bluuebunny/crossref_metadata_2025 huggingface dataset:
Doping dependent NH3 sensing of indium oxide nanowires
Authors: Zhang Daihua , Li Chao , Liu Xiaolei , Han Song , Tang Tao , Zhou Chongwu
Published: August 2003

NH 3 gas sensing properties of In2O3 nanowires were carefully studied. Change of conductance in opposite directions was observed with different nanowire sensors. We suggest that this differential response is caused by various doping concentrations in the semiconducting In2O3 nanowires. In addition, we have also investigated a “gate-screening effect” exhibited in our nanowire chemical sensors at high NH3 concentrations, which is induced by adsorbed NH3 molecules working as charge traps. Both the doping-dependent response and the gate-screening effect will be especially valuable and helpful for understanding the detailed sensing mechanism of semiconducting metal oxide materials.
Can potentially useful dynamics to solve complex problems emerge from constrained chaos and/or chaotic itinerancy?
Authors: Nara Shigetoshi
Published: August 2003

Complex dynamics including chaos in systems with large but finite degrees of freedom are considered from the viewpoint that they would play important roles in complex functioning and controlling of biological systems including the brain, also in complex structure formations in nature. As an example of them, the computer experiments of complex dynamics occurring in a recurrent neural network model are shown. Instabilities, itinerancies, or localization in state space are investigated by means of numerical analysis, for instance by calculating correlation functions between neurons, basin visiting measures of chaotic dynamics, etc. As an example of functional experiments with use of such complex dynamics, we show the results of executing a memory search task which is set in a typical ill-posed context. We call such useful dynamics “constrained chaos,” which might be called “chaotic itinerancy” as well. These results indicate that constrained chaos could be potentially useful in complex functioning and controlling for systems with large but finite degrees of freedom typically observed in biological systems and may be such that working in a delicate balance between converging dynamics and diverging dynamics in high dimensional state space depending on given situation, environment and context to be controlled or to be processed.
Dancing “atoms” and “molecules” of luminous gas-discharge spots
Authors: Nasuno Satoru
Published: August 2003

Highly localized, dynamic particle-like excitations are observed in a dc-driven, quasi-twodimensional gas-discharge system. These localized excitations undergo a transition from isolated to aggregated state as the discharge current is increased. Although they provide us a macroscopic analogue of microscopic atoms and molecules, they are quite distinct from the latter in the point that they exhibit a rich variety of complex dynamics. The fact that these localized excitations can show synchronous dynamics even in distant places, together with recent theoretical studies, indicates that a global coupling plays an important role on the dynamics of such localized excitations.
Effect of hydrogen in dilute InNxSb1−x alloys grown by molecular beam epitaxy
Authors: Veal T. D. , Mahboob I. , McConville C. F. , Burke T. M. , Ashley T.
Published: August 2003

The electronic properties and nitrogen bonding configurations are investigated in dilute InNxSb1−x alloys grown by molecular beam epitaxy using a mixed nitrogen and hydrogen plasma. High-resolution electron-energy-loss spectroscopy is used to observe annealing-induced changes in the conduction band electron plasma frequency and plasmon lifetime. X-ray photoelectron spectroscopy of the N 1s core level indicates that a large proportion of the nitrogen in the InNxSb1−x alloy is contained within neutral N–H complexes. Annealing at 300 °C removes hydrogen from these complexes, increasing the concentration of isoelectronic nitrogen acceptors. This increases the ionized impurity scattering and reduces the background conduction electron density.
Indication of hysteresis in AlMnN
Authors: Frazier R. , Thaler G. , Overberg M. , Gila B. , Abernathy C. R. , Pearton S. J.
Published: August 2003

AlN films grown by gas-source molecular beam epitaxy were doped with different levels of Mn during growth. High resolution x-ray diffraction characterization revealed good crystallinity in single phase material, with lattice constant decreasing with increasing Mn concentration. Single phase AlMnN was found to be p type while AlMnN/AlMn mixed phase material was found to be highly conductive n type. Magnetization measurements performed with a superconducting quantum interference device magnetometer indicated ferromagnetism in single phase material persisting to 300 K and showed no evidence of room temperature magnetization in multiphase material. In particular, it was shown that Mn4N second phases are not contributing to the magnetization in the AlMnN under optimized growth conditions.
Itinerancy of money
Authors: Yasutomi Ayumu
Published: August 2003

Previously, I studied [Physica D 82, 180–194 (1995)] the emergence and collapse of money in a computer simulation model. In this paper I will revisit the same topic, building a model in the same line. I discuss this problem from the viewpoint of chaotic itinerancy. Money is the most popular system for evading the difficulty of exchange under division of labor. It emerges autonomously from exchanges among selfish agents which behave as automata. And such emergent money collapses autonomously. I describe money as a structure in economic space, explaining its autonomous emergence and collapse as two phases of the same phenomenon. The key element in this phenomenon is the switch of the meaning of strategies. This is caused by the drastic change of environment caused by the emergence of a structure. This dynamics shares some aspects with chaotic itinerancy.
Discontinuous tracks in relaxed Si0.5Ge0.5 alloy layers: A velocity effect
Authors: Gaiduk P. I. , Larsen A. Nylandsted , Hansen J. Lundsgaard , Trautmann C. , Toulemonde M.
Published: August 2003

Strain-relaxed crystalline Si0.5Ge0.5 layers were irradiated with U ions of various energies (0.8–2.64 GeV) but of approximately identical electronic stopping power. Transmission electron microscopy reveals tracks of different morphology depending on the velocity of the projectiles. For decreasing beam energy, individual dotted defect structures form aligned discontinuous tracks including a large number of dislocation loops. No indication for track amorphization is found.
Self-assembled NiSi quantum-dot arrays on epitaxial Si0.7Ge0.3 on (001)Si
Authors: Wu W. W. , He J. H. , Cheng S. L. , Lee S. W. , Chen L. J.
Published: August 2003

Self-assembled NiSi quantum-dot arrays have been grown on relaxed epitaxial Si0.7Ge0.3 on(001)Si. The formation of the one-dimensional ordered structure is attributed to the nucleation of NiSi nanodots on the surface undulations induced by step bunching on the surface of SiGe film owing to the miscut of the wafers from normal to the (001)Si direction. The two-dimensional pseudohexagonal structure was achieved under the influence of repulsive stress between nanodots. Since the periodicity of surface bunching can be tuned with appropriate vicinality and misfit, the undulated templates promise to facilitate the growth of ordered silicide quantum dots with selected periodicity and size.
Substrate dependent ultrafast dynamics in thin NiFe films
Authors: Bosco C. A. C. , Azevedo A. , Acioli L. H.
Published: August 2003

We have studied the ultrafast electronic response of thin NiFe films by femtosecond transient reflectivity measurements. The experiments were performed on films with varying thicknesses, substrates, and pump fluences. It has been observed that for high excitation densities the electron cooling time depends strongly on the nature of the underlying substrate and we attribute our results to transport of hot carriers out of the excited region. In particular, we have observed that for NiFe over NiO, carrier transport should be less important than for NiFe over Si.
Ordering oscillation of local composition in melt-processed REBa2Cu3O7−δ superconductors (RE: Sm, Eu, Gd)
Authors: Hu A. , Murakami M. , Zhou H.
Published: August 2003

High quality melt-processed GdBa2Cu3O7−δ, (Sm0.5Eu0.5)Ba2Cu3O7−δ and (Sm0.33Eu0.33Gd0.33)Ba2Cu3O7−δ single domains were grown in air and the microstructural origin of their distinguished superconducting performance was investigated by a transmission electron microscope equipped with energy-dispersive spectroscopy. A periodic array of the chemical composition with tens of micrometer correlation was unveiled due to the spatial oscillation of RE (RE: Sm, Eu, and Gd) and Ba around the stoichiometric ratio with a nanometer scale wavelength. For (Sm0.5Eu0.5)Ba2Cu3O7−δ, the temperature dependence of the reversible magnetic moment at certain fields showed two kinks. The first transition followed three-dimensional scaling and yielded dBc2/dT=1.98 T/K, corresponding to thermal fluctuation of the bulk superconductivity while the second displayed a linear temperature function with slightly lower Tc than bulk Tc and a slope of 0.6 T/K. This may indicate a field-induced pinning scenario with low Tc stripes.
Terahertz electroluminescence from boron-doped silicon devices
Authors: Adam T. N. , Troeger R. T. , Ray S. K. , Lv P.-C. , Kolodzey J.
Published: August 2003

Terahertz emission was observed from electrically pumped boron-doped p-type silicon structures at cryogenic temperatures. At a current of 1.5 A and temperature of 4.4 K, we achieved a pulsed peak power of 31 μW from a single mesa facet, integrated over three closely spaced spectral lines centered about 8.1 THz. The radiation was slightly transverse magnetically polarized with respect to the plane of the substrate and was still detectable at temperatures as high as 150 K. These findings suggest that moderate power THz sources can be fabricated without epitaxially grown quantum wells using techniques compatible with silicon integrated circuit technology.
Unidirectional lasing from InGaN multiple-quantum-well spiral-shaped micropillars
Authors: Chern G. D. , Tureci H. E. , Stone A. Douglas , Chang R. K. , Kneissl M. , Johnson N. M.
Published: August 2003

We report unidirectional emission from lasing in In0.09Ga0.91N/In0.01Ga0.99N multiple-quantum-well spiral micropillars. Our imaging technique shows that the maximum emission comes from the notch of the spiral microcavities at an angle about 40° from the normal of the notch. At room temperature, the spiral microcavity lases near 400 nm when optically pumped with 266 or 355 nm light. A reduction in the lasing threshold and an improvement in unidirectionality occurs when the microcavity is selectively pumped near its boundary.
In situ growth rate measurements and length control during chemical vapor deposition of vertically aligned multiwall carbon nanotubes
Authors: Geohegan D. B. , Puretzky A. A. , Ivanov I. N. , Jesse S. , Eres G. , Howe J. Y.
Published: August 2003

Time-resolved reflectivity is employed as an in situ diagnostic in thermal chemical vapor deposition of vertically aligned arrays of multiwall carbon nanotubes (VAA–MWNT). Fabry–Ṕerot interference fringes and attenuation of a reflected HeNe laser beam are used to measure the length of VAA–MWNT throughout the first 3–8 μm of growth yielding in situ measurements of growth rates and kinetics and the capability to observe the onset and termination of growth. VAA–MWNT growth is characterized between 565 and 750 °C on Si substrates with evaporated Al/Fe/Mo multilayer catalysts and acetylene feedstock. Nanotube lengths were controlled by rapid evacuation of the chamber at predetermined reflectivities, and it was demonstrated that growth can be restarted at later times. The extinction coefficients of the VAA–MWNT were studied and correlated with nanotube wall structure. Growth rates for VAA–MWNT are found to vary depending on the catalyst preparation, temperature, and time. Both the highest growth rates (0.3 μm/s) and the tallest VAA–MWNT (0.75 mm long) were achieved at 730 °C.
Voltage-variable microwave delay line using ferroelectric liquid crystal with aligned submicron polymer fibers
Authors: Fujikake Hideo , Kuki Takao , Kamoda Hirokazu , Sato Fumio , Nomoto Toshihiro
Published: August 2003

This letter describes a variable microwave delay line that uses a thick ferroelectric liquid crystal film stabilized by aligned polymer fibers of submicron diameter. The delay line consists of a microstrip transmission line with a 50-μm-thick liquid crystal film as the dielectric material. The conical axis of the helical alignment of the ferroelectric liquid crystal molecules can be anchored in the microwave propagation direction by aligned polymer fibers dispersed in the liquid crystal film without conventional alignment layers coated on substrates. The phase shift of microwaves passed through the device could be varied by applying a voltage across the film to loosen the liquid crystal helical alignment.
Controlling the transmission at the surface plasmon resonance of nanocomposite films using photonic structures
Authors: Suarez-Garcia A. , del Coso R. , Serna R. , Solis J. , Afonso C. N.
Published: August 2003

The transmission of nanocomposite thin films formed by Cu nanocrystals embedded in an amorphous aluminum oxide (Al2O3) matrix has been enhanced in the vicinity of the surface plasmon resonance wavelength, by the design of one-dimensional photonic structures. The nanocrystals are distributed in layers, whose separation and periodicity are used to control the optical response of the films. It is found that at the SPR a transmission enhancement up to 36% can be achieved with respect to a film with an homogeneous distribution of the nanocrystals. These photonic structures have been successfully produced by pulsed laser deposition.
Mechanical characterization of trilayer thin films by the microbridge testing method
Authors: Xu Wei-Hua , Zhang Tong-Yi
Published: August 2003

Microbridge tests were conducted on two sets of trilayer samples, consisting of SiO2/Si3N4/SiO2 and Si3N4/SiO2/Si3N4, which were fabricated on 4 in. p-type (100) silicon wafers by means of the microelectromechanical fabrication technique. The experimental results were analyzed using the small deformation formula including substrate deformation. By changing the sample length, we determined the bending stiffness and the resultant residual force per unit width in each of the trilayer thin films. A simplification is proposed to estimate residual stress and Young’s modulus in each layer. Young’s moduli and residual stresses are estimated to be 54.59 GPa and −429.49 MPa for the silicon oxide films and 270.54 GPa and 550.75 MPa for the silicon nitride films, respectively. The microbridge testing method enables us to characterize the mechanical properties of each layer of sandwich-structured thin films.
Rectifying electrical characteristics of La0.7Sr0.3MnO3/ZnO heterostructure
Authors: Tiwari Ashutosh , Jin C. , Kumar D. , Narayan J.
Published: August 2003

We have fabricated a p–n junction, consisting of p-type manganite (La0.7Sr0.3MnO3) and n-type ZnO layers grown on sapphire substrate. This junction exhibits excellent rectifying behavior over the temperature range 20–300 K. Electrical characteristics of La0.7Sr0.3MnO3 (LSMO) film in this heterostructure are found to be strongly modified by the built-in electric field at the junction. It has been shown that by applying the external bias voltage, the thickness of the depletion layer, and hence, the electrical and magnetic characteristics of LSMO film can precisely be modified.
Effects of power truncation on the insertion loss and crosstalk of arrayed-waveguide grating devices
Authors: Bernussi A. A. , de Peralta L. Grave , Frisbie S. , Temkin H.
Published: August 2003

A measurement of the insertion loss and crosstalk in folded, 100 GHz, arrayed waveguide multiplexers as a function of the number of grating waveguides is described. The number of grating waveguides can be varied in a single device to evaluate power truncation effects with high accuracy. We show that the central peak insertion loss decreases exponentially with the number of grating waveguides. The crosstalk decreases with increased number of waveguides and the measured dependence shows valleys and peaks associated with the passband spectrum of individual channels. These observations are in good agreement with simulation results. With the arrayed waveguide design used in this work the crosstalk performance becomes phase error limited for the number of waveguides greater than 250.
Aluminum-doped zinc oxide films as transparent conductive electrode for organic light-emitting devices
Authors: Jiang X. , Wong F. L. , Fung M. K. , Lee S. T.
Published: August 2003

Highly transparent conductive, aluminum-doped zinc oxide (ZnO:Al) films were deposited on glass substrates by midfrequency magnetron sputtering of metallic aluminum-doped zinc target. ZnO:Al films with surface work functions between 3.7 and 4.4 eV were obtained by varying the sputtering conditions. Organic light-emitting diodes (OLEDs) were fabricated on these ZnO:Al films. A current efficiency of higher than 3.7 cd/A, was achieved. For comparison, 3.9 cd/A was achieved by the reference OLEDs fabricated on commercial indium–tin–oxide substrates.
Field-effect transistor on SrTiO3 with sputtered Al2O3 gate insulator
Authors: Ueno K. , Inoue I. H. , Akoh H. , Kawasaki M. , Tokura Y. , Takagi H.
Published: August 2003

A field-effect transistor has been constructed that employs a perovskite-type SrTiO3 single crystal as the semiconducting channel. This device functions as an n-type accumulation-mode device. The device was fabricated at room temperature by sputter-deposition of amorphous Al2O3 films as a gate insulator on the SrTiO3 substrate. The field-effect (FE) mobility is 0.1 cm2/V s and on-off ratio exceeds 100 at room temperature. The temperature dependence of the FE mobility down to 2 K shows a thermal-activation-type behavior with an activation energy of 0.6 eV.
Preparation and structure investigation of nanoparticle-assembled titanium dioxide microtubes
Authors: Ma Dongling , Schadler Linda S. , Siegel Richard W. , Hong Jung-Il
Published: August 2003

Nanoparticle-assembled capped TiO2 microtubes were synthesized by means of a recently discovered freeze-drying method. Most tubes are about 1 μm in diameter and have a high aspect ratio. The resulting structure was observed using field emission scanning electron microscopy and transmission electron microscopy. The crystal structure and chemical nature of the nanoparticle constituents and the resulting structures were investigated by x-ray diffraction, selected area energy dispersive x-ray spectroscopy, and Raman spectroscopy. The formation of the observed TiO2 microtubes occurs through the physical rearrangement or self-organization of nanoparticles, most likely driven by capillarity and aided by hydrogen bonding between nanoparticles and the water molecules during freeze-drying.
Optical properties of the ZnO nanotubes synthesized via vapor phase growth
Authors: Xing Y. J. , Xi Z. H. , Xue Z. Q. , Zhang X. D. , Song J. H. , Wang R. M. , Xu J. , Song Y. , Zhang S. L. , Yu D. P.
Published: August 2003

A large quantity of nanosized ZnO tubular structures was prepared using a very simple thermal evaporation of mixed Zn–ZnO powders under a wet oxidation condition. The ZnO nanotubes have a hollow core with crystalline wall of 8–20 nm in thickness. Optical properties of ZnO nanotubes were studied at room temperature. Raman peaks arising from the ZnO nanotubes were analyzed, which correspond well to that of the bulk ZnO sample. The photoluminescence measurements of ZnO nanotubes revealed an intensive UV peak at 377 nm corresponding to the free exciton emission, and a broad peak at about 500 nm arising from defect-related emission.
Triggered polarization-correlated photon pairs from a single CdSe quantum dot
Authors: Ulrich S. M. , Strauf S. , Michler P. , Bacher G. , Forchel A.
Published: August 2003

We report on the generation of polarization-correlated photon pairs by the radiative biexciton (XX)-exciton (X) cascade of single CdSe quantum dots (QDs). Under nonresonant optical pulsed excitation (76 MHz) at low temperature (4 K), a high collinear correlation degree of 74.5% was observed from cross-correlation measurements between single XX and X emissions, which reflects an asymmetry-induced exciton fine-structure splitting. In consideration of the excitonic radiative lifetime (250 ps) this effect allows for direct conclusions about the relaxation time (T⩾480 ps) between the corresponding sublevels. Our results also suggest that the biexciton-exciton cascade in a CdSe QD is well suited for triggered single photon and/or photon pair generation rates above 1 GHz.
Good rectifying characteristic in p–n junctions composed of La0.67Ca0.33MnO3−δ/Nb–0.7 wt %-doped SrTiO3
Authors: Hu F. X. , Gao J. , Sun J. R. , Shen B. G.
Published: August 2003

Simple p–n junctions have been fabricated using a simple heteroepitaxial structure of La0.67Ca0.33MnO3−δ/Nb-doped SrTiO3. In such junctions, the La0.67Ca0.33MnO3−δ exhibits semiconductor behavior due to oxygen deficiency, whereas the Nb–0.7 wt %-doped SrTiO3 shows a metal behavior. These junctions demonstrate good rectifying characteristic in a wide temperature range from 5 to 350 K. An intriguing observation is that the rectifying behavior is nearly independent of temperature.
Photoconductivity of single-wall carbon nanotubes under continuous-wave near-infrared illumination
Authors: Levitsky I. A. , Euler W. B.
Published: August 2003

The photoconductivity of films of single-wall carbon nanotubes has been studied under continuous-wave near-infrared illumination. The photocurrent exhibits a linear response with the light intensity and with bias voltage up to 5 V. The temporal photoresponse of on/off step illumination shows a relatively slow relaxation time (4.3 s for films with a thickness of ∼500 nm), which can be interpreted in terms of a kinetic model that takes into account the binding of photoelectrons with adsorbed oxygen. Possible applications of this photosensitive material are discussed.
Birefringence of GaN/AlGaN optical waveguides
Authors: Hui R. , Wan Y. , Li J. , Jin S. X. , Lin J. Y. , Jiang H. X.
Published: August 2003

We have experimentally studied the birefringence of wurtzite GaN grown on a sapphire substrate. The measurements were done with single-mode GaN/AlGaN planar optical waveguides on c-plane grown heterostructure films. The refractive indices were found to be different for signal optical field perpendicular or parallel to the crystal c axis (n⊥≠n∥). More importantly, we found an approximately 10% change in index difference Δn=n∥−n⊥ with variation of the waveguide orientation in the a–b plane, and a 60° periodicity was clearly observed. This is attributed to the hexagonal structure of nitride materials.
Spin polarization and magnetotransport of Mn–Sb alloys in magnetic tunnel junctions
Authors: Panchula Alex F. , Kaiser Christian , Kellock Andrew , Parkin Stuart S. P.
Published: August 2003

The spin polarization of MnxSb1−x for x=0.35–0.45 has been explored via magnetic tunnel junctions using CoFe counterelectrodes and via superconducting tunneling spectroscopy using Al counterelectrodes. MnxSb1−x with x∼0.45 shows a tunneling spin polarization of ∼30% at 0.25 K, and a tunneling magnetoresistance of ∼18% at 10 K both of which are very similar to previously reported data on NiMnSb alloys. These results support the notion that surface segregation of Mn and Sb reduces the spin polarization of the purported half-metal NiMnSb.
What caliber pore is like a pipe? Nanotubes as modulators of ionic gradients
Authors: Fernández Ariel
Published: August 2003

Carbon nanotubes are shown to be modulators of ionic gradients possibly impacting artificial nanophysiology. In contrast with wider nanotubes, the ion traffic inside narrow nanotubes (∼9.45 Å diameter) eventually reaches steady stoichiometries which may be dramatically different from bulk ratios. The narrow nanotube reduces the polarizability of confined water so that charge imbalances inside cannot be sustained.
Determination of absolute photoionization cross sections for vinyl and propargyl radicals
Authors: Robinson Jason C. , Sveum Niels E. , Neumark Daniel M.
Published: August 2003

Photofragment translational spectroscopy experiments employing tunable vacuum ultraviolet photoionization yielded absolute photoionization cross sections for vinyl and propargyl radicals at 10 eV of 11.1±2.2 and 8.3±1.6 Mb, respectively. From these values, the photoionization efficiency curves from 7.8–10.8 eV for these radicals were placed on an absolute scale.
Thermal transport in stacked superconductor–normal metal–superconductor Josephson junctions
Authors: Chong Yonuk , Dresselhaus P. D. , Benz S. P.
Published: August 2003

Nb/MoSi 2 / Nb stacked superconductor–normal metal–superconductor (SNS) Josephson junctions has proven to be a good candidate for high-density series arrays for Josephson voltage-standard applications. As the junction density increases, self-heating becomes an issue because the high power density per junction (1 W/cm2) generates significant power dissipation under typical operating conditions. In this letter, we analyze the heating effect of these sandwich-type SNS junctions using a model to quantitatively estimate and predict thermal-transport properties of the stacked structures. We describe several strategies that reduce heating and demonstrate improved properties of stacked-junction arrays with enhanced cooling capacity.
Atomic-resolution noncontact atomic force microscopy in air
Authors: Seo Yongho , Choe Hwansung , Jhe Wonho
Published: August 2003

Atomic resolution by noncontact atomic force microscopy with a self-sensing quartz tuning fork is achieved in ambient conditions. We have employed a trident-shape quartz crystal resonator oscillating at 1 MHz with a high spring constant of 4×105 N/m, allowing subnanometer resolution of the cleaved mica in air.
Electronic structure of cubic Li(Fe0.1Mn1.9)O4 studied with Mössbauer spectroscopy and first-principles calculation
Authors: Wei Y. J. , Xu X. G. , Wang C. Z. , Li C. , Chen G. , Wu F.
Published: August 2003

Mössbauer spectrum was collected in Fe3+ doped cubic LiMn2O4 (LiFe0.1Mn1.9O4) by using Fe57 as the radiation source. In the model of the crystal-field theory, the energy gaps between different d orbitals, ΔE(b1g−a1g) and ΔE(b2g−eg), characterize the strength of the Jahn–Teller effect in the crystal. A relationship between the Mössbauer quardrupole splitting and the energy gaps was established, based on which both ΔE(b1g−a1g) and ΔE(b2g−eg) of the [MnO6] octahedron in LiFe0.1Mn1.9O4 are estimated to be about 0.41 and 0.30 eV, respectively. Electronic structure of LiMn2O4 was studied theoretically via ab initio calculation based on the density-functional theory. Calculation shows that a gap about 0.28 eV between the filled Mn d bands is equivalent to ΔE(b2g−eg). It also shows that the first unoccupied states are dominated by Mn 3d contribution essentially from both a1g and b1g of Mn d states. Distance between the two peaks in the first unoccupied band was used to calculate ΔE(b1g−a1g), which is about 0.36 eV. The Mössbauer quardrupole splitting characterizes Jahn–Teller distortion and its effect on the fine structure of Mn 3d bands.
Pinned synthetic ferrimagnets with perpendicular anisotropy and tuneable exchange bias
Authors: Sort J. , Rodmacq B. , Auffret S. , Dieny B.
Published: August 2003

Pinned synthetic ferrimagnets (syFerri) with perpendicular-to-plane magnetic anisotropy, of the form AP1/Ru/AP2/FeMn [where AP1 and AP2 are (Co/Pt) multilayers], have been prepared and characterized. The magnitudes of the exchange bias fields of both AP1 and AP2 can be tuned at room temperature by simply varying the relative number of (Co/Pt) repeats in each multilayer. This effect can be quantitatively interpreted by considering the different energy contributions involved during magnetization reversal. Moreover, from the values of these fields, the characteristic parameters of the system (i.e., coupling strength through the Ru and AP2/FeMn pinning energy), can be evaluated. Interestingly, an extended plateau with a virtually constant magnetization is observed around zero field when the number of Co/Pt repeats in AP1 is equal or larger than in AP2. This is very appealing for field sensor or memories applications using spin valves or tunnel junctions with perpendicular anisotropy, since it offers a large dynamic range over which the magnetic configuration of the syFerri remains stable.
Preparation of MgB2 tapes using a nanocrystalline partially reacted precursor
Authors: Fischer C. , Rodig C. , Häßler W. , Perner O. , Eckert J. , Nenkov K. , Fuchs G. , Wendrock H. , Holzapfel B. , Schultz L.
Published: August 2003

Fe-cladded MgB2 conductors have been prepared by the powder-in-tube method using mechanically alloyed nanocrystalline Mg+2B powder mixtures consisting of the constituents Mg, B, and MgB2 as precursor. Despite low Tc values of about 31 K, maximum critical current densities (Jc) of 22 and 7 kA/cm2 in external magnetic fields of 7.5 and 10 T, respectively, are achieved at 4.2 K. These excellent values can mainly be attributed to the very fine-grained microstructure of the superconducting phase. Higher values have only been reported for tapes doped with SiC nanoparticles. The irreversibility fields Hirr of these tapes are 9.5 and 4.2 T at 10 and 20 K, respectively.
Room-temperature dephasing time of intersubband transitions in heavily-doped InGaAs/AlAs/AlAsSb coupled quantum wells
Authors: Venu Gopal Achanta , Yoshida Haruhiko , Simoyama Takasi , Kasai Junichi , Mozume Teruo , Ishikawa Hiroshi
Published: August 2003

Room-temperature dephasing time (T2) estimates of 1.55 μm intersubband transitions (ISBT) are presented in InGaAs/AlAs/AlAsSb coupled double quantum wells. In this material, optimized for ISBT at communication wavelengths for all-optic switching application, we also studied the doping-density dependence of the four-wave-mixing decay time as a function of incident excitation intensity. A T2 value of about 292±40 fs is estimated at an intensity corresponding to the saturation intensity in samples that are doped to 2×1019 cm−3. This value is in good agreement with the value estimated from earlier calculations.
Lasing characteristics of InAs quantum-dot lasers on (001) InP substrate
Authors: Qiu Yueming , Uhl David , Chacon Rebecca , Yang Rui Q.
Published: August 2003

Single-stack InAs self-assembled quantum-dot (QD) lasers based on (001) InP substrates have been grown by metalorganic vapor-phase epitaxy. The narrow ridge waveguide lasers lased up to 260 K in continuous-wave operation, and near room temperature in pulsed mode, with emission wavelengths between 1.59 to 1.74 μm. Above 200 K, a very low wavelength temperature sensitivity of 0.09 nm/K was observed. Lasing spectra at different temperatures suggests that the ground states and the excited states almost overlap, and form a quasicontinuous band due to the large size of the InAs dots and their inhomogeneous broadening. These results will provide guidance for further development of long wavelength InAs QD lasers based on InP substrates.
Proton beam writing of low-loss polymer optical waveguides
Authors: Sum T. C. , Bettiol A. A. , van Kan J. A. , Watt F. , Pun E. Y. B. , Tung K. K.
Published: August 2003

Proton beam writing is a direct-write micromachining technique capable of producing three-dimensional microstructures with straight and smooth sidewalls. Low-loss channel waveguides in SU-8, a chemically amplified negative tone resist, were fabricated using a focused submicron beam of 2.0 MeV protons with a dose of 30 nC/mm2 and a beam current of approximately 2 pA. Propagation losses of approximately (0.19±0.03) dB/cm were measured at 632.8 nm wavelength. Waveguides of arbitrary design can be easily fabricated using proton beam writing, making the technique ideal for the rapid prototyping of optical circuits.
Influence of ultraviolet illumination on the poling characteristics of lithium niobate crystals
Authors: Müller Manfred , Soergel Elisabeth , Buse Karsten
Published: August 2003

The poling characteristics of lithium niobate crystals under ultraviolet laser illumination are investigated. Light with a wavelength around 334 nm permanently decreases the coercive field of the crystal and is used to demonstrate optical control of domain patterning.
Heteroepitaxial growth of CuInS2 thin films on sapphire by radio frequency reactive sputtering
Authors: He Y. B. , Kriegseis W. , Meyer B. K. , Polity A. , Serafin M.
Published: August 2003

Direct heteroepitaxial growth of uniform stoichiometric CuInS2 (CIS) thin films on sapphire (0001) substrates has been achieved by radio frequency reactive sputtering. X-ray ω–2θ scans reveal that the sputtered layers grow in a (112) orientation with a chalcopyrite structure. A rocking curve full width at half maximum of about 0.05° (180 arc sec) for the (112) peak demonstrates a nearly perfect out-of-plane arrangement of CIS (112)∥sapphire (0001). X-ray diffraction Phi scans further illustrate an excellent in-plane ordering of CIS [1̄10]∥sapphire (101̄0). The sputtered thin CIS epilayers had a smooth surface with a typical root-mean-square roughness of about 3.3 nm as evaluated by atomic force microscopy. The epitaxial growth of tetragonal CIS on hexagonal sapphire provides evidence that heteroepitaxial growth may be realized between structures of different symmetry, such as films of cubic or tetragonal structures on hexagonal substrates or vice versa.
Superior Schottky electrode of RuO2 for deep level transient spectroscopy on anatase TiO2
Authors: Miyagi Takahira , Kamei Masayuki , Mitsuhashi Takefumi , Yamazaki Atsushi
Published: August 2003

An anatase TiO2 film was epitaxially grown on a conductive Nb-doped single-crystalline SrTiO3 (100) substrate by metalorganic chemical vapor deposition. RuO2 Schottky electrode was fabricated on the epitaxial anatase film by reactive dc magnetron sputtering. The dark I–V and capacitance–voltage characteristics indicated the good rectification and thermal stability of the RuO2/anatase junction. This RuO2/anatase junction enables the stable measurements of deep level transient spectroscopy in the high-temperature region and is a promising Schottky electrode to examine the origins of deep levels in the band gap of anatase.
Mesoscopic ferroelectric cell arrays prepared by imprint lithography
Authors: Harnagea Catalin , Alexe Marin , Schilling Jörg , Choi Jinsub , Wehrspohn Ralf B. , Hesse Dietrich , Gösele Ulrich
Published: August 2003

Arrays of mesoscopic ferroelectric (Pb,Zr)TiO3 cells with lateral sizes from several micrometers down to below 300 nm were prepared applying nanoimprint lithography. The ferroelectric properties of the mesoscopic cells were investigated by scanning force microscopy in piezoresponse mode. The best chemical route to obtain ferroelectric cells was found to be the sol-gel method. Using Nb-doped SrTiO3 single crystals as bottom electrodes, the crystallization into the ferroelectric phase was uniform with grain sizes in the 35 nm range.
Charge storage in undoped hydrogenated amorphous silicon by ambient atomic force microscopy
Authors: Rezek B. , Mates T. , Stuchlı́k J. , Kočka J. , Stemmer A.
Published: August 2003

Hydrogenated amorphous silicon (a-Si:H) layers are prepared by plasma-enhanced chemical vapor deposition on metallized glass substrates. Ambient atomic force microscopy (AFM) is employed for both modification and characterization of a-Si:H films. Voltage pulses of up to 35 V are applied as a cantilever scans the amorphous silicon surface in contact mode AFM. Subsequent detection by Kelvin probe microscopy reveals a persistent negative charge stored in the a-Si:H layers. The stored charge is always negative independent of voltage polarity and results in an upward shift of the Fermi level by as much as 0.1 eV. Only at higher negative voltages (<−15 V) a positively charged oxide layer grows on the surface due to anodic oxidation. A model of the observed phenomena is proposed considering charge transport across a biased contact junction, metastable trapping in band-gap states of a-Si:H, as well as the influence of silicon oxide.
Spin lifetimes of electrons injected into GaAs and GaN
Authors: Krishnamurthy Srinivasan , van Schilfgaarde Mark , Newman Nathan
Published: August 2003

The spin relaxation times of electrons in GaAs and GaN are determined with a model that includes momentum scattering by phonons and ionized impurities, and spin scattering by the Elliot–Yafet, D’yakonov–Perel, and Bir–Aronov–Pikus mechanisms. Accurate bands generated using a long-range tight-binding Hamiltonian obtained from empirical pseudopotentials are used. The inferred temperature dependence of the spin relaxation lifetime agrees well with measured values in GaAs. We further show that the spin lifetimes decrease rapidly with injected electron energy and reach a local maximum at the longitudinal optical phonon energy. Our calculation predicts that electron spin lifetime in pure GaN is about three orders of magnitude longer than in GaAs at all temperatures, primarily as a result of the lower spin-orbit interaction and higher conduction band density of states.
“Plug and play” quantum key distribution system with differential phase shift
Authors: Zhou Chunyuan , Wu Guang , Chen Xiuliang , Zeng Heping
Published: August 2003

We propose a “plug and play” scheme for the long-distance fiber-based cryptosystem based on the differential phase shift quantum key distribution, where any birefringence effects and polarization-dependent losses in the transmission fiber are automatically compensated by using a Faraday mirror. This system not only has stable performance but also creates keys 8/3 times more efficiently than the conventional cryptosystem based on the BB84 protocol.
Isotopically enriched designer-diamond anvil
Authors: Baker Paul A. , Vohra Yogesh K. , Peterson Randolph S. , Weir Samuel T.
Published: August 2003

Isotopically enriched (42 at. % C13) designer-diamond anvils were grown by microwave plasma chemical vapor deposition using methane/hydrogen/oxygen chemistry. These isotopically enriched diamond layers can modify the thermal properties of the culet of high-pressure anvils and also allow the use of a C13/C12 Raman pressure sensor system for high-pressure, high-temperature measurements. Raman spectroscopy clearly revealed the isotopically mixed nature of the culet while the photoluminescence spectra at 80 K demonstrate the nitrogen-based defect center near the culet of the diamond anvil. The ability to isotopically modify the diamond culets offers yet another functionality for the embedded electric and magnetic sensors in a designer-diamond anvil.
Field emission from carbon black and carbon black/silica nanoparticles dispersed in photoresist
Authors: Busta H. , Boldridge D. , Myers R. , Edwards E. , Feinerman A.
Published: August 2003

Carbon black and carbon black mixed with silica nanoparticles were dispersed in Shipley 1818 photoresist and spun onto copper-coated silicon substrates. By using an appropriate amount of carbon black, the films are made conductive, at least in the vertical direction, and exhibit electron emitting properties with macroscopic current densities of about 1 mA/cm2 and turn-on fields ranging from 3 to 5 V/μm. These results are compared to carbon nanotubes (CNTs) also dispersed in photoresist and prepared in a similar manner. The current-extraction field curves exhibit a pronounced peak for the C/SiO2 sample and saturation regions for the C and CNT samples. It is conjectured that resonance tunneling caused by the presence of impurities gives rise to this behavior. The maximum deposition temperature these films experience is 120 °C, which makes them ideal candidates for low-cost, large-area electron and x-ray sources. The role of the photoresist is mainly for providing adhesion of the carbonaceous material to the substrate.
Ultrafast and highly sensitive photodetectors fabricated on high-energy nitrogen-implanted GaAs
Authors: Mikulics M. , Marso M. , Kordoš P. , Stanček S. , Kováč P. , Zheng X. , Wu S. , Sobolewski Roman
Published: August 2003

We have fabricated and tested metal–semiconductor–metal (MSM) photodetectors based on nitrogen-ion-implanted GaAs. Nitrogen ions with energy of 700 and 880 keV, respectively, were implanted into epitaxial GaAs films at an ion concentration of 3×1012 cm−2. Ti/Au MSM photodetectors with 1-μm-wide fingers were fabricated on top of the implanted GaAs. In comparison to low-temperature-grown GaAs photodetectors, produced in parallel in identical MSM geometry, the 880 keV N+-implanted photodetectors exhibited almost two orders of magnitude lower dark current (10 nA at 1 V bias) and the responsivity more than doubled (>20 mA/W at 20 V bias). Illumination with 100-fs-wide, 810 nm wavelength laser pulses, generated ∼2.5-ps-wide photoresponse signals with amplitudes as high as 2 V. The 2.5 ps relaxation time was the same for both the ion-implanted and low-temperature-grown devices and was limited by the MSM capacitance time constant.
Exciton diffusion in light-emitting organic thin films studied by photocurrent spectra
Authors: Yang C. L. , Tang Z. K. , Ge W. K. , Wang J. N. , Zhang Z. L. , Jian X. Y.
Published: August 2003

Photocurrent spectroscopy is employed to obtain the exciton diffusion coefficient and diffusion length in two typical light-emitting organic thin films, the hole transport material [N,N′-diphenyl-N,N′-bis(3-methyl-phenyl)-1,1′biphenyl-4,4′diamine] (TPD) and the electron transport material tris(8-hydroxyquinolinolato) aluminum III (Alq3). The photocurrent spectra of both films at higher biases exhibit either symbatic or antibatic responses. However, complicated responses are observed for smaller biases. At certain small biases, the direction of the photocurrent even alters with the energy of illuminating photons. A model based on exciton diffusion and dissociation at film/electrode interfaces is used to explain these experimental results. Good agreement is achieved between experiments and theory by taking into account both symbatic and antibatic responses, especially at zero or small biases. The diffusion coefficient and the diffusion length of excitons derived are 1.53×10−3 cm2 s−1 and 17 nm for TPD and 4×10−5 cm2 s−1 and 8 nm for Alq3, respectively.
Regulating the size and position of deposited Zn nanoparticles by optical near-field desorption using size-dependent resonance
Authors: Yatsui T. , Takubo S. , Lim J. , Nomura W. , Kourogi M. , Ohtsu M.
Published: August 2003

We report that optical near-field desorption can dramatically regulate the growth of Zn nanoparticles during optical chemical vapor deposition. The trade off between the deposition due to 3.81 eV optical near-field light and desorption due to 2.54 eV optical near-field light allowed the fabrication of a single 15 nm Zn dot, while regulating its size and position.
Direct observation of microscopic photoinduced charge redistribution on TiO2 film sensitized by chloroaluminum phthalocyanine and perylenediimide
Authors: Cao Jian , Sun Jing-Zhi , Hong Jian , Yang Xin-Guo , Chen Hong-Zheng , Wang Mang
Published: August 2003

The microscopic photoinduced charge redistribution on the surface of an organic dye composite layer was directly observed using Kelvin probe force microscopy (KFM) coupled with a laser source. The organic layer composed of chloroaluminum phthalocyanine and N,N′-dioctadecyl-3,4,9,10-perylene tetracarboxylic-diimide was cast onto the surface of nano-TiO2 film. Under illumination, the surface potential morphologies of the organic layer changed from large fluctuation to relatively uniform features. Such a phenomenon was associated with the photoinduced charge transfer between the dye composite and nano-TiO2, which was rationalized in terms of energy level alignment. Considering that the KFM have been proved to be a visual technique in the characterization of nanomaterials, we have presented a method to visualize the photoinduced charge transfer in microscopic scale.
Al
2
O
3
prepared by atomic layer deposition as gate dielectric on 6H-SiC(0001)
Authors: Gao K. Y. , Seyller Th. , Ley L. , Ciobanu F. , Pensl G. , Tadich A. , Riley J. D. , Leckey R. G. C.
Published: August 2003

Al 2 O 3 films were deposited as alternative gate dielectric on hydrogen-terminated 6H-SiC(0001) by atomic layer chemical vapor deposition and characterized by photoelectron spectroscopy (PES) and admittance measurements. The PES results indicate an abrupt interface free of significant Si–suboxide contributions where the Al2O3 layer is connected to SiC by bridging oxygen atoms. The admittance measurements yield an interface state density which is lower than that of the thermally formed oxide and show in particular no increase toward the conduction band edge. Furthermore, a nearly symmetrical band alignment of Al2O3 on 6H-SiC with offsets of 2.2 and 1.8 eV is determined for the valence and conduction bands, respectively. This makes Al2O3 a serious competitor to thermal oxides as gate insulator in SiC devices.
Cartan-decomposition subgroups of 𝑆𝑂(2,𝑛)
Authors: Oh Hee , Morris Dave
Published: September 2003

For

G
=
SL
⁡
(
3
,

R

)

G = \operatorname {SL} (3,\mathbb {R})

and

G
=
SO
⁡
(
2
,
n
)

G = \operatorname {SO}(2,n)

, we give explicit, practical conditions that determine whether or not a closed, connected subgroup

H
H

of

G
G

has the property that there exists a compact subset

C
C

of

G
G

with

C
H
C
=
G

CHC = G

. To do this, we fix a Cartan decomposition

G
=
K

A
+

K

G = K A^+ K

of

G
G

, and then carry out an approximate calculation of

(
K
H
K
)
∩

A
+

(KHK) \cap A^+

for each closed, connected subgroup

H
H

of

G
G

.
Investigation by 13C-NMR and tricarboxylic acid (TCA) deletion mutant analysis of pathways for succinate formation in Saccharomyces cerevisiae during anaerobic fermentation
Authors: Camarasa Carole , Grivet Jean-Philippe , Dequin Sylvie
Published: August 2003

NMR isotopic filiation of13C-labelled aspartate and glutamate was used to explore the tricarboxylic acid (TCA) pathway inSaccharomyces cerevisiaeduring anaerobic glucose fermentation. The assimilation of [3-13C]aspartate led to the formation of [2,3-13C]malate and [2,3-13C]succinate, with equal levels of13C incorporation, whereas site-specific enrichment on C-2 and C-3 of succinate was detected only with [3-13C]glutamate. The non-random distribution of13C labelling in malate and succinate demonstrates that the TCA pathway operates during yeast fermentation as both an oxidative and a reductive branch. The observed13C distribution suggests that the succinate dehydrogenase (SDH) complex is not active during glucose fermentation. This hypothesis was tested by deleting theSDH1gene encoding the flavoprotein subunit of the SDH complex. The growth, fermentation rate and metabolite profile of thesdh1mutant were similar to those of the parental strain, demonstrating that SDH was indeed not active. Filiation experiments indicated the reductive branch of the TCA pathway was the main pathway for succinate production if aspartate was used as the nitrogen source, and that a surplus of succinate was produced by oxidative decarboxylation of 2-oxoglutarate if glutamate was the sole nitrogen source. Consistent with this finding, akgd1mutant displayed lower levels of succinate production on glutamate than on other nitrogen sources, and higher levels of oxoglutarate dehydrogenase activity were observed on glutamate. Thus, the reductive branch generating succinate via fumarate reductase operates independently of the nitrogen source. This pathway is the main source of succinate during fermentation, unless glutamate is the sole nitrogen source, in which case the oxidative decarboxylation of 2-oxoglutarate generates additional succinate.