and almost 20000 cm−1 above the band edge. The technique thus allows precise determination of the strong internal fields that originate from strain-induced polarization and differences in spontaneous polarization. The fields measured on functioning diodes vary between 1.1 and 1.4 MV/cm for indium concentrations in InGaN quantum wells ranging from 7% to 9%.
Influence of exciton localization on the quantum efficiency of GaN/(In,Ga)N multiple quantum wells grown by molecular-beam epitaxy
Authors: Dhar S. , Jahn U. , Brandt O. , Waltereit P. , Ploog K. H.
Published: July 2002

Using cathodoluminescence spectroscopy, we investigate the dependence of the transition energy and quantum efficiency of a GaN/(In,Ga)N multiple quantum-well structure on both the temperature and excitation density. A coupled rate-equation model is introduced to explain the experimental results. Polarization field screening has been incorporated in a realistic manner by solving these coupled rate equations self-consistently along with the Schrödinger and Poisson equations. Our study suggests that exciton localization increases the internal quantum efficiency significantly.
Band anticrossing in dilute InNxSb1−x
Authors: Murdin B. N. , Adams A. R. , Murzyn P. , Pidgeon C. R. , Bradley I. V. , Wells J.-P. R. , Matsuda Y. H. , Miura N. , Burke T. , Johnson A. D.
Published: July 2002

Dilute nitrogen alloys of InSb exhibit extremely strong band gap bowing with nitrogen composition that has been associated with anticrossing between the localized resonant states of the nitrogen within the conduction band and the extended states of the conduction band itself. This also results in the conduction band dispersion having an enhanced nonparabolicity. We have measured the electron effective mass near the anticrossing by cyclotron resonance in InNxSb1−x alloys with absorption edge near 15 μm, using pulsed fields up to 150 T. The results directly demonstrate the band anticrossing and quantitatively confirm the increase of effective mass versus x predicted for InNxSb1−x by a tight binding calculation for low nitrogen concentration (x<0.01).
Long-lasting phosphorescence in Sn2+–Cu2+ codoped silicate glass and its high-pressure treatment effect
Authors: Qiu Jianbei , Miyauchi Koichi , Kawamoto Yoji , Kitamura Naoyuki , Qiu Jianrong , Hirao Kazuyuki
Published: July 2002

Long-lasting phosphorescence was observed at 510 nm in a Sn2+–Cu2+ codoped Na2O–CaO–SiO2 glass at room temperature under UV illumination of 254 nm. When the glass was compressed under 3, 6, and 9 GPa, the phosphorescence shifted to 465 nm and its decay rate became shorter. The optical absorption spectra of the samples changed after compression, showing that the cupric ions were reduced to the cuprous ions. The high-pressure treatment also resulted in a lower-energy shift in the absorption edge. It was suggested that Sn2+ ions act as hole trapping centers, while oxygen vacancies surrounding by Ca2+ ions as well as active sites in the glass matrix, i.e., as electron trapping centers.
Exciton spectra of an AlN epitaxial film on (0001) sapphire substrate grown by low-pressure metalorganic vapor phase epitaxy
Authors: Onuma T. , Chichibu S. F. , Sota T. , Asai K. , Sumiya S. , Shibata T. , Tanaka M.
Published: July 2002

Exciton resonance energies in an AlN epilayer on (0001) sapphire substrate grown by low-pressure metalorganic vapor phase epitaxy were determined as a function of temperature by means of optical reflectance (OR) and cathodoluminescence measurements. The OR spectra exhibited distinct reflectance anomalies at the photon energies just above the multiple internal reflection fringes, and the spectral line shape was fitted considering A (Γ7vu→Γ7c) and BC (Γ9v,Γ7vl→Γ7c) exciton transitions. The fitting gave the values of them at 0 K to be 6.211 and 6.266 eV, giving the crystal- field splitting (Δcr) of approximately 55 meV. The AlN film exhibited an excitonic emission even at 300 K, which is due to the small Bohr radius of excitons and large longitudinal optical phonon energies. The Einstein characteristic temperature ΘE was estimated to be 580 K.
Study of domain stability on (Pb0.76Ca0.24)TiO3 thin films using piezoresponse microscopy
Authors: Guo H. Y. , Xu J. B. , Wilson I. H. , Xie Z. , Luo E. Z. , Hong Seungbum , Yan H.
Published: July 2002

We introduce a method to investigate the domain stability issue based on piezoresponse microscopy. The switching and back-switching processes of sol–gel derived (Pb0.76Ca0.24)TiO3 thin films were investigated by comparing the local piezoelectric hysteresis loops in continuous mode and pulse mode. It was found that the local hysteresis loops acquired by the pulse mode in shorter pulse width were dominated by the back-switching process while the continuous mode piezoelectric hysteresis loop provides information on the domain switching process.
Zn-induced features at the GaAs(110) surface and its importance in the growth of ZnSe on GaAs(110)
Authors: Miotto R. , Ferraz A. C.
Published: July 2002

A possible model for the ZnSe growth on GaAs(110) is proposed based on a first-principles pseudopotential method. Our calculations suggest that ZnSe growth on GaAs(110) could be understood in a two-step process: (i) Zn atoms will be adsorbed over Ga and As sites of the GaAs(110) surface, and (ii) the Zn atom over the Ga site will be replaced by a Se atom, followed by layer-by-layer ZnSe growth. We have also investigated Zn-induced features at the GaAs(110) surface, during the initial Zn interaction with the surface. Zn was found to adsorb preferentially at Ga substitutional sites at the subsurface layer and over Ga and As surface atoms. Theoretical STM images show the presence of bright features related to the Zn at Ga substitutional sites in the subsurface layers in agreement with recent experimental works.
Cobalt–oxide underlayers for cobalt–ferrite pinned spin valves
Authors: Maat S. , Carey M. J. , Fullerton Eric E. , Le T. X. , Rice P. M. , Gurney B. A.
Published: July 2002

We report on the advantages of employing insulating Co–oxide underlayers for spin valves utilizing Co–ferrite as a pinning layer. These underlayers provide for improved crystalline growth of Co-ferrite pinning layers, which can be reduced to as little as 3 nm in thickness while maintaining high coercivity and thermal stability. This allows a typical antiferromagnetically pinned spin valve to fit into a 50 nm gap, which is anticipated for recording densities >100 Gbit/in2. Magnetoresistance values ∼7%, excellent stability, and free layer properties are observed and pinned which is comparable to present PtMn based sensors of similar thickness.
Direct transformation of cubic diamond to hexagonal diamond
Authors: He Hongliang , Sekine T. , Kobayashi T.
Published: July 2002

For a long time, hexagonal diamond has been formed only by static and shock wave compression of well-crystallized graphites. Here, we demonstrate that cubic diamond loses its structure stability and transforms to hexagonal diamond in massive. This transformation has been completed in nanoseconds under a shock wave compression of cubic diamond, in which the shock pressure and temperature are only tens of giga pascal and hundreds of kelvin, thermodynamically being within the stability of cubic diamond. The formation of hexagonal diamond is interpreted as a direct transition (solid to solid) of cubic diamond by a kinetic mechanism due to the shear stress and enhanced temperature induced by the rapid shock wave compression.
Structural characterization of a porous low-dielectric-constant thin film with a non-uniform depth profile
Authors: Lin Eric K. , Lee Hae-Jeong , Lynn Gary W. , Wu Wen-li , O’Neill Mark L.
Published: July 2002

High-resolution x-ray reflectivity (XR) and small-angle neutron scattering (SANS) are applied to characterize both the nonuniform depth profile and pore structure of a low-dielectric-constant (low-k) thin film as prepared on a silicon substrate. The XR data show that the density depth profile has a multilayered structure with a dense, nonporous top layer and a less dense, porous bulk layer. A scattering invariant analysis of the SANS data is used to determine the average chord length of the pores, (14.8±2.0) nm, independent of the depth profile. Given the elemental composition of the film, the XR and SANS data are combined to calculate the mass density of the top layer, (1.13±0.05) g/cm3, the porosity of the less dense layer, (0.28±0.10), and the wall density, (0.92±0.15) g/cm3.
Initial growth of GaN on α-Al2O3(0001) by molecular beam epitaxy
Authors: Davidsson S. K. , Andersson T. G. , Zirath H.
Published: July 2002

The sapphire surface, used for growth of GaN by molecular beam epitaxy, was studied by reflection high-energy electron diffraction during the in situ pretreatment and initial growth. We investigated the effect of Ga cleaning, nitridation, and growth of the AlN nucleation layer on the lattice constant. One Ga-cleaning cycle restored the surface with a higher diffraction density. Nitridation changed the lattice constant instantaneously. The lattice constant was restored if the nitridation was switched off. For the AlN nucleation layer, the first monolayer was strained, followed by first a rapid and then a slow relaxation.
Spin-polarized light-emitting diode using metal/insulator/semiconductor structures
Authors: Manago T. , Akinaga H.
Published: July 2002

We have succeeded in growing ferromagnetic metals (Co, Fe, and NiFe)/Al2O3/AlGaAs heterostructures with homogeneous flat interfaces. The electroluminescence from a light-emitting diode with a metal/insulator/semiconductor (MIS) structure depends on the magnetization direction of the ferromagnetic electrode. This fact shows that a spin injection from the ferromagnetic metal to the semiconductor is achieved. The spin-injection efficiency is estimated to be the order of 1% at room temperature.
Plasticity of orientation preference maps in the visual cortex of adult cats
Authors: Godde Ben , Leonhardt Ralph , Cords Sven M. , Dinse Hubert R.
Published: July 2002

In contrast to the high degree of experience-dependent plasticity usually exhibited by cortical representational maps, a number of experiments performed in visual cortex suggest that the basic layout of orientation preference maps is only barely susceptible to activity-dependent modifications. In fact, most of what we know about activity-dependent plasticity in adults comes from experiments in somatosensory, auditory, or motor cortex. Applying a stimulation protocol that has been proven highly effective in other cortical areas, we demonstrate here that enforced synchronous cortical activity induces major changes of orientation preference maps (OPMs) in adult cats. Combining optical imaging of intrinsic signals and electrophysiological single-cell recordings, we show that a few hours of intracortical microstimulation (ICMS) lead to an enlargement of the cortical representational zone at the ICMS site and an extensive restructuring of the entire OPM layout up to several millimeters away, paralleled by dramatic changes of pinwheel numbers and locations. At the single-cell level, we found that the preferred orientation was shifted toward the orientation of the ICMS site over a region of up to 4 mm. Our results show that manipulating the synchronicity of cortical activity locally without invoking training, attention, or reinforcement, OPMs undergo large-scale reorganization reminiscent of plastic changes observed for nonvisual cortical maps. However, changes were much more widespread and enduring. Such large-scale restructuring of the visual cortical networks indicates a substantial capability for activity-dependent plasticity of adult visual cortex and may provide the basis for cognitive learning processes.
Truncated hemoglobin HbN protects
Mycobacterium bovis
from nitric oxide
Authors: Ouellet Hugues , Ouellet Yannick , Richard Christian , Labarre Marie , Wittenberg Beatrice , Wittenberg Jonathan , Guertin Michel
Published: July 2002

Mycobacterium tuberculosis
, the causative agent of human tuberculosis, and
Mycobacterium
bovis
each express two genes,
glbN
and
glbO
, encoding distantly related truncated hemoglobins (trHbs), trHbN and trHbO, respectively. Here we report that disruption of
M. bovis
bacillus Calmette–Guérin
glbN
caused a dramatic reduction in the NO-consuming activity of stationary phase cells, and that activity could be restored fully by complementing knockout cells with
glbN
. Aerobic respiration of knockout cells was inhibited markedly by NO in comparison to that of wild-type cells, indicating a protective function for trHbN. TyrB10, which is highly conserved in trHbs and interacts with the bound oxygen, was found essential for NO consumption. Titration of oxygenated trHbN (trHbN⋅O
2
) with NO resulted in stoichiometric oxidation of the protein with nitrate as the major product of the reaction. The second-order rate constant for the reaction between trHbN⋅O
2
and NO at 23°C was 745 μM
−1
⋅s
−1
, demonstrating that trHbN detoxifies NO 20-fold more rapidly than myoglobin. These results establish a role for a trHb and demonstrate an NO-metabolizing activity in
M. tuberculosis
or
M. bovis
. trHbN thus might play an important role in persistence of mycobacterial infection by virtue of trHbN′s ability to detoxify NO.
Genetic complementation in apicomplexan parasites
Authors: Striepen Boris , White Michael W. , Li Catherine , Guerini Michael N. , Malik S.-Banoo , Logsdon John M. , Liu Chang , Abrahamsen Mitchell S.
Published: July 2002

A robust forward genetic model for Apicomplexa could greatly enhance functional analysis of genes in these important protozoan pathogens. We have developed and successfully tested a genetic complementation strategy based on genomic insertion in
Toxoplasma gondii
. Adapting recombination cloning to genomic DNA, we show that complementing sequences can be shuttled between parasite genome and bacterial plasmid, providing an efficient tool for the recovery and functional assessment of candidate genes. We show complementation, gene cloning, and biological verification with a mutant parasite lacking hypoxanthine-xanthine-guanine phosphoribosyltransferase and a
T. gondii
cDNA library. We also explored the utility of this approach to clone genes based on function from other apicomplexan parasites using
Toxoplasma
as a surrogate. A heterologous library containing
Cryptosporidium parvum
genomic DNA was generated, and we identified a
C. parvum
gene coding for inosine 5-monophosphate-dehydrogenase (IMPDH). Interestingly, phylogenetic analysis demonstrates a clear eubacterial origin of this gene and strongly suggests its lateral transfer from ɛ-proteobacteria. The prokaryotic origin of this enzyme might make it a promising target for therapeutics directed against
Cryptosporidium
.
Kinesin–microtubule binding depends on both nucleotide state and loading direction
Authors: Uemura Sotaro , Kawaguchi Kenji , Yajima Junichiro , Edamatsu Masaki , Toyoshima Yoko Yano , Ishiwata Shin'ichi
Published: July 2002

Kinesin is a motor protein that transports organelles along a microtubule toward its plus end by using the energy of ATP hydrolysis. To clarify the nucleotide-dependent binding mode, we measured the unbinding force for one-headed kinesin heterodimers in addition to conventional two-headed kinesin homodimers under several nucleotide states. We found that both a weak and a strong binding state exist in each head of kinesin corresponding to a small and a large unbinding force, respectively; that is, weak for the ADP state and strong for the nucleotide-free and adenosine 5′-[β,γ-imido]triphosphate states. Model analysis showed that (
i
) the two binding modes in each head could be explained by a difference in the binding energy and (
ii
) the directional instability of binding, i.e., dependence of unbinding force on loading direction, could be explained by a difference in the characteristic distance for the kinesin–microtubule interaction during plus- and minus-end-directed loading. Both these factors must play an important role in the molecular mechanism of kinesin motility.
Decreased expression of the transcription factor NURR1 in dopamine neurons of cocaine abusers
Authors: Bannon Michael J. , Pruetz Barb , Manning-Bog Amy B. , Whitty Christopher J. , Michelhaugh Sharon K. , Sacchetti Paola , Granneman James G. , Mash Deborah C. , Schmidt Carl J.
Published: July 2002

Chronic exposure to cocaine induces long-term adaptations that are likely to involve changes in transcription factor expression. This possibility has not been examined in the cocaine-exposed human brain. The transcription factor nurr1 is highly expressed in rodent midbrain dopamine neurons and is essential for their proper phenotypic development. Here we show that human NURR1 gene expression is robust within control subjects and reduced markedly within the dopamine neurons of human cocaine abusers. NURR1 is known to regulate transcription of the gene encoding the cocaine-sensitive dopamine transporter (DAT). We show here that DAT gene expression also is reduced markedly in the dopamine neurons of NURR1-deficient cocaine abusers, suggesting that NURR1 plays a critical role
in vivo
in controlling human DAT gene expression and adaptation to repeated exposure to cocaine.
Folate deficiency disturbs hepatic methionine metabolism and promotes liver injury in the ethanol-fed micropig
Authors: Halsted Charles H. , Villanueva Jesus A. , Devlin Angela M. , Niemelä Onni , Parkkila Seppo , Garrow Timothy A. , Wallock Lynn M. , Shigenaga Mark K. , Melnyk Stepan , James S. Jill
Published: July 2002

Alcoholic liver disease is associated with abnormal hepatic methionine metabolism and folate deficiency. Because folate is integral to the methionine cycle, its deficiency could promote alcoholic liver disease by enhancing ethanol-induced perturbations of hepatic methionine metabolism and DNA damage. We grouped 24 juvenile micropigs to receive folate-sufficient (FS) or folate-depleted (FD) diets or the same diets containing 40% of energy as ethanol (FSE and FDE) for 14 wk, and the significance of differences among the groups was determined by ANOVA. Plasma homocysteine levels were increased in all experimental groups from 6 wk onward and were greatest in FDE. Ethanol feeding reduced liver methionine synthase activity,
S
-adenosylmethionine (SAM), and glutathione, and elevated plasma malondialdehyde (MDA) and alanine transaminase. Folate deficiency decreased liver folate levels and increased global DNA hypomethylation. Ethanol feeding and folate deficiency acted together to decrease the liver SAM/
S
-adenosylhomocysteine (SAH) ratio and to increase liver SAH, DNA strand breaks, urinary 8-oxo-2′-deoxyguanosine [oxo(8)dG]/mg of creatinine, plasma homocysteine, and aspartate transaminase by more than 8-fold. Liver SAM correlated positively with glutathione, which correlated negatively with plasma MDA and urinary oxo(8)dG. Liver SAM/SAH correlated negatively with DNA strand breaks, which correlated with urinary oxo(8)dG. Livers from ethanol-fed animals showed increased centrilobular CYP2E1 and protein adducts with acetaldehyde and MDA. Steatohepatitis occurred in five of six pigs in FDE but not in the other groups. In summary, folate deficiency enhances perturbations in hepatic methionine metabolism and DNA damage while promoting alcoholic liver injury.
Liquid–liquid separation in solutions of normal and sickle cell hemoglobin
Authors: Galkin Oleg , Chen Kai , Nagel Ronald L. , Hirsch Rhoda Elison , Vekilov Peter G.
Published: July 2002

We show that in solutions of human hemoglobin (Hb)—oxy- and deoxy-Hb A or S—of near-physiological pH, ionic strength, and Hb concentration, liquid–liquid phase separation occurs reversibly and reproducibly at temperatures between 35 and 40°C. In solutions of deoxy-HbS, we demonstrate that the dense liquid droplets facilitate the nucleation of HbS polymers, whose formation is the primary pathogenic event for sickle cell anemia. In view of recent results that shifts of the liquid–liquid separation phase boundary can be achieved by nontoxic additives at molar concentrations up to 30 times lower than the protein concentrations, these findings open new avenues for the inhibition of the HbS polymerization.
Adeno-associated virus type-2 expression of pigmented epithelium-derived factor or Kringles 1–3 of angiostatin reduce retinal neovascularization
Authors: Raisler Brian J. , Berns Kenneth I. , Grant Maria B. , Beliaev Denis , Hauswirth William W.
Published: July 2002

Neovascular diseases of the retina include age-related macular degeneration and diabetic retinopathy, and together they comprise the leading causes of adult-onset blindness in developed countries. Current surgical, pharmaceutical, and laser therapies for age-related macular degeneration (AMD) rarely result in improved vision, do not significantly prevent neovascularization (NV), and often result in at least some vision loss. To address this therapeutic gap, we determined the efficacy of recombinant adeno-associated viral (rAAV) serotype-2-mediated expression of pigment epithelium-derived factor (PEDF) or Kringle domains 1–3 of angiostatin (K1K3) in reducing aberrant vessel formation in a mouse model of ischemia-induced retinal NV. Both PEDF and K1K3 are potent inhibitors of NV when injected directly, hence expression of these therapeutic factors from rAAV may provide long-term protection from neovascular eye disease. rAAV vectors expressing the therapeutic gene were injected into one eye of postnatal day 0 (P0) newborn mouse pups. Retinal NV was induced in P7 mice by exposure to elevated oxygen for 5 days followed by room air for another five days. Retinal NV was quantified by the number of vascular-endothelial-cell nuclei above the inner-limiting membrane in P17 eyes. The number of such vascular endothelial cell nuclei in eyes treated with rAAV-PEDF or rAAV-K1K3 was significantly reduced (both
P
< 0.0000002) compared with control eyes. Ocular protein levels detected by ELISA correlate well with the reduction in NV and confirm that expression of antineovascular agents from rAAV vectors may be a therapeutically useful treatment of retinal or choroidal neovascular disease.
Isolation of receptor–ligand pairs by capture of long-lived multivalent interaction complexes
Authors: de Wildt Ruud M. T. , Tomlinson Ian M. , Ong Jennifer L. , Holliger Philipp
Published: July 2002

We have combined phage display and array screening for the rapid isolation of pairs of interacting polypeptides. Our strategy, named SAC (selection by avidity capture), is based on the avidity effect, the formation of highly stable complexes formed by multivalent interactions; in our case, between a receptor (multivalently displayed on phage) and a ligand (coexpressed as a multimeric fusion protein). Capture of the long-lived interaction complex allows the isolation of phage bearing cognate interaction pairs, as we demonstrate for a range of interactions, including Ab–antigen pairs and the rapamycin-dependent interaction of FKBP-12 and FRAP. Cognate phage are enriched by SAC up to 1000-fold and interacting pairs can be identified by array screening. Application of SAC to Ab–antigen interactions as a model system yielded over 140 specific Abs to a single antigen and 92 Abs to three different fetal human brain antigens in a single round of SAC each. Our results suggest that SAC should prove useful for the identification and study of receptor–ligand interactions in particular among extracellular proteins, as well as for the rapid generation of specific Abs to multiple antigens.