bluuebunny/crossref_metadata_2025 huggingface dataset:
An eastward propagating two‐day wave: Evidence for nonlinear planetary wave and tidal coupling in the mesosphere and lower thermosphere
Authors: Palo S. E. , Forbes J. M. , Zhang X. , Russell J. M. , Mlynczak M. G.
Published: April 2007

Temperature observations from the SABER instrument on the TIMED spacecraft are used to investigate the structure and evolution of an eastward propagating zonal wavenumber 2 disturbance with a period near two days. This oscillation obtains a maximum amplitude of nearly 10 K in the southern hemisphere mid‐latitudes during late January. The timing and location of this planetary wave is coincident with the regular quasi two‐day wave intensification that occurs annually in late January. The period, wavenumber and spatial structure of the eastward propagating two‐day wave are consistent with a wave that results from a nonlinear interaction between the quasi two‐day wave and the migrating diurnal tide. The existence of an eastward propagating wave with a period near two days coincident with the westward propagating two day wave will have an impact on the interpretation of ground based observations. Analysis of the SABER temperature observations are utilized to determine the structure and evolution of both this eastward propagating two‐day wave and the classic westward propagating zonal wavenumber 3 quasi two‐day wave during January 2005.
Predicting the breaking onset of surface water waves
Authors: Babanin Alexander , Chalikov Dmitry , Young Ian , Savelyev Ivan
Published: April 2007

Why do ocean waves break? Understanding this important and obvious property of the ocean surface has been elusive for decades. This paper investigates causes which lead deep‐water two‐dimensional initially monochromatic waves to break. Individual wave steepness is found to be the single parameter which determines whether the wave will break immediately, never break or take a finite number of wave lengths to break. The breaking will occur once the wave reaches the Stokes limiting steepness. The breaking probability and the location of breaking onset can be predicted, properties of incipient breakers measured. Potential applications to field conditions are discussed.
Observations of daytime F2‐layer stratification under the southern crest of the equatorial ionization anomaly region
Authors: Fagundes P. R. , Klausner V. , Sahai Y. , Pillat V. G. , Becker‐Guedes F. , Bertoni F. C. P. , Bolzan M. J. A. , Abalde J. R.
Published: April 2007

Ionospheric vertical sounding observations are being carried out at Sao Jose dos Campos (23.2°S, 45.9°W; dip latitude 17.6°S), Brazil, under the southern crest of the equatorial ionization anomaly (EIA) since August 2000. In this paper, we present and discuss the observations of daytime F2‐layer stratification near the crest of EIA, for the first time, under magnetically quiet high solar activity conditions. Three examples and a year of statistics are presented. The F2‐layer stratification and F3‐layer were observed between 10:40 and 11:45 UT on 31 December 2000, between 13:30 and 14:30 UT on 1 January 2001, and between 13:15 and 15:15 UT on 11 February 2001. The statistics during September 2000 to August 2001 shows that the F3‐layer occurs only for 66 days (18% occurrence), and it occurs only during September–February (spring–summer), with maximum occurrence in September–October and longest duration in February. The F2‐layer stratification seems to be associated with gravity waves (GWs), which have periods of about 30–60 min, downward phase velocities of about 60–140 m/s, and vertical wavelengths of about 200–500 km. The presence of powerful gravity waves in a vertically extended F‐layer seems to stratify the F2‐layer and produce the F3‐layer. Because the stratifications are observed during geomagnetically quiet periods, the source of the gravity waves are most likely to be associated with local tropospheric disturbances and not with high‐latitude disturbances.
Large‐scale experiments on the mechanics of pyroclastic flows: Design, engineering, and first results
Authors: Dellino Pierfrancesco , Zimanowski Bernd , Büttner Ralf , La Volpe Luigi , Mele Daniela , Sulpizio Roberto
Published: April 2007

A newly designed apparatus for experimental studies of pyroclastic flows consists of a cylindrical conduit that is filled with samples of natural volcanic products (tephra). Blowing nozzles in the base plate of the conduit are connected to a volume of highly pressurized gas. Opening of fast solenoid valves results in impulse‐like coupling of the released gas to the sample. The system was designed so that the range of mechanical energy transferred to the particle mass in the conduit reflects the mechanical energy observed and measured during fragmentation experiments with melts of similar composition. Depending on the specific mechanical energy (SME) of the system, which results from ΔPV/m, where ΔP is gas overpressure (i.e., pressure > atmospheric), V is gas volume, and m is sample mass, different behaviors are observed. If SME > 2.6 kJ/kg, a dilute plume develops, and particles are sedimented by fallout exclusively. If SME < 1.5 kJ/kg, the exiting column collapses and develops a shear current similar to a pyroclastic flow. The Reynolds number of the shear currents is >106, implying that flows are fully turbulent and that particle coupling to gas turbulence of natural pyroclastic flows is replicated by the experiments. The measured shear current velocities are proportional to the impact mass flow rate, i.e., the product of mixture density and impact velocity. Experimental data and grain‐size analysis of the produced particle deposits suggest that the scale of the experiment is large enough to reproduce the transport dynamics of natural pyroclastic flows.
Correlations between earthquakes and large mud volcano eruptions
Authors: Mellors R. , Kilb D. , Aliyev A. , Gasanov A. , Yetirmishli G.
Published: April 2007

We examine the potential triggering relationship between large earthquakes and methane mud volcano eruptions. Our data set consists of a 191‐year catalog (1810–2001) of eruptions from 77 volcanoes in Azerbaijan, central Asia, supplemented with reports from mud volcano eruptions in Japan, Romania, Pakistan, and the Andaman Islands. We compare the occurrence of historical regional earthquakes (M > 5) with the occurrence of Azerbaijan mud volcano eruptions and find that the number of same‐day earthquake/eruption pairs is significantly higher than expected if the eruptions and earthquakes are independent Poisson processes. The temporal correlation between earthquakes and eruptions is most pronounced for nearby earthquakes (within ∼100 km) that produce seismic intensities of Mercalli 6 or greater at the location of the mud volcano. This assumed magnitude/distance relationship for triggering observed in the Azerbaijan data is consistent with documented earthquake‐induced mud volcano eruptions elsewhere. We also find a weak correlation that heightened numbers of mud volcano eruptions occur within 1 year after large earthquakes. The distribution of yearly eruptions roughly approximates a Poisson process, although the repose times somewhat favor a nonhomogenous failure rate, which implies that the volcanoes require some time after eruption to recharge. The volcanic triggering likely results from some aspect of the seismic wave's passage, but the precise mechanism remains unclear.
Formation and structure of refrozen cracks in land‐fast first‐year sea ice
Authors: Petrich Chris , Langhorne Pat J. , Haskell Tim G.
Published: April 2007

This study characterizes the healing process and structure of undeformed, linear, parallel‐sided, flooded cracks in land‐fast sea ice. Field investigations and refreezing experiments were performed in McMurdo Sound, Ross Sea, Antarctica, between 1998 and 2002. Data from a two‐dimensional thermistor array are used to show that the ice‐water interface of freezing cracks is arch‐shaped due to bidirectional heat flow to the surrounding ice cover and to the atmosphere. Ice growing laterally into the crack is found to desalinate over a prolonged period of time, until the isotherms are approximately horizontal. Superposition of heat flow to the atmosphere and to the host sea ice sheet allows the refreezing progress to be modeled analytically. Close to the ice–air interface, the salinity is higher at the sides of wide refrozen cracks than it is at the center. However, deeper down and in narrower cracks in general, the salinity is higher at the center than at the sides. A finite volume, computational fluid dynamics (CFD) model reproduces the generally arch‐shaped alignment of brine pockets. This pattern is attributed to convection in the mushy layer. Crystals are found to grow upstream into the crack due to a salinity gradient in the buoyant convective flow.
A two‐dimensional atmospheric chemistry modeling investigation of Earth's Phanerozoic O3 and near‐surface ultraviolet radiation history
Authors: Harfoot Michael B. J. , Beerling David J. , Lomax Barry H. , Pyle John A.
Published: April 2007

We use the Cambridge two‐dimensional (2‐D) chemistry‐radiation transport model to investigate the implications for column O3 and near‐surface ultraviolet radiation (UV), of variations in atmospheric O2 content over the Phanerozoic (last 540 Myr). Model results confirm some earlier 1‐D model investigations showing that global annual mean O3 column increases monotonically with atmospheric O2. Sensitivity studies indicate that changes in temperature and N2O exert a minor influence on O3 relative to O2. We reconstructed Earth's O3 history by interpolating the modeled relationship between O3 and O2 onto two Phanerozoic O2 histories. Our results indicate that the largest variation in Phanerozoic column O3 occurred between 400 and 200 Myr ago, corresponding to a rise in atmospheric O2 to ∼1.5 times the present atmospheric level (PAL) and subsequent fall to ∼0.5 PAL. The O3 response to this O2 decline shows latitudinal differences, thinning most at high latitudes (30–40 Dobson units (1 DU = 0.001 atm cm) at 66°N) and least at low latitudes (5–10 DU at 9°N) where a “self‐healing” effect is evident. This O3 depletion coincides with significant increases in the near‐surface biologically active UV radiation at high latitudes, +28% as weighted by the Thimijan spectral weighting function. O3 and UV changes were exacerbated when we incorporated a direct feedback of the terrestrial biosphere on atmospheric chemistry, through enhanced N2O production as the climate switched from an icehouse to a greenhouse mode. On the basis of a summary of field and laboratory experimental evidence, we suggest that these UV radiation increases may have exerted subtle rather than catastrophic effects on ecosystem processes.
Forces on stationary particles in near‐bed turbulent flows
Authors: Schmeeckle Mark W. , Nelson Jonathan M. , Shreve Ronald L.
Published: April 2007

In natural flows, bed sediment particles are entrained and moved by the fluctuating forces, such as lift and drag, exerted by the overlying flow on the particles. To develop a better understanding of these forces and the relation of the forces to the local flow, the downstream and vertical components of force on near‐bed fixed particles and of fluid velocity above or in front of them were measured synchronously at turbulence‐resolving frequencies (200 or 500 Hz) in a laboratory flume. Measurements were made for a spherical test particle fixed at various heights above a smooth bed, above a smooth bed downstream of a downstream‐facing step, and in a gravel bed of similarly sized particles as well as for a cubical test particle and 7 natural particles above a smooth bed. Horizontal force was well correlated with downstream velocity and not correlated with vertical velocity or vertical momentum flux. The standard drag formula worked well to predict the horizontal force, but the required value of the drag coefficient was significantly higher than generally used to model bed load motion. For the spheres, cubes, and natural particles, average drag coefficients were found to be 0.76, 1.36, and 0.91, respectively. For comparison, the drag coefficient for a sphere settling in still water at similar particle Reynolds numbers is only about 0.4. The variability of the horizontal force relative to its mean was strongly increased by the presence of the step and the gravel bed. Peak deviations were about 30% of the mean force for the sphere over the smooth bed, about twice the mean with the step, and 4 times it for the sphere protruding roughly half its diameter above the gravel bed. Vertical force correlated poorly with downstream velocity, vertical velocity, and vertical momentum flux whether measured over or ahead of the test particle. Typical formulas for shear‐induced lift based on Bernoulli's principle poorly predict the vertical forces on near‐bed particles. The measurements suggest that particle‐scale pressure variations associated with turbulence are significant in the particle momentum balance.
Modeling banded vegetation patterns in semiarid regions: Interdependence between biomass growth rate and relevant hydrological processes
Authors: Ursino N.
Published: April 2007

Vegetation patterns, such as regular spots and bands, have been observed in arid and semiarid lands. One of the most common explanations for vegetation banding is that the homogeneous steady state solution of soil moisture and vegetation biomass density balance, expressed in the form of a bucket model, may be unstable under conditions of scarce mean annual rainfall. Even though the theory seems to support our intuitive explanation of the phenomenon, there are still unresolved questions concerning soil parameterization, relevant hydrological processes, and the way plant physiology should be modeled in arid and semiarid environments where vegetation patterns have been observed. This paper examines the interrelation between hydrological processes and plant physiology. The biomass growth rate resembles plant physiology within the bucket model and determines the survival plant strategy given a limited soil moisture availability. The fact that very different hypotheses concerning the biomass growth rate have been formulated has not yet been given the important consideration it deserves. Different models for vegetation banding will be considered here. They are formulated by introducing different growth rates within the same soil moisture and vegetation balance equations. Linear stability analysis and numerical integration of the different models showed some relevant interrelation between hydrological and physiological features. It was demonstrated that the relation between biomass growth rate and biomass density determines which hydrological process enables vegetation pattern initiation. The discussion of this result leads to a critical review of previously published hypotheses on plant physiology and hydrological processes inducing vegetation organization.
The Atlantic Meridional Mode and hurricane activity
Authors: Vimont Daniel J. , Kossin James P.
Published: April 2007

Connections between the Atlantic Meridional Mode (AMM) and seasonal hurricane activity are investigated. The AMM, a dynamical “mode” of variability intrinsic to the tropical coupled ocean‐atmosphere system, is strongly related to seasonal hurricane activity on both decadal and interannual time scales. The connection arises due to the AMM's relationship with a number of local climatic conditions that all cooperate in their influence on hurricane activity. Further analysis indicates that the Atlantic Multi‐decadal Oscillation (AMO) can excite the AMM on decadal time scales. As such, it is suggested that the AMO's influence on seasonal hurricane activity manifests itself through the AMM. This relationship between the AMM, AMO, and seasonal hurricane activity refocuses our understanding of how climate variations relate to seasonal hurricane activity in the Atlantic, and offers an improved framework beyond purely thermodynamic arguments that relates hurricanes to large‐scale climate variations.
Derivation of a modified Fermi-Dirac distribution for quantum dot ensembles under nonthermal conditions
Authors: Summers Huw D. , Rees Paul
Published: April 2007

Semiconductor quantum dot lasers offer significant advantages over traditional quantum well devices. However, the advantages due to the discrete density of states of a structure confined in all three spatial dimensions are usually not fully realized because of problems associated with the capture of carriers into the discrete states. In this paper we use a simple rate equation model to identify the processes that limit the performance of a quantum dot laser. This simplistic approach, while lacking the rigor of more complex models, allows us to develop a physical understanding of how the properties of the quantum dot electronic states effect the operation of a laser. The existence of a thermal, Fermi-Dirac distribution of carriers is shown to exist only when there are no recombination processes (either radiative or nonradiative). In a quantum well laser the rate of thermalization is much faster than the carrier loss processes and therefore the distribution appears to be close to Fermi-Dirac; however, in a quantum dot structure the slower capture/escape rates can cause nonthermal carrier distributions. The interplay of the radiative recombination and capture and escape rates in the dots is shown to define the mode of operation of the laser. An identity, derived simply in terms of the rates of carrier escape and spontaneous recombination and a confinement energy, predicts whether the carrier population is coupled across the dot ensemble. This will determine whether a semiconductor quantum dot laser exhibits single mode operation.
Multiferroic nanoparticulate Bi3.15Nd0.85Ti3O12–CoFe2O4 composite thin films prepared by a chemical solution deposition technique
Authors: Zhong X. L. , Wang J. B. , Liao M. , Huang G. J. , Xie S. H. , Zhou Y. C. , Qiao Y. , He J. P.
Published: April 2007

Multiferroic xBi3.15Nd0.85Ti3O12–(1−x)CoFe2O4 composite thin films with x=0.5, 0.6, and 0.7 were fabricated on Pt∕Ti∕SiO2∕Si(100) substrates by a chemical solution deposition technique. X-ray diffraction shows that there are no other phases but bismuth-layered perovskite Bi3.15Nd0.85Ti3O12 and spinel CoFe2O4 phases in the films. Scanning electron microscopy reveals that CoFe2O4 aggregates locally into nanoparticles and embeds in the Bi3.15Nd0.85Ti3O12 matrix. The composite films exhibit both good ferroelectric and magnetic properties at room temperature, as well as distinct magnetoelectric coupling behaviors, which are comparable with those of multiferroic composite films with conventional Pb-based ferroelectric as a ferroelectric component.
Domain wall width and velocity behaviors in notched magnetic devices
Authors: Kim Se Dong , Chun Byong Sun , Kim Young Keun
Published: April 2007

Current-induced domain wall motion on a magnetic strip has been studied for its potential application to magnetic logic and memory devices. In this study, we carried out a series of calculations to investigate the changes of the domain wall velocity and width as a function of the notch shape using the Landau-Lifschitz-Gilbert solver. The domain wall had a uniform velocity when the notches were shallow and wide, while the width of the domain wall became narrow when the notches were deep and narrow. To produce a domain wall structure with both a uniform velocity and a narrow width, an asymmetric notch shape was devised.
Hollow oxide formation by oxidation of Al and Cu nanoparticles
Authors: Nakamura R. , Tokozakura D. , Nakajima H. , Lee J.-G. , Mori H.
Published: April 2007

The formation of hollow metal oxide nanoparticles through the oxidation process at low temperatures from 295 to 423 K has been studied by transmission electron microscopy for Cu, Al, and Pb. For Cu and Al, hollow oxide nanoparticles are obtained as a result of vacancy aggregation in the oxidation processes, resulting from the rapid outward diffusion of metal ions through the oxide layer during the oxidation process. On the other hand, Pb nanoparticles turn to solid PbO because the diffusivity difference DPb<DO in PbO does not lend itself to the formation of vacancy clusters. The oxide growth behavior of Cu and Al nanoparticles of a larger size at 423 K are summarized as follows: (i) for Al, the rapidly forming oxide layer on its surface stops growing once it reaches a critical thickness of about 1.5 nm, (ii) the growth of Cu2O continues until hollow Cu2O of a certain thickness is formed. This suggests the occurrence of two different diffusion processes in the formation of hollow oxides: the rapid outward diffusion of metal ions based on the Cabrera–Mott theory plays an important role in the formation of hollow Al-oxides, whereas the Kirkendall effect at the Cu∕Cu2O interface, where Cu diffuses much faster than oxygen, brings about the formation of hollow Cu2O.
Annealing temperature dependence of the electronic structure of La0.7Ca0.3MnO3−δ perovskite
Authors: Park J. S. , Lee Y. P. , Lee Youn-Seoung
Published: April 2007

The local atomic and electronic structures of La0.7Ca0.3MnO3 (LCMO) prepared by solid-state reaction was investigated at different annealing temperatures (TA). We employed x-ray absorption spectroscopy and x-ray photoemission spectroscopy to understand the local atomic and electronic structures of the LCMO system. Both the atomic and electronic structures due to the oxygen deficiency in LCMO turn out to change with increasing TA. Especially, the eg ↑ states, leading to the ferromagnetic interaction, are reduced by the diminution of Mn 3d–O 2p hybridization with increasing TA.
Further studies of a simple atomistic model of silica: Thermodynamic stability of zeolite frameworks as silica polymorphs
Authors: Ford Matthew H. , Auerbach Scott M. , Monson P. A.
Published: April 2007

We have applied our previously reported model of silica based on low coordination and strong association [J. Chem. Phys. 121, 8415 (2004)], to the calculation of phase stability of zeolite frameworks SOD, LTA, MFI, and FAU as silica polymorphs. We applied the method of Frenkel and Ladd for calculating free energies of these solids. Our model predicts that the MFI framework structure has a regime of thermodynamic stability at low pressures and above ∼1400 K, relative to dense phases such as quartz. In contrast, our calculations predict that the less dense frameworks SOD, LTA, and FAU exhibit no regime of thermodynamic stability. We have also used our model to investigate whether templating extends the MFI regime of thermodynamic stability to lower temperatures, by considering templates with hard-sphere repulsions and mean-field attractions to silica. Within the assumptions of our model, we find that quartz remains the thermodynamically stable polymorph at zeolite synthesis temperatures (∼400 K) unless unphysically large template-silica attractions are assumed. These predictions suggest that some zeolites such as MFI may have regimes of thermodynamic stability even without template stabilization.
Generalized Weierstrass representations of surfaces with the constant Gauss curvature in pseudo-Riemannian three-dimensional space forms
Authors: Xia Qiaoling
Published: April 2007

We know that there exists the correspondence between one of the nonumbilical Riemannian surface with constant Gauss curvature K in M¯3(K¯)(K≠K¯), the nonumbilical spacelike or timelike surfaces with constant Gauss curvature K in M¯13(K¯)(K≠K¯), and a solution of one of the following differential equations: the sine-Gordon, sinh-Gordon, sine-laplace, sinh-laplace, and cosh-Gordon equations. In this paper, we consider the initial value problems of these equations and give the generalized Weierstrass representations of these surfaces that depend only on the initial values of these equations.
Mode identification of high-quality-factor single-defect nanocavities in quantum dot-embedded photonic crystals
Authors: Shirane Masayuki , Kono Shunsuke , Ushida Jun , Ohkouchi Shunsuke , Ikeda Naoki , Sugimoto Yoshimasa , Tomita Akihisa
Published: April 2007

We investigate the quality (Q) factor and the mode dispersion of single-defect nanocavities based on a triangular-lattice GaAs photonic-crystal (PC) membrane, which contain InAs quantum dots (QDs) as a broadband emitter. To obtain a high Q factor for the dipole mode, we modulate the radii and positions of the air holes surrounding the nanocavity while keeping sixfold symmetry. A maximum Q of 17 000 is experimentally demonstrated with a mode volume of V=0.39(λ∕n)3. We obtain a Q∕V of 44 000(n∕λ)3, one of the highest values ever reported with QD-embedded PC nanocavities. We also observe ten cavity modes within the first photonic band gap for the modulated structure. Their dispersion and polarization properties agree well with the numerical results.
X-ray diffraction analysis of the structure of antisite arsenic point defects in low-temperature-grown GaAs layer
Authors: Fukushima S. , Otsuka N.
Published: April 2007

A method for the structure analysis of point defects in a semiconductor layer is developed by combining x-ray diffraction and growth of a superlattice where the concentration of point defects is periodically varied in the growth direction. Intensities of satellite reflections from the superlattice depend predominantly on the atomic structure of point defects, and hence this method can be applicable to the case of a low concentration of point defects. By using this method, the atomic structure of antisite As point defects in GaAs layers grown by molecular-beam epitaxy at low temperatures has been analyzed. Measured intensity ratios of the first-order satellite reflection in the lower angle side to that in the higher angle side for a number of (hkl) reflections are compared with those calculated based on structure models. The analysis has shown that experimental intensity ratios cannot be reproduced by models which include only a uniform tetragonal lattice distortion and local atomic displacements around an antisite As atom. A fairly good agreement between measured and calculated intensity ratios is obtained with a model which account for both gradual change in the tetragonal lattice distortion in the (0 0 1) plane and displacements of neighboring atoms away from the antisite As atom.
Unusual halogen-bonded complex FBrδ+⋯Brδ+F and hydrogen-bonded complex FBrδ+⋯Hδ+F formed by interactions between two positively charged atoms of different polar molecules
Authors: Wang Fang-Fang , Hou Jian-Hua , Li Zhi-Ru , Wu Di , Li Ying , Lu Zhong-Yuan , Cao Wei-Liang
Published: April 2007

Using ab initio calculations, the authors’ predicted for the first time that the halogen-bonded complex FBrδ+⋯Brδ+F and hydrogen-bonded complex FBrδ+⋯Hδ+F formed by the interactions between two positively charged atoms of different polar molecules can be stable in gas phase. It shows that halogen bond or hydrogen bond not only exists between oppositely charged atoms but also between like-charged atoms. That the attraction arising from the special halogen bond or hydrogen bond can exceed the electrostatic repulsion between two contact positively charged atoms stabilizes the complex. Of course, from the point of view of physics they can consider the interactions in FBrδ+⋯Brδ+F and FBrδ+⋯Hδ+F as mainly the sum of the long range molecular interactions, namely, electrostatic, induction, and dispersion with some short-range repulsion. They found that the intermolecular electron correlation contribution representing dispersion interaction plays a crucial role in the stabilities of seemingly repulsive complexes FBrδ+⋯Brδ+F and FBrδ+⋯Hδ+F.
Improving the accuracy of density-functional theory calculation: The genetic algorithm and neural network approach
Authors: Li Hui , Shi LiLi , Zhang Min , Su Zhongmin , Wang XiuJun , Hu LiHong , Chen GuanHua
Published: April 2007

The combination of genetic algorithm and neural network approach (GANN) has been developed to improve the calculation accuracy of density functional theory. As a demonstration, this combined quantum mechanical calculation and GANN correction approach has been applied to evaluate the optical absorption energies of 150 organic molecules. The neural network approach reduces the root-mean-square (rms) deviation of the calculated absorption energies of 150 organic molecules from 0.47to0.22eV for the TDDFT∕B3LYP∕6-31G(d) calculation, and the newly developed GANN correction approach reduces the rms deviation to 0.16eV.
Vibrational dynamics of DNA: IV. Vibrational spectroscopic characteristics of A-, B-, and Z-form DNA’s
Authors: Lee Chewook , Cho Minhaeng
Published: April 2007

Linear and nonlinear IR spectroscopic studies of nucleic acids can provide crucial information on solution conformations of DNA double helix and its complex with other molecules. Carrying out density functional theory calculations of A-, B-, and Z-form DNA’s, the authors obtained vibrational spectroscopic properties as well as coupling constants between different basis modes. The vibrational couplings that determine the extent of exciton delocalization are strongly dependent on DNA conformation mainly because the interlayer distance between two neighboring base pairs changes with respect to the DNA conformation. The Z-DNA has comparatively small interlayer vibrational coupling constants so that its vibrational spectrum depends little on the number of base pairs, whereas the A-DNA shows a notable dependency on the size. Furthermore, it is shown that a few distinctively different line shape changes in both IR and two-dimensional IR spectra as the DNA conformation changes from B to A or from B to Z can be used as marker bands and characteristic features distinguishing different DNA conformations.
Low energy ion beam assisted deposition of a spin valve
Authors: Quan J. J. , Wolf S. A. , Wadley H. N. G.
Published: April 2007

The spin dependent electron transport in giant magnetoresistive (GMR) multilayers is significantly affected by the atomic scale structure of their interfaces. Devices with atomically flat and chemically sharp interfaces are preferred for magnetic sensor and memory applications. Recent atomic simulations of the atom-by-atom assembly of these devices indicate that near optimal interfacial structures can be created using low energy, ion assisted vapor deposition techniques with ion energies in the 5–10eV range. A recently developed biased target ion beam deposition system has been used to experimentally test this hypothesis. Prototypical Ta∕NiFe∕Co∕Cu∕Co∕FeMn∕Cu spin valve structures were first grown using (simultaneous) argon ion assistance during deposition of the Co∕Cu∕Co trilayer part of the spin valve multilayer. Assisting ion energies of around 10eV resulted in structures with a 30% higher magnetoresistance ratio and significantly reduced coupling field compared to samples grown with no ion assistance or with ion energies above 15eV. These results are consistent with the atomistic simulation predictions. Other promising ion assistance schemes identified by the simulations were then used to deposit the Ta, NiFe, FeMn, and the top copper layer. A near optimal strategy was identified that resulted in the further improvement of the GMR ratio.
Geometry optimization based on linear response free energy with quantum mechanical/molecular mechanical method: Applications to Menshutkin-type and Claisen rearrangement reactions in aqueous solution
Authors: Higashi Masahiro , Hayashi Shigehiko , Kato Shigeki
Published: April 2007

The authors present a method based on a linear response theory that allows one to optimize the geometries of quantum mechanical/molecular mechanical (QM/MM) systems on the free energy surfaces. Two different forms of linear response free energy functionals are introduced, and electronic wave functions of the QM region, as well as the responses of electrostatic and Lennard-Jones potentials between QM and MM regions, are self-consistently determined. The covariant matrix relating the QM charge distribution to the MM response is evaluated by molecular dynamics (MD) simulation of the MM system. The free energy gradients with respect to the QM atomic coordinates are also calculated using the MD trajectory results. They apply the present method to calculate the free energy profiles of Menshutkin-type reaction of NH3 with CH3Cl and Claisen rearrangement of allyl vinyl ether in aqueous solution. For the Menshutkin reaction, the free energy profile calculated with the modified linear response free energy functional is in good agreement with that by the free energy perturbation calculations. They examine the nonequilibrium solvation effect on the transmission coefficient and the kinetic isotope effect for the Claisen rearrangement.
Image analysis system for evaluating sliding behavior of a liquid droplet on a hydrophobic surface
Authors: Sakai Munetoshi , Hashimoto Ayako , Yoshida Naoya , Suzuki Shunsuke , Kameshima Yoshikazu , Nakajima Akira
Published: April 2007

An analysis system was developed to evaluate the sliding behavior of a liquid droplet on a hydrophobic surface. This system enables continuous and simultaneous measurement of both the sliding acceleration and shape deformation during the sliding of a liquid droplet. Moreover, the velocity vector of the internal fluidity of a sliding droplet was obtained by employing particle image velocimetry in the analysis system. This evaluation method pioneers the measurement of the dynamic wettability of a hydrophobic solid surface.
Comparison of two approaches to the study of filamentation in plasmas
Authors: Sodha Mahendra Singh , Sharma Ashutosh
Published: April 2007

This communication presents a comparison of two approaches to the study of filamentation in a plasma; viz., the study of the spatial growth of a sinusoidal instability, superposed on a uniform electromagnetic beam and the study of self-focusing of a Gaussian ripple, superposed on a uniform electromagnetic beam. The results of the two approaches have been expressed in a form whereby they can be compared and the similarities/differences highlighted. It has been pointed out that the growth of the instability in the first approach is equivalent to the self-focusing of the ripple in the second approach. The critical condition for the onset of the growth of a sinusoidal instability is seen to depend only on the irradiance of the main beam, while that for the self-focusing of the Gaussian ripple depends on the irradiance of the main beam, the irradiance of the ripple, and the phase difference between the two.
Glass transition in an isostatically compressed calcium metaphosphate glass
Authors: Yue Yuanzheng , Wondraczek Lothar , Behrens Harald , Deubener Joachim
Published: April 2007

The authors report an ambient-pressure differential scanning calorimetric study of a calcium metaphosphate glass that has been isostatically compressed slightly above its glass transition temperature and was frozen-in under pressure. It is shown that the enthalpy overshoot of the calorimetric glass transition is enhanced by this treatment. This enhancement is associated with a decrease in the apparent fictive temperature TfA that is determined using the enthalpy-matching approach. The origin of this correlation is discussed.
Extreme ultraviolet source using a forced recombination process in lithium plasma generated by a pulsed laser
Authors: Nagano Akihisa , Inoue Takahiro , Nica Petru-Edward , Amano Sho , Miyamoto Shuji , Mochizuki Takayasu
Published: April 2007

An extreme ultraviolet source having a tamper has been studied. This target scheme recombines forcedly lithium ions by low temperature electrons from the tamper, converting Li3+ rapidly to excited Li2+ which emit intense 1s-2p Lyman α emissions at 13.5nm. A strong 13.5nm emission appeared at 20–30ns after the time of laser peak within a small space volume near the tamper. The authors obtained an enhancement of extreme ultraviolet conversion efficiency by a factor of about 2 with the tamper against that of a target without the tamper at the same laser irradiation condition.
Operational characteristics and plasma measurements in cylindrical Hall thrusters
Authors: Shirasaki Atsushi , Tahara Hirokazu
Published: April 2007

The cylindrical Hall thruster (CHT) is an attractive approach to achieve a long lifetime thruster operation especially in low power space applications. Because of the larger volume-to-surface ratio than conventional coaxial Hall thrusters, the cylindrical Hall thrusters are characterized by a reduced heating of the thruster parts and potential lower erosion. Existing CHTs can feature a short coaxial channel in order to sustain a high ionization in the thruster discharge. A 5.6 cm diameter cylindrical Hall thruster was developed and operated with and without a short coaxial region of the thruster channel, in the power range of 70–300 W. It is shown that the CHT without coaxial region can operate stable and achieve higher thrust efficiency, 22%–32% more than that with a coaxial region. Plasma probe measurements inside the thruster channel and ion energy measurements in the plasma plume suggest that the ionization/acceleration region in the CHT is located near the anode region where a radial magnetic field is stronger.
Co location and valence state determination in ferromagnetic ZnO:Co thin films by atom-location-by-channeling-enhanced-microanalysis electron energy-loss spectroscopy
Authors: Liu Yuzi , Xu Qingyu , Schmidt Heidemarie , Hartmann Lars , Hochmuth Holger , Lorenz Michael , Grundmann Marius , Han Xiaodong , Zhang Ze
Published: April 2007

The Co valence state and the location of Co dopant atoms in ZnO:Co thin films revealing anomalous Hall effect below 50K have been determined by electron energy-loss spectroscopy (EELS) measurements and atom location by channeling enhanced microanalysis (ALCHEMI), respectively. The method of ALCHEMI-EELS to investigate the dopant in materials was brought. It was used to determine that the intrinsic ferromagnetism in ZnO:Co thin films derives from the +2 Co which substitute for Zn atoms at Zn sites in the ZnO matrix. The divalent Co state in ZnO:Co has been unambiguously proven by modeling the corresponding EELS data.
Threading dislocations in In-polar InN films and their effects on surface morphology and electrical properties
Authors: Wang Xinqiang , Che Song-Bek , Ishitani Yoshihiro , Yoshikawa Akihiko
Published: April 2007

In-polar InN films with atomically flat surface are grown on Ga-polar GaN templates by molecular beam epitaxy. Densities of threading dislocations with screw and edge components in these films are about 108 and low 1010cm−2, respectively. It is found that the screw-component threading dislocation is the dominant cause for macroscopic surface defects appearing as growth-spiral hillocks; their densities (their growth temperature dependences) are almost the same (similar) with each other. Further, it is shown that the residual electron concentration in InN is almost the same with the density of dangling bonds at the edge-component threading dislocations.
Electrical transport through monatomic titania chains
Authors: Yilmaz Bilge , Sacco Albert , Deng Jiangdong
Published: April 2007

A crystalline titanosilicate material, Engelhard titanosilicate-4 (ETS-4), contains monatomic (i.e., one-atom-thick) semiconductor ⋯Ti–O–Ti–O–Ti⋯ (titania) chains in its framework, which are hypothesized to be “intrinsic” quantum wires. Monolithic ETS-4 crystals were individually embedded between gold electrodes using a photolithography based fabrication methodology, forming a device architecture in microcircuit configuration. Electrical transport measurements were performed, and current-voltage characteristics of the monatomic titania chains in ETS-4 were investigated. A non-Ohmic behavior with higher conductivities at higher bias voltages was observed.
Dynamics of quantum dot photonic crystal lasers
Authors: Ellis Bryan , Fushman Ilya , Englund Dirk , Zhang Bingyang , Yamamoto Yoshihisa , Vučković Jelena
Published: April 2007

Quantum dot photonic crystal membrane lasers were fabricated and the large-signal modulation characteristics were studied. The authors find that the modulation characteristics of quantum dot lasers can be significantly improved using cavities with large spontaneous emission coupling factor. Their experiments show, and simulations confirm, that the modulation rate is limited by the rate of carrier capture into the dots to around 30GHz in their present system.
Dependence of the electron beam parameters on the stability of laser propagation in a laser wakefield accelerator
Authors: Hafz N. M. , Choi I. W. , Sung J. H. , Kim H. T. , Hong K.-H. , Jeong T. M. , Yu T. J. , Kulagin V. , Suk H. , Noh Y.-C. , Ko D.-K. , Lee J.
Published: April 2007

Characteristics of electron beams produced by the laser wakefield acceleration are presented. The dependence of the electron beam parameters on the laser focal spot size is investigated. The experimental result shows the generation of quasimonoenergetic electron beam although the laser spot size was several times larger than the plasma wavelength. Stable electron beam generation at large laser spots was owing to the stable laser propagation in plasma channels. At a small laser spot, the beam quality is poor and this is attributed to the the filamentation instability of the laser beam.
Electron nanodiffraction using sharply focused parallel probes
Authors: Dwyer Christian , Kirkland Angus I. , Hartel Peter , Müller Heiko , Haider Maximilian
Published: April 2007

The authors describe an electron-optical configuration for producing a nanometer-scale sharply focused parallel electron probe in the transmission electron microscope. The configuration utilizes one of the round lenses in an objective prefield aberration corrector and generates a sharply focused parallel probe of 10nm in diameter, with better than 0.2nm edge acuity. Such a probe makes it possible to obtain electron diffraction patterns from nanometer-scale volumes of the specimen with unprecedented precision. A method for measuring the transverse coherence of the probe is also described.
Control of InN quantum dot density using rare gases in metal organic vapor phase epitaxy
Authors: Ruffenach S. , Briot O. , Moret M. , Gil B.
Published: April 2007

Indium nitride (InN) quantum dots have been grown on gallium nitride (GaN) templates with heights of 10 and 20nm. The authors demonstrate that the surface densities of the dots are strongly affected by the nature of the carrier gas used during the growth, which can be used to modulate the surface density. The authors show here that replacing nitrogen by helium leads to a decrease of the dot surface density, while argon induces a strong increase of the density. Although validated for the InN∕GaN system, this approach has a more general scope and can be extended to other material systems.
Electric field tunable electron g factor and high asymmetrical Stark effect in InAs1−xNx quantum dots
Authors: Zhang X. W. , Fan W. J. , Li S. S. , Xia J. B.
Published: April 2007

The electronic structure, electron g factor, and Stark effect of InAs1−xNx quantum dots are studied by using the ten-band k∙p model. It is found that the g factor can be tuned to be zero by the shape and size of quantum dots, nitrogen (N) doping, and the electric field. The N doping has two effects on the g factor: the direct effect increases the g factor and the indirect effect decreases it. The Stark effect in quantum ellipsoids is high asymmetrical and the asymmetry factor may be 319.
Electrical control for anisotropy in charge-orbital order state of Bi1−xSrxMnO3 (x∼0.5) at room temperature
Authors: Taniguchi K. , Nishiyama S. , Arima T. , Konno S. , Yamada S. , Sugano E.
Published: April 2007

The electric perturbation effect to the charge-orbital ordering state has been investigated for Bi1−xSrxMnO3 (x∼0.5). The alignment and rotation of the charge-orbital stripes have been observed in the resistance and the polarizing microscope images at room temperature. These results indicate the possibility of electrical control of the anisotropic properties in the charge-orbital ordering state through the orbital degree of freedom.
Optical characterization of CO2-laser-ablated Si-rich SiOx
Authors: Lin Gong-Ru , Lin Chun-Jung , Chang Yia-Chung
Published: April 2007

Anomalous absorption and the corresponding change in the optical band gap of a CO2-laser-ablated Si-rich SiO2 (SiOx) film are studied. The optical band gap energy of as-grown nonstoichiometric SiOx is slightly reduced by increasing Si–Si bonds as compared to quartz. After rapid thermal annealing using a CO2 laser, the dehydrogenation of SiOx film further increases the Si–Si bonding states and redshifts the optical band gap by 1eV. Laser ablation is initiated at a laser intensity of >7.5kW∕cm2, leaving numerous luminescent centers that are related to neutral oxygen vacancy defects, increasing the absorption coefficient and related optical band gap energy, and reducing the refractive index in partially annealed SiOx.
Ab initio studies of arsenic and boron related defects in silicon mesa diodes
Authors: Janke C. , Jones R. , Öberg S. , Briddon P. R.
Published: April 2007

E centers are known to diffuse around 400K in Si and may then form larger donor-vacancy defects such as As2V in heavily doped n-type Si doped with As or AsBV if they diffuse into p-type regions. Ab initio methods are used to explore these possibilities. The AsV defect possesses electrical levels in agreement with experiments. The AsBV defect is found to exhibit a charge-dependent structure, has a barrier to dissociation of ∼1.4eV, and possesses an acceptor level at 0.27 or 0.47eV above the valence band top depending on the defect structure. The As2V defect possesses only an acceptor level at 0.22eV below the conduction band. Comparison is made with recent experiments carried out on mesa diodes.
Surface-morphology evolution and strain relaxation during heteroepitaxial growth of GaN films without low-temperature nucleation layers
Authors: Lee Sanghwa , Choe Hyeokmin , Oh Taegeon , Jean Jai Weon , Shin Boa , Sohn Yuri , Kim Chinkyo , Choi Jaewan , Moon Yong-Tae , Lee Jeong Soo
Published: April 2007

With no low-temperature nucleation layers, heteroepitaxial GaN films were grown at 1050°C on c-plane sapphire substrates by hydride vapor phase epitaxy. Not like for the films grown on low-temperature buffer layers, upon island merging no immediate smoothing of a surface was observed, but through several intermediate stages the surface morphology evolved from three-dimensional islands to terrace-and-step structures. From synchrotron x-ray diffraction measurements, it was revealed that the density of screw-or mixed-type threading dislocations greatly increased when the islands merged, but that of edge-type dislocations did not (edge-type threading dislocations are known to be dominantly formed in the GaN films grown on low-temperature buffer layers). This implies that the evolution of surface morphology sensitively depends on the type of threading dislocations introduced during island merging. Despite the absence of intentional nucleation layers, the strain was found to be fully relaxed even before the nucleated islands began to merge.
Field-driven gel actuator with versatile long-range locomotion in air
Authors: Liang Songmiao , Weng Lihui , Tan Shuaixia , Xu Jian , Zhang Xiaoli , Zhang Lina
Published: April 2007

An approach to fabricate gel actuator using nonionic polymer gels and dielectric liquid pattern was developed. The actuator could be operated in air using a dc electric field and exhibited a rotation-slide complex motion on the horizontal and a snail-like or sliding motion on the slope with controlled path. The average rotational speed and the average climbing speed of the gel could reach 0.35rad∕s and 3.8mm∕s, respectively. The driving force of the gel originated from the electrohydrodynamic flow of the dielectric solvent inside and outside the gel induced by the ion-dragging mechanism under the electric field.