bluuebunny/crossref_metadata_2025 huggingface dataset:
A mechanistic model for permeability evolution in fractured sorbing media
Authors: Wang Shugang , Elsworth Derek , Liu Jishan
Published: May 2012

A mechanistic model is presented to represent the evolution of permeability in fractured sorbing media such as coal beds and organic‐rich shales. This model accommodates key competing processes of poromechanical dilation and sorption‐induced swelling. We show that the significant difference in stiffness between fracture and matrix transforms the composite system from globally unconstrained to locally constrained by the development of a virtual “stiff shell” that envelops the perimeter of a representative elementary volume containing a fracture. It is this transformation that results in swelling‐induced permeability reduction at low (sorbing) gas pressures and self consistently allows competitive dilation of the fracture as gas pressures are increased. Importantly, net dilation is shown to require a mismatch in the Biot coefficients of fracture and matrix with the coefficient for the fracture exceeding that for the matrix—a condition that is logically met. Permeability evolution is cast in terms of series and parallel models with the series model better replicating observational data. The model may be cast in terms of nondimensional parameters representing sorptive and poromechanical effects and modulated by the sensitivity of the fracture network to dilation or compaction of the individual fractures. This latter parameter encapsulates the effects of fracture spacing and initial permeability and scale changes in permeability driven by either sorption or poromechanical effects. This model is applied to well‐controlled observational data for different ranks of coals and different gases (He, CO2) and satisfactory agreement is obtained.
Seasonal variation of CH4 emissions from central California
Authors: Jeong Seongeun , Zhao Chuanfeng , Andrews Arlyn E. , Bianco Laura , Wilczak James M. , Fischer Marc L.
Published: May 2012

We estimate seasonal variations in methane (CH4) emissions from central California from December 2007 through November 2008 by comparing CH4 mixing ratios measured at a tall tower with transport model predictions based on a global 1° a priori CH4emissions map (EDGAR32) and a 10 km seasonally varying California‐specific map, calibrated to statewide by CH4emission totals. Atmospheric particle trajectories and surface footprints are computed using the Weather Research and Forecasting and Stochastic Time‐Inverted Lagrangian Transport models. Uncertainties due to wind velocity and boundary layer mixing depth are evaluated using measurements from radar wind profilers. CH4signals calculated using the EDGAR32 emission model are larger than those based on the California‐specific model and in better agreement with measurements. However, Bayesian inverse analyses using the California‐specific and EDGAR32 maps yield comparable annually averaged posterior CH4emissions totaling 1.55 ± 0.24 times and 1.84 ± 0.27 times larger than the California‐specific prior emissions, respectively, for a region of central California within approximately 150 km of the tower. If these results are applicable across California, state total CH4 emissions would account for approximately 9% of state total greenhouse gas emissions. Spatial resolution of emissions within the region near the tower reveal seasonality expected from several biogenic sources, but correlations in the posterior errors on emissions from both prior models indicate that the tower footprints do not resolve spatial structure of emissions. This suggests that including additional towers in a measurement network will improve the regional specificity of the posterior estimates.
Problems associated with uncertain parameters and missing physics for long‐term ionosphere‐thermosphere forecasting
Authors: Schunk R. W. , Gardner L. , Scherliess L. , Zhu L.
Published: May 2012

Data assimilation models like the Utah State University (USU) Global Assimilation of Ionospheric Measurements (GAIM) models use physics‐based models of the ionosphere, ionosphere‐plasmasphere, or thermosphere and a Kalman filter as a basis for assimilating a diverse set of measurements. With a sufficient amount of data and with multiple data types, the data assimilation models can provide reliable specifications and near‐term forecasts. However, for long‐term forecasts (5 days or longer) stand‐alone or coupled physics‐based models are needed. Unfortunately, the various physics‐based models contain several uncertain parameters and processes as well as missing physics. Further complications arise for coupled physics‐based models because of coupling issues and error propagation from model to model. Some of the problems are associated with the magnetosphere and lower atmosphere drivers, the adopted set of physics‐based equations, the parameterization of physical processes, the values adopted for the transport coefficients, the numerical techniques used, the spatial and temporal resolutions adopted, and the uncertainties in the initial and boundary conditions. Examples of the type of problems the space weather community faces in its attempt at long‐term ionosphere‐thermosphere forecasting are given.
F2‐region atmospheric gravity waves due to high‐power HF heating and subauroral polarization streams
Authors: Mishin E. , Sutton E. , Milikh G. , Galkin I. , Roth C. , Förster M.
Published: May 2012

We report the first evidence of atmospheric gravity waves (AGWs) generated in the F2 region by high‐power HF heating and subauroral polarization streams. Data come from the CHAMP and GRACE spacecraft overflying the High‐frequency Active Auroral Research Program (HAARP) heating facility. These observations facilitate a new method of studying the ionosphere‐thermosphere coupling in a controlled fashion by using various HF‐heating regimes. They also reveal the subauroralF2 region to be a significant source of substorm AGWs, in addition to the well‐known auroralE region.
Numerical investigation of turbulence generation in non‐breaking potential waves
Authors: Babanin Alexander V. , Chalikov Dmitry
Published: May 2012

Theoretically, potential waves cannot generate the vortex motion, but scale considerations indicate that if the steepness of waves is not too small, Reynolds number can exceed critical values. This means that in presence of initial non‐potential disturbances the orbital velocities can generate the vortex motion and turbulence. In the paper, this problem was investigated numerically on basis of full two‐dimensional (x‐z) equations of potential motion with the free surface in cylindrical conformal coordinates. It was assumed that all variables are a sum of the 2D potential orbital velocities and 3D non‐potential disturbances. The non‐potential motion is described directly with 3D Euler equations, with very high resolution. The interaction between potential orbital velocities and non‐potential components is accounted through additional terms which include the components of vorticity. Long‐term numerical integration of the system of equations was done for different wave steepness. Vorticity and turbulence usually occur in vicinity of wave crests (where the velocity gradients reach their maximum) and then spread over upwind slope and downward. Specific feature of the wave turbulence at low steepness (steepness was kept low in order to avoid wave breaking) is its strong intermittency: the turbulent patches are mostly isolated and intermittency grows with decrease of wave amplitude. Maximum values of energy of turbulence are in agreement with available experimental data. The results suggest that even non‐breaking potential waves can generate turbulence, which thus enhances the turbulence created by the shear current.
Thermally responsive core–shell microparticles and cross-linked networks based on nitrone chemistry
Authors: Barner Leonie , Quick Alexander S. , Vogt Andrew P. , Winkler Volker , Junkers Tanja , Barner-Kowollik Christopher
Published: May 2012

Complex cross-linked soluble architectures are generated using nitrone mediated chemistry and are subsequently cleaved into network fragments. Nitrone mediated reactions are additionally applied for the synthesis of stimuli-responsive microspheres.
Working Memory Capacity and Its Relation to Stroop Interference and Facilitation Effects in Individuals With Mild Cognitive Impairment
Authors: Sung Jee Eun , Kim Jin Hee , Jeong Jee Hyang , Kang Heejin
Published: May 2012

Purpose

The purposes of the study were to investigate (a) the task-specific differences in short-term memory (STM) and working memory capacity (WMC) in individuals with mild cognitive impairment (MCI) and normal elderly adults (NEAs), (b) the Stroop interference and facilitation effects, and (c) the relationship of STM and WMC to the Stroop effects.

Method

Thirty-two individuals participated in the study (n= 16 for each group). WMC demands were increased using a computerized Stroop-like token task to add more linguistic units. Six STM and WMC measures were administered overall.

Results

Digit-related tasks and an alphabet span task sensitively differentiated individuals with MCI from the NEA group. The group with MCI exhibited greater Stroop interference effects than the NEA group, but the 2 groups did not exhibit different Stroop facilitation effects. WMC significantly predicted performance on the response time analyses but not on the error rate analyses.

Conclusion

Task-specific differences emerged in the group with MCI, and a reduced WMC accounts for the impaired inhibitory and goal maintenance processes. It is critical that WMC demands be systematically manipulated to tax individuals' WMC in a way that can clearly demonstrate their deficits, especially in individuals who are at risk for clinically demented states.
Near infrared up-conversion in organic photovoltaic devices using an efficient Yb3+:Ho3+ Co-doped Ln2BaZnO5 (Ln = Y, Gd) phosphor
Authors: Adikaari A. A. Damitha , Etchart Isabelle , Guéring Paul-Henri , Bérard Mathieu , Silva S. Ravi P. , Cheetham Anthony K. , Curry Richard J.
Published: May 2012

The first detailed study that combines the use of a new generation of high-efficiency Yb3+:Ho3+ co-doped Y2BaZnO5 near-infrared up-converting phosphors with organic photovoltaic devices is reported. We show that it is possible to obtain a Jsc of 16 μA cm−2 under 986 nm illumination (∼390 mW cm−2 corresponding to ∼37 suns) leading to an up-conversion external quantum efficiency (ηEQEUC) of 0.0052%. Through modification of the organic photovoltaic devices to incorporate transparent electrodes we show that ηEQEUC could be increased to 0.031 %, matching that achieved in amorphous-Si:H PV cells. Accounting for the full spectral range that may be absorbed by the phosphor (∼870–1030 nm) yields an up-conversion power conversion efficiency (ηPCEUC) of 0.073% which again could be improved to 0.45% using transparent electrodes. This technique for utilizing the near-infrared spectral region may therefore offer a potential route to improving the performance of organic photovoltaic devices as research into discovering high-efficiency up-converting phosphors continues to provide improved materials.
Influence of oxygen vacancies on the electronic structure and magnetic properties of NiFe2O4 thin films
Authors: Jaffari G. Hassnain , Rumaiz A. K. , Woicik J. C. , Shah S. Ismat
Published: May 2012

We report stabilization of magnetic glassy state in non-stoichiometric nickel ferrite thin films prepared by pulse laser deposition. Details of electronic structure of the films are presented and compared with stoichiometric bulk counterpart. Hard x-ray photoelectron spectroscopy shows significant amount of oxygen vacancies and enhanced cationic inversion for thin films. Films show spin glass (SG) features which is contrary to the usual ferrimagnetic response of the bulk nickel ferrite. Films exhibit spin freezing temperature which is above room temperature in low fields (0.1 T) and shifts to lower temperature (∼250 K) in the presence of a large applied field of 3 T. An exceptionally large exchange bias (EB) of 170 Oe at a significantly higher temperature (∼50 K) is measured in cooling field of 3 T. In comparison, bulk samples do not show exchange bias and magnetic irreversibility vanishes in significantly weaker fields (i.e., few kOe). Role of oxygen vacancies is to induce spin canting by destabilizing indirect super exchange interaction. Consequently, the spin-glass like behavior occurs that is coupled with huge suppression in saturation magnetization in the thin films. Observation of exchange bias is explained to be due to oxygen vacancies (hence non-stoichiometry) which generates random anisotropy in exchange coupled grains.
Electric field tuning of spin splitting in a quantum dot coupled to a semimagnetic quantum dot
Authors: Lyanda-Geller Y. , Reinecke T. L. , Bacher G.
Published: May 2012

We develop an approach for tuning the spin splitting and g-factor of a quantum dot by coupling it to semi-magnetic quantum dot and tuning the electric field. We show that spin splittings and g-factors of the states of a non-magnetic quantum dot coupled to semimagnetic quantum dot can be enhanced orders of magnitude. Evaluations are made for coupled CdTe/CdMnTe quantum dots. These effects are caused by electric field control of repulsion of spin sublevels in the non-magnetic dot due to tunnel coupling of quantum dots. Electric field control of spin splittings in quantum dots is of potential interest in connection with spin qubit rotations for quantum computation.
Importance of semicore states in GW calculations for simulating accurately the photoemission spectra of metal phthalocyanine molecules
Authors: Umari P. , Fabris S.
Published: May 2012

The quasi-particle energy levels of the Zn-Phthalocyanine (ZnPc) molecule calculated with the GW approximation are shown to depend sensitively on the explicit description of the metal-center semicore states. We find that the calculated GW energy levels are in good agreement with the measured experimental photoemission spectra only when explicitly including the Zn 3s and 3p semicore states in the valence. The main origin of this effect is traced back to the exchange term in the self-energy GW approximation. Based on this finding, we propose a simplified approach for correcting GW calculations of metal phthalocyanine molecules that avoids the time-consuming explicit treatment of the metal semicore states. Our method allows for speeding up the calculations without compromising the accuracy of the computed spectra.
Quantum dynamics of H + LiH reaction and its isotopic variants
Authors: Roy Tanmoy , Mahapatra S.
Published: May 2012

Time-dependent quantum wave packet dynamics study is carried out to investigate the initial state selected channel specific reactivity of H + LiH collisional system on a new and more accurate ab initio potential energy surface developed by Wernli et al. [J. Phys. Chem. A 113, 1121 (2009)]. The H + LiH reaction proceeds through LiH depletion and H-exchange paths. While the former path is highly exoergic (by ∼2.258 eV), the latter path is thermoneutral. State selected and energy resolved integral reaction cross sections and thermal rate constants are reported and compared with the literature data. The reactivity of the LiH depletion channel is found to be greater than the H-exchange channel. Rotational excitation of the reagent LiH molecule causes a decrease of reactivity of both the channels. On the other hand, the vibrational excitation of the reagent LiH decreases the reactivity of the LiH depletion channel and increases the reactivity of the H-exchange channel. The effect of isotopic substitution (H by D) on the reaction dynamics is also examined.
Quasi-matched propagation of ultra-short, intense laser pulses in plasma channels
Authors: Benedetti C. , Schroeder C. B. , Esarey E. , Leemans W. P.
Published: May 2012

The propagation of an ultrashort and relativistically intense laser pulse in a preformed plasma channel is investigated. The nonlinear paraxial wave equation describing the laser propagation in the plasma is solved both analytically and numerically. For any arbitrary temporal laser pulse profile with a given power (less then the critical power for self-focusing) and any prescribed transverse density profile in the channel, we determine the laser intensity distribution along the pulse ensuring quasi-matched propagation, neglecting non-paraxial effects. For the case of a Gaussian laser with an initially uniform spot throughout the pulse, we determine the optimal channel depth that minimizes laser evolution (e.g., minimizes spot size oscillations). The analytical and semi-analytical results obtained for both cases in the weakly relativistic regime are presented and validated through comparison with numerical simulations.
Experiment and theoretical modeling of the luminescence of silver nanoclusters dispersed in oxyfluoride glass
Authors: Cuong Ngo T. , Tikhomirov Victor K. , Chibotaru Liviu F. , Stesmans Andre , Rodríguez Vicente D. , Nguyen Minh T. , Moshchalkov Victor V.
Published: May 2012

Density functional theory (DFT) and complete active space perturbation theory (CASPT2) have been applied for modeling the configuration, charge, energy states, and spin of luminescent Ag nanoclusters dispersed within the bulk of oxyfluoride glass host. The excitation spectra of luminescence of the Ag nanoclusters have been measured and simulated by means of the DFT and CASPT2. Electron spin resonance spectra have been recorded and suggest diamagnetic state of Ag nanoclusters. The silver nanoclusters have been argued to consist mostly of pairs of Ag2+ dimers, or Ag42+ tetramers, with different extent of distortion along the tetramer diagonal. The sites for the Ag nanoclusters have been suggested where the pairs of Ag ions substitute onto metal and hole cation sites and are surrounded by fluorine ions within a fluorite-type lattice.
Strong nonlinear growth of energy coupling during laser irradiation of transparent dielectrics and its significance for laser induced damage
Authors: Duchateau Guillaume , Feit Michael D. , Demos Stavros G.
Published: May 2012

The interaction of high power nanosecond laser pulses with absorbing defects, located in the bulk of transparent dielectric materials and having a multilevel electronic structure, is addressed. The model assumes a moderate localized initial absorption that is strongly enhanced during the laser pulse via excited state absorption and thermally driven generation of new point defects in surrounding material. This model is applied to laser induced damage initiation in the bulk of potassium dihydrogen phosphate crystals (KH2PO4 or KDP) and addresses how during a fraction of the pulse duration the host material around the defect cluster is transformed into a strong absorber that leads to the sufficiently large energy coupling resulting in a damage event. This scenario is supported by time resolved imaging of material modification during the initial phases of laser induced damage in KDP and fused silica.
Spectroscopic identification of benzyl-type radicals generated by corona discharge of 2-chloro-4-fluorotoluene
Authors: Huh Chang Soon , Yoon Young Wook , Lee Sang Kuk
Published: May 2012

By means of a technique of corona excited supersonic expansion coupled with a pinhole-type glass nozzle, we generated vibronically excited but jet-cooled benzyl-type radicals from precursor 2-chloro-4-fluorotoluene seeded in a large amount of inert carrier gas He. From an analysis of the visible vibronic emission spectrum, we found evidence of the formation of the 2-chloro-4-fluorobenzyl and 4-fluorobenzyl radicals. A possible pathway for the formation of these benzyl-type radicals is herein proposed. Also, the electronic energy in the D1 → D0 transition and the vibrational mode frequencies of the 2-chloro-4-fluorobenzyl radical in the ground electronic state were accurately determined, for the first time, by comparison with ab initio calculations and the known vibrational data of the precursor.
Designing interlayers to improve the mechanical reliability of transparent conductive oxide coatings on flexible substrates
Authors: Kim Eun-Hye , Yang Chan-Woo , Park Jin-Woo
Published: May 2012

In this study, we investigate the effect of interlayers on the mechanical properties of transparent conductive oxide (TCO) on flexible polymer substrates. Indium tin oxide (ITO), which is the most widely used TCO film, and Ti, which is the most widely used adhesive interlayer, are selected as the coating and the interlayer, respectively. These films are deposited on the polymer substrates using dc-magnetron sputtering to achieve varying thicknesses. The changes in the following critical factors for film cracking and delamination are analyzed: the internal stress (σi) induced in the coatings during deposition using a white light interferometer, the crystallinity using a transmission electron microscope, and the surface roughness of ITO caused by the interlayer using an atomic force microscope. The resistances to the cracking and delamination of ITO are evaluated using a fragmentation test. Our tests and analyses reveal the important role of the interlayers, which significantly reduce the compressive σi that is induced in the ITO and increase the resistance to the buckling delamination of the ITO. However, the relaxation of σi is not beneficial to cracking because there is less compensation for the external tension as σi further decreases. Based on these results, the microstructural control is revealed as a more influential factor than σi for improving crack resistance.
Waveguide terahertz time-domain spectroscopy of ammonium nitrate polycrystalline films
Authors: Khachatrian Ani , Melinger Joseph S. , Qadri Syed B.
Published: May 2012

We report temperature-dependent measurements of the terahertz (THz) vibrational spectrum of ammonium nitrate (AN) films and mixed potassium nitrate (KN)–ammonium nitrate films using waveguide THz time domain spectroscopy. The experiments were performed on polycrystalline films on the metal surface of a parallel plate waveguide. At cryogenic temperature and with frequency resolution as high as 7 GHz, our measurements produce a complex vibrational spectrum for AN, and show vibrational resonances not observed in previous far infrared and Raman measurements. We investigate potential interactions between AN and the metal surface by measuring THz spectra of films on aluminum, gold, and a gold surface coated with an organic self-assembled monolayer. Measurements are also performed on a deuterated AN film and indicate that the observed THz modes are due largely to the motion of the nitrate ions in the AN crystal. Finally, the effect of introducing small amounts of an impurity into the AN lattice is examined. We find that introduction of as little as 1%-2% by weight of potassium nitrate into the AN lattice causes line broadening of the THz modes, which is consistent with increased disorder introduced by the impurity.
Imaging a spatially confined photoacoustic source defined by a distribution of plasmonic nanoparticles
Authors: Norton Stephen J. , Vo-Dinh Tuan
Published: May 2012

This paper describes the use of plasmonic nanoparticles in photoacoustic imaging. When acoustic waves are generated by thermoacoustic expansion in the fluid medium surrounding a distribution of these particles and the acoustic signals are recorded over a planar aperture, a bandlimited image of this distribution can be reconstructed. It is shown that the accessible portion of the three-dimensional spatial Fourier transform of the unknown source distribution is a spherical shell in k-space, with the core representing missing low-frequency Fourier components of the source density. When the source arises from an isolated distribution of nanoparticles, the iterative Gerchberg-Papoulis procedure can be applied to recover the low-frequency Fourier components. It is shown that this version of the photoacoustic source reconstruction problem is well suited for the use of this procedure. In this way, the fidelity of the image of the photoacoustic-generated source defined by the particle concentration can be enhanced. The procedure is illustrated using simulated data derived from a hypothetical source distribution.
Full dimension Rb2He ground triplet potential energy surface and quantum scattering calculations
Authors: Guillon Grégoire , Viel Alexandra , Launay Jean-Michel
Published: May 2012

We have developed a three-dimensional potential energy surface for the lowest triplet state of the Rb2He complex. A global analytic fit is provided as in the supplementary material [see supplementary material at 10.1063/1.4709433E-JCPSA6-136-034218 for the corresponding Fortran code]. This surface is used to perform quantum scattering calculations of 4He and 3He colliding with 87Rb2 in the partial wave J = 0 at low and ultralow energies. For the heavier helium isotope, the computed vibrational relaxation probabilities show a broad and strong shape resonance for a collisional energy of 0.15 K and a narrow Feshbach resonance at about 17 K for all initial Rb2 vibrational states studied. The broad resonance corresponds to an efficient relaxation mechanism that does not occur when 3He is the colliding partner. The Feshbach resonance observed at higher collisional energy is robust with respect to the isotopic substitution. However, its effect on the vibrational relaxation mechanism is faint for both isotopes.
Excitation power and temperature dependence of excitons in CuInSe2
Authors: Luckert F. , Yakushev M. V. , Faugeras C. , Karotki A. V. , Mudryi A. V. , Martin R. W.
Published: May 2012

Excitonic recombination processes in high quality CuInSe2 single crystals have been studied by photoluminescence (PL) and reflectance spectroscopy as a function of excitation powers and temperature. Excitation power dependent measurements confirm the identification of well-resolved A and B free excitons in the PL spectra and analysis of the temperature quenching of these lines provides values for activation energies. These are found to vary from sample to sample, with values of 12.5 and 18.4 meV for the A and B excitons, respectively, in the one showing the highest quality spectra. Analysis of the temperature and power dependent PL spectra from the bound excitonic lines, labelled M1, M2, and M3 appearing in multiplets points to a likely assignment of the hole involved in each case. The M1 excitons appear to involve a conduction band electron and a hole from the B valence band hole. In contrast, an A valence band hole appears to be involved for the M2 and M3 excitons. In addition, the M1 exciton multiplet seems to be due to the radiative recombination of excitons bound to shallow hydrogenic defects, whereas the excitons involved in M2 and M3 are bound to more complex defects. In contrast to the M1 exciton multiplet, the excitonic lines of M2 and M3 saturate at high excitation powers suggesting that the concentration of the defects involved is low.
Efficiency enhancement of blue InGaN/GaN light-emitting diodes with an AlGaN-GaN-AlGaN electron blocking layer
Authors: Xia Chang Sheng , Li Z. M. Simon , Lu Wei , Zhang Zhi Hua , Sheng Yang , Hu Wei Da , Cheng Li Wen
Published: May 2012

Blue InGaN/GaN multiple quantum well light-emitting diodes (LEDs) with the conventional AlGaN and AlGaN-GaN-AlGaN (AGA) electron blocking layer (EBL) are investigated numerically. The simulation results show that the LEDs with the AGA EBL exhibit much higher output power and smaller efficiency droop at high current as compared to those with the conventional EBL due to the enhancement of the electron confinement and improvement of the hole injection from p-type region, which are induced by the strong electrostatic fields and tunneling effect in the AGA EBL.
Experimental investigation on focusing characteristics of a He-Ne laser using circular Fresnel zone plate for high-precision alignment of linear accelerators
Authors: Suwada Tsuyoshi , Satoh Masanori , Telada Souichi , Minoshima Kaoru
Published: May 2012

We experimentally investigate the focusing characteristics of a He–Ne laser at the focal region for the high-precision alignment of long-distance linear accelerators using a circular Fresnel zone plate. The laser wave passing through the Fresnel zone plate having a focal length of 66.7 m propagates for a 268-m-long distance at atmospheric pressure. A new laser-based alignment system using Fresnel zone plates as the alignment targets is discussed. The transverse displacement of the focused spot of the laser is measured as a function of the displacement of the target by a detector installed at the focal point. Systematic studies on the focusing characteristics and alignment precision have been successfully conducted in this experiment. The experimental results are in good agreement with theoretical calculations, and the alignment precision of the target is determined to be less than ±30 μm. In this study, we perform a detailed experimental investigation on the laser propagation and focusing characteristics using the circular Fresnel zone plate at the focal region along with theoretical calculations.
Texture-induced magnetic interactions in ferrofluids
Authors: Urtizberea A. , Arizaga A. , Silva N. J. O. , Millán A. , Palacio F. , Luis F.
Published: May 2012

We report a method for reversibly controlling the strength of dipole-dipole interactions in maghemite ferrofluids. In order to induce some magnetic texture, the ferrofluid is exposed to a strong magnetic field while it is cooled from room temperature to below its freezing temperature. The experimental data show that the average strength of dipolar interactions increases with increasing texture and that the magnetic relaxation becomes slower.
Vibrating piezoelectric nanofilms as sandwich nanoplates
Authors: Zhang Jin , Wang Chengyuan
Published: May 2012

Vibrating piezoelectric nanofilms (PNFs) play an important role in developing electromechanical nanodevices. To accurately characterize their dynamic behavior a sandwich-plate model is developed by integrating the surface effect and piezoelectric effect into the elastic plate theory. It is then used to analyze the vibration of PNFs with an emphasis on the effect of piezoelectricity and the surface layers. Analytic formulae are derived to identify the key factors that determine the structural responses of PNFs. Their influence on the vibration of PNFs is then evaluated qualitatively in a numerical study. It is found that the surface effect is significant and originates primarily from the residual surface stresses and the equivalent pre-stresses induced by an electrical voltage via the surface piezoelectricity.
Interface and oxide quality of CoFeB/MgO/Si tunnel junctions
Authors: Shaw Jonathan T. , Tseng H. W. , Rajwade Shantanu , Tung Lieh-Ting , Buhrman R. A. , Kan Edwin C.
Published: May 2012

CoFeB/MgO/Si MOS capacitors were characterized to study the oxide and interface quality of very thin MgO layer (< 2 nm) after various annealing protocols. The improvement in tunneling magneto-resistance (TMR) ratio of a CoFeB/MgO/CoFeB composite layer is believed to result from boron (B) diffusion into the MgO layer to form a polycrystalline Mg-B-O layer, which has a much sharper interface after annealing. By studying the stress-induced-leakage current and the oscillating component of the tunneling current in these MOS structures, we were able to monitor the trap density, interface quality, and endurance of the dielectric. With the CoFeB capping layer, the dielectric defect density is indeed smaller after annealing, which may help to explain the enhancement in TMR. While thermal annealing results in a sharper interface and superior endurance quality with the capping layer, the interface trap density is only marginally improved with annealing.
The extended Koopmans’ theorem: Vertical ionization potentials from natural orbital functional theory
Authors: Piris Mario , Matxain Jon M. , Lopez Xabier , Ugalde Jesus M.
Published: May 2012

The Piris natural orbital functional, PNOF5, has been used to predict vertical ionization potentials of a selected set of 30 organic and inorganic spin-compensated molecules by means of the extended Koopmans’ theorem. Electron affinities of 10 selected radicals have also been estimated as the inverse of the ionization potentials of the anionic species, calculated at the experimental geometries of the neutral radicals. The basis set limit effects have been assessed by inspecting the data obtained for the Dunning's basis set series cc-pVXZ and aug-cc-pVXZ (X = D, T, Q, 5). The performance of the PNOF5 is established by carrying out a statistical analysis of the mean absolute errors (MAEs) with respect to the experiment values. The calculated PNOF5 ionization potentials and electron affinities agree satisfactorily with the corresponding experimental data, with MAEs smaller than 0.5 eV.
Dynamic observations of dislocation behavior in SrTiO3 by in situ nanoindentation in a transmission electron microscope
Authors: Kondo Shun , Shibata Naoya , Mitsuma Tasuku , Tochigi Eita , Ikuhara Yuichi
Published: May 2012

The plastic deformation process of a SrTiO3 single crystal has been dynamically and microscopically observed by in situ nanoindentation in a transmission electron microscope. The plastic deformation of SrTiO3 was found to proceed by dislocation slips even at room temperatures under the present experimental condition, and the dynamic behavior of dislocations such as nucleation, propagation, and annihilation have been clearly captured. The detailed dislocation analyses revealed that the activated slip system is 〈110〉{11¯0} type, which is consistent with the results of macroscopic deformation experiments reported so far. The mechanisms of dislocation behavior are discussed based on the experimentally obtained results.
Nonlinear graphene metamaterial
Authors: Nikolaenko Andrey E. , Papasimakis Nikitas , Atmatzakis Evangelos , Luo Zhiqiang , Shen Ze Xiang , De Angelis Francesco , Boden Stuart A. , Di Fabrizio Enzo , Zheludev Nikolay I.
Published: May 2012

We demonstrate that the broadband nonlinear optical response of graphene can be resonantly enhanced by more than an order of magnitude through hybridization with a plasmonic metamaterial, while retaining an ultrafast nonlinear response time of ∼1 ps. Transmission modulation close to ∼1% is seen at a pump fluence of ∼30 μJ/cm2 at the wavelength of ∼1.6 μm. This approach allows to engineer and enhance graphene’s nonlinearity within a broad wavelength range enabling applications in optical switching, mode-locking, and pulse shaping.
Effect of amorphous carbon coatings on the mechanical behavior of silicon carbide nanowire
Authors: Li L. , Solá F. , Xia Z. H. , Yang Y. Q.
Published: May 2012

Silicon carbide nanowires (NWs) are promising candidates for structural applications owing to their excellent mechanical, thermal, and electronic properties. The effect of amorphous carbon coatings on the mechanical behavior of the nanowires was studied via molecular dynamics methods at room temperature. The results show that the amorphous carbon coatings can shield opening cracks on silicon carbide nanowires, making them damage-tolerant. With increasing the defect size, the tensile strength and fracture energy of uncoated silicon carbide nanowires rapidly decrease; however, the properties of coated nanowires maintain nearly constant. Increasing the coating thickness leads to a brittle-to-ductile transition for the nanowires. Careful tailoring of the coatings permits engineering of these nanostructures for higher strength and damage tolerance at submicron scales.
Partially disordered photonic-crystal thin films for enhanced and robust photovoltaics
Authors: Oskooi Ardavan , Favuzzi Pedro A. , Tanaka Yoshinori , Shigeta Hiroaki , Kawakami Yoichi , Noda Susumu
Published: May 2012

We present a general framework for the design of thin-film photovoltaics based on a partially disordered photonic crystal that has both enhanced absorption for light trapping and reduced sensitivity to the angle and polarization of incident radiation. The absorption characteristics of different lattice structures are investigated as an initial periodic structure is gradually perturbed. We find that an optimal amount of disorder controllably introduced into a multi-lattice photonic crystal causes the characteristic narrow-band, resonant peaks to be broadened resulting in a device with enhanced and robust performance ideal for typical operating conditions of photovoltaic applications.
The charge trapping characteristics of Si3N4 and Al2O3 layers on amorphous-indium-gallium-zinc oxide thin films for memory application
Authors: Sim Jung Ji , Rha Sang-Ho , Ki Kim Un , Jang Chung Yoon , Soo Jung Yoon , Choi Jung-Hae , Seong Hwang Cheol
Published: May 2012

The charge trapping characteristics of 30-nm-thick Si3N4 and 3-nm-thick Al2O3 layers between amorphous In-Ga-Zn-O thin films and 100-nm-thick blocking oxides made of thermal SiO2 were examined. The Si3N4 layer showed several discrete trap levels with relatively low density, while the Al2O3 layer showed a higher trap density with continuous distribution for electron trapping. When no tunneling oxide was adopted, the trapped carriers were easily detrapped, even at room temperature. Adoption of a 6-nm-thick SiO2 tunneling layer grown by atomic layer deposition largely improved the retention of the trapped charges and retained ∼60% of the trapped charges even after 10 000 s.
A liftoff process of GaN layers and devices through nanoporous transformation
Authors: Zhang Yu , Leung Benjamin , Han Jung
Published: May 2012

A process to slice and separate GaN device layers for vertical light emitting diodes (LEDs) is presented through a developed electrochemical anodization process to create nanoporous (NP) GaN of designed porosity profiles. The NP GaN serves dual purposes of supporting subsequent overgrowth of LED structures while undergoing, during growth, shape transformation into a largely voided morphology. It is shown that this voided region decreases the lateral fracture resistance and enables large-area separation of the LED structures after appropriate wafer bonding. The separated LED layers are shown to have comparable material quality before and after the liftoff process. Blue emitting GaN LEDs are transferred to silicon substrates with vertical configuration by this unique process.
Room-temperature ferromagnetism in epitaxial Mg-doped SnO2 thin films
Authors: Wu Ping , Zhou Baozeng , Zhou Wei
Published: May 2012

The magnetic behavior of epitaxial Mg-doped SnO2 thin films prepared by radio-frequency magnetron sputtering was investigated in this work. Room-temperature ferromagnetism with the saturation magnetization of about 6.9 emu/cm3 was observed in 6% Mg-doped SnO2 samples. And the saturation magnetization decreases when further doped to 8%, while the optical band-gap increases. The room-temperature ferromagnetism was induced by the holes created by Mg on the substitutional site. Additionally, Mg interstitials and oxygen vacancies play an important role in reducing the magnetic moments.
Experimental investigation of convective structure evolution and heat transfer in quasi-steady evaporating liquid films
Authors: Kimball J. T. , Hermanson J. C. , Allen J. S.
Published: May 2012

The stability, convective structure, and heat transfer characteristics of upward-facing, evaporating, thin liquid films were studied experimentally. Dichloromethane, chloroform, methanol, and acetone films with initial thicknesses of 2–5 mm were subjected to constant levels of superheating until film rupture occurred (typically at a thickness of around 50 μm). The films resided on a temperature controlled, polished copper plate incorporated into a closed pressure chamber free of non-condensable gasses. The dynamic film thickness was measured at multiple points using a non-intrusive ultrasound ranging system. Instability wavelength and convective structure information was obtained using double-pass schlieren imaging. The sequence of the convective structures as the film thins due to evaporation is observed to be as follows: (1) large, highly variable cells, (2) concentric rings and spirals, and (3) apparent end of convection. The transition from large, variable cells to concentric rings and spirals occurs at a Rayleigh number of 4800 ± 960. The apparent end of convection occurs at a Rayleigh number of 1580 ± 180. At the cessation of convection, the Nusselt number is nearly unity, indicating that there is little heat transfer in the film due to convection. In films where the Rayleigh number is above this transitional value, the Nusselt number increases with increasing Rayleigh number. The current results suggest that the equilibrium condition at the evaporating surface suppresses surface temperature variation, effectively eliminating thermocapillary-driven instability.
Low-dimensional modeling of streaks in a wedge flow boundary layer
Authors: Higuera María , Vega José M.
Published: May 2012

This paper is concerned with the low dimensional structure of optimal streaks in a wedge flow boundary layer, which have been recently shown to consist of a unique (up to a constant factor) three-dimensional streamwise evolving mode, known as the most unstable streaky mode. Optimal streaks exhibit a still unexplored/unexploited approximate self-similarity (not associated with the boundary layer self-similarity), namely the streamwise velocity re-scaled with their maximum remains almost independent of both the spanwise wavenumber and the streamwise coordinate; the remaining two velocity components instead do not satisfy this property. The approximate self-similar behavior is analyzed here and exploited to further simplify the description of optimal streaks. In particular, it is shown that streaks can be approximately described in terms of the streamwise evolution of the scalar amplitudes of just three one-dimensional modes, providing the wall normal profiles of the streamwise velocity and two combinations of the cross flow velocity components; the scalar amplitudes obey a singular system of three ordinary differential equations (involving only two degrees of freedom), which approximates well the streamwise evolution of the general streaks.
A novel driving principle by means of the parasitic motion of the microgripper and its preliminary application in the design of the linear actuator
Authors: Huang Hu , Zhao Hongwei , Yang Zhaojun , Mi Jie , Fan Zunqiang , Wan Shunguang , Shi Chengli , Ma Zhichao
Published: May 2012

This paper presents a novel driving principle by means of the parasitic motion of the microgripper. Actuators based on this principle can realize the large displacement range and high speed easily. Also the structure can be simple. A parasitic motion principle linear actuator mainly consisting of two piezoelectric stacks, two microgrippers and a mover was designed. Experimental results indicate that at a low driving frequency of 5 Hz, large velocity over 40 μm/s is obtained with the driving voltage of 100 V. Backward motion was observed and analyzed. Experimental results verify the feasibility of the new principle and it can be used to design new linear or rotary actuators.
Transfer printing and nanomanipulating luminescent photonic crystal membrane nanocavities
Authors: Wang Bowen , Siahaan Timothy , Dündar Mehmet A. , Nötzel Richard , He Sailing , van der Heijden Rob W.
Published: May 2012

The release of photoluminescent InGaAsP photonic crystal nanocavity chiplets from the host chip for creating autonomous functional microparticles is demonstrated. A transfer printing method using a soft polymeric material as a stamp is used to transfer cavity arrays to other substrates. Alternatively, cavities are transferred individually by a nanomanipulation technique. The chiplets can be fully deterministically positioned on both the host chip and another substrate (glass) with the nanomanipulator. The chiplets have the striking property of spontaneously orienting themselves with their plane perpendicular to the receiving surface. At each stage of the process, the condition of the cavities as dependent on their immediate surroundings is monitored from their photoluminescence spectrum.
Possible contribution of triboelectricity to snow - air interactions
Authors: Tkachenko Ekaterina Y. , Kozachkov Sergey G.
Published: May 2012

Environmental contextPolar near-surface snow can act as a chemical reactor that alters the composition and chemistry of snow and the overlying air. Although the mechanisms and driving forces of these reactions have long been debated, triboelectrification (production of electrostatic charges by friction) of snow by wind has not yet been considered as a factor. It is proposed that in polar regions, triboelectrification could significantly influence the composition and chemistry of snow.
AbstractReactions that proceed in polar snow cover may significantly affect the chemistry of the overlying atmosphere, but mechanisms and driving forces of these reactions are still under discussion. The proposed hypothesis attempts to explain some experimental data that cannot be fully understood (e.g. the effect of wind on OH radicals, ozone and persistent organic pollutants levels) by taking into account the influence of electrical phenomena on the snow surface. We assumed that a combination of such factors as low humidity, high wind speed and low temperatures makes the influence of triboelectrification of snow significant for polar areas, where purity and the depth of snow cover prevent fast charge dissipation. The major points of the hypothesis are: (1) when the electric field reaches a value sufficient for the onset of corona discharge, various free radical processes are initiated resulting in changes in the concentrations of ozone, OH radical, nitrate, etc., and the decomposition of pollutant molecules; (2) the high electric field can stimulate transport of ions (such as bromide and nitrate) from the condensed phase to the gas phase; and (3) the ageing of charged snow can increase its electrical potential.
Modelling of arsenate retention from aqueous solutions by living coryneform double-mutant bacteria
Authors: Ordoñez Efren , Villadangos Almudena F. , Fiuza María , Pereira Fernando J. , Gil Jose A. , Mateos Luis M. , Aller A. Javier
Published: May 2012

Environmental contextIndustrial development has favoured the release of toxic elements to the environment and monitoring and assessment their environmental impact are key points. An important aspect of understanding these concerns is to evaluate how toxic substances interact with microorganisms, which has critical implications in the environment. Current studies show that heavy metals have the potential to affect bacterial viability, although a great deal remains to be understood concerning metal speciation using engineered bacterial cells. AbstractModelling of the arsenate (AsV) retention from aqueous solutions by a living, genetically modified coryneform bacterium (Corynebacterium glutamicum ArsC1–C2) was evaluated. The bacterium used was a double mutant strain unable to reduce arsenate to arsenite. Batch experiments were carried out to study the effects of high initial AsV concentrations, retention times and temperatures on the retention process. Arsenate retention kinetics was modelled using pseudo-second-order and Elovich models. Both models provided high coefficients of determination, but better applicability of the Elovich model was confirmed using the Z function. A useful generalised predictive equation, allowing evaluation of the simultaneous effects of time and the initial AsV concentration on the retention process, was proposed. The retention equilibrium for a wide concentration range of arsenate showed a mechanistic process underlying chemical-nature retention with the experimental data strongly consistent with the Langmuir isotherm. Thermodynamic studies defined the negative free energy changes and demonstrated the spontaneity of the retention process. Positive values for both enthalpy and entropy were indicative of endothermic retention and a high affinity for AsV by the bacteria. The high maximum retained quantity, 2.0 mg AsV g–1 bacteria, confirmed the bacterium’s high affinity for this arsenic species.