bluuebunny/crossref_metadata_2025 huggingface dataset:
Simulation of high power broadband cyclotron autoresonance maser amplifier and electron beam experiments
Authors: Speirs D. C. , Phelps A. D. R. , Konoplev I. V. , Cross A. W. , He W.
Published: March 2004

The design, simulation, and preliminary experimental implementation of an efficient, broadband cyclotron autoresonance maser (CARM) amplifier operating over the 9–13 GHz frequency band is presented. The amplifier is based on a high current accelerator capable of generating a ∼35 A pencil electron beam at an accelerating voltage of ∼450 kV. A full three-dimensional numerical model of the CARM amplifier has been constructed within the particle-in-cell code KARAT taking into account electron beam parameters derived from simulation and experiment. An electron beam current of 32A at an accelerating voltage of 400 kV was measured. Numerical simulations demonstrate the possibility of obtaining 37 dB gain and an interaction efficiency of 17%. In addition a viable amplification bandwidth of 9–13 GHz is apparent, with a minimum gain and efficiency of 25 dB and 10%, respectively, at the boundaries of the amplification band. The peak modeled efficiency and gain (17%, 37 dB) were obtained at a frequency of 12 GHz. Computational simulations have also revealed correlation between the instantaneous amplification bandwidth and the spectral width of cyclotron superradiant emission within the system.
Thinning of silicon-on-insulator wafers by numerically controlled plasma chemical vaporization machining
Authors: Mori Yuzo , Yamamura Kazuya , Sano Yasuhisa
Published: March 2004

Silicon-on-insulator (SOI) wafers are semiconductor substrates that can be used in next-generation high-speed and low-power devices. An ultrathin SOI wafer with a SOI layer of less than 20 nm is required for the dynamic random access memory half-pitch node of 100 nm or lower. In this report, the thinning of the SOI layer by numerically controlled plasma chemical vaporization machining is described. A commercial 6 in. SOI wafer was used for the demonstration thinning. Spectroscopic ellipsometry was used to measure the thickness distribution of the SOI layer. The SOI layer thickness of ∼200±4 nm was successfully thinned to 13±2 nm.
Real space and Fourier microscopy of colloidal suspensions confined to a parallel plate geometry
Authors: Biehl Ralf , Palberg Thomas
Published: March 2004

Optical microscopy is known as a versatile tool to investigate the structure and dynamics of ordered colloidal suspensions. We present a version facilitating the quasisimultaneous access to global structure information’s via Fourier microscopy and local real-space analysis on the level of individual particles. For the latter the sampling volume was significantly enhanced in z direction manipulating the cover glass correction. The resulting image is a projection of x–y particle positions at different z into a two-dimensional plane of which two-dimensional projection correlation diagrams (PCDs) and radial averages were obtained. For crystals and layered structures the PCDs contain valuable information on in-plane order and the relative position of neighboring planes. Time resolved measurements give access to statistically significant average particle (layer) trajectories, which in principle are directly comparable to simulation results. The instrument is tested on a variety of ordered colloidal structures under equilibrium conditions and under linear shear in a plate–plate optical cell.
Electron mobility in ultrathin silicon-on-insulator layers at 4.2 K
Authors: Prunnila M. , Ahopelto J. , Gamiz F.
Published: March 2004

Low temperature mobility measurements of silicon-on-insulator (SOI) metal-oxidefield-effect-transistors are reported. The batch of devices fabricated in this work includes both ultrathin and thick devices for which the SOI film thicknesses are in the ranges of 10–15 nm and 56–61 nm, respectively. The 4.2 K peak mobility of the thick devices is 1.9 m2/V s. The ultrathin devices show mobility degradation at low electron densities where the mobility is also observed to decrease with decreasing the SOI film thickness. The peak mobilities of these devices are in the range of 1.35–1.57 m2/V s. Numerical calculations show that ultrathin devices are in the limit where the electrons are confined by the quantum well defined by gate oxide and buried oxide, which is interpreted to lead to the observed mobility degradation.
Highly directional emission from colloidally synthesized nanocrystals in vertical cavities with small mode spacing
Authors: Roither J. , Heiss W. , Talapin D. V. , Gaponik N. , Eychmuller A.
Published: March 2004

The optical properties of light emitting devices based on colloidally synthesized CdSe/CdS core/shell semiconductor nanocrystals embedded in vertical cavities are investigated. The cavities are several micrometers, thick and formed by a metallic mirror and a dielectric Bragg interference mirror deposited on the opposite surfaces of cleaved pristine mica sheets. Due to the large cavity length, up to 30 resonances are found within the Bragg mirror stop band. The corresponding small mode spacing allows one to extract a large portion of the broad nanocrystal luminescence band from the cavity upon optical excitation. The spontaneous emission of these cavities is highly forward directed with a beam divergence smaller than 1.3°. Furthermore, the emission is linearly polarized which is a result of the birefringent properties of the mica sheets.
Current conduction processes in high-κ Gd0.31Ga0.1O0.59/Ga2O3 gate dielectric stacks on GaAs
Authors: Chen A. , Passlack M. , Medendorp N. , Braddock D.
Published: March 2004

Current conduction processes in high-κ (κ=20.2) Gd0.31Ga0.1O0.59/Ga2O3 dielectric stacks grown on n-type GaAs by molecular beam epitaxy have been investigated. Metal-oxide-semiconductor capacitors have been characterized by current density (j) versus electric field (E) measurements at temperatures ranging from 90 to 450 K. For temperatures T⩽200 K, the high field (4.5⩽E⩽6.2 MV/cm) current is temperature independent and a Fowler–Nordheim tunneling slope of 1.75 eV3/2 is obtained. Frenkel–Poole emission is found to dominate at temperatures of 300 K and above at moderate electric fields (1.3⩽E⩽2.2 MV/cm). For Frenkel–Poole emission, a barrier height of 1.1 eV and a dynamic dielectric constant of 7.95 is derived from ln(j/E) vs 1/T and ln(j/E) vs E1/2 plots, respectively.
Energetic stability and magnetic properties of Mn dimers in silicon
Authors: Bernardini F. , Picozzi S. , Continenza A.
Published: March 2004

We present an accurate first-principles study of magnetism and energetics of single Mn impurities and Mn dimers in Si. Our results, in general agreement with available experiments, show that (i) Mn atoms tend to aggregate, the formation energy of dimers being lower than the sum of the separate constituents, (ii) ferromagnetic coupling is favored between the Mn atoms constituting the dimers in p-type Si, switching to an antiferromagnetic coupling in n-type Si, (iii) Mn atoms show donors (acceptor) properties in p-type (n-type) Si, therefore they tend to compensate doping, while dimers being neutral or acceptors allow for Si to be doped p-type.
Single carrier localization in InxGa1−xAs1−yNy investigated by magnetophotoluminescence
Authors: Polimeni A. , Masia F. , Vinattieri A. , Baldassarri Höger von Högersthal G. , Capizzi M.
Published: March 2004

We investigated the origin of radiative recombination in InxGa1−xAs1−yNy/GaAs quantum wells by photoluminescence (PL) after picosecond excitation and under a magnetic field, B. Continuous wave and time-resolved PL show that at low temperature T localized states are mainly involved in the radiative recombination processes. Most importantly, the shift of the PL peak position induced by B depends dramatically on temperature, being higher at lower T. This result indicates that the PL emission at low temperature is determined by the recombination of loosely bound electron-hole pairs in which one carrier is localized by N-induced potential fluctuations, and the other carrier is delocalized.
A vacuum-UV laser-induced fluorescence experiment for measurement of rotationally and vibrationally excited H2
Authors: Vankan P. , Heil S. B. S. , Mazouffre S. , Engeln R. , Schram D. C. , Döbele H. F.
Published: March 2004

An experimental setup is built to detect spatially resolved rovibrationally excited hydrogen molecules via laser-induced fluorescence. To excite the hydrogen molecules, laser radiation is produced in the vacuum UV part of the spectrum. The laser radiation is tunable between 120 nm and 230 nm and has a bandwith of 0.15 cm−1. The wavelength of the laser radiation is calibrated by simultaneous recording of the two-photon laser induced fluorescence spectrum of nitric oxide. The excited hydrogen populations are calibrated on the basis of coherent anti-Stokes Raman scattering measurements. A population distribution is measured in the shock region of a pure hydrogen plasma expansion. The higher rotational levels (J>5) show overpopulation compared to a Boltzmann distribution determined from the lower rotational levels (J⩽5).
Robust photodetector for vacuum ultraviolet laser light
Authors: Kitahara Tadashi , Nomoto Yoshiro , Ichikawa Norio
Published: March 2004

An evacuated photocell including a pair of planar interdigitated electrodes fabricated on a sapphire substrate is proposed. Photocurrent in the photocell was found to be originated from both a photoemission effect on the gold cathode and an internal photoelectric effect in the sapphire substrate. Because of the robust nature of the sapphire substrate and the gold electrodes, the photocell reliably tolerated more than 105 shots of fluorine laser pulses as strong as 0.13 J/cm2/pulse on the 7×7 mm2 active area. The photocell was successfully employed for measuring the pulse shapes of F2 laser light with a temporal resolution shorter than 6 ns.
Preparation of gold tips suitable for tip-enhanced Raman spectroscopy and light emission by electrochemical etching
Authors: Ren Bin , Picardi Gennaro , Pettinger Bruno
Published: March 2004

We describe a method of preparing gold scanning tunneling microscopy (STM) tips by direct current electrochemical etching in concentrated HCl and ethanol solution. Gold tips with tip apex radius lower than 30 nm can be reproducibly prepared by this method. The influence of the solution composition, etching voltage on the surface structure, and sharpness has been investigated. These tips can be efficiently used for STM imaging, tip-enhanced Raman spectroscopy, and light emission investigations on the same sample.
Photoelectronic transport imaging of individual semiconducting carbon nanotubes
Authors: Balasubramanian Kannan , Fan Yuwei , Burghard Marko , Kern Klaus , Friedrich Marcel , Wannek Uli , Mews Alf
Published: March 2004

Photoconductivity in individual semiconducting single-wall carbon nanotubes was investigated using a confocal scanning optical microscope. The magnitude of the photocurrent was found to increase linearly with the laser intensity, and to be maximum for parallel orientation between the light polarization and the tube axis. Larger currents were obtained upon illuminating the tubes at 514.5 nm in comparison to those at 647.1 nm, consistent with the semiconducting tubes having a resonant absorption energy at the former wavelength. Moreover, the determination of the photoresponse as a function of position along single nanotubes has proven to be a useful tool to monitor local electronic structure effects.
Energy band alignment at the (100)Ge/HfO2 interface
Authors: Afanas’ev V. V. , Stesmans A.
Published: March 2004

The Ge/HfO2 interface band diagram was directly determined using internal photoemission of electrons and holes from Ge into the Hf oxide. The inferred offsets of the conduction and valence band at the interface, 2.0±0.1 and 3.0±0.1 eV, respectively, suggest the possibility to apply the deposited HfO2 as an insulator on Ge. The post-deposition annealing of the Ge/HfO2 structures in oxygen results in ∼1 eV reduction of the valence band offset attributed to the growth of GeO2 interlayer.
Ultracompact, multifunctional, and highly integrated 3×2 photonic switches
Authors: Li Baojun , Li Jing , Zhao Yuzhou , Lin Xubin , Chua Soo-Jin , Miao Lingyun , Fitzgerald Eugene A. , Lee Minjoo L. , Chaudhari Bharat S.
Published: March 2004

An ultracompact, multifunctional, and highly integrated photonic switch with a 3×2 configuration has been designed and fabricated with SiGe/Si material by using silicon-optical bench technology. This kind of switch can be used in fiber optic communications systems, photonic integrated circuits and wavelength division multiplexed networks as an optical power splitter, optical crossconnect, optical add-drop multiplexer, and wavelength division multiplexer simultaneously or individually. The function of the device is to combine multiwavelengths from different input channels and to switch them to different output channels. The operating wavelength range of the device is designed in C band, i.e., 1530–1570 nm. The device was characterized at 1540, 1550, and 1560 nm wavelengths. The performance at these wavelengths is found satisfactory. The measured insertion loss is less than 2 dB, ON/OFF ratio is greater than 30 dB, crosstalk is between −20 and −25 dB, and switching speed is 100–200 ns.
Direct observation of metal complex nanoparticles doped in sol-gel silica glasses using transmission electron microscopy
Authors: Baba M. , Ichihara M. , Ganeev R. A. , Suzuki M. , Kuroda H. , Morita M. , Rau D. , Ishii T. , Iwamura M.
Published: March 2004

Sol-gel silica glasses are good nanoporous matrices and are able to confine various phosphors as dopants, so these are quite interesting and strategic materials and show phenomena such as surface-enhanced quantum confinement effects: high luminescence quantum efficiency and ultrafast decay times. We observed directly clear images of metal complex nanoparticles in those with transmission electron microscopy. This can easily improve the quality of uniformly doped sol-gel silica glasses.
Dependence of hard x-ray yield on laser pulse parameters in the wavelength-cubed regime
Authors: Hou Bixue , Nees John A. , Theobald Wolfgang , Mourou Gérard A. , Chen L. M. , Kieffer Jean-Claude , Krol Andrzej , Chamberlain C. C.
Published: March 2004

Conversion efficiency and electron temperature scaling laws are experimentally studied in the wavelength-cubed (λ3) regime, where a single-wavelength focus allows low energy pulses incident on a Mo target to produce x rays with excellent efficiency and improved spatial coherence. Focused intensity is varied from 2×1016 to 2×1018 W/cm2. Conversion efficiency and electron temperature are best described by a power law for energy scaling while an exponential law best describes the scaling of these parameters with pulse duration.
Super-resolution in laser annealing and ablation
Authors: Avrutsky Ivan , Georgiev Daniel G. , Frankstein Dmitry , Auner Gregory , Newaz Golam
Published: March 2004

This letter reports observation of ablated holes as small as 0.7 μm fabricated by single 25 ns pulses of KrF (λ=248 nm) laser focused onto a 5.6 μm spot. Samples with high thermal conductivity films with respect to that of the substrate (Si/silica, Al/glass) repeatedly showed considerable reduction in the size of the ablated spot (0.7- and 1.2-μm-diam holes, respectively). This letter also presents a likely mechanism of the observed super-resolution and the criteria necessary to achieve super-resolution. Due to the nonoptical origin of this effect it is expected that a tightly focused (<0.5 μm) laser beam can be used to ablate with nanoscale (<100 nm) resolution.
Tuning the electrical resistivity of pulsed laser deposited TiSiOx thin films from highly insulating to conductive behaviors
Authors: Brassard D. , Sarkar D. K. , El Khakani M. A. , Ouellet L.
Published: March 2004

We report on the successful growth of amorphous TiSiOx thin films by means of pulsed-laser ablation of a TiO2/SiO2 composite target in a high-vacuum chamber. The room-temperature resistivity of the TiSiOx films is found to decrease by more than 6 orders of magnitude (i.e., from ∼2×104 to 10−2 Ω cm) when their substrate deposition temperature (Td) is increased from 20 to 600 °C. On the other hand, by subjecting these films to a post-deposition annealing at 600 °C in oxygen atmosphere, they become highly insulating with a resistivity level as high as 2×1010 Ω cm, regardless of the Td value. The presence of conductive titanium silicide and titanium sub-oxide local phases in the as-deposited TiSiOx films, as revealed by photoelectron spectroscopy analyses, appears to be the cause of the observed tremendous change in the film resistivity. In particular, it is shown that the resistivity of the TiSiOx films is strongly correlated with their oxygen content.
Application of the compatibility factor to the design of segmented and cascaded thermoelectric generators
Authors: Snyder G. Jeffrey
Published: March 2004

Using thermoelectric compatibility, efficient thermoelectric generators are rationally designed. With examples, compatible and incompatible systems are explained and materials proposed for targeted development. The compatibility factor explains why segmentation of (AgSbTe2)0.15(GeTe)0.85 (TAGS) with SnTe or PbTe produces little extra power, while filled skutterudite increases the efficiency from 10.5% to 13.6%. High efficiency generators are designed with compatible n-type La2Te3, and similar p-type material, while incompatible SiGe alloys actually reduce the efficiency. A refractory metal with high p-type thermopower (>100 μV/K) is required for development. Cascaded generators avoid the compatibility problem. The thermoelectric potential provides a simple derivation of the cascading ratio.
Space-resolved photoluminescence of ZnS:Cu,Al nanocrystals fabricated by sequential ion implantation
Authors: Ishizumi Atsushi , White C. W. , Kanemitsu Yoshihiko
Published: March 2004

We report on photoluminescence (PL) properties of Cu- and Al-doped ZnS nanocrystals fabricated by sequential implantation of Zn+, S+, Cu+, and Al+ ions into Al2O3 matrices. The spatially resolved PL spectrum has been studied by a scanning near-field optical microscope (SNOM). In the SNOM image, bright spots are observed on the sample surface. The PL spectrum at each bright spot is broad and is not sensitive to the monitored positions. The broad SNOM-PL spectrum at each spot is very similar to the macroscopic PL spectrum measured by conventional optics. The donor-acceptor pair luminescence process in nanocrystals is discussed.
Mapping of multiple-quantum-well layers and structure of V defects in InGaN/GaN diodes
Authors: Saijo H. , Hsu J. T. , Tu R. C. , Yamada M. , Nakagawa M. , Yang J. R. , Shiojiri M.
Published: March 2004

Cathodoluminescence mapping reveals threading defects, frequently formed by the lattice misfit between GaN and sapphire substrate, as a dark contrast connected with changes in the energy state. Multiple quantum wells, 2.5 nm In0.25Ga0.75N and 13.9 nm GaN layers, are resolved in the secondary electron image as well as in the backscattered electron image. The backscattered electron image, providing compositional mapping without surface effects such as cleaved steps, reveals the presence of V defects and confirms the thin six-walled structure of the V defect with InGaN/GaN {1011̄} layers. These scanning electron microscopy observations can be performed after very simple specimen preparation, namely just cleaving the sapphire substrate with the epilayers.
In-plane-type channel drop filter in a two-dimensional photonic crystal slab
Authors: Takano Hitomichi , Akahane Yoshihiro , Asano Takashi , Noda Susumu
Published: March 2004

An in-plane-type channel-drop filtering device with input/output waveguides and a point defect cavity in a two-dimensional photonic crystal slab is investigated. The in-plane operation becomes possible by employing a point defect cavity with an extremely high Q factor to suppress the radiation loss for the out-of-plane direction. 60° bends are also introduced in the output waveguide to avoid interference between the input/output waveguides. The transmission frequency range of the output waveguide with bends is tuned by changing the size of the air holes at the apex of the corner so that the resonant frequency of the point defect cavity is within the transmission band. A channel drop operation with a very high resolution of 0.12 nm is successfully observed at 1.55 μm wavelengths.
Nanoscale thin single-crystal silicon and its application to electronics
Authors: Avci Uygar , Tiwari Sandip
Published: March 2004

We demonstrate a method for achieving thin (tens of nanometers) single-crystal silicon films with electronic-quality bulk and surface properties, atomic-scale surface roughness, and suitable for use in complex multilayered structures. The thin silicon film is achieved by defining thickness through the use of oxidation depth as a reference, transfer of film structure by exfoliation, and chemical-mechanical polishing for selective thin film definition. Such structures are useful in a variety of directions: in electronic devices such as field-effect transistors, in characterization studies such as those utilizing vertical transport using thicknesses of the order of mean free path length, etc. We demonstrate its use in a silicon metal-oxide-semiconductor transistor with two gates, one on each side of the thin silicon film on a planar substrate.
Single graphene sheet detected in a carbon nanofilm
Authors: Horiuchi Shigeo , Gotou Takuya , Fujiwara Masahiro , Asaka Toru , Yokosawa Tadahiro , Matsui Yoshio
Published: March 2004

In order to clarify the existence of a single sheet of carbon six-membered-ring plane (graphene) this letter presents a method by which the stacking number of the sheets in a carbon nanofilm (CNF) can be exactly counted, based on the quantitative analysis of electron diffraction intensity. Using the method we can detect a single graphene sheet in a CNF.
Rewritable phase-change optical recording in Ge2Sb2Te5 films induced by picosecond laser pulses
Authors: Siegel J. , Schropp A. , Solis J. , Afonso C. N. , Wuttig M.
Published: March 2004

The phase transformation dynamics induced in Ge2Sb2Te5 films by picosecond laser pulses were studied using real-time reflectivity measurements with subnanosecond resolution. Evidence was found that the thermal diffusivity of the substrate plays a crucial role in determining the ability of the films to crystallize and amorphize. A film/substrate configuration with optimized heat flow conditions for ultrafast phase cycling with picosecond laser pulses was designed and produced. In this system, we achieved reversible phase transformations with large optical contrast (>20%) using single laser pulses with a duration of 30 ps within well-defined fluence windows. The amorphization (writing) process is completed within less than 1 ns, whereas crystallization (erasing) needs approximately 13 ns to be completed.
Photonic bandgaps in patterned waveguides of silicon-rich silicon dioxide
Authors: Neal R. T. , Zoorob M. E. , Charlton M. D. , Parker G. J. , Finlayson C. E. , Baumberg J. J.
Published: March 2004

We describe waveguides of photoluminescent silicon-rich silicon dioxide, which have been patterned by triangular two-dimensional (2D) photonic crystals to give higher-order photonic bandgaps occurring within the luminescence band of the core material. Photonic crystal modification of the photoluminescence spectrum allows identification of angle-tuned photonic bandgaps, in close agreement with 2D plane wave expansion and finite-difference time domain simulations. We discuss the importance of these findings for the development of integrated optical circuitry based on silicon-compatible microelectronics.
Thickness-dependent metal–insulator transition in V2O3 ultrathin films
Authors: Luo Qiang , Guo Qinlin , Wang E. G.
Published: March 2004

In this study, V2O3 ultrathin films about 5–20 nm thick were prepared on Al2O3 (0001) substrates through a reactive evaporation process. Auger electron spectroscopy and x-ray photoelectron spectroscopy have been used in situ to characterize their compositions and chemical states. Electric resistance measurements show that V2O3 films transform from metallic to semiconducting with the decrease of film thickness, which results from the a1g level rising because the lattice mismatch between the substrate and the film expands the c/a parameter ratio. No temperature-induced metal–insulator transition (like that in bulk V2O3) was observed in V2O3 thin films at low temperature. We conclude that stress plays a major role in suppressing the temperature-induced metal–insulator transition.
Effects of high-dose Mn implantation into ZnO grown on sapphire
Authors: Heo Y. W. , Ivill M. P. , Ip K. , Norton D. P. , Pearton S. J. , Kelly J. G. , Rairigh R. , Hebard A. F. , Steiner T.
Published: March 2004

ZnO films grown by pulsed-laser deposition on c-plane Al2O3 substrates were annealed at temperatures up to 600 °C to produce n-type carrier concentrations in the range 7.5×1015–1.5×1020 cm−3. After high-dose (3×1016 cm−2) Mn implantation and subsequent annealing at 600 °C, all the films show n-type carrier concentrations in the range 2–5×1020 cm−3 and room temperature hysteresis in magnetization loops. The saturation magnetization and coercivity of the implanted single-phase films were both strong functions of the initial anneal temperature, suggesting that carrier concentration alone cannot account for the magnetic properties of ZnO:Mn, and that factors such as crystalline quality and residual defects play a role.
Two-photon fluorescence-guided laser tweezers for study of cluster growth and gelation process
Authors: Kuzmin A. N. , Kachynski A. V. , Ohulchanskyy T. Y. , Roy I. , Prasad P. N. , Bruckenstein S.
Published: March 2004

Laser tweezers trapping technology has been used to monitor the bulk solution viscosity during the sol-gel gelation process at different depths from an interface. The gelation rate is the same in depth ranges 2–20 μm from the bounding surface. Simultaneously with the laser tweezers study, a microviscosity kinetic measurement of the sol-gel process was performed by fluorescent anisotropy and quantum yield methods. The large differences found between the bulk and microviscosities obtained in the experiment reflect the intrinsic differences in solution environment sensed by the laser tweezers on the macro level and by other optical techniques on the microlevels.
Magnetic domain structure of a La0.7Sr0.3MnO3 (001) surface observed by a spin-polarized scanning electron microscope
Authors: Konoto M. , Kohashi T. , Koike K. , Arima T. , Kaneko Y. , Tomioka Y. , Tokura Y.
Published: March 2004

The magnetization vector distribution at a cleaved surface of La0.7Sr0.3MnO3 (001) crystal has been quantitatively analyzed by using a newly developed low-temperature spin-polarized scanning electron microscope. The magnetic structure essentially consists of two kinds of domains, where magnetizations are parallel or antiparallel to the [110] direction with no surface-normal component. The rhombus-shaped domains range from a few to several tens of micrometers across. The domain structure can be considered to be made by laying down the magnetization from the out-of-surface-plane easy axis to the surface plane to reduce the magnetostatic energy without forming closure domains.
Transparent matrix structures for detection of neutron particles based on di-ureasil xerogels
Authors: Im Hee-Jung , Willis Carl , Stephan Andrew C. , Pawel Michelle D. , Saengkerdsub Suree , Dai Sheng
Published: March 2004

An important element in the development of advanced neutron detectors is the synthesis and characterization of improved, highly efficient neutron-scintillating materials. The research described here concerns the development of elastic, transparent, and thick film neutron scintillators with high Li+6 loading through room-temperature sol-gel processing. The room-temperature sol-gel processing allows an easy integration of such scintillating materials into electronic detecting devices. The lithium-6 salicylate di-ureasil xerogels developed here show promise for the fabrication of large area neutron detectors and have high potential for use in in situ monitoring and imaging of fissile materials and radioactive contaminants.
Partial focusing of radiation by a slab of indefinite media
Authors: Smith David R. , Schurig David , Mock Jack J. , Kolinko Pavel , Rye Patrick
Published: March 2004

Negative refraction can occur at the interface between vacuum and an indefinite medium—an anisotropic medium for which not all elements of the permittivity and permeability tensors have the same sign. We show experimentally and via simulations that a metamaterial composed of split ring resonators, designed to provide a permeability equal to −1 along the longitudinal axis, will redirect s-polarized electromagnetic waves from a nearby source to a partial focus. The dispersion characteristics of indefinite media prohibit the possibility of true aplanatic points for a planar slab; however, by contouring the surfaces aplanatic points may be realized, as well as other geometrical optical behavior.
Magneto-optical properties of a highly transparent cadmium ferrite-based magnetic fluid
Authors: Bakuzis A. F. , Skeff Neto K. , Gravina P. P. , Figueiredo L. C. , Morais P. C. , Silva L. P. , Azevedo R. B. , Silva O.
Published: March 2004

The magneto-optical properties of a highly stable ionic magnetic fluid sample containing CdFe2O4 nanoparticles were investigated using static magnetic birefringence, zero-field optical transmissivity, and transmission electron microscopy. From our measurements we found that the transmittivity and the birefringence of the CdFe2O4-based sample is several times greater than a typical magnetic fluid sample (γ-Fe2O3 based), giving this magnetic material great potential for magneto-optics applications. We also found that the birefringence can be increased by several orders of magnitude, allowing full manipulation of the observed negative differential transmitted optical intensity feature.
Band-stop magnetostatic waves micromachined resonators
Authors: Marcelli Romolo , Sajin George , Cismaru Alina , Craciunoiu Florea
Published: March 2004

Band-stop magnetostatic wave (MSW) resonators based on microstrip transducers realized on low-resistivity silicon (LRS) wafers and on micromachined silicon membranes have been studied. Frequency tunable MSW straight edge resonators made of yttrium–iron–garnet film have been used as selective frequency components. The frequency tunability ranged between 5 and 7.5 GHz for bulk devices and between 3.5 and 9.5 GHz for the membrane supported ones, with better rejection and wider bandwidth with respect to the bulk configuration. From measurements of the transmission parameter suppression of more than 20 dB of high-order modes and a rejection ratio better than 20 dB for the membrane supported resonators have been obtained. As a result, the possibility to manufacture microwave resonators characterized by high rejection ratios on low-cost, LRS membranes is demonstrated.
Electron transport properties in AlGaN/InGaN/GaN double heterostructures grown by metalorganic vapor phase epitaxy
Authors: Wang C. X. , Tsubaki K. , Kobayashi N. , Makimoto T. , Maeda N.
Published: March 2004

Electron transport properties in AlGaN/InGaN/GaN double heterostructures have been investigated. Samples were grown by metalorganic vapor phase epitaxy and evaluated using x-ray diffraction and variable temperature Hall effect measurements. Much higher two-dimensional electron gas density of up to 50% has been obtained in AlGaN/InGaN/GaN structure than in a typical AlGaN/GaN structure due to the larger polarization effect while the mobilities are comparable at room temperature and above in these structures, which demonstrates the suitability of an AlGaN/InGaN/GaN structure for high-power device applications. Theoretical simulations were done to investigate the carrier transport mechanism, and they suggest that alloy disorder and interface roughness scattering have a very strong impact on the electron transport properties in AlGaN/InGaN/GaN structures.
Ferromagnetism and magnetoresistance of Co–ZnO inhomogeneous magnetic semiconductors
Authors: Yan Shi-shen , Ren C. , Wang X. , Xin Y. , Zhou Z. X. , Mei L. M. , Ren M. J. , Chen Y. X. , Liu Y. H. , Garmestani H.
Published: March 2004

Co–ZnO inhomogeneous magnetic semiconductor thin films were synthesized on the subnanometer scale by sputtering. Room temperature ferromagnetism with high magnetization was found. Large negative magnetoresistance of 11% was found at room temperature, and its value increased with a decrease in temperature up to 36% at 4.8 K. The mechanism for large negative magnetoresistance is discussed.
Mature InAs quantum dots on the GaAs(114)A surface
Authors: Xu M. C. , Temko Y. , Suzuki T. , Jacobi K.
Published: March 2004

InAs quantum dots (QDs), grown by molecular-beam epitaxy on GaAs(114)A surfaces, were studied in situ by atomically resolved scanning tunneling microscopy. At their mature stage, the QDs present a complicated but regular shape being bound by flat {110}, (111)A, and {2 5 11}A facets, and a steep part composed of rather variable combinations of {110}, (111)A, {1̄1̄1̄}B, and {2 5 11} surfaces. The QD shape can be derived from mature InAs QDs on GaAs(001).
Polarization-induced electron island in semiconductor grain placed into pyroelectric matrix
Authors: Kachorovskii V. Yu. , Shur M. S.
Published: March 2004

We consider the electron distribution in an n-type semiconductor grain inserted into a pyroelectric matrix. We show that for typical values of parameters, the electric field induced in the grain by the pyroelectric spontaneous polarization is very strong and cannot be screened by the electrons in the grain. This field removes the electrons from donors and leads to the formation of a two-dimensional electron island on the surface of the grain with the size much smaller than the grain size. This effect can be used for creation of small two-dimensional quantum dots.
Clear quantum-confined luminescence from crystalline silicon/SiO2 single quantum wells
Authors: Cho Eun-Chel , Green Martin A. , Xia James , Corkish Richard , Reece Peter , Gal Mike
Published: March 2004

Crystalline silicon single quantum wells (QWs) were fabricated by high-temperature thermal oxidation of ELTRAN® (Epitaxial Layer TRANsfer) silicon-on-insulator (SOI) wafers. The Si layer thicknesses enclosed by thermal SiO2 range from 0.8 to 5 nm. Luminescence energies from such QWs vary from 1.77 to 1.35 eV depending on the Si layer thickness, without evidence for interface-mediated transition seen in earlier work. The ability to detect quantum-confined luminescence seems to arise from the use of ELTRAN SOI wafers, from suppressed interface state luminescence by high-temperature oxidation and, possibly, from interface matching by crystalline silicon oxide.
Electron counting of single-electron tunneling current
Authors: Fujisawa T. , Hayashi T. , Hirayama Y. , Cheong H. D. , Jeong Y. H.
Published: March 2004

Single-electron tunneling through a quantum dot is detected by means of a radio-frequency single-electron transistor. Poisson statistics of single-electron tunneling events are observed from frequency domain measurements, and individual tunneling events are detected in the time-domain measurements. Counting tunneling events gives an accurate current measurement in the saturated current regime, where electrons tunnel into the dot only from one electrode and tunnel out of the dot only to the other electrode.
Interpretation of anomalous temperature dependence of anti-Stokes photoluminescence at GaInP2/GaAs interface
Authors: Xu S. J. , Li Q. , Dong J.-R. , Chua S. J.
Published: March 2004

In this letter, we report on temperature-dependent anti-Stokes photoluminescence (ASPL) at an interface between partially ordered GaInP2 epilayer and GaAs substrate. It is found that the intensity of the ASPL depends strongly on temperature accompanying with a clear blueshift in energy. A localized-state luminescence model was employed to quantitatively interpret temperature dependence of the ASPL. Excellent agreement between the theory and experiment was obtained. Radiative recombination mechanism of the up-converted carriers was discussed.
Electromagnetic radiation produced by avalanches in the magnetization reversal of Mn12-acetate
Authors: Tejada J. , Chudnovsky E. M. , Hernandez J. M. , Amigó R.
Published: March 2004

Electromagnetic radiation produced by avalanches in the magnetization reversal of Mn12-acetate has been measured. Short bursts of radiation have been detected, with intensity significantly exceeding the intensity of the blackbody radiation from the sample. A model based upon superradiance from inversely populated spin levels has been suggested.
Local switching properties of dense nanocrystalline BaTiO3 ceramics
Authors: Mitoseriu Liliana , Harnagea Catalin , Nanni Paolo , Testino Andrea , Buscaglia Maria Teresa , Buscaglia Vincenzo , Viviani Massimo , Zhao Zhe , Nygren Mats
Published: March 2004

The switching properties of dense BaTiO3 ceramics with 50 nm average grain size were investigated at local scale by piezoresponse force microscopy. Large areas with low piezoelectrical activity beside islands with strong piezoresponse were found. The application of electric fields induces stable domain structures and changes in the polarization state far away from the probing area, probably via trans-granular dipole interactions. Piezoelectric hysteresis loops were recorded on various positions, even in regions with initial zero piezoresponse, which possibly showed a superparaelectric behavior. The results are incontestable proof that 50 nm BaTiO3 ceramics retain ferroelectricity at a local scale.
Ghrelin stimulation of growth hormone release and appetite is mediated through the growth hormone secretagogue receptor
Authors: Sun Yuxiang , Wang Pei , Zheng Hui , Smith Roy G.
Published: March 2004

Synthetic agonists of the growth hormone secretagogue receptor (GHSR) rejuvenate the pulsatile pattern of GH-release in the elderly, and increase lean but not fat mass in obese subjects. Screening of tissue extracts in a cell line engineered to overexpress the GHSR led to the identification of a natural agonist called ghrelin. Paradoxically, this hormone was linked to obesity. However, it had not been directly shown that the GHSR is a physiologically relevant ghrelin receptor. Furthermore, ghrelin's structure is significantly different from the synthetic agonist (MK-0677) used to expression-clone the GHSR. To address whether the GHSR mediates ghrelin's stimulatory effects on GH release and appetite, we generatedGhsr-null mice. In contrast to wild-type mice, acute treatment ofGhsr-null mice with ghrelin stimulated neither GH release nor food intake, showing that the GHSR is a biologically relevant ghrelin receptor. Nevertheless,Ghsr-null mice are not dwarfs; their appetite and body composition are comparable to that of wild-type littermates. Furthermore, in contrast to suggestions that ghrelin regulates leptin and insulin secretion, fasting-induced changes in serum levels of leptin and insulin are identical in wild-type and null mice. Serum insulin-like growth factor 1 levels and body weights of matureGhsr-null mice are modestly reduced compared to wild-type littermates, which is consistent with ghrelin's property as an amplifier of GH pulsatility and its speculated role in establishing an insulin-like growth factor 1 set-point for maintaining anabolic metabolism. Our results suggest that chronic treatment with ghrelin antagonists will have little effect on growth or appetite.
A latitudinal cline in flowering time in
Arabidopsis thaliana
modulated by the flowering time gene
FRIGIDA
Authors: Stinchcombe John R. , Weinig Cynthia , Ungerer Mark , Olsen Kenneth M. , Mays Charlotte , Halldorsdottir Solveig S. , Purugganan Michael D. , Schmitt Johanna
Published: March 2004

A latitudinal cline in flowering time in accessions of
Arabidopsis thaliana
has been widely predicted because the environmental cues that promote flowering vary systematically with latitude, but evidence for such clines has been lacking. Here, we report evidence of a significant latitudinal cline in flowering time among 70 Northern European and Mediterranean ecotypes when grown under ecologically realistic conditions in a common garden environment. The detected cline, however, is found only in ecotypes with alleles of the flowering time gene
FRIGIDA
(
FRI
) that lack major deletions that would disrupt protein function, whereas there is no relationship between flowering time and latitude of origin among accessions with
FRI
alleles containing such deletions. Analysis of climatological data suggests that late flowering in accessions with putatively functional
FRI
was associated with reduced January precipitation at the site of origin, consistent with previous reports of a positive genetic correlation between water use efficiency and flowering time in
Arabidopsis
, and the pleiotropic effects of
FRI
of increasing water use efficiency. In accessions collected from Southern latitudes, we detected that putatively functional
FRI
alleles were associated with accelerated flowering relative to accessions with nonfunctional
FRI
under the winter conditions of our experiment. These results suggest that the ecological function of the vernalization requirement conferred by
FRI
differs across latitudes. More generally, our results indicate that by combining ecological and molecular genetic data, it is possible to understand the forces acting on life history transitions at the level of specific loci.
Bmp4 signaling is required for outflow-tract septation and branchial-arch artery remodeling
Authors: Liu Wei , Selever Jennifer , Wang Degang , Lu Mei-Fang , Moses Kelvin A. , Schwartz Robert J. , Martin James F.
Published: March 2004

The
Bmp4
signaling molecule is expressed in ventral splanchnic and branchial-arch mesoderm and outflow-tract (OFT) myocardium, suggesting a role for
Bmp4
in OFT development. Inactivation of
Bmp4
in the caudal branchial arch and splanchnic mesoderm and OFT myocardium by using a conditional null allele of
Bmp4
and the

Nkx2.5
cre
recombinase

allele resulted in abnormal morphogenesis of branchial-arch arteries (BAAs) and defective OFT septation. Expression of aortic-sac myocardial markers was reduced and expression of the

sm22
LacZ

transgene, a smooth-muscle marker, was attenuated in BAAs and conotruncus of

Nkx2.5
cre
; Bmp4

conditional mutants. Moreover, we found tissue-specific functions for
Bmp4
in the regulation of cellular proliferation and apoptosis. We also demonstrate a strong genetic interaction between
Bmp4
and
Bmp7
in OFT development. Our findings uncover a previously uncharacterized function for
Bmp4
in vascular remodeling of the BAAs, and they show definitively that
Bmp4
, in cooperation with
Bmp7
, has a central role in OFT septation.
Characterization of a regiospecific epivancosaminyl transferase GtfA and enzymatic reconstitution of the antibiotic chloroeremomycin
Authors: Lu Wei , Oberthür Markus , Leimkuhler Catherine , Tao Junhua , Kahne Daniel , Walsh Christopher T.
Published: March 2004

Chloroeremomycin, a vancomycin family glycopeptide antibiotic has three sugars, one
d
-glucose and two
l
-4-
epi
-vancosamines, attached to the crosslinked heptapeptide backbone by three glycosyltransferases, GtfA, -B, and -C. Prior efforts have revealed that GtfB and -C in tandem build an epivancosaminyl-1,2-glucosyldisaccharide chain on residue 4 of the aglycone; however, the characterization of GtfA and glycosylation sequence of chloroeremomycin have been lacking. Here, we report the expression and purification of GtfA, as well as synthesis of its sugar donor, 2′-deoxy-thymidine 5′-diphosphate (dTDP)-β-
l
-4-
epi
-vancosamine. GtfA transfers 4-
epi
-vancosamine from the chemically synthesized dTDP-4-
epi
-vancosamine to the β-OH-Tyr
6
residue of the aglycone, preferentially after GtfB action, to generate chloroorienticin B. With the preferred kinetic order of GtfB, then GtfA, then GtfC established, we have succeeded in reconstitution of chloroeremomycin from the heptapeptide aglycone by the sequential actions of the three enzymes.
The structure of the RNA-dependent RNA polymerase from bovine viral diarrhea virus establishes the role of GTP in
de novo
initiation
Authors: Choi Kyung H. , Groarke James M. , Young Dorothy C. , Kuhn Richard J. , Smith Janet L. , Pevear Daniel C. , Rossmann Michael G.
Published: March 2004

The bovine viral diarrhea virus (BVDV) RNA-dependent RNA polymerase can initiate RNA replication by a
de novo
mechanism without a primer. The structure of BVDV polymerase, determined to 2.9-Å resolution, contains a unique N-terminal domain, in addition to the fingers, palm, and thumb domains common to other polymerases. The structure of BVDV polymerase complexed with GTP, which is required for
de novo
(primer-independent) initiation, shows that GTP binds adjacent to the initiation NTP, suggesting that the GTP mimics a vestigial RNA product. Comparison of five monomers in two different crystal forms showed conformational changes in the fingertip region and in the thumb domain that may help to translocate the RNA template and product strands during elongation. The putative binding sites of previously reported BVDV inhibitors are also discussed.
TGF-β regulatesin vivoexpansion of Foxp3-expressing CD4+CD25+regulatory T cells responsible for protection against diabetes
Authors: Peng Yufeng , Laouar Yasmina , Li Ming O. , Green E. Allison , Flavell Richard A.
Published: March 2004

CD4+CD25+regulatory T cells are essential in the protection from organ-specific autoimmune diseases. In the pancreas, they inhibit actions of autoreactive T cells and thereby prevent diabetes progression. The signals that control the generation, the maintenance, or the expansion of regulatory T cell poolin vivoremain poorly understood. Here we show that a transient pulse of transforming growth factor β (TGF-β) in the islets during the priming phase of diabetes is sufficient to inhibit disease onset by promoting the expansion of intraislet CD4+CD25+T cell pool. Approximately 40–50% of intraislet CD4+T cells expressed the CD25 marker and exhibited characteristics of regulatory T cells including small size, high level of intracellular CTLA-4, expression of Foxp3, and transfer of protection against diabetes. Results fromin vivoincorporation of BrdUrd revealed that the generation of a high frequency of regulatory T cells in the islets is due toin situexpansion upon TGF-β expression. Thus, these findings demonstrate a previously uncharacterized mechanism by which TGF-β inhibits autoimmune diseases via regulation of the size of the CD4+CD25+regulatory T cell poolin vivo.