bluuebunny/crossref_metadata_2025 huggingface dataset:
Transmittance and optical constants of Ho films in the 3–1340 eV spectral range
Authors: Fernández-Perea Mónica , Larruquert Juan I. , Aznárez José A. , Méndez José A. , Poletto Luca , Frassetto Fabio , Malvezzi A. Marco , Bajoni Daniele , Giglia Angelo , Mahne Nicola , Nannarone Stefano
Published: April 2011

The optical constants n and k of holmium (Ho) films were obtained in the 3–1340−eV range from transmittance measurements performed at room temperature. Thin films of Ho with various thicknesses were deposited by evaporation in ultra high vacuum conditions and their transmittance was measured in situ. Ho films were deposited onto thin C-film substrates supported on high transmittance grids. Transmittance measurements were used to obtain the extinction coefficient k of Ho films. The refractive index n of Ho was calculated with Kramers−Krönig analysis; in order to do this, k data were extrapolated both on the high and on the low energy parts of the spectrum by using experimental and calculated k values available in the literature. Ho, similar to other lanthanides, has a low-absorption band below the O2,3 edge onset; the lowest absorption was measured at ∼22 eV. Therefore, Ho is a promising material for filters and multilayer coatings in the energy range below the O2,3 edge in which most materials have a large absorption. Good consistency of the data resulted from the application of f and inertial sum rules.
Analysis of three-wave mixing in one-dimensional nonlinear multilayer structures with pump depletion
Authors: Ren Ming-Liang , Li Zhi-Yuan
Published: April 2011

We present a general theoretical approach that combines the iterative technique with the nonlinear transfer matrix method to solve the general problems of three-wave mixing with pump depletion in nonlinear multilayer structures. The theoretical model is directly derived from nonlinear coupled wave equations, shows fast numerical convergence and maintains conservation of total energy perfectly for general structures. We have employed this approach to discuss second harmonic generation in photonic bandgap crystals with slow light effect and obtained a conversion efficiency of about 25% for both the forward and backward second harmonic waves. The detailed wave evolution process and energy conversion mechanism of sum frequency generation and difference frequency generation in perfect and imperfectly quasiphase-matching structures have been systematically investigated and their subtle and strong dependence on the initial conditions of incident waves has been revealed. The developed theoretical approach can be very useful for efficient and quantitative analysis of various nonlinear optical problems and help to design optimum nonlinear multilayer structures for specific high-efficiency nonlinear three-wave mixing processes.
Simulation of bubble migration in a turbulent boundary layer
Authors: Mattson M. , Mahesh K.
Published: April 2011

This paper presents the results from a one-way coupled, Euler–Lagrangian, direct numerical simulation of bubbles injected into a turbulent boundary layer. The Reynolds number of the turbulent boundary layer varies from 420<Reθ<1800, and the bubble Reynolds number Reb∼1. Simulation parameters were chosen to match the experiment of Sanders et al. [J. Fluid Mech. 552, 353 (2006)] investigating bubble-induced skin-friction drag reduction in a turbulent boundary layer, although the Reynolds number of the simulation is lower than the experiment. After injection, bubbles move away from the wall as they travel downstream with the flow. Mean bubble diffusion is compared to Sanders et al. and the passive scalar diffusion results given by Poreh and Cermak [Int. J. Heat Mass Transfer 7, 1083 (1964)]. The mean diffusion profiles in the Sanders experiment and the simulation are comparable to the passive scalar results. Except very near the wall, the profiles of bubble concentration are also found to be similar to passive scalar results. The forces on a bubble were analyzed through budgets and the carrier-fluid acceleration was found to be the reason for moving the bubbles away from the wall.
Omnidirectional reflectance and vanishing Brewster angle of disordered semiconductor nanopillar surface with gradually changed refractive index
Authors: Meng Fan-Shuen , Lin Yung-Hsiang , Lin Gong-Ru
Published: April 2011

The geometrical-factor dependent Brewster angle shift in angular reflection spectra of the subwavelength nanopillar roughened semiconductor antireflective surface is investigated. Modeling of the nanopillar roughened surface evidently accounts for the shift and vanishing of the high-contrast transverse magnetic (TM) TM-mode angular reflectance spectrum with enlarging nanopillar height. The Brewster angle down-shifts by 20° with corresponding reflectance minimum increasing from <0.01% to 5.3% as the height of nanopillar increases from 0 to 240 nm, and the Brewster angle vanishing phenomenon is persistently observed until its disappearance with the nanopillar height enlarging up to 240 nm, leaving a slightly increased reflectance at larger incident angles.
Composition and structure of Pd nanoclusters in SiOx thin film
Authors: Thøgersen A. , Mayandi J. , Vines L. , Sunding M. F. , Olsen A. , Diplas S. , Mitome M. , Bando Y.
Published: April 2011

The nucleation, distribution, composition, and structure of Pd nanocrystals in SiO2 multilayers containing Ge, Si, and Pd are studied using high resolution transmission electron microscopy (HRTEM) and x-ray photoelectron spectroscopy (XPS), before and after heat treatment. The Pd nanocrystals in the as deposited sample (sample ASD) seem to be capped by a layer of PdOx. A 1–2 eV shift in binding energy was found for the Pd-3d XPS peak, due to initial state Pd to O charge transfer in this layer. The heat treatment results in a decomposition of PdO and Pd into pure Pd nanocrystals and SiOx.
Voltage dependent capacitances in CaCu3Ti4O12
Authors: Luo Xiao-jing , Yang Chang-ping , Song Xue-ping , Huang Chang , Wang Rui-long , Xu Ling-fang , Bärner K.
Published: April 2011

The capacitance–dc voltage relation has been investigated in the giant dielectric constant material CaCu3Ti4O12. A capacitance drop, rise and even maxima, minima have been found with increasing dc voltage, which could not simply be explained by the back-to-back Schottky barrier theory. A trap charges repositioning between field perpendicular boundaries and field parallel boundaries has been put forward to explain the strange C-V relations. In addition, a first order calculation of C-V has been done based on the charge repositioning model and has led to a successful fitting of the experimental curves, extracting parameters such as a polarisability of about 1.5×10−39 for the induced dipoles.
Rupture threshold characterization of polymer-shelled ultrasound contrast agents subjected to static overpressure
Authors: Chitnis Parag V. , Lee Paul , Mamou Jonathan , Allen John S. , Böhmer Marcel , Ketterling Jeffrey A.
Published: April 2011

Polymer-shelled micro-bubbles are employed as ultrasound contrast agents (UCAs) and vesicles for targeted drug delivery. UCA-based delivery of the therapeutic payload relies on ultrasound-induced shell rupture. The fragility of two polymer-shelled UCAs manufactured by Point Biomedical or Philips Research was investigated by characterizing their response to static overpressure. The nominal diameters of Point and Philips UCAs were 3 μm and 2 μm, respectively. The UCAs were subjected to static overpressure in a glycerol-filled test chamber with a microscope-reticule lid. UCAs were reconstituted in 0.1 mL of water and added over the glycerol surface in contact with the reticule. A video-microscope imaged UCAs as glycerol was injected (5 mL/h) to vary the pressure from 2 to 180 kPa over 1 h. Neither UCA population responded to overpressure until the rupture threshold was exceeded, which resulted in abrupt destruction. The rupture data for both UCAs indicated three subclasses that exhibited different rupture behavior, although their mean diameters were not statistically different. The rupture pressures provided a measure of UCA fragility; the Philips UCAs were more resilient than Point UCAs. Results were compared to theoretical models of spherical shells under compression. Observed variations in rupture pressures are attributed to shell imperfections. These results may provide means to optimize polymeric UCAs for drug delivery and elucidate associated mechanisms.
Floating bodies in two dimensions without gravity
Authors: Kemp Todd M. , Siegel David
Published: April 2011

In this paper we study the stability of equilibrium configurations for two-dimensional smooth convex bodies in the absence of gravity, focusing on one example in particular, the ellipse. We also begin to consider the floating configurations of bodies which are not strictly convex; that is, bodies that have one or more linear sides. We will show that a body cannot float in a stable equilibrium with the fluid interface intersecting the interior of a straight side in a single point, and then using this result we will proceed to study the stability of convex bodies comprised of only straight sides consequently deriving a necessary and sufficient condition for stable equilibrium of polygonal bodies. We illustrate our findings using the square as an example. Finally, we prove several results concerning the number of stable and unstable configurations for an n-sided regular polygon (n≥3) including a result guaranteeing the existence of a stable global energy minimum provided the contact angle γ is not 0 or π.
Pattern transformations in periodic cellular solids under external stimuli
Authors: Zhang K. , Zhao X. W. , Duan H. L. , Karihaloo B. L. , Wang J.
Published: April 2011

The structural patterns of periodic cellular materials play an important role in their properties. Here, we investigate how these patterns transform dramatically under external stimuli in simple periodic cellular structures that include a nanotube bundle and a millimeter-size plastic straw bundle. Under gradual hydrostatic straining up to 20%, the cross-section of the single walled carbon nanotube bundle undergoes several pattern transformations, while an amazing new hexagram pattern is triggered from the circular shape when the strain of 20% is applied suddenly in one step. Similar to the nanotube bundle, the circular plastic straw bundle is transformed into a hexagonal pattern on heating by conduction through a baseplate but into a hexagram pattern when heated by convection. Besides the well-known elastic buckling, we find other mechanisms of pattern transformation at different scales; these include the minimization of the surface energy at the macroscale or of the van der Waals energy at the nanoscale and the competition between the elastic energy of deformation and either the surface energy at the macroscale or the van der Waals energy at the nanoscale. The studies of the pattern transformations of periodic porous materials offer new insights into the fabrication of novel materials and devices with tailored properties.
Diode and photocurrent effect in ferroelectric BaTiO3−δ
Authors: Won C. J. , Park Y. A. , Lee K. D. , Ryu H. Y. , Hur N.
Published: April 2011

The leakage current has been regarded as one of the major problems in ferroelectric memories. However, recent studies on the unidirectional electric transport through a ferroelectric single crystal and the giant tunnel electroresistance in ferroelectric tunnel junctions suggest a possibility to utilize this undesirable leakage current. Here, we present the diodelike transport and the significantly enhanced photocurrent effects in oxygen-deficient BaTiO3−δ single crystals, which are mainly dependent on the direction of ferroelectric polarization. Diode effects in the carrier doped conventional ferroelectrics may suggest an alternative way for the nondestructive readout of polarization states in common ferroelectric memories.
The influence of pyridine ligand onto the structure and phonon spectra of CdSe nanocrystals
Authors: Dzhagan V. M. , Lokteva I. , Himcinschi C. , Kolny-Olesiak J. , Valakh M. Ya. , Schulze S. , Zahn D. R. T.
Published: April 2011

The influence of ligand exchange for pyridine onto the structure and phonon spectra of oleic acid-stabilized CdSe nanocrystals (NCs) is studied by resonant Raman and optical absorption spectroscopy, nuclear magnetic resonance and transmission electron microscopy. The removal of oleic acid ligand by pyridine treatment results in change of intensity ratio of the longitudinal optical (LO) phonon peak to its overtones. The latter effect is attributed to a changed electron-phonon coupling in NCs upon introduction of the hole-capturing ligand (pyridine). The upward shift and broadening of the LO phonon peak are also observed and supposed to be the result of interplay between partial oxidation of the NC and strain induced by surface reconstruction. The relative contribution of these two effects is found to be dependent on the NC size. The activation of two additional Raman features, in the low-frequency range and above the LO band, for pyridine-treated NCs is supposed to be related with induced disorder or reconstruction on the NC surface. No noticeable effect of the surface treatment and concomitant NC aggregation onto the surface optical phonon mode was observed.
Implosion and explosion of electrostatic cylindrical and spherical shocks in asymmetric pair-ion plasmas
Authors: Masood W. , Rizvi H.
Published: April 2011

Nonlinear electrostatic shock waves are studied in unmagnetized, dissipative pair-ion plasmas. The dissipation in the system is taken into account by considering the effect of kinematic viscosity of both positive and negative ions in plasmas. The system of fluid equations for asymmetric pair-ion plasma is reduced to Korteweg–deVries–Burgers equation in the limit of small amplitude perturbation. It is observed that the system under consideration admits rarefactive shocks. Keeping in view the practical applications, the nonlinear propagation of both the exploding and imploding shocks is investigated and the differences are expounded in detail. The present study may have relevance in the study of the formation of electrostatic shocks in laser-induced implosion devices, star formation, supernovae explosion, etc.
Skin depth criterion for foil thickness in laser pressure acceleration of monoenergetic ions
Authors: Tripathi Vipin K. , Liu Chuan S. , Shao Xi , Sharma Anamika
Published: April 2011

An analytical formalism is developed to study the nonlinear laser penetration through radiation pressure accelerated thin foils, employed to produce high energy ions. We include the effects of relativistic mass increase and nonuniform electron density compression due to the ponderomotive force in plasma permittivity. For foils like diamond the usual optimum foil thickness lopt=λLncra0/πn0 (where λL and a0 are the wavelength and normalized amplitude of the laser), at which the ponderomotive force on electrons balances the space charge force due to the ions left behind, is significantly below the skin depth unless the laser intensity is excessively large and significant laser transmission through the foil reduces the radiation pressure on it. The reflection coefficient decreases with laser field strength, though the ponderomotive force led electron compression tends to raise it. The reflection coefficient also decreases significantly with foil velocity, which is a sensitive function of laser field strength and foil thickness.
Soliton propagation, reflection, and transmission in an inhomogeneous plasma with trapped electrons
Authors: Aziz Farah , Malik Hitendra K. , Stroth Ulrich
Published: April 2011

Transmission and reflection of solitons from a semitransparent grid in an inhomogeneous plasma in the presence of trapped electrons is studied analytically. Conditions are obtained for the obliqueness of the propagation and the drift velocity of ions for the soliton transmission and reflection. Also, a transmission–reflection conservation law is derived. The contribution of trapped electrons to the solitons’ propagation and their reflection and transmission is examined through energy, amplitude, and width of the solitons, in addition to the effect of temperature and drift of the ions.
Effect of surface preparation and interfacial layer on the quality of SiO2/GaN interfaces
Authors: Alam E. Al , Cortés I. , Besland M.-P. , Goullet A. , Lajaunie L. , Regreny P. , Cordier Y. , Brault J. , Cazarré A. , Isoird K. , Sarrabayrouse G. , Morancho F.
Published: April 2011

In this work, SiO2/GaN MOS structures have been fabricated using Electron Cyclotron Resonance Plasma Enhanced Chemical Vapor Deposition (ECR-PECVD) for deposition of silicon dioxide (SiO2) at low temperature (300 °C) on N-type, NID and P-type GaN epitaxial layers. Surface preparation involving chemical, UV-Ozone oxidation and oxygen plasma oxidation have been investigated by XPS analysis of the GaN surfaces prior to SiO2 deposition. The association of UV ozone and plasma oxidation allows a complete removal of carbon contamination and has a huge beneficial effect on the quality of the SiO2/GaN interface. Electrical C-V characterizations put into evidence the improved quality of the SiO2/GaN interface with a low interface trap density of 1010 cm−2 eV−1. The advantage of this soft interface treatment is thus specially observed for the N-type samples without annealing step, whereas improvements are still needed in the case of NID and P-type samples.
Nitrogen plasma treatment of fluorine-doped tin oxide for enhancement of photo-carrier collection in amorphous Si solar cells
Authors: Baik Seung Jae , Lim Koeng Su
Published: April 2011

Nitrogen plasma treatment was performed on fluorine-doped tin oxide (SnO2:F) front electrodes, and its impact on the performance of pin type amorphous Si (a-Si) solar cells was investigated. Nitrogen plasma treatment reverses the surface band bending of SnO2:F from accumulation to depletion, thus in turn reversing the band bending of the p type amorphous silicon carbide (p-a-SiC) window layer. The reversal of band bending leads to the collection of carriers generated in p-a-SiC, and quantum efficiency in the short wavelength regime is thereby enhanced. On the other hand, surface depletion of SnO2:F causes a reduction of the diode built-in voltage and increased series resistance, which could degrade the open circuit voltage (Voc) and fill factor (FF), the degradation of which is strongly affected by the deposition time of p-a-SiC.
Photoluminescence of nanocrystalline titanium dioxide films loaded with silver nanoparticles
Authors: Preclíková Jana , Galář Pavel , Trojánek František , Rezek Bohuslav , Němcová Yvonne , Malý Petr
Published: April 2011

We report on a systematic study of the photoluminescence properties of nanocrystalline titanium dioxide films loaded with silver nanoparticles under various ambient conditions: in the temperature interval of 10 to 300 K, under different values of ambient air pressure (5 to 105 Pa), and under visible light irradiation that causes the photochromic transformation of the film. Our results enable us to follow the electron transfer from titanium dioxide into silver nanoparticles and to observe the oxidation of silver nanoparticles during the photochromic transformation. We propose a microscopic model explaining the behavior of extinction and photoluminescence of the Ag–TiO2 film under different values of ambient air pressure.
Size-dependent elastic properties of single-walled ZnO nanotubes: A first-principles study
Authors: Wen Yu-Hua , Zhang Yang , Wu Shun-Qing , Zhu Zi-Zhong
Published: April 2011

By means of first-principles calculations, we have investigated a size dependence of elastic modulus in single-walled ZnO nanotubes with armchair and zigzag forms. It is found that for these tubes the Young’s modulus is increased dramatically with the increased diameters and inversely proportional to the Zn−O bond length. Further, the amount of charge transfer, calculated by the Bader analysis, is introduced to elucidate the strength of bonding between Zn and O atoms in these tubes.
A first principles theoretical examination of graphene-based field effect transistors
Authors: Champlain James G.
Published: April 2011

This paper presents an in-depth theoretical examination of graphene-based field effect transistors, looking at thermal statistics, electrostatics, and electrodynamics. Using a first principles approach, the unique behavior observed in graphene-based field effect transistors, such as the V-shaped transfer characteristic, limited channel pinch-off, and lack of off-state (under gate modulation), are described. Unlike previous attempts, a description of both drift and diffusion currents in the device is presented. The effect of external resistance on steady-state and high-frequency performance is examined. Comparisons of the theoretical results to experimental results are made and show good agreement. Finally, the theoretical work in this paper is used as a basis to discuss the possible source of some observed behavior in practical graphene-based field effect transistors.
All-inorganic spin-cast quantum dot based bipolar nonvolatile resistive memory
Authors: Kannan V. , Chae Y. S. , Ramana CH. V. V. , Ko Dong-Sik , Rhee J. K.
Published: April 2011

We introduce an all-inorganic solution processed bipolar nonvolatile resistive memory device with quantum dot/metal-metal oxide/quantum dot structure. The two terminal device exhibits excellent switching characteristics with ON/OFF ratio >103. The device maintained its state even after removal of the bias voltage. The switching time is faster than 50 ns. Device did not show degradation after 1-h retention test at 150 °C. The memory functionality was consistent even after multiple cycles of operation and the device is reproducible. The switching mechanism is discussed on the basis of charge trapping in quantum dots with metal oxide serving as the barrier.
Experimental and simulation study of missing matching peaks in Nb thin films with square pinning arrays
Authors: Cao R. , Horng Lance , Wu T. C. , Lin J. C. , Wu J. C. , Yang T. J. , Koláček J.
Published: April 2011

Square arrays of circular pinning centers of various diameters were patterned in Nb thin films to explore their vortex pinning behavior. Periodic critical current matching peaks and some “missing peaks” were observed in magnetotransport measurements. The larger the diameter of the pinning centers, the higher the index of the missing matching fields observed. This phenomenon is explained by molecular dynamics simulations and is caused by the interaction between interstitial vortices and vortices occupying the pinning centers.
On the dynamic behavior of three readily available soft tissue simulants
Authors: Appleby-Thomas G. J. , Hazell P. J. , Wilgeroth J. M. , Shepherd C. J. , Wood D. C. , Roberts A.
Published: April 2011

Plate-impact experiments have been employed to investigate the dynamic response of three readily available tissue simulants for ballistic purposes: gelatin, ballistic soap (both subdermal tissue simulants), and lard (adipose layers). All three materials exhibited linear Hugoniot equations-of-state in the US-uP plane. While gelatin behaved hydrodynamically under shock, soap and lard appeared to strengthen under increased loading. Interestingly, the simulants under test appeared to strengthen in a material-independent manner on shock arrival (tentatively attributed to a rearrangement of the amorphous molecular chains under loading). However, material-specific behavior was apparent behind the shock. This behavior appeared to correlate with microstructural complexity, suggesting a steric hindrance effect.
Magnetoresistance of substituted lanthanum manganites La0.7Ca0.3MnO3 upon nonequilibrium overheating of carriers
Authors: Shaykhutdinov K. A. , Semenov S. V. , Popkov S. I. , Balaev D. A. , Bykov A. A. , Dubrovskiy A. A. , Petrov M. I. , Volkov N. V.
Published: April 2011

Current–voltage characteristics of the polycrystalline substituted lanthanum manganite La0.7Ca0.3MnO3 were experimentally studied at T = 77.4 K in magnetic fields up to 13 kOe. In these characteristics, a portion of negative differential resistivity was observed above a certain threshold value of critical current density j caused, in our opinion, by nonequilibrium heating of the electron gas due to low thermal conductivity of the manganite material. Because of the nonlinearity of the current–voltage characteristics, the field dependences of resistivity ρ(H) appear extremely sensitive to the value of a transport current. In this case, the ρ(H) dependences reveal both ordinary negative and positive magnetoresistance.
Atomic vacancy defects in the electronic properties of semi-metallic carbon nanotubes
Authors: Zeng Hui , Zhao Jun , Hu Huifang , Leburton Jean-Pierre
Published: April 2011

We investigate the electronic properties of semimetallic (12,0) carbon nanotubes in the presence of a variety of monovacancy, divacancy, and hexavacancy defects, by using first principle density functional theory combined with nonequilibrium Green’s function technique. We show that defect states related to the vacancies hybridize with the extended states of the nanotubes to modify the band edge, and change the energy gap. As a consequence, the nanotube conductance is not a monotonic function of the defect size and geometry. Paradoxically, tetravacancy and hexavacancy nanotubes have higher conductance than divacancy nanotubes, which is due to the presence of midgap states originating from the defect, thereby enhancing the conductance.
Target-heating effects on the Kα1,2-emission spectrum from solid targets heated by laser-generated hot electrons
Authors: Nilson P. M. , Theobald W. , Mileham C. , Stoeckl C. , Myatt J. F. , Delettrez J. A. , MacFarlane J. , Begishev I. A. , Zuegel J. D. , Betti R. , Sangster T. C. , Meyerhofer D. D.
Published: April 2011

Target-heating effects on the Kα1,2-emission spectrum from small-mass Cu targets irradiated with 1-ps pulses focused to intensities >1018 W/cm2 have been observed. A collisional-radiative atomic physics model is unable to reproduce the time-integrated Kα1,2-emission spectrum from the smallest-mass targets when calculated with a single, time-independent thermal-electron temperature. When time-dependent heating to several hundred electron volts is included in the model, the synthetic spectra better reproduce the main observed spectral features.
Electronic transport and I-V characteristics of polycrystalline LuFe2O4
Authors: Fisher B. , Genossar J. , Patlagan L. , Reisner G. M.
Published: April 2011

We report on electronic transport at low electric fields and I-V measurements on a polycrystalline sample of LuFe2O4. The range of temperatures of our measurements (limited by the high resistivity of the sample at low temperatures) includes the range where dramatic nonlinear conductivity and phase transitions were reported in the past. The temperature dependence of the absolute thermopower, measured up to 400 K, exhibits the characteristics of the 3-D charge-ordering and the magnetic-ordering transitions; the temperature dependence of the resistivity, measured only up to room temperature, indicates activated transport. Using single, short current pulses we obtained perfectly linear I-V characteristics up to fields of ∼500 V/cm. The large nonlinearity of the d.c. I-V characteristics at high fields is typical of self-heating in samples with activated conductivity.
Phase stability of intermetallic PtIn2 under pressure: An in-situ transport, structural and first principles investigations
Authors: Garg Alka B. , Modak P. , Vijayakumar V.
Published: April 2011

The high pressure phase stability of intermetallic PtIn2 at ambient temperature has been investigated using the synchrotron based angle dispersive x-ray diffraction technique, electrical resistance measurements, and first principles electronic structure calculations. The electrical resistance falls continuously up to 5 GPa and then remains almost constant up to 10 GPa. Beyond 10 GPa, the resistance increases and continues to increase up to 21 GPa which is the highest pressure of the present measurements. Consistent with this, the x-ray diffraction measurements show a structural phase transition from the ambient cubic phase to a low symmetry orthorhombic phase beyond 10 GPa. Upon pressure release, the ambient cubic phase is obtained. Ab initio structural relaxations and total energy calculations predict the cubic to orthorhombic phase transitions at 10.5 GPa.
AlGaN/GaN high-electron mobility transistors with low thermal resistance grown on single-crystal diamond (111) substrates by metalorganic vapor-phase epitaxy
Authors: Hirama Kazuyuki , Taniyasu Yoshitaka , Kasu Makoto
Published: April 2011

AlGaN/GaN heterostructures with a wurtzite structure were epitaxially grown on single-crystal diamond (111) with a diamond structure by metalorganic vapor phase epitaxy. In the AlGaN/GaN heterostructure, two-dimensional electron gas with sheet carrier density of 1.0×1013 cm−2 and mobility of 730 cm2/V s was obtained. The 3-μm-gate-length AlGaN/GaN high-electron mobility transistors (HEMTs) show maximum drain current of 220 mA/mm, cut-off frequency of 3 GHz, and maximum frequency of oscillation of 7 GHz. The thermal resistance of the AlGaN/GaN HEMTs on diamond substrates is 4.1 K mm/W, the lowest ever reported for AlGaN/GaN HEMTs, due to the high thermal conductivity of single-crystal diamond.
Transition between positive and negative permittivity in field-dependent metamaterial
Authors: Sakai Osamu
Published: April 2011

Permittivity in a metamaterial, which depends on the electric field of a propagating electromagnetic wave, induces anomalous responses against electromagnetic waves. When we consider discharge plasma an example of a metamaterial with field-dependent permittivity and negative permeability, a transition takes place between positive and negative permittivity, leading to reflection of waves in a low-field branch and transmission by the creation of a negative-refractive-index medium in a high-field branch. Nonlinearity in the phase space of the permittivity and the electric field in this system trigger this dynamic transition, which is suggested in the numerical results and clarified in an analytical model.
Ab initio investigation on the magnetic ordering in Gd doped ZnO
Authors: Bantounas Ioannis , Goumri-Said Souraya , Benali Kanoun Mohammed , Manchon Aurélien , Roqan Iman , Schwingenschlögl Udo
Published: April 2011

The current study investigates the magnetic properties of the GdxZn1-xO, with x=0.0625 and 0.0185, doped semiconductor using the full potential (linearized) augmented plane wave plus local orbital method. We show that in contrast to the findings of Shi et al. [J. Appl. Phys. 106, 023910 (2009)], the implementation of the Hubbard U parameter to the Gd f states favors an antiferromagnetic phase in both wurtzite GdO and GdxZn1-xO. Spin polarized calculations on GdxZn1-xO indicate that, even if a ferromagnetic ground state were favored, the magnetic influence of Gd in a perfect ZnO wurtzite lattice is highly localized and limited to the first three nearest neighboring O atoms. Increasing the supercell size and thus diluting the concentration of Gd within the ZnO matrix does not show any changes in the net magnetic moment between these three O atoms nor in the remaining lattice sites, indicating that sizing effects do not influence the range of matrix polarization. We conclude that the localized Gd induced polarization can not account for long range magnetic ordering in a defect-free ZnO wurtzite lattice.
Method for fabricating third generation photovoltaic cells based on Si quantum dots using ion implantation into SiO2
Authors: Yedji M. , Demarche J. , Terwagne G. , Delamare R. , Flandre D. , Barba D. , Koshel D. , Ross G. G.
Published: April 2011

In this paper, we report on the synthesis of silicon quantum dots for photovoltaic applications by means of ion implantation followed by annealing. Nucleation was achieved by implanting Si+ ions into SiO2 thin films, previously thermally grown on a Si(100) substrate, and annealing to 1100 °C. Passivation was used for photoluminescence (PL) measurements. The thickness of the oxide layer, the stoichiometry of the implanted layer, and the depth profiles of the implanted ions were determined for all samples by both Rutherford backscattering spectroscopy (RBS) and ellipsometry techniques. Characterization by transmission electron microscopy (TEM) indicates that the diameter of the silicon quantum dots (Si-QDs) varies from 2 to 4 nm, which is less than the Bohr radius of bulk crystalline Si(∼5 nm). Optical and electrical properties have been investigated by PL and I-V measurements. When passivated silicon nanocrystals (Si-nc) embedded into SiO2 are excited using a 450 nm diode laser, they exhibit a strong PL emission in the range of 650-1000 nm. Based on these investigations, p-type Si-QDs/n-type c-Si junctions were fabricated and electrically characterized in the dark as well as under an AM1.5G terrestrial solar spectrum for nonimplanted, as-implanted, and implanted-annealed samples for different implantation fluences. The electrical curves of the structures under illumination demonstrate the photovoltaic behavior of the Si-QDs. Despite the weak light conversion of these devices, these results remain very promising and offer potentially unprecedented, vast improvements to third generation solar cells.
The susceptibility diagrams of a multipactor discharge on a dielectric: Effects of rf magnetic field
Authors: Sazontov A. G. , Nechaev V. E. , Vdovicheva N. K.
Published: April 2011

A statistical theory is constructed to evaluate the multipactor induced breakdown on a dielectric. The calculation employs the integral equation allowing predicting the threshold of multipactor growth. It is found that the action of the rf magnetic field considerably changes the shape of the upper susceptibility curve and, depending on the secondary electron yield, leads either to rising of the slope of the upper boundary or to its complete elimination. The critical value of the secondary yield above which the region of multipactor existence is not restricted from above by any limiting rf electric field strength is estimated.
Phase transition in porous electrodes
Authors: Kiyohara Kenji , Sugino Takushi , Asaka Kinji
Published: April 2011

It is shown by Monte Carlo simulation that electrochemical thermodynamics of electrolytes in a porous electrode is qualitatively different from that in the bulk with a planar electrode. In particular, first order phase transitions occur in porous electrodes when the pore size is comparable to the ion size of the electrolytes: as the voltage is increased from zero, the surface charge density and the ion density in the porous electrodes discontinuously change at a specific voltage. The critical points for those phase transitions are identified.
On the hyperbolicity of Maxwell's equations with a local constitutive law
Authors: Perlick Volker
Published: April 2011

Maxwell's equations are considered in metric-free form, with a local but otherwise arbitrary constitutive law. After splitting Maxwell's equations into evolution equations and constraints, we derive the characteristic equation and we discuss its properties in detail. We present several results that are relevant for the question of whether the evolution equations are hyperbolic, strongly hyperbolic, or symmetric hyperbolic. In particular, we give a convenient characterization of all constitutive laws for which the evolution equations are symmetric hyperbolic. The latter property is sufficient, but not necessary, for well-posedness of the initial-value problem. By way of example, we illustrate our results with the constitutive laws of biisotropic media and of Born–Infeld theory.
Exciton-polariton microphotoluminescence and lasing from ZnO whispering-gallery mode microcavities
Authors: Dai J. , Xu C. X. , Sun X. W. , Zhang X. H.
Published: April 2011

Hexagonal ZnO microrods were employed as whispering-gallery mode (WGM) optical microcavities to investigate exciton-polariton microphotoluminescence and lasing emission. Using a confocal microphotoluminescence system, the exciton-polariton emission with a large Rabi splitting of about 90 meV was observed from a ZnO microrod with the diameter of 9.38 μm. The spatial-resolved spectra demonstrated a collective nonlinear blueshift in the WGM resonance peaks along a tapered microrod and proved the anticross dispersion property of the exciton-polariton. Furthermore, the exciton-polariton WGM lasing was stimulated and blueshifted in the strong coupling region under the excitation of a 355 nm nanosecond pulsed laser.
The effect of electronegative difference on the electronic structure and visible light photocatalytic activity of N-doped anatase TiO2 by first-principles calculations
Authors: Zhao Dongqiu , Huang Xiaowei , Tian Baoli , Zhou Shaomin , Li Yuncai , Du Zuliang
Published: April 2011

The effect of electronegative difference between nitrogen and oxygen on electronic properties of N-doped anatase TiO2 has been studied using first-principles calculations. The results indicate that the valence band maximum (VBM) shifts to high energy by 0.27 eV and the band gap states composed of N 2p, O 2p, and Ti 3d states are formed through the three states entering into the gap after N doping. The interactions of three states widen and delocalize the band gap states. The raised VBM and the wide band gap states can improve the visible light photocatalytic activity.
Whole field decoupling of predistortion on polymeric cell force transducer
Authors: Zheng Xiaoyu , Zhang Xin
Published: April 2011

Microfabricated polymer transducers have been developed to study cell mechanics. The key principle is to quantify the deformations on the sensor arrays induced by cell contractions and convert them into force distributions. The simplifications in deformation measurements come from the basic assumption that the deformation is solely attributed to cell contractions triggered by chemical or electrical stimuli. The diffraction moiré fringes via two polymer gratings provide whole field evolutions of distortion/strain on soft-lithography fabricated substrates. We found that the moiré patterns are able to decouple predistortions which were traditionally thought to be solely caused by cell contractile forces.
Minority carrier transport length of electrodeposited Cu2O in ZnO/Cu2O heterojunction solar cells
Authors: Liu Yingchi , Turley Hubert K. , Tumbleston John R. , Samulski Edward T. , Lopez Rene
Published: April 2011

The minority carrier transport length is a critical parameter limiting the performance of inexpensive Cu2O–ZnO photovoltaic devices. In this letter, this length is estimated to be ∼430 nm for electrochemically deposited Cu2O by linking the cell’s carrier generation profile with back and front incident photon-to-electron conversion efficiency measurements to a one-dimensional transport model. This critical length explains the losses typically presented by these devices and appears to correlate well with the microcrystalline film structure. The consequences of the magnitude of the length on device design with the aim of improving solar cell performance are described.
Generating infinite-dimensional algebras from loop algebras by expanding Maurer–Cartan forms
Authors: Caroca R. , Merino N. , Salgado P. , Valdivia O.
Published: April 2011

It is shown that the expansion methods developed in de Azcarraga et al.[Nucl. Phys. B 662, 185 (2003); Class. Quantum Grav. 21, S1375 (2004)]; can be generalized so that they permit to study the expansion of algebras of loops, both when the compact finite-dimensional algebra and the algebra of loops have a decomposition into two subspaces.
The stability and work function of TaCxN1−x alloy surfaces
Authors: Zhu H. , Ramprasad R.
Published: April 2011

The stability and work function of the (001), (110), and (111) surfaces of ordered TaCxN1−x crystals with various possible terminations were studied using density functional theory calculations. Among all surfaces considered, those with (001) orientations were found to be the most stable. The work function of these (001) stable surfaces increases monotonically from the pure TaN value of 3.3 eV to the pure TaC value of 3.8 eV. However, this variation is far surpassed by the strong dependence of the work function on the surface orientation and chemistry, regardless of the bulk alloy composition.
Photorotamerization of matrix-isolated acrylic acid revisited
Authors: Kuş Nihal , Ai Yue-Jie , Fang Wei-Hai , Fausto Rui
Published: April 2011

In this study, the conformational preferences and photochemistry of acrylic acid (AA, CH2=CHCOOH) monomer isolated in cryogenic argon and krypton matrices were interpreted, based on results of quantum chemical calculations. Natural bond orbital analysis allowed to shed light on the main electronic effects determining the relative stability of the conformers of the molecule in the ground electronic state. The conformational isomerization taking place upon UV-irradiation of the matrix-isolated compound (λ ∼ 243 nm) was explained, based on theoretical complete active space self-consistent field/complete active space with second order perturbation theory (CASSCF/CASPT2) and time-dependent density functional theory (TD-DFT) results, allowing to rationalize the nearly equal populations of the two lowest energy conformers of the molecule observed in the photostationary state. Besides, details of the infrared spectra of the compound were reinterpreted based on the calculated spectra for the two most stable conformers of the molecule. In particular, the assignments for the out-of-plane A″ symmetry vibrations were revised.
On the influence of nonlocal molecular vibrations and charge transfer on the spectra of aggregated push–pull chromophores
Authors: Rodriguez Justo J. , Salam A.
Published: April 2011

Through their fluorescence spectrum, aggregates of push–pull chromophores are good reporters of their microenvironment temperature and polarity. The understanding of the fluorescence and charge-separation dynamics in arrays composed of this type of species is consequently of considerable interest. In this article, we study the effect of charge fluctuations induced by molecular nonlocal vibrations on the electronic coupling between a pair of linear push–pull chromophores, for side-to-side or head-to-tail orientations, using a valence-bond charge-transfer (VB-CT) model and the Redfield equation. The results show that the exciton–vibrational dynamics along the bond length alternation coordinate can significantly modify the inter-molecular electronic coupling, which determines the fluorescence spectral band redshift due to aggregation. Numerical results for the electronic and exciton–vibrational contributions to the Coulombic coupling between two of these chromophores are obtained using experimentally based parameters for polyene linker species. The exciton–vibrational contribution is significant relative to the electronic contribution at room temperature in some ranges of the energy gap between the VB and CT states, and it is more important for the side-to-side than for the head-to-tail configuration. Our calculations also show that, even without including solvation effects, the spectral band associated with an S0 → S1 transition is redshifted with increasing temperature.
Hot spot analysis in integrated circuit substrates by laser mirage effect
Authors: Perpiñà X. , Jordà X. , Vellvehi M. , Altet J.
Published: April 2011

This work shows an analytical and experimental technique for characterizing radial heat flow present in integrated circuits (ICs) when power is dissipated by integrated devices. The analytical model comes from the resolution of the Fermat equation for the trajectory of rays and supposing a spherical heat source dissipating a time-periodic power. An application example is presented; hence demonstrating how hot spots and heat transfer phenomena in the IC substrate can be characterized. The developed method may become a practical alternative to usual off-chip techniques for inspecting hot spots in ICs and to experimentally characterize heat flow in the semiconductor substrate.
Scalings of strongly coupled collisionless q = 2 triple tearing modes in a tokamak plasma
Authors: Wei Lai , Wang Zheng-Xiong , Fan Dong-Mei , Wang Feng , Liu Yue
Published: April 2011

Within the framework of a reduced collisionless magnetohydrodynamics (MHD) model containing the effects of electron inertia and electron pressure gradient, scaling laws of the strongly coupled collisionless q=2 triple tearing instabilities with broad linear spectra in a tokamak plasma are studied numerically. It is found that as the poloidal mode number m increases, the scaling power laws of the linear growth rate of triple tearing modes (TTMs) on SHp, de, and ρs change gradually from the strongly coupled tearing mode scalings to the standard single tearing mode (STM) scalings, where SHp is the magnetic Reynolds number, de is the electron inertial skin depth, and ρs is the ion sound gyroradius. For example, de-scaling of the linear growth rate γ~deαd changes gradually from γ~de1 to γ~de3 as the m number increases in the case of ρs≪de, while ρs-scaling γ~deαρ shifts gradually from γ~ρs2/3 to γ~ρs1 as m increases in the case of de≪ρs. Furthermore, it is observed that the increase of ρs gradually weakens the dependence on de in both large and small Δ' regimes, such as, ~de1→~de1/3 in the large Δ' regime, where Δ' is the linear instability parameter of tearing modes. The numerical results about the scaling laws are validated by the previous relevant analytical theories in terms of the similarity of physical characteristics in the same Δ' regime. In addition, the effects of de and ρs on the spectrum structures of the unstable TTMs are discussed. Finally, the spatial characteristics of the eigenmode structures for different mode numbers are analyzed in detail. It is interestingly found that the structure of the most unstable eigenmode of the TTMs is quite similar to the eigenmode structure of a standard double tearing mode (DTM).
An integrated laser Raman optical sensor for fast detection of nitrogen and oxygen in a cryogenic mixture
Authors: Tiwari Vidhu S. , Luanje Appolinaire T. , Kalluru Rajamohan R. , Yueh Fang Y. , Singh Jagdish P.
Published: April 2011

An integrated fiber optic Raman sensor was designed for real-time, nonintrusive detection of liquid nitrogen (LN2) in liquid oxygen (LO2) at high pressures and high flow rates. This was intended to monitor the quality of LO2 in oxidizer feed lines during the ground testing of rocket engines. Various issues related to optical diagnosis of cryogenic fluids (LN2/LO2) in supercritical environment of rocket engine test facility, such as fluorescence from impurity in optical window of feed line, signal-noise ratio, and fast data acquisition time, etc., are well addressed. The integrated sensor employed a frequency doubled 532-nm continuous wave Nd:YAG laser as an excitation light source. The other optical components included were InPhotonics Raman probes, spectrometers, and photomultiplier tubes (PMTs). The spectrometer was used to collect the Raman spectrum of LN2 and LO2. The PMT detection unit was integrated with home-built LABVIEW software for fast monitoring of concentration ratios LN2 and LO2. Prior to designing an integrated sensor system, its optical components were also tested with gaseous nitrogen (GN2) and oxygen (GO2).
Steady transcritical flow over an obstacle: Parametric map of solutions of the forced extended Korteweg–de Vries equation
Authors: Ee Bernard K. , Grimshaw R. H. J. , Chow K. W. , Zhang D-H.
Published: April 2011

Transcritical flow of a stratified fluid over an obstacle is often modeled by the forced Korteweg–de Vries equation, which describes a balance among weak nonlinearity, weak dispersion, and small forcing effects. However, in some special circumstances, it is necessary to add an additional cubic nonlinear term, so that the relevant model is the forced extended Korteweg–de Vries equation. Here we seek steady solutions with constant, but different amplitudes upstream and downstream of the forcing region. Our main interest is in the case when the forcing has negative polarity, which represents a hole. The effects of the width of the hole and the amplitude of the hole on these steady solutions are then investigated.