bluuebunny/crossref_metadata_2025 huggingface dataset:
Pair correlations of the leVeque sequence on the polydisc
Authors: NAIR R.
Published: April 2008

We consider a system of “forms” defined for ẕ = (zij) on a subset of $\Bbb C^d$ by

where d = d1 + ⋅ ⋅ ⋅ + dl and for each pair of integers (i,j) with 1 ≤ i ≤ l, 1 ≤ j ≤ di we denote by $(v_{ij}(k))_{k=1}^{\infty}$ a strictly increasing sequence of natural numbers. Let ${\Bbb C}_1$ = {z ∈ ${\Bbb C}$ : |z| < 1} and let ${\underline X} \ = \ \times _{i=1}^l \times _{j=1}^{d_i}X_{ij}$ where for each pair (i, j) we have Xij = ${\Bbb C}\backslash {\Bbb C}_1$. We study the distribution of the sequence on the l-polydisc $({\Bbb C}_1)^l$ defined by the coordinatewise polar fractional parts of the sequence Xk(ẕ) = (L1(ẕ)(k),. . ., Ll(ẕ)(k)) for typical ẕ in ${\underline X}$ More precisely for arcs I1, . . ., I2l in $\Bbb T$, let B = I1 × ⋅ ⋅ ⋅ × I2l be a box in $\Bbb T^{2l}$ and for each N ≥ 1 define a pair correlation function by

and a discrepancy by ΔN = $\sup_{B \subset \Bbb T^{2l}}${VN(B) − N(N−1)leb(B)}, where the supremum is over all boxes in $\Bbb T^{2l}$. We show, subject to a non-resonance condition on $(v_{ij}(k))_{k=1}^{\infty}$, that given ε > 0 we have ΔN = o(N$(log N)^{l + {1\over 2}}$(log log N)1+ε) for almost every $\underline x(\underline z)\in \Bbb T^{2l}$. Similar results on extremal discrepancy are also proved. Our results complement those of I. Berkes, W. Philipp, M. Pollicott, Z. Rudnick, P. Sarnak, R Tichy and the author in the real setting.
A note on finite determinacy for corank 2 map germs from surfaces to 3-space
Authors: MARAR W. L. , NUÑO–BALLESTEROS J. J.
Published: April 2008

We study properties of finitely determined corank 2 quasihomogeneous map germs f:($\C^2$, 0) → ($\C^3$, 0). Examples and counter examples of such map germs are presented.
Falls
Authors: Davison John , Marrinan Sarah
Published: April 2008

In the context of an aging population, the importance of falls and their prevention has arguably never been greater. Both injurious and non-injurious falls are a frequent occurrence resulting in potentially devastating physical and social consequences. In recent years, there has been a marked expansion in falls research, meta-analysis, and local and national management guidelines, reflecting increased multidisciplinary professional and public awareness of falls.
Asymptotic and numerical homogenization
Authors: Engquist B. , Souganidis P. E.
Published: April 2008

Homogenization is an important mathematical framework for developing effective models of differential equations with oscillations. We include in the presentation techniques for deriving effective equations, a brief discussion on analysis of related limit processes and numerical methods that are based on homogenization principles. We concentrate on first- and second-order partial differential equations and present results concerning both periodic and random media for linear as well as nonlinear problems. In the numerical sections, we comment on computations of multi-scale problems in general and then focus on projection-based numerical homogenization and the heterogeneous multi-scale method.
Interior-point methods for optimization
Authors: Nemirovski Arkadi S. , Todd Michael J.
Published: April 2008

This article describes the current state of the art of interior-point methods (IPMs) for convex, conic, and general nonlinear optimization. We discuss the theory, outline the algorithms, and comment on the applicability of this class of methods, which have revolutionized the field over the last twenty years.
North Pacific Gyre Oscillation links ocean climate and ecosystem change
Authors: Di Lorenzo E. , Schneider N. , Cobb K. M. , Franks P. J. S. , Chhak K. , Miller A. J. , McWilliams J. C. , Bograd S. J. , Arango H. , Curchitser E. , Powell T. M. , Rivière P.
Published: April 2008

Decadal fluctuations in salinity, nutrients, chlorophyll, a variety of zooplankton taxa, and fish stocks in the Northeast Pacific are often poorly correlated with the most widely‐used index of large‐scale climate variability in the region ‐ the Pacific Decadal Oscillation (PDO). We define a new pattern of climate change, the North Pacific Gyre Oscillation (NPGO) and show that its variability is significantly correlated with previously unexplained fluctuations of salinity, nutrients and chlorophyll. Fluctuations in the NPGO are driven by regional and basin‐scale variations in wind‐driven upwelling and horizontal advection – the fundamental processes controlling salinity and nutrient concentrations. Nutrient fluctuations drive concomitant changes in phytoplankton concentrations, and may force similar variability in higher trophic levels. The NPGO thus provides a strong indicator of fluctuations in the mechanisms driving planktonic ecosystem dynamics. The NPGO pattern extends beyond the North Pacific and is part of a global‐scale mode of climate variability that is evident in global sea level trends and sea surface temperature. Therefore the amplification of the NPGO variance found in observations and in global warming simulations implies that the NPGO may play an increasingly important role in forcing global‐scale decadal changes in marine ecosystems.
Non‐contact infrared temperature measurements in dry permafrost boreholes
Authors: Junker Ralf , Grigoriev Mikhail N. , Kaul Norbert
Published: April 2008

While planning the COAST Expedition to the Siberian Laptev Sea in 2005, the question of how to make a short equilibrium temperature measurement in a dry borehole arose. As a result, an infrared borehole tool was developed and used in three dry boreholes (up to 60.2 m deep) in the coastal transition zone from terrestrial to sub‐sea permafrost near Mamontovy Klyk in the western Laptev Sea. A depth versus temperature profile was acquired with equilibration times of 50 × 10−3 s at each depth interval. Comparison with a common resistor string revealed an offset due to limitations of accuracy of the infrared technique and the influence of the probe's massive steel housing. Therefore it was necessary to calibrate the infrared sensor with a high precision temperature logger in each borehole. The results of the temperature measurements show a highly dynamic transition zone with temperature gradients up to −0.092°C/m and heat flow of −218 mW/m. A period of submergence of only 600 years the drilled sub‐sea permafrost is approaching the overlying seawater temperature at −1.61°C with a temperature gradient of 0.021°C/m and heat flow of 49 mW/m. Further offshore, 11 km from the coastline, a temperature gradient of 0.006°C/m and heat flow of 14 mW/m occur. Thus the sub‐sea permafrost in the Mamontovy Klyk region has reached a critical temperature for the presence of interstitial ice. The aim of this article is to give a brief feasibility study of infrared downhole temperature measurements and to present experiences and results of its successful application.
Equatorial Undercurrent and North Equatorial Countercurrent at 38°W: A new perspective from direct velocity data
Authors: Urbano D. F. , De Almeida R. A. F. , Nobre P.
Published: April 2008

The western Equatorial Undercurrent (EUC) and North Equatorial Countercurrent (NECC) are investigated at 38°W using contemporaneous subsurface (ADCP) and high‐resolution near‐surface (drifters + satellite altimeter) velocity measurements, together with hydrographic (CTDO2) data that were collected from 1998 to 2006. The observations reveal an EUC with a strong semiannual pattern of intensification. Direct measurements also confirm the existence of a northern branch of the NECC (nNECC), observed here for the first time in the western tropical Atlantic. The NECC displays an annual cycle of northward migration on the basin, driven by the Sverdrup transport generated by the wind field. In this cycle the nNECC is a semipersistent feature fed by waters from the Northern Hemisphere and the residual nNECC flow from the previous year.
A climatology of the Northern Hemisphere winter anticyclones
Authors: Ioannidou L. , Yau M. K.
Published: April 2008

A climatology of the Northern Hemisphere winter anticyclonic activity is presented based on the European Centre for Medium‐Range Weather Forecasts reanalysis data set for the 1957–2002 period (ERA‐40) and a system‐centered method of tracking diagnostics. The method tracks the anticyclonic systems and monitors their evolution along their paths thus allowing documentation of aspects that have been previously overlooked due to the common perception of anticyclones as semi‐stationary, relatively uniform, slowly evolving structures. The anticyclonic signal is examined in a wide range of meteorological fields and at multiple levels, over the continental masses, over the subtropical ocean basins, and over the sectors where blocking highs tend to develop. Structural features, thermal and vorticity properties, and mobility characteristics of the corresponding anticyclones are highlighted. Over continental regions an association with low level pools of cold air is found to be prevalent for the Eurasian anticyclones but less so for the North American anticyclones. In the subtropical zone the anticyclonic signal in the regions affected by the winter monsoon, including southeastern Asia and the western Pacific features prominently in the upper levels. In the eastern sectors of the subtropical ocean basins the anticyclonic signal is less deep. Intense anticyclonic development at low levels is also identified over the western sectors of the subtropical continents where it appears associated to the effect of orography and of low level cooling. The dominant type of anticyclone observed in the regions of blocking is characterized by 5–6 days timescales and considerable eastward mobility.
Validation of the Aura MLS cloud ice water content measurements
Authors: Wu Dong L. , Jiang Jonathan H. , Read William G. , Austin Richard T. , Davis Cory P. , Lambert Alyn , Stephens Graeme L. , Vane Deborah G. , Waters Joe W.
Published: April 2008

Retrieval and validation of upper tropospheric ice water content (IWC) measurements with the Aura Microwave Limb Sounder (MLS) are described. The MLS version 2.2 (V2.2) IWC, derived from 240‐GHz cloud‐induced radiances (Tcir) at high tangent heights, is scientifically useful at 215–83 hPa. The V2.2 IWC represents a bulk cloud property averaged over a ∼300 × 7 × 4 km3 volume near the pointing tangent height. Precision, accuracy, and spatial resolution of the V2.2 IWC are determined through model simulations and comparisons with CloudSat observations. Comparisons of MLS V2.2 and CloudSat R03 IWCs are made for the months of January and July in terms of normalized probability density function (PDF). The differences between MLS and CloudSat IWC PDFs are generally less than 50% over the IWC range where the MLS technique is valid. At pressures <177 hPa and extratropical latitudes, the MLS V2.2 IWC exhibits a slightly low bias compared to CloudSat, part of which can be attributed to systematic errors in the MLS retrieval. Cloud inhomogeneity and particle size distribution are the leading sources of uncertainties in the V2.2 IWC.
Downscale linkage of global model output for regional chemical transport modeling: Method and general performance
Authors: Song C.‐K. , Byun D. W. , Pierce R. B. , Alsaadi J. A. , Schaack T. K. , Vukovich F.
Published: April 2008

The goal of this study is to assess the impact of the downscale linkage of global chemistry model output on the simulated regional scale O3 concentration and its vertical and horizontal structure over the continental US. For the global chemistry model, the NASA LaRC‐University of Wisconsin Real‐time Air Quality Modeling System (RAQMS) with a global ozone assimilation framework was used. RAQMS simulation results were downscaled with a RAQMS‐CMAQ linkage tool (RAQ2CMAQ) to provide dynamic lateral boundary conditions for regional air quality simulations with the EPA Models‐3 Community Multiscale Air Quality (CMAQ) Model. We performed two sets of CMAQ simulations; (1) with the predefined lateral boundary condition profile representing relatively clean air conditions in the eastern‐half of the United States, and (2) with the lateral boundary conditions generated from RAQMS 6 hourly analyses, for the 1999 Southern Oxidant Study (SOS‐99) period (15 June–14 July 1999). When compared and evaluated with available ozone soundings and nation wide surface observations (AIRS/EPA) in U.S., CMAQ simulations with the lateral boundary condition from the predefined profile failed to simulate the so‐called “chemical tropopause” above which O3 concentration rapidly increases with height. On the other hand, the simulations with the lateral boundary condition provided with the global RAQMS output not only improved diurnal variations and daily maxima of surface O3 concentrations, but also produced better agreement with the vertical structure of O3 measured in the middle and upper troposphere. The tracer mode CMAQ simulations showed that, at the upper troposphere, more than 85% of the O3 concentration difference associated with the lateral BCs was caused by the transport and diffusion processes.
Recent global sea level acceleration started over 200 years ago?
Authors: Jevrejeva S. , Moore J. C. , Grinsted A. , Woodworth P. L.
Published: April 2008

We present a reconstruction of global sea level (GSL) since 1700 calculated from tide gauge records and analyse the evolution of global sea level acceleration during the past 300 years. We provide observational evidence that sea level acceleration up to the present has been about 0.01 mm/yr2 and appears to have started at the end of the 18th century. Sea level rose by 6 cm during the 19th century and 19 cm in the 20th century. Superimposed on the long‐term acceleration are quasi‐periodic fluctuations with a period of about 60 years. If the conditions that established the acceleration continue, then sea level will rise 34 cm over the 21st century. Long time constants in oceanic heat content and increased ice sheet melting imply that the latest Intergovernmental Panel on Climate Change (IPCC) estimates of sea level are probably too low.
Possible linkages between Saharan dust and tropical cyclone rain band invigoration in the eastern Atlantic during NAMMA‐06
Authors: Jenkins Gregory S. , Pratt Aaron S. , Heymsfield Andrew
Published: April 2008

The Saharan Air Layer (SAL) is a dominant feature that influences the large‐scale environment from West Africa to the western tropical North Atlantic. While the SAL can create hostile thermodynamic and kinematic environmental conditions for tropical cyclogenesis, it also provides an infusion of cloud condensation and ice nuclei which can potentially invigorate convection. Here we show that these mechanisms may have been involved with the development of Tropical Storm (TS) Debby and Tropical Depression (TD) 8 (later Hurricane Helene) in 2006. Satellite imagery and rawinsondes indicate SAL outbreaks just prior to the emergence of these disturbances over the extreme Eastern tropical Atlantic Ocean. Here we examine the invigoration of convective bands associated with TS Debby and TD‐8 based on satellite and direct aircraft measurement. In‐situ aircraft measurements show enhanced cloud water content, cloud and precipitation sized particles, lightning and a 26 ms‐1 updraft just south of the SAL with TD‐8.
Concerted action of two novel tRNA mtDNA point mutations in chronic progressive external ophthalmoplegia
Authors: Kornblum Cornelia , Zsurka Gábor , Wiesner Rudolf J. , Schröder Rolf , Kunz Wolfram S.
Published: April 2008

CPEO (chronic progressive external ophthalmoplegia) is a common mitochondrial disease phenotype in adults which is due to mtDNA (mitochondrial DNA) point mutations in a subset of patients. Attributing pathogenicity to novel tRNA mtDNA mutations still poses a challenge, particularly when several mtDNA sequence variants are present. In the present study we report a CPEO patient for whom sequencing of the mitochondrial genome revealed three novel tRNA mtDNA mutations: G5835A, del4315A, T1658C in tRNATyr, tRNAIle and tRNAVal genes. In skeletal muscle, the tRNAVal and tRNAIle mutations were homoplasmic, whereas the tRNATyr mutation was heteroplasmic. To address the pathogenic relevance, we performed two types of functional tests: (i) single skeletal muscle fibre analysis comparing G5835A mutation loads and biochemical phenotypes of corresponding fibres, and (ii) Northern-blot analyses of mitochondrial tRNATyr, tRNAIle and tRNAVal. We demonstrated that both the G5835A tRNATyr and del4315A tRNAIle mutation have serious functional consequences. Single-fibre analyses displayed a high threshold of the tRNATyr mutation load for biochemical phenotypic expression at the single-cell level, indicating a rather mild pathogenic effect. In contrast, skeletal muscle tissue showed a severe decrease in respiratory-chain activities, a reduced overall COX (cytochrome c oxidase) staining intensity and abundant COX-negative fibres. Northern-blot analyses showed a dramatic reduction of tRNATyr and tRNAIle levels in muscle, with impaired charging of tRNAIle, whereas tRNAVal levels were only slightly decreased, with amino-acylation unaffected. Our findings suggest that the heteroplasmic tRNATyr and homoplasmic tRNAIle mutation act together, resulting in a concerted effect on the biochemical and histological phenotype. Thus homoplasmic mutations may influence the functional consequences of pathogenic heteroplasmic mtDNA mutations.
Reconciling semiclassical and Bohmian mechanics. V. Wavepacket dynamics
Authors: Poirier Bill
Published: April 2008

In previous articles [B. Poirier J. Chem. Phys. 121, 4501 (2004); C. Trahan and B. Poirier, ibid. 124, 034115 (2006); 124, 034116 (2006); B. Poirier and G. Parlant, J. Phys. Chem. A 111, 10400 (2007)] a bipolar counterpropagating wave decomposition, ψ=ψ++ψ−, was presented for stationary states ψ of the one-dimensional Schrödinger equation, such that the components ψ± approach their semiclassical Wentzel–Kramers–Brillouin analogs in the large action limit. The corresponding bipolar quantum trajectories are classical-like and well behaved, even when ψ has many nodes, or is wildly oscillatory. In this paper, the method is generalized for time-dependent wavepacket dynamics applications and applied to several benchmark problems, including multisurface systems with nonadiabatic coupling.
Excited-state relaxation in PbSe quantum dots
Authors: An Joonhee M. , Califano Marco , Franceschetti Alberto , Zunger Alex
Published: April 2008

In solids the phonon-assisted, nonradiative decay from high-energy electronic excited states to low-energy electronic excited states is picosecond fast. It was hoped that electron and hole relaxation could be slowed down in quantum dots, due to the unavailability of phonons energy matched to the large energy-level spacings (“phonon-bottleneck”). However, excited-state relaxation was observed to be rather fast (⩽1ps) in InP, CdSe, and ZnO dots, and explained by an efficient Auger mechanism, whereby the excess energy of electrons is nonradiatively transferred to holes, which can then rapidly decay by phonon emission, by virtue of the densely spaced valence-band levels. The recent emergence of PbSe as a novel quantum-dot material has rekindled the hope for a slow down of excited-state relaxation because hole relaxation was deemed to be ineffective on account of the widely spaced hole levels. The assumption of sparse hole energy levels in PbSe was based on an effective-mass argument based on the light effective mass of the hole. Surprisingly, fast intraband relaxation times of 1–7ps were observed in PbSe quantum dots and have been considered contradictory with the Auger cooling mechanism because of the assumed sparsity of the hole energy levels. Our pseudopotential calculations, however, do not support the scenario of sparse hole levels in PbSe: Because of the existence of three valence-band maxima in the bulk PbSe band structure, hole energy levels are densely spaced, in contradiction with simple effective-mass models. The remaining question is whether the Auger decay channel is sufficiently fast to account for the fast intraband relaxation. Using the atomistic pseudopotential wave functions of Pb2046Se2117 and Pb260Se249 quantum dots, we explicitly calculated the electron-hole Coulomb integrals and the P→S electron Auger relaxation rate. We find that the Auger mechanism can explain the experimentally observed P→S intraband decay time scale without the need to invoke any exotic relaxation mechanisms.
Analyses of laser-plasma interactions in National Ignition Facility ignition targets
Authors: Hinkel D. E. , Callahan D. A. , Langdon A. B. , Langer S. H. , Still C. H. , Williams E. A.
Published: April 2008

A capability to analyze laser-plasma interactions (LPI) for ignition targets to be fielded at the National Ignition Facility has been developed and exercised. LPI in these targets may cause direct energy loss (backscatter) or energy redirection (beam spray, deflection, and energy transfer). These analyses range from analyzing the gain exponents for backscatter and beam spray to performing massively parallel, three-dimensional simulations of laser beam propagation in the most promising candidate ignition target designs. In the former assessment, ignition designs are iterated to reduce the gain exponent values. In the latter, beam propagation simulations are performed to analyze the reflectivity and beam transmission of speckled laser beams in the computed plasma profiles of the ignition targets. In current ignition designs, laser reflectivity is calculated to be well below 10%.
Interference of surface plasmon polaritons controlled by the phase of incident light
Authors: Ren Xi-Feng , Guo Guo-Ping , Huang Yun-Feng , Wang Zhi-Wei , Zhang Pei , Guo Guang-Can
Published: April 2008

Interference patterns of surface plasmon polaritons (SPPs) are observed in the extraordinary optical transmission through subwavelength holes in an optically thick metal plate. It is found that the phase of incident light can be transferred to SPPs. We can control the destructive and constructive interferences of SPPs by modulating the relative phase between two incident beams. Using a slightly displaced Mach–Zehnder interferometer, we also observe a SPP interference pattern composed of bright and dark stripes.
A multilayer multiconfigurational time-dependent Hartree approach for quantum dynamics on general potential energy surfaces
Authors: Manthe Uwe
Published: April 2008

The multiconfigurational time-dependent Hartree (MCTDH) approach facilitates multidimensional quantum dynamics calculations by representing the wavepacket in an optimal set of time-dependent basis functions, called single-particle functions. Choosing these single-particle functions to be themselves multidimensional wavefunctions which are represented using a MCTDH representation, a multilayer MCTDH scheme has been constructed and used for quantum dynamics calculations treating up to 1000degrees of freedom rigorously [Wang and Thoss, J. Chem. Phys. 199, 1289 (2003)]. The present work gives a practical scheme which facilitates the application of the multilayer MCTDH approach, which previously has only been employed to study systems described by model-type Hamiltonians, to molecular systems described by more complicated Hamiltonians and general potential energy surfaces. A multilayer extension of the correlation discrete variable representation (CDVR) scheme employed in MCTDH calculations studying quantum dynamics on general potential energy surfaces is developed and tested in a simple numerical application. The resulting multilayer MCTDH/CDVR approach might offer a perspective to rigorously describe the quantum dynamics of larger polyatomic systems.
Effects of specimen preparation on the cathodoluminescence properties of ZnO nanoparticles
Authors: Dierre B. , Yuan X. L. , Ohashi N. , Sekiguchi T.
Published: April 2008

To establish a reliable procedure for the characterization of luminescence from nanomaterials, the cathodoluminescence (CL) observation conditions, such as the packing density of particles and the electron beam energy for irradiation, were examined by using ZnO nanoparticles. The evolutions of the intensities and peak position with the accelerating voltage are strongly affected by the packing density of particles. For the low-density specimen, the band edge emission reaches a maximum at 4kV where the excitation of each nanoparticle becomes the most effective. On the other hand, the position of band edge emission does not shift for low-density specimen while it changes for high-density specimen. Such artifact may bring serious modifications to the CL data. This work suggests that the specimen preparation and the optimum excitation conditions are the keys for the reliable CL characterization of nanomaterials.
Diverse mechanism of refractive index modification in neodymium-doped KGd(WO4)2 crystal induced by MeV He+ or C3+ ion implantation for waveguide construction
Authors: Chen Feng , Tan Yang , Wang Lei , Wang Xue-Lin , Wang Ke-Ming , Lu Qing-Ming
Published: April 2008

Nd-doped KGd(WO4)2 laser crystals are implanted by either He+ [at energies of (1.9+2.0+2.1) MeV and fluences of (3.24+3.24+5.4)×1015 cm−2] or C3+ ions (at an energy of 6.0 MeV and a dose of 1.0×1015 cm−2). The two implantations induce diverse refractive index modifications for waveguide construction in the near surface regions, which implies different formation mechanisms between the He+ and C3+ ion implanted waveguides. The surface region of the He+ implanted sample remains crystalline, while an optically isotropic layer with a large refractive index decrease is generated at the surface of the C3+ ion implanted crystal. The results show that the implantation of C3+ ions through electron excitations is considered to be responsible for the heavy lattice damage during the trajectory of the incident ions inside the substrate, creating an amorphouslike layer with a thickness of ∼3 μm, whereas the nuclear energy deposition mechanism is dominant for a typical barrier confined waveguide construction in the case of He+ ion implantation.
Formation of thin bifurcated current sheets by quasisteady compression
Authors: Schindler Karl , Hesse Michael
Published: April 2008

Thin current sheets with half-widths in the range of about 10 or less ion inertial lengths (or ion gyroradii) have been identified as the sites of important dynamical phenomena in space plasmas. Recent space observations established that thin current sheets often have a bifurcated (double-peaked) current density. Earlier suggestions of possible bifurcation mechanisms are based on the presence of microfluctuations, magnetic reconnection, or a normal magnetic field component or assumed simplified models of adiabatic dynamics. Despite these efforts, the cause (or causes) of the formation of thin bifurcated current sheets and the conditions under which they form have remained unclear. In this paper, we identify quasisteady compression of a plane, initially wide collisionless current sheet as an effective physical mechanism for the formation of thin bifurcated current sheets. Our main tool is electromagnetic particle simulation. The initial sheet has a singly peaked current and a half-width that is five times larger than the ion inertial length. This sheet is quasisteadily compressed by external forces. A three-scale structure develops and the current bifurcates during compression. It is shown that the bifurcation, pressure anisotropy, and other major properties of the embedded current sheet can be understood in terms of basic physical principles, such as electric field shielding and momentum conservation. Sufficient conditions for bifurcation of symmetric current sheets are presented. They suggest that bifurcation must generally occur by quasisteady compression if the (singly peaked) initial current sheet is sufficiently wide.
Ab initio study of the one- and two-photon circular dichroism of R-(+)-3-methyl-cyclopentanone
Authors: Rizzo Antonio , Lin Na , Ruud Kenneth
Published: April 2008

One- and two-photon circular dichroism spectra of R-(+)-3-methyl-cyclopentanone, a system that has been the subject of recent experimental studies of (2+1) resonance-enhanced multiphoton ionization circular dichroism, have been calculated with an origin-invariant density functional theory approximation in the region of the lowest electronic excited states, both for the gas phase and for a selection of solvents. A polarizable continuum model is used in the calculations performed on the solvated system. Two low-lying conformers are analyzed, and a comparison of the intensities and characteristic features is made with the corresponding two-photon absorption for each species, also for the Boltzmann-averaged spectra. The effect of the choice of geometry, basis set, and exchange-correlation functional is carefully analyzed. It is found that a density functional theory approach using the Coulomb attenuating method variant of Becke’s three-parameter exchange and the Lee–Yang–Parr correlation functionals with correlation-consistent basis sets of double-zeta quality can reproduce the experimental electronic circular dichroism spectra very well. The features appearing in experiment are characterized in terms of molecular excitations, and the differences in the response of each state in the one- and two-photon processes are highlighted.
Electrostatic depletion forces between planar surfaces
Authors: Hatlo M. M. , Curtis R. A. , Lue L.
Published: April 2008

The interaction between two dielectric plates immersed in an electrolyte solution is examined by using a variational perturbation approximation for the grand partition function. This approach differs from previous treatments in that the screening length between the plates is treated as a variational parameter. A key finding is that adjacent to each plate is a layer of ion depletion with thickness given by about one-half of a Bjerrum length. Consequently, for plate-plate separations less than the Bjerrum length, nearly all the electrolyte is excluded from between the plates, and the interaction is given by the sum of a van der Waals interaction and an attractive osmotic depletion force. In contrast to the predictions of previous theories, the interaction between the plates at short range increases with increasing electrolyte concentration and may provide an important contribution to the salt-induced attraction, commonly referred to as salting out. Because the range of the osmotic depletion force is roughly equal to the Bjerrum length, it increases with the square of the valency of the electrolyte. At larger plate-plate separations, the van der Waals interaction is screened as electrolyte enters the space between the plates, leading to an exponential decay of the interactions, as has been previously observed. However, this interaction is slightly stronger than that previously predicted, due to ion depletion from the surface of the interface, also this effect increases with increasing electrolyte concentration.
Möbius and twisted graphene nanoribbons: Stability, geometry, and electronic properties
Authors: Caetano E. W. S. , Freire V. N. , dos Santos S. G. , Galvão D. S. , Sato F.
Published: April 2008

Results of classical force field geometry optimizations for twisted graphene nanoribbons with a number of twists Nt varying from 0 to 7 (the case Nt=1 corresponds to a half-twist Möbius nanoribbon) are presented in this work. Their structural stability was investigated using the Brenner reactive force field. The best classical molecular geometries were used as input for semiempirical calculations, from which the electronic properties (energy levels, HOMO, LUMO orbitals) were computed for each structure. CI wavefunctions were also calculated in the complete active space framework taking into account eigenstates from HOMO−4 to LUMO+4, as well as the oscillator strengths corresponding to the first optical transitions in the UV-VIS range. The lowest energy molecules were found less symmetric than initial configurations, and the HOMO-LUMO energy gaps are larger than the value found for the nanographene used to build them due to electronic localization effects created by the twisting. A high number of twists leads to a sharp increase of the HOMO→LUMO transition energy. We suggest that some twisted nanoribbons could form crystals stabilized by dipolar interactions.
Electronic structure and zone-center phonon modes in multiferroic bulk BiFeO3
Authors: Tütüncü H. M. , Srivastava G. P.
Published: April 2008

We present ab initio investigations of the electronic structure and zone-center phonon modes in bulk BiFeO3 with rhombohedral R3c symmetry. The electronic structure has been studied using the plane wave pseudopotential method, with electron-electron interaction treated within the local spin density approximation (LSDA), a spin-polarized generalized gradient approximation (σGGA), and the LSDA+U and σGGA+U schemes. It is found that with a reasonable choice of a rotationally invariant value of U both the LSDA+U and σGGA+U schemes produce electronic band gaps in the range of values derived from optical spectroscopy. The computed electronic structure results are utilized to determine the zone-center phonon modes by applying a linear response technique. We provide a clearer analysis of the energy values and polarization characteristics of the Raman active modes than is presently available from theoretical and experimental studies.
Bistability of left- and right-handed π-twist states in a pixel-isolated dual-frequency nematic liquid crystal cell
Authors: Lee Seong Ryong , Lee Joong Ha , Yoon Tae-Hoon , Kim Jae Chang
Published: April 2008

The authors propose a bistable nematic liquid crystal cell which has two stable states of left- and right-handed π-twist. The switching between the two states is achieved by using a dual-frequency liquid crystal material and sequential waveform of low and high frequencies. Each state is stabilized by a multidimensional anchoring effect of a pixel-isolating polymer wall without any chiral additives. Therefore, the elastic free energies of left- and right-handed π-twist states are almost the same. As a result, the two states show superior retention time in spite of the topological equivalence.
Hot sputtering of barium strontium titanate on nickel foils
Authors: Aygün Seymen M. , Daniels Patrick , Borland William , Maria Jon-Paul
Published: April 2008

The relationships linking temperature and voltage dependent dielectric response, grain size, and thermal budget during synthesis are illustrated. In doing so, it was found that maximizing thermal budgets within experimental bounds leads to electrical properties comparable to the best literature reports irrespective of the processing technique or microstructure. The optimal film properties include a bulk transition temperature, a room temperature permittivity of 1800, a voltage tuning ratio of 10:1 at 450 kV/cm, and a loss tangent less than 1.5% at 450 kV/cm. The sample set illustrates the well-known relationship between permittivity and crystal dimension, and the onset of a transition temperature shifts at very fine grain sizes. A brick wall model incorporating a high permittivity grain and a low permittivity grain boundary is used to interpret the dielectric data. However, the data show that high permittivity and tunability values can be achieved at grain sizes or film thicknesses that many reports associate with dramatic reductions in the dielectric response. These differences are discussed in terms of crystal quality and maximum processing temperature. The results collectively suggest that scaling effects in ferroelectric thin films are in many cases the result of low thermal budgets and the consequently high degree of structural imperfection and are not from the existence of low permittivity phases at the dielectric-electrode interface.
Size-dependent interface phonon transmission and thermal conductivity of nanolaminates
Authors: Liang L. H. , Wei Y. G. , Li Baowen
Published: April 2008

An analytical model for size-dependent interface phonon transmission and thermal conductivity of nanolaminates is derived based on the improved acoustic mismatch theory and the Lindemann melting theory by considering the size effect of phonon velocity and the interface lattice mismatch effect. The model suggests that the interface phonon transmission is dominant for the cross-plane thermal conductivity of nanolaminates and superlattices, and the intrinsic variety of size effect of thermal conductivity for different systems is proposed based on the competition mechanism of size effect of phonon transport between two materials constituting the interfaces. The model’s prediction for thermal conductivity of nanolaminates agrees with the experimental results.
Variational calculation of second-order reduced density matrices by strong N-representability conditions and an accurate semidefinite programming solver
Authors: Nakata Maho , Braams Bastiaan J. , Fujisawa Katsuki , Fukuda Mituhiro , Percus Jerome K. , Yamashita Makoto , Zhao Zhengji
Published: April 2008

The reduced density matrix (RDM) method, which is a variational calculation based on the second-order reduced density matrix, is applied to the ground state energies and the dipole moments for 57 different states of atoms, molecules, and to the ground state energies and the elements of 2-RDM for the Hubbard model. We explore the well-known N-representability conditions (P, Q, and G) together with the more recent and much stronger T1 and T2′ conditions. T2′ condition was recently rederived and it implies T2 condition. Using these N-representability conditions, we can usually calculate correlation energies in percentage ranging from 100% to 101%, whose accuracy is similar to CCSD(T) and even better for high spin states or anion systems where CCSD(T) fails. Highly accurate calculations are carried out by handling equality constraints and/or developing multiple precision arithmetic in the semidefinite programming (SDP) solver. Results show that handling equality constraints correctly improves the accuracy from 0.1to0.6mhartree. Additionally, improvements by replacing T2 condition with T2′ condition are typically of 0.1–0.5mhartree. The newly developed multiple precision arithmetic version of SDP solver calculates extraordinary accurate energies for the one dimensional Hubbard model and Be atom. It gives at least 16 significant digits for energies, where double precision calculations gives only two to eight digits. It also provides physically meaningful results for the Hubbard model in the high correlation limit.
Effect of nanoparticle size on sessile droplet contact angle
Authors: Munshi A. M. , Singh V. N. , Kumar Mukesh , Singh J. P.
Published: April 2008

We report a significant variation in the static contact angle measured on indium oxide (IO) nanoparticle coated Si substrates that have different nanoparticle sizes. These IO nanoparticles, which have well defined shape and sizes, were synthesized by chemical vapor deposition in a horizontal alumina tube furnace. The size of the IO nanoparticles was varied by changing the source material, substrate temperature, and the deposition time. A sessile droplet method was used to determine the macroscopic contact angle on these IO nanoparticle covered Si substrate using two different liquids: de-ionized water and diethylene glycol (DEG). It was observed that contact angle depends strongly on the nanoparticle size. The contact angle was found to vary from 24° to 67° for de-ionized water droplet and from 15° to 60° for DEG droplet, for the nanoparticle sizes varying from 14 to 620 nm. The contact angle decreases with a decrease in the particles size. We have performed a theoretical analysis to determine the dependence of contact angle on the nanoparticle size. This formulation qualitatively shows a similar trend of decrease in the contact angle with a decrease in nanoparticle size. Providing a rough estimate of nanoparticle size by sessile droplet contact angle measurement is the novelty in this work.
Laser nanotrapping and manipulation of nanoscale objects using subwavelength apertured plasmonic media
Authors: Baev Alexander , Furlani Edward P. , Prasad Paras N. , Grigorenko Alexander N. , Roberts Nicholas W.
Published: April 2008

We study the net radiation force on dielectric nanowires above a planar plasmonic medium consisting of a nanostructured gold film on a glass substrate. The film has a subwavelength aperture and the medium is illuminated by a continuous plane wave. We compute the steady-state field distribution by using two-dimensional finite element analysis, and we evaluate the force on the nanowires above the aperture by using the Maxwell stress tensor. We show that the vertical position of a nanowire can be controlled by either tuning the wavelength of the laser source or tapering the aperture. We experimentally check the conclusions of the theory by realizing laser nanotrapping of dielectric beads near the surface of nanostructured metal. Our results demonstrate the potential of using nanostructured plasmonic media for the controlled optical manipulation of nanowires and nanoparticles, including their application as “plasmonic sieve” for separation based on size and/or refractive index.
Natural quantum dots in the InAs∕GaAs wetting layer
Authors: Babiński A. , Borysiuk J. , Kret S. , Czyż M. , Golnik A. , Raymond S. , Wasilewski Z. R.
Published: April 2008

We report on microphotoluminescence study of excitons localized by potential fluctuations in a wetting layer (WL), which accompanies InAs∕GaAs quantum dots (QDs). Linear polarization of spectral lines due to localized excitons enable us to identify a neutral excitonic and biexcitonic emission. A charged exciton has also been identified. High resolution transmission electron microscopy measurements of the investigated structure reveal lateral fluctuations of In content in the WL, as well as its broadening. Both effects give rise to potential fluctuations, which can confine excitons observed in our measurements. The potential fluctuations can be regarded as “natural” QDs in the WL.
Toward high output-power nanogenerator
Authors: Liu Jin , Fei Peng , Zhou Jun , Tummala Rao , Wang Zhong Lin
Published: April 2008

In this paper, the factors that determine the power output of a piezoelectric nanowire (NW) nanogenerator (NG) have been analyzed. The output current is the sum of those contributed by all of the NWs while the output voltage is determined by the voltage generated by a single NW, the capacitance of the NW array and the system, and the contact resistance. By growing uniform ZnO NWs with diameters of ∼100nm and lengths of ∼5μm, the output current density and output voltage of the NG was improved to ∼8.3μA∕cm2 and 10mV, respectively, which are 20–30 times higher than that we previously reported. A power generation density of ∼83nW∕cm2 is achieved by using a single layer NW NG.
Phonon polariton of InN observed by infrared synchrotron radiation
Authors: Inushima Takashi , Fukui Kazutoshi , Lu Hai , Schaff William J.
Published: April 2008

Temperature dependence of the reflectivity of InN with a carrier concentration of (3.5–4.7)×1017cm3 is investigated from 50to750cm−1 using infrared synchrotron radiation. E1 phonon is separately observed from plasma oscillation, and in the energy range below E1(TO), phonon polariton is observed up to 104cm−1. The lifetime of the E1(TO) phonon is directly determined by the reflectivity measurements. From the temperature dependence of the lifetime, the E1(TO) phonon primarily decays into phonons with a renormalized frequency of 177cm−1. From the plasma edge position the electron effective mass is estimated to be me⊥=0.076m0 for the intrinsic InN.
Selective targeting of ITK blocks multiple steps of HIV replication
Authors: Readinger Julie A. , Schiralli Gillian M. , Jiang Jian-Kang , Thomas Craig J. , August Avery , Henderson Andrew J. , Schwartzberg Pamela L.
Published: April 2008

Treatment for HIV has relied on the use of antiretroviral agents that can be subject to the development of resistant viruses. The study of inhibitors directed against cellular proteins required for HIV replication is therefore of growing interest. Inducible T cell kinase (ITK) is a Tec family tyrosine kinase that regulates T cell receptor (TCR)-induced activation of PLCγ-1, Ca
2+
mobilization and transcription factor activation, and actin rearrangement downstream of both TCR and chemokine receptors. Because productive infection of T cells with HIV requires T cell activation, chemokine receptors and actin reorganization, we asked whether ITK affects HIV infection using ITK-specific siRNA, a kinase-inactive ITK mutant or an ITK inhibitor. We demonstrate that loss of ITK function resulted in marked reductions in intracellular p24 levels upon HIV infection. Loss of ITK function after establishment of HIV infection also decreased virus spread within the culture. Inhibition of ITK did not affect expression of the HIV coreceptors CD4 or CXCR4 but partially blocked HIV viral entry, an effect that correlated with decreased actin polarization to gp120. Additionally, ITK was required for efficient HIV transcription, and overexpression of ITK increased both viral transcription and virus-like particle formation. Our data suggest that inhibition of ITK blocks HIV infection by affecting multiple steps of HIV replication.
Bone marrow-derived circulating endothelial precursors do not contribute to vascular endothelium and are not needed for tumor growth
Authors: Purhonen Susanna , Palm Jarmo , Rossi Derrick , Kaskenpää Nina , Rajantie Iiro , Ylä-Herttuala Seppo , Alitalo Kari , Weissman Irving L. , Salven Petri
Published: April 2008

The mechanisms by which bone marrow (BM)-derived stem cells might contribute to angiogenesis and the origin of neovascular endothelial cells (ECs) are controversial. Neovascular ECs have been proposed to originate from VEGF receptor 2-expressing (VEGFR-2+) stem cells mobilized from the BM by VEGF or tumors, and it is thought that angiogenesis and tumor growth may depend on such endothelial precursors or progenitors. We studied the mobilization of BM cells to circulation by inoculating mice with VEGF polypeptides, adenoviral vectors expressing VEGF, or tumors. We induced angiogenesis by syngeneic melanomas, APCmin adenomas, adenoviral VEGF delivery, or matrigel plugs in four different genetically tagged universal or endothelial cell-specific chimeric mouse models, and subsequently analyzed the contribution of BM-derived cells to endothelium in a wide range of time points. To study the existence of circulating ECs in a nonmyeloablative setting, pairs of genetically marked parabiotic mice with a shared anastomosed circulatory system were created. We did not observe specific mobilization of VEGFR-2+ cells to circulation by VEGF or tumors. During angiogenesis, abundant BM-derived perivascular cells were recruited close to blood vessel wall ECs but did not form part of the endothelium. No circulation-derived vascular ECs were observed in the parabiosis experiments. Our results show that no BM-derived VEGFR-2+ or other EC precursors contribute to vascular endothelium and that cancer growth does not require BM-derived endothelial progenitors. Endothelial differentiation is not a typical
in vivo
function of normal BM-derived stem cells in adults, and it has to be an extremely rare event if it occurs at all.