bluuebunny/crossref_metadata_2025 huggingface dataset:
On the solitary wave paradigm for tsunamis
Authors: Madsen Per A. , Fuhrman David R. , Schäffer Hemming A.
Published: December 2008

Since the 1970s, solitary waves have commonly been used to model tsunamis especially in experimental and mathematical studies. Unfortunately, the link to geophysical scales is not well established, and in this work, we question the geophysical relevance of this paradigm. In part 1, we simulate the evolution of initial rectangular‐shaped humps of water propagating large distances over a constant depth. The objective is to clarify under which circumstances the front of the wave can develop into an undular bore with a leading soliton. In this connection, we discuss and test various measures for the threshold distance necessary for nonlinear and dispersive effects to manifest in a transient wave train. In part 2, we simulate the shoaling of long smooth transient and periodic waves on a mild slope and conclude that these waves are effectively non‐dispersive. In this connection, we discuss the relevance of finite amplitude solitary wave theory in laboratory studies of tsunamis. We conclude that order‐of‐magnitude errors in effective temporal and spatial duration occur when this theory is used as an approximation for long waves on a sloping bottom. In part 3, we investigate the phenomenon of disintegration of long waves into shorter waves, which has been observed, for example, in connection with the Indian Ocean tsunami in 2004. This happens if the front of the tsunami becomes sufficiently steep, and as a result, the front turns into an undular bore. We discuss the importance of these very short waves in connection with breaking and runup and conclude that they do not justify a solitary wave model for the bulk tsunami.
Microlidar observations of biomass burning aerosol over Djougou (Benin) during African Monsoon Multidisciplinary Analysis Special Observation Period 0: Dust and Biomass‐Burning Experiment
Authors: Pelon J. , Mallet M. , Mariscal A. , Goloub P. , Tanré D. , Bou Karam D. , Flamant C. , Haywood J. , Pospichal B. , Victori S.
Published: December 2008

Microlidar observations have been performed at the Djougou‐Nangatchori site in northern Benin during the African Monsoon Multidisciplinary Analysis (AMMA) Special Observation Period 0 in the dry season, combined with the Dust and Biomass‐Burning Experiment (DABEX) from mid‐January to mid‐February 2006. During the dry season, the Djougou area is a region where biomass burning aerosols are heavily produced from agriculture fires. The aerosol vertical distribution is also controlled by dynamics, and the penetration of the winter monsoon flux to the north and northern winds bringing mineral dust to the South leads to a frontal discontinuity location close to Djougou latitude. During the early dry season, the aerosol vertical distribution was observed to be structured in two layers, the lower being the boundary layer reaching altitudes up to 2 km and the upper one corresponding to the trade wind layer extending up to 5 km. Lidar data are used to retrieve the time evolution and vertical profile of extinction and discuss transport processes during the period analyzed. As the monsoon flux during the dry season is steadily progressing to the north but also moving back and forth according to shorter timescale forcings, biomass burning particles are transported from the boundary layer into the upper troposphere. This transport has a strong impact on the distribution of aerosol particles on the vertical, and extinction values larger than 0.3 km−1 have been retrieved at altitudes close to 3 km. A particular event of biomass burning air mass outbreak associated with a synoptic forcing is studied, where satellite observations are used to discuss observations of biomass burning particles over Djougou and at the regional scale.
Temporal moments in geoelectrical monitoring of salt tracer experiments
Authors: Pollock Davina , Cirpka Olaf A.
Published: December 2008

Monitoring of salt tracer experiments by electrical resistivity tomography (ERT) has been shown to be a valuable tool for characterizing the hydraulics of an aquifer, but efficient approaches of determining the spatial hydraulic conductivity distribution from ERT data are still missing. Standard inversion of ERT data obtained during salt tracer tests may even lead to estimates of the concentration distribution that are in contradiction to flow and transport of conservative compounds in porous media. In order to avoid nonphysical behavior, we consider the governing equations of groundwater flow, solute transport, and geoelectrics as a coupled system. While the tracer passes through the part of the domain that is sensitive for a particular electrode configuration, the measured electrical potential differences are perturbed. We characterize these perturbations by their temporal moments and relate them to the temporal moments of concentration, which themselves depend on hydraulic conductivity. We present temporal moment‐generating equations leading from the hydraulic conductivity field via heads and velocities to the temporal moments of concentration and electrical potential perturbations. The approach makes use of a linearized version of the Poisson equation. On the basis of this system of coupled steady state equations, we compute the sensitivity of electrical potential perturbations with respect to the log hydraulic conductivity distribution by the continuous adjoint state method for coupled systems. For demonstration, we simulate salt tracer experiments in a virtual quasi‐two‐dimensional sandbox, monitored by ERT. We show that the ratio of the first over the zeroth temporal moment of potential perturbation is less affected by the linearization of the Poisson equation than the zeroth and first moments themselves. Thus, it appears recommendable to use the ratio of first to zeroth moments also as data in inversion. We compare sensitivity patterns resulting from different electrode configurations. The methods of forward simulations and sensitivity calculations presented in this paper can be combined with any inverse kernel to develop a complete inverse model. Altogether, using temporal moments of potential perturbation appears promising for fully coupled hydrogeophysical inversion of ERT surveys during salt tracer tests.
Cellular senescence and organismal ageing in the absence of p21CIP1/WAF1 in ku80−/− mice
Authors: Zhao Bo , Benson Erica K , Qiao Ruifang , Wang Xing , Kim Sunchin , Manfredi James J , Lee Sam W , Aaronson Stuart A
Published: December 2008

Ku80 is important in the repair of DNA double‐strand breaks by its essential function in non‐homologous end‐joining. The absence of Ku80 causes the accumulation of DNA damage and leads to premature ageing in mice. We showed that mouse embryonic fibroblasts (MEFs) from ku80−/− mice senesced rapidly with elevated levels of p53 and p21. Deletion of p21 delayed the early senescence phenotype in ku80−/− MEFs, despite an otherwise intact response of p53. In contrast to ku80−/−p53−/− mice, which die rapidly primarily from lymphomas, there was no significant increase in tumorigenesis in ku80−/−p21−/− mice. However, ku80−/−p21−/− mice showed no improvement with respect to rough fur coat or osteopaenia, and even showed a shortened lifespan compared with ku80−/− mice. These results show that the increased lifespan of ku80−/− MEFs owing to the loss of p21 is not associated with an improvement of the premature ageing phenotypes of ku80−/− mice observed at the organismal level.
21 Tesla MRI Microimaging of Rat Heart by Superparamagnetic Antimyoglobin Bound Nanoparticles
Authors: Sharma Rakesh
Published: December 2008

Superparamagnetic iron-oxide bound antimyoglobin (SPIOM) characteristics were analyzed at different pH. After injecting SPIOM in heart, ex vivo magnetic resonance microimaging (MRM) technique was used to visualize microvasculature of rat heart including cardiac arteries, veins, cordate tendons attached with valves. The measurement accuracy of ventricles, aorta, vasculature and dimensions of cordate tendons were close to coregistered histology measurements.
Defect energy levels in Ta2O5 and nitrogen-doped Ta2O5
Authors: Shin Hyunho , Park Sang Yeup , Bae Shin-Tae , Lee Sangwook , Hong Kug Sun , Jung Hyun Suk
Published: December 2008

Crystalline Ta2O5 has very rich defect energy levels as compared to the previously known ones. Thermal treatment of Ta2O5 in a reducing atmosphere increases the photoluminescence (PL) intensity of the deep energy levels in the band gap as compared to the shallow ones. Nitrogen doping creates no new defect energy levels, but rather shifts the maximum PL intensity position of the deep energy levels to even deeper states. The doped nitrogen is interpreted to fill the oxygen vacancy at “in-plane” lattice sites when it is doped to TaO6 octahedron.
Universality for distances in power-law random graphs
Authors: van der Hofstad Remco , Hooghiemstra Gerard
Published: December 2008

We survey the recent work on phase transition and distances in various random graph models with general degree sequences. We focus on inhomogeneous random graphs, the configuration model, and affine preferential attachment models, and pay special attention to the setting where these random graphs have a power-law degree sequence. This means that the proportion of vertices with degree k in large graphs is approximately proportional to k−τ for some τ>1. Since many real networks have been empirically shown to have power-law degree sequences, these random graphs can be seen as more realistic models for real complex networks than classical random graphs such as the Erdős–Rényi random graph. It is often suggested that the behavior of random graphs should have a large amount of universality, meaning, in this case, that random graphs with similar degree sequences share similar behavior. We survey the available results on graph distances in power-law random graphs that are consistent with this prediction.
Loss and recovery of Gibbsianness for XY models in external fields
Authors: van Enter A. C. D. , Ruszel W. M.
Published: December 2008

We consider planar rotors (XY spins) in Zd, starting from an initial Gibbs measure and evolving with infinite-temperature stochastic (diffusive) dynamics. At intermediate times, if the system starts at low temperature, Gibbsianness can be lost. Due to the influence of the external initial field, Gibbsianness can be recovered after large finite times. We prove some results supporting this picture.
Light absorption in the near field around surface plasmon polaritons
Authors: Luque Antonio , Martí Antonio , Mendes Manuel J. , Tobías Ignacio
Published: December 2008

A semiclassical method is developed to calculate the energy absorption of an electronic system located in the near field of a metal nanoparticle sustaining surface plasmons. The results are found to be similar to those of photon absorption from ordinary transversal radiation. However, they are affected by a geometrical factor that can increase the absorption by several orders of magnitude. As example, we investigate ellipsoidal-shaped metal nanoparticles which, under favorable conditions, may provide near field aborption enhancements almost as large as 104, and in many cases above 10.
Theoretical and numerical estimates of the gas-liquid critical point of a primitive model for silica
Authors: Bianchi Emanuela , Tartaglia Piero , Sciortino Francesco
Published: December 2008

We present a numerical evaluation of the critical point location for a primitive model for silica recently introduced by Ford et al. [J. Chem. Phys. 121, 8415 (2004)]. We complement the numerical estimate with a theoretical description of the system free energy (and related thermodynamic quantities) by solving (i) the standard parameter-free first order thermodynamic perturbation Wertheim theory and (ii) an ad hoc modeling of the temperature and density dependences of the bonding free energy, inspired by the Wertheim theory but requiring one fitting parameter α(ρ). This parameter takes into account the correlation between adjacent bonding induced by excluded volume effects. We compare the predicted critical point location in the temperature-density plane with the “exact” numerical Monte Carlo value. The critical temperature is correctly predicted by both theoretical approaches, while only approach (ii) is able to accurately predict the critical density.
Engineering the free vacancy and active donor concentrations in phosphorus and arsenic double donor-doped germanium
Authors: Chroneos A. , Grimes R. W. , Bracht H. , Uberuaga B. P.
Published: December 2008

In germanium, donor atoms migrate or form larger immobile clusters via their interaction with lattice vacancies. By engineering the concentration of free vacancies, it is possible to control the diffusion of the donor atoms and the formation of those larger clusters that lead to the deactivation of a significant proportion of the donor atoms. Electronic structure calculations in conjunction with mass action analysis are used to predict the concentrations of free vacancies and deactivated donor atoms in germanium doped with different proportions of arsenic and phosphorous. We find, for example, that at low temperatures, the concentration of free vacancies is partially suppressed by increasing the proportion of arsenic doping, whereas at high temperatures (above 1000 K), the concentration of free vacancies is relatively constant irrespective of the donor species. It is predicted that the free vacancy and active donor concentrations vary linearly with the arsenic to phosphorous ratio across a wide range of temperatures.
Quantum dynamics of the O+OH→H+O2 reaction at low temperatures
Authors: Quéméner Goulven , Balakrishnan Naduvalath , Kendrick Brian K.
Published: December 2008

We report quantum dynamics calculations of the O+OH→H+O2 reaction on two different representations of the electronic ground state potential energy surface (PES) using a time-independent quantum formalism based on hyperspherical coordinates. Calculations show that several excited vibrational levels of the product O2 molecule are populated in the reaction. Rate coefficients evaluated using both PESs were found to be very sensitive to the energy resolution of the reaction probability, especially at temperatures lower than 100 K. It is found that the rate coefficient remains largely constant in the temperature range of 10–39 K, in agreement with the conclusions of a recent experimental study [Carty et al., J. Phys. Chem. A 110, 3101 (2006)]. This is in contrast with the time-independent quantum calculations of Xu et al. [J. Chem. Phys. 127, 024304 (2007)] which, using the same PES, predicted nearly two orders of magnitude drop in the rate coefficient value from 39 to 10 K. Implications of our findings to oxygen chemistry in the interstellar medium are discussed.
Enhancement of ferroelectricity and ferromagnetism in rare earth element doped BiFeO3
Authors: Cheng Z. X. , Wang X. L. , Dou S. X. , Kimura H. , Ozawa K.
Published: December 2008

Rare earth element doped BiFeO3 thin films were fabricated using the pulsed laser deposition method and various targets made from different starting Fe2O3 and Fe3O4 iron source materials. The films fabricated using the targets made from Fe3O4 exhibit great enhancement in their ferroelectricity, due to greatly reduced electrical leakage, as well as enhanced magnetization compared to those films deposited using targets from Fe2O3. It is suggested that the Fe2+ ion plays an important role in compensating for the charge imbalance and reducing current leakage, as well as enhancing the magnetic moment through the introduction of antiferrimagnetic ordering at Fe2+ site.
Optical measurement of the thermal diffusivity of intact thermal barrier coatings
Authors: Heeg Bauke , Clarke David R.
Published: December 2008

The thermal diffusivity of vapor deposited thermal barrier coatings is measured using an all-optical approach. The method combines CO2 laser heating of the coating surface with detection of the temperature variation in the laser induced Cr3+ luminescence signal from the thermally grown oxide (TGO) formed under the coating during service. The thermal diffusivity is determined from the delay between a surface heating laser pulse and the heating of the TGO. Data analysis is performed using a three layer heat equation model. Various locations on several coatings were investigated with different average heating laser powers and modulation frequencies. The normalized standard deviation was found to be of the order of 20%, which is thought to be largely due to localized variations in coating thickness and density. The advantage of the method described here is that it does not require knowledge of either the thermal properties of the alloy, its temperature, or thickness, and is therefore well suited for measurement on coated parts of complex shape, such as a hot section gas turbine blade.
Anomalous electronic structure of ionic liquids determined by soft x-ray emission spectroscopy: Contributions from the cations and anions to the occupied electronic structure
Authors: Kanai K. , Nishi T. , Iwahashi T. , Ouchi Y. , Seki K. , Harada Y. , Shin S.
Published: December 2008

Soft x-ray emission spectroscopy was used for elucidating the electronic structure of ionic liquids [C4mim]+PF6− and [C4mim]+OTf−, where [C4mim]+ stands for methylbutylimidazolium cation and OTf− for the trifluoromethanesulfonate anion. Nonresonant spectra measured above N, O, and F 1s edges selectively probed the molecular orbitals (MOs) of the cation and anions. They give a clear evidence that the highest occupied molecular orbital of the [C4mim]+ cation contributes to the topmost occupied states of the ionic liquids [C4mim]+PF6−, while both cationic and anionic MOs contribute for the case of [C4mim]+OTf−. Resonant soft x-ray emission spectra at the N 1s edge of these ionic liquids revealed that the energy gap of [C4mim]+PF6− is solely determined by the [C4mim]+ cation, in contrast to usual ionic crystals. The ionic liquids form a new class of the ionic materials from the viewpoint of the electronic structure.
Nonlinear periodic solutions for isothermal magnetostatic atmospheres
Authors: Khater A. H. , Callebaut D. K. , Kamel E. S.
Published: December 2008

Magnetohydrodynamic equilibria for a plasma in a gravitational field are investigated analytically. For equilibria with one ignorable spatial coordinate, the equations reduce to a single nonlinear elliptic equation for the magnetic potential A, known as the Grad–Shafranov equation. Specifying the arbitrary functions in the latter equation, one obtains three types of nonlinear elliptic equations (a Liouville equation, a sinh Poisson equation, and a generalization of those with a sum of exponentials). Analytical solutions are obtained using the tanh method; this is elaborated in the Appendix. The solutions are adequate to describe an isothermal atmosphere in a uniform gravitational field showing parallel filaments of diffuse, magnetized plasma suspended horizontally in equilibrium.
First principles study of bulk CrSe and CrSe/ZnSe(001) interface
Authors: Hazrati Ebrahim , Hashemifar S. Javad , Akbarzadeh Hadi
Published: December 2008

Density functional theory calculations are performed to study the structural, electronic, and magnetic properties of hexagonal NiAs type and cubic zinc blende type CrSe structures and interface of zinc blende CrSe with ZnSe(001). We observe that the local density approximation to the exchange correlation energy leads an antiferromagnetic ground state for bulk CrSe, while within generalized gradient approximation, a ferromagnetic state is more stable. The zinc blende structure of CrSe is found to be ferromagnetic half-metal with a total moment of 4μB per formula unit. The results show that the induced tetragonal distortion on ZnSe(001) substrate does not destroy the half-metallicity of zinc blende CrSe. We argue that interface effects in CrSe/ZnSe(001) heterojunctions are not harmful for half-metallicity and are highly localized around interface layer. The Schottky barrier height and band offsets of this interface are calculated and compared with some Heusler alloys/GaAs(001) heterostructures.
Electron and hole states and the exciton diamagnetic shifts in an InAs/InP rectangular quantum wire in a magnetic field
Authors: Duan Xiu-Zhi , Kong Xiao-Jun
Published: December 2008

We present a perturbation theory to calculate the ground state of an electron and a hole in the presence of a magnetic field, which is the basis of the exciton binding energies obtained by using the one-dimensional effective potential model in InAs/InP self-assembled rectangular quantum wires (QWRs). We study the exciton diamagnetic shift for the case of a magnetic field applied along the wire, taking into account the strain in the sample and the different masses of the various particles inside and outside the QWR. The calculations are carried out in the single band effective-mass approximation. Better agreement is obtained between our results and experimental data than has been found in early work.
Pretilt angle control of liquid crystal molecules by photoaligned films of azobenzene-containing polyimide with a different content of side-chain
Authors: Usami Kiyoaki , Sakamoto Kenji , Yokota Junichiro , Uehara Yoichi , Ushioda Sukekatsu
Published: December 2008

We have investigated the pretilt angle of liquid crystal (LC) molecules induced by photoaligned films of a series of polyimides. The polyimides were random copolymers synthesized from pyromellitic dianhydride and a mixture of 4,4′-diaminoazobenzene and 4-[4′-propylbi(cyclohexan)-4-yl]phenyl 3,5-diaminobenzoate (PBCP-DABA). PBCP-DABA is a diamine to introduce a side-chain structure into polyimide. We found that the pretilt angle of LC molecules can be controlled from 0° to 90° by varying the molar fraction (x) of PBCP-DABA from 0 to 0.5. Defect-free uniform LC alignment was observed for x≤0.125 and x≥0.3, but threadlike textures appeared for 0.15≤x≤0.25. Since the interaction between the polyimide backbone structure and the LC molecule may be blocked by relatively dense side-chains, the appearance of threadlike texture is tentatively attributed to weak azimuthal anchoring strength of the photoaligned polyimide films with x≥0.15.
On an example of the space charge limited conduction breakdown in relation to the current-voltage characteristics of a single layer metal/organic structure
Authors: Cvikl B.
Published: December 2008

The room-temperature current-voltage data of the hole conducting single layer poly-p-phenylene-vinylene indium tin oxide/poly-p-phenylene-vinylene (200 nm)/Al organic structure of Karg et al. [J. Appl. Phys. 82, 1951 (1997)], representing an archetype for (shallow) trap-controlled space charge limited conduction characterized by the current density proportional to the second power of the applied bias, j∝Va2, are used to calculate the spatial dependence of the internal electric field, the internal electric potential, and the total (free and trap) hole charge density within the organic layer. In contrast to the usual space charge limited current results, it is determined that these quantities are linearly distributed throughout the above structure. The breakdown of the Mott–Gurney approach is assigned to the usual, but in this case inappropriate, assumption of the boundary condition at the hole injecting metal/organic junction. It is shown that for the above stated structure the electric field at the anode/organic boundary is nonzero and linearly increases with the forward bias. As a consequence of this fact the product of the hole mobility and the trapping parameter turns out to be considerably larger than the value deduced on the basis of the Mott–Gurney law. The organic bulk differential capacitance is small in magnitude and exhibits a nonlinear decrease with the increasing value of the applied voltage Va.
Floquet analysis of secondary instability of boundary layers distorted by Klebanoff streaks and Tollmien–Schlichting waves
Authors: Liu Yang , Zaki Tamer A. , Durbin Paul A.
Published: December 2008

Previous studies of the interaction between boundary layer streaks and Tollmien–Schlichting (TS) waves have shown puzzling effects. Streaks were shown to reduce the growth rate of primary TS waves and, thereby, to delay transition; however, they can also promote transition by inducing a secondary instability. The outcome of the interaction depends on the spanwise wavelength and intensity of the streaks as well as on the amplitude of the TS waves. A Floquet analysis of secondary instability is able to explain many of these features. The base state is periodic in two directions: it is an Ansatz composed of a saturated TS wave (periodic in x) and steady streaks (periodic in z). Secondary instability analysis is extended to account for the doubly periodic base flow. Growth rate computations show that, indeed, the streak can either enhance or diminish the overall stability of the boundary layer. The stabilizing effect is a reduction in the growth rate of the primary two-dimensional TS wave; the destabilizing effect is a secondary instability. Secondary instability falls into two categories, depending on the spanwise spacing of the streaks. The response of one category to perturbations is dominated by fundamental and subharmonic instability; the response of the other is a detuned instability.
Chemical patterning of Ag(111): Spatially confined oxide formation induced by electron beam irradiation
Authors: Günther S. , Reichelt R. , Wintterlin J. , Barinov A. , Menteş T. O. , Niño M. Á. , Locatelli A.
Published: December 2008

Low energy electron irradiation of a Ag(111) surface during NO2 adsorption at 300 K induces formation of Ag oxide. Using a spatially confined electron beam, small Ag2O spots could be grown with a sharp, ∼100 nm wide, boundary to the nonirradiated metallic surface. Since the structure size will mainly depend on the sharpness of the irradiating electron beam, this process has the potential of a single step nanostructuring process. Temperature treatment offers an easy way to manipulate the boundary between oxide and metallic silver by steering a chemical front.
Feasibility study for sidewall fluorination of SWCNTs in CF4 plasma
Authors: Shoda K. , Kohno H. , Kobayashi Y. , Takagi D. , Takeda S.
Published: December 2008

Fluorination of single-wall carbon nanotubes (SWCNTs) was performed by using the CF4 radio-frequency plasma technique. The structural and bonding properties of the plasma-processed SWCNTs were investigated by transmission electron microscopy, x-ray photoelectron spectroscopy, and Raman spectroscopy. The plasma parameters, i.e., self-biased voltage, plasma ion density, and ion dose, substantially affected the structural and bonding properties of plasma-processed SWCNTs. We show that sidewall fluorination of SWCNTs occurs in CF4 plasma at low self-biased voltages. Plasma conditions for the sidewall fluorination of SWCNTs are discussed.
Signature of Jahn–Teller distortion and oxygen stoichiometry in Raman spectra of epitaxial LaMnO3+δ thin films
Authors: Dubey Aditi , Sathe V. G. , Rawat R.
Published: December 2008

Epitaxial thin films of LaMnO3 were grown by pulsed laser deposition in identical deposition conditions but in situ annealed in different oxygen environments. All the films showed an insulator to metal transition. It also showed a magnetic order that correlates well with an insulator to metal behavior. The transition temperature critically depends on annealing oxygen pressure and increases with increasing oxygen content. It is observed that instead of strain, oxygen content plays a major role in deciding the transport properties. X-ray diffraction and Raman spectroscopy measurements revealed that the increasing oxygen content results in a decrease in unit cell volume along with a reduction in Jahn–Teller distortion. It is shown that the intensities of the two Jahn–Teller activated modes in Raman spectra are highly sensitive to the oxygen stoichiometry of the films and that the intensity ratio can be effectively used to characterize oxygen content in these films.
Composition dependence of Schottky barrier heights and bandgap energies of GaNxAs1−x synthesized by ion implantation and pulsed-laser melting
Authors: Kim Taeseok , Alberi Kirstin , Dubon Oscar D. , Aziz Michael J. , Narayanamurti Venkatesh
Published: December 2008

We present a systematic investigation on the band structure of the GaNxAs1−x alloys synthesized using nitrogen ion implantation followed by pulsed-laser melting and rapid thermal annealing. The evolution of the nitrogen-concentration depth profile is consistent with liquid-phase diffusion, solute trapping at the rapidly moving solidification front, and surface evaporation. The reduction of the Schottky barrier height of the Γ-like threshold at nitrogen composition up to x=0.016 is studied by ballistic electron emission microscopy (BEEM) and determined quantitatively using the second voltage derivative BEEM spectra to be −191±63 meV per x=0.01, which is close to the corresponding slope for samples grown by low-temperature molecular beam epitaxy. This slope is also consistent with the bandgap narrowing measured on the same samples by photomodulated reflectance and is consistent with the band anticrossing model for the splitting of the conduction band in the GaNxAs1−x alloys. Lithographically patterned GaNxAs1−x dots are imaged by BEEM. Analysis of BEEM spectra of the locally confined dots indicates an alloying-induced decrease in the Schottky barrier height of four times the thermal energy at room temperature.
The effect of cross-sectional geometry and size on magnetostatic modes in nanorods
Authors: Krawczyk M. , Puszkarski H.
Published: December 2008

We study the effect of cross-sectional geometry and size on the spectrum of magnetostatic modes in dipole-coupled magnetic nanorods of circular or square cross section. The scope of this study is confined only to excitations propagating along the central axis of the nanorod; at the same time, the direction of the central axis is assumed to correspond to that of the magnetization of the sample. We find that a modification of the cross-sectional geometry results in nonuniform changes in the local field profile and the effect proves especially strong in the range of nanometer lateral dimensions. Moreover, the effect of cross-sectional geometry on the magnetostatic spectrum of a nanorod is found to be the most significant in peripheral regions of elongated rods. This is due to the occurrence of dipolar local field wells in these very regions. The shape of these peripheral wells affects first of all the bulk-dead and surface modes, which are “stuck” in them; frequencies of these modes prove to increase when the cross-sectional shape changes from square to circle.
An analytical model for bending angle in metal/ceramic bilayer system of laser forming
Authors: Shen Hong , Yao Zhenqiang , Hu Jun
Published: December 2008

An analytical model based on force and moment balances is developed to predict the bending angle in metal/ceramic bilayer material systems when applying a laser forming process. The formula is derived based on the assumptions that the plastic deformation is generated only during heating, while during cooling the plate undergoes only elastic deformation. Results are obtained for a bilayer plate of an Al 6061 metallic layer and a SiC ceramic layer. Moreover, numerical simulation is carried out using the finite element analysis method to estimate the accuracy of the presented model, which shows that the results of analytical model are in good agreement with the numerical simulation.
Effects of disorder and scaling of optical conductivity in Nd0.5Ca0.5−xBaxMnO3 (x= and 0.02) thin films as observed by terahertz time-domain spectroscopy
Authors: Mavani K. R. , Nagai M. , Rana D. S. , Yada H. , Kawayama I. , Tonouchi M. , Tanaka K.
Published: December 2008

We have explored the low energy charge dynamics of charge-ordered Nd0.5Ca0.5MnO3 and Ba-doped Nd0.5Ca0.48Ba0.02MnO3 manganite thin films by recording the complex optical spectra using terahertz time-domain spectroscopy. The extracted frequency- and temperature-dependent optical conductivity (σ) and dielectric constant (ε1) show distinct features at Néel’s ordering and the charge-ordering temperatures (TN and TCO). We found that the Ba-doping induced cation disorder effects on σ and ε1 are more pronounced above TCO, but these effects are subtle in a large temperature region below TCO. Both ε1 and σ increase as a function of temperature and form a scaling relationship ε1∝σ.
Restoration of high spatial frequency at the image formed by stimulated Brillouin scattering with a prepulse
Authors: Beak Du Hyun , Yoon Jin Woo , Shin Jae Sung , Kong Hong Jin
Published: December 2008

We have found experimentally that it is possible to restore the high spatial frequency of optical images by using stimulated Brillouin scattering (SBS) with a prepulse. The Stokes wave usually has to lose the high spatial frequency because of the necessary energy to generate the acoustic grating. However, this problem has been resolved by using a prepulse method. We have achieved an amplitude increase of ∼41% compared to the normal SBS scheme at a spatial frequency of ∼0.027 mm−1. This method is easy to apply to a system using the optical image by the SBS process.
Bandgap engineering by tuning particle size and crystallinity of SnO2–Fe2O3 nanocrystalline composite thin films
Authors: Sahana M. B. , Sudakar C. , Setzler G. , Dixit A. , Thakur J. S. , Lawes G. , Naik R. , Naik V. M. , Vaishnava P. P.
Published: December 2008

We report the structural and optical properties of xSnO2–yFe2O3 nanocrystalline composite thin films. SnO2 and Fe2O3 exhibit strong phase separation instability and their particle size and crystallinity are tunable by changing their composition and annealing temperature. The bandgap for these composites continuously increases from 2.3 to 3.89 eV. We discuss the increasing bandgap values in terms of the quantum confinement effect manifested by the decreasing size of Fe2O3 crystallites. The method provides a generic approach for the tuning of the bandgap in nanocomposite systems.
Three-dimensional filamentary structures of a relativistic electron beam in fast ignition plasmas
Authors: Karmakar Anupam , Kumar Naveen , Pukhov Alexander , Polomarov O. , Shvets G.
Published: December 2008

The filamentary structures and associated electromagnetic fields of a relativistic electron beam have been studied by three-dimensional particle-in-cell simulations in the context of fast ignition fusion. The simulations explicitly include collisions in return plasma current and distinctly examine the effects of beam temperature and collisions on the growth of filamentary structures generated.
Single-sided radio-frequency field gradient with two unsymmetrical loops: Applications to nuclear magnetic resonance
Authors: Guendouz Laouès , Leclerc Sébastien , Retournard Alain , Hedjiedj Ahcène , Canet Daniel
Published: December 2008

Magnetic field gradients are nowadays indispensable to most nuclear magnetic resonance experiments and are at the basis of magnetic resonance imaging (MRI). Most of the time, gradients of the static magnetic field are employed. Gradients of the radio-frequency (rf) field may constitute an interesting alternative. Until now, they were produced by a single loop. We demonstrate in this paper how two unsymmetrical series loops can be optimized to produce rf gradients of much better performances. This optimization is based on a thorough theoretical approach and the gradient uniformity is studied through accurate simulations. Two prototypes were devised: one for a 2.34 T horizontal magnet (used in MRI), and the other for a 4.7 T vertical magnet (used for pure spectroscopic applications). These two-loop systems were designed for proton resonance frequencies (100 and 200 MHz, respectively). Performances of both systems were verified (versus theoretical predictions) by means of experiments employing gradients in view of the determination of the self-diffusion coefficients of liquids.
High-resolution Rydberg tagging time-of-flight measurements of atomic photofragments by single-photon vacuum ultraviolet laser excitation
Authors: Jones Brant , Zhou Jingang , Yang Lei , Ng C. Y.
Published: December 2008

By coupling a comprehensive tunable vacuum ultraviolet (VUV) laser system to a velocity-mapped ion imaging apparatus, we show that high-resolution high-n Rydberg tagging time-of-flight (TOF) measurements of nascent atomic photofragments formed by laser photodissociation can be made using single-photon VUV laser photoexcitation. To illustrate this single-photon Rydberg tagging TOF method, we present here the results of the VUV laser high-n Rydberg tagging TOF measurements of O(P32) and S(P32) formed in the photodissociation of SO2 and CS2 at 193.3 and 202.3 nm, respectively. These results are compared to those obtained by employing the VUV laser photoionization time-sliced velocity-mapped ion imaging technique. The fact that the kinetic energy resolutions achieved in the VUV laser high-n Rydberg tagging TOF measurements of O and S atoms are found to be higher than those observed in the VUV laser photoionization, time-sliced velocity-mapped ion imaging studies show that the single-photon VUV laser high-n Rydberg tagging TOF method is useful and complementary to state-of-the-art time-sliced velocity-mapped ion imaging measurements of heavier atomic photofragments, such as O and S atoms. Furthermore, the general agreement observed between the VUV laser high-n Rydberg tagging TOF and velocity-mapped ion imaging experiments supports the conclusion that the lifetimes of the tagged Rydberg states of O and S atoms are sufficiently long to allow the reliable determination of state-resolved UV photodissociation cross sections of SO2 and CS2 by using the VUV laser high-n Rydberg tagging TOF method.
Digital control of force microscope cantilevers using a field programmable gate array
Authors: Jacky Jonathan P. , Garbini Joseph L. , Ettus Matthew , Sidles John A.
Published: December 2008

This report describes a cantilever controller for magnetic resonance force microscopy based on a field programmable gate array, along with the hardware and software used to integrate the controller into an experiment. The controller is assembled from a low-cost commercially available software defined radio device and libraries of open-source software. The controller includes a digital filter comprising two cascaded second-order sections (“biquads”), which together can implement transfer functions for optimal cantilever controllers. An appendix in this report shows how to calculate filter coefficients for an optimal controller from measured cantilever characteristics. The controller also includes an input multiplexer and adder used in calibration protocols. Filter coefficients and multiplexer settings can be set and adjusted by control software while an experiment is running. The input is sampled at 64 MHz; the sampling frequency in the filters can be divided down under software control to achieve a good match with filter characteristics. Data reported here were sampled at 500 kHz, chosen for acoustic cantilevers with resonant frequencies near 8 kHz. Inputs are digitized with 12 bit resolution, and outputs are digitized with 14 bits. The experiment software is organized as a client and server to make it easy to adapt the controller to different experiments. The server encapsulates the details of controller hardware organization, connection technology, filter architecture, and number representation. The same server could be used in any experiment, while a different client encodes the particulars of each experiment.
Nanoscale lithography with frequency-modulation atomic force microscopy
Authors: Hamada Masayuki , Eguchi T. , Akiyama K. , Hasegawa Y.
Published: December 2008

A lithographic method to draw nanoscale structures by repetitive depositions of the tip material by field evaporation has been developed based on frequency-modulation atomic force microscopy (FM-AFM). Because of high stiffness of quartz tuning forks, a force sensor in the AFM, unwanted mechanical contact of the AFM tip with the substrate was prevented. Precise control of the tip-substrate gap distance with FM-AFM and a gold tip sharpened with focused ion beam enable us to deposit gold dots as small as ∼20 nm in size and construct nanoscale patterns.
The enhancement of ZnO nanowalls photoconductivity induced by CdS nanoparticle modification
Authors: Fang F. , Zhao D. X. , Li B. H. , Zhang Z. Z. , Zhang J. Y. , Shen D. Z.
Published: December 2008

10 nm sized hexagonal CdS nanoparticles were decorated on the surface of well-aligned ZnO nanowall through a facile hydrothermal approach. The effects of CdS-cap layer on the optical and photoelectrical properties of ZnO nanowalls have been studied. It was found the CdS acted not only as a passivation layer to suppress the detrimental surface states of ZnO nanowalls, which reduced the deep-level emissions, but also as an interfacial carrier transport layer to reduce the probability of carrier recombination. Due to this effect a significant enhancement in ultraviolet photoconductivity of ZnO nanowalls was observed by the surface modification with CdS nanoparticles.
Enhanced and partially polarized output of a light-emitting diode with its InGaN/GaN quantum well coupled with surface plasmons on a metal grating
Authors: Shen Kun-Ching , Chen Cheng-Yen , Chen Hung-Lu , Huang Chi-Feng , Kiang Yean-Woei , Yang C. C. , Yang Ying-Jay
Published: December 2008

The enhanced and partially polarized output of a green light-emitting diode (LED), in which its InGaN/GaN quantum well (QW) couples with surface plasmons (SPs) on a surface Ag grating structure, is demonstrated. Compared with a LED sample without (flat) Ag coating, the total output intensity of an LED of SP-QW coupling can be enhanced by ∼59% (∼200)% when the grating period and groove depth are 500 and 30 nm, respectively. Also, a bottom-emission polarization ratio of 1.7 can be obtained under the condition of 15 nm in groove depth.
Optimal design of miniaturized thin-film helical resonators
Authors: Zhu Jiwen , Hao Tong , Stevens Christopher J. , Edwards David J.
Published: December 2008

This paper investigates the optimal miniaturization of a helical resonator, which is in the form of a two turn helix. We theoretically demonstrate that its outmost dimension can be reduced to below 1% of the free space wavelength at resonance. Maximum miniaturization is achieved at an optimal metallic fill ratio of around 0.2 independent of diameter. Simulation and experimental results were used to validate the theoretical model and explore the miniaturization level achievable. The broadside coupled split ring resonators have the same optimal properties, but for identically sized resonator the helical design provides a factor of 2 greater miniaturization.
The historical range of variability of fires in the Andean - Patagonian Nothofagus forest region
Authors: Veblen Thomas T. , Kitzberger Thomas , Raffaele Estela , Mermoz Mónica , González Mauro E. , Sibold Jason S. , Holz Andrés
Published: December 2008

The present synthesis addresses key questions about several extreme fire events that occurred in the Nothofagus forest region of southern Argentina and Chile in the late 1990s and early 2000s: (1) are there historical precedents for the extent and severity of these recent wildfires? (2) To what extent can large, severe fires be attributed to influences from modern humans, either indirectly through land-use practices or directly through ignition? (3) What are the relationships of these fire events to interannual climatic variability and trends? (4) What are the medium-term ecological consequences of these fire events, particularly in terms of the resiliency of the burned ecosystems? Historic fire regimes vary greatly across the different ecosystem types in the southern Andean region, and the tree-ring record shows that before the 20th century, large severe fires also played a significant ecological role in shaping even the wettest forests. Recent severe droughts at an annual time scale have been facilitated by a trend towards higher temperatures since the mid-1970s. In large parts of the region, the risk of wildfire ignition and spread has been exacerbated by increases in lightning associated with higher temperatures, increased ignitions associated with exurban development, and conversion of less flammable native vegetation to more flammable plantations of exotic conifers.
Effects of large fires on biodiversity in south-eastern Australia: disaster or template for diversity?
Authors: Bradstock Ross A.
Published: December 2008

Large fires coincident with drought occurred in south-eastern Australia during 2001–2007. Perceptions of large, intense fires as being ecologically ‘disastrous’ are common. These are summarised by four hypotheses characterising large fires as: (i) homogenous in extent and intensity; (ii) causing large-scale extinction due to perceived lack of survival and regeneration capacity among biota; (iii) degrading due to erosion and related edaphic effects; (iv) unnatural, as a consequence of contemporary land management. These hypotheses are examined using available evidence and shown to inadequately account for effects of large fires on biodiversity. Large fires do not burn homogeneously, though they may produce intensely burnt patches and areas. The bulk of biota are resilient through a variety of in situ persistence mechanisms that are reinforced by landscape factors. Severe erosive episodes following fire tend to be local and uncertain rather than global and inevitable. Redistribution of soil and nutrients may reinforce habitat variation in some cases. Signals of fire are highly variable over prehistoric and historic eras, and, in some cases, contemporary and pre-European signal levels are equivalent. The most important effects of large fires in these diverse ecological communities and landscapes stem from their recurrence rate. Adaptive management of fire regimes rather than fire events is required, based on an understanding of risks posed by particular regimes to biota.
Large fires in Australian alpine landscapes: their part in the historical fire regime and their impacts on alpine biodiversity
Authors: Williams Richard J. , Wahren Carl-Henrik , Tolsma Arn D. , Sanecki Glenn M. , Papst Warwick A. , Myers Bronwyn A. , McDougall Keith L. , Heinze Dean A. , Green Ken
Published: December 2008

The fires of summer 2003 in south-eastern Australia burnt tens of thousands of hectares of treeless alpine landscape. Here, we examine the environmental impact of these fires, using data from the Bogong High Plains area of Victoria, and the Snowy Mountains region of New South Wales. Historical and biophysical evidence suggests that in Australian alpine environments, extensive fires occur only in periods of extended regional drought, and when severe local fire weather coincides with multiple ignitions in the surrounding montane forests. Dendrochronological evidence indicates that large fires have occurred approximately every 50–100 years over the past 400 years. Post-fire monitoring of vegetation in grasslands and heathlands indicates that most alpine species regenerate rapidly after fire, with >90% of species present 1 year after fire. Some keystone species in some plant communities, however, had not regenerated after 3 years. The responses of alpine fauna to the 2003 fires were variable. The core habitat (closed heathland) of several vulnerable small mammals was extensively burnt. Some mammals experienced substantial falls in populations, others experienced substantial increases. Unburnt patches of vegetation are critical to faunal recovery from fire. There was, however, no evidence of local extinction. We conclude that infrequent extensive fires are a feature of alpine Australia. For both the flora and fauna, there is no quantitative evidence that the 2003 fires were an ecological disaster, and we conclude that the flora and fauna of alpine Australia are highly resilient to infrequent, large, intense fires.