bluuebunny/crossref_metadata_2025 huggingface dataset:
NVU dynamics. II. Comparing to four other dynamics
Authors: Ingebrigtsen Trond S. , Toxvaerd Søren , Schrøder Thomas B. , Dyre Jeppe C.
Published: September 2011

In the companion paper [T. S. Ingebrigtsen, S. Toxvaerd, O. J. Heilmann, T. B. Schrøder, and J. C. Dyre, “NVU dynamics. I. Geodesic motion on the constant-potential-energy hypersurface,” J. Chem. Phys. (in press)] an algorithm was developed for tracing out a geodesic curve on the constant-potential-energy hypersurface. Here, simulations of NVU dynamics are compared to results for four other dynamics, both deterministic and stochastic. First, NVU dynamics is compared to the standard energy-conserving Newtonian NVE dynamics by simulations of the Kob-Andersen binary Lennard-Jones liquid, its WCA version (i.e., with cut-off's at the pair potential minima), and the Lennard-Jones Gaussian liquid. We find identical results for all quantities probed: radial distribution functions, incoherent intermediate scattering functions, and mean-square displacement as function of time. Arguments are presented for the equivalence of NVU and NVE dynamics in the thermodynamic limit; in particular, to leading order in 1/N these two dynamics give identical time-autocorrelation functions. In the final part of the paper, NVU dynamics is compared to Monte Carlo dynamics, to a diffusive dynamics of small-step random walks on the constant-potential-energy hypersurface, and to Nos\documentclass[12pt]{minimal}\begin{document}$\acute{e}$\end{document}é-Hoover NVT dynamics. If time is scaled for the two stochastic dynamics to make single-particle diffusion constants identical to that of NVE dynamics, the simulations show that all five dynamics are equivalent at low temperatures except at short times.
Valence isoelectronic substitution in the B8− and B9− molecular wheels by an Al dopant atom: Umbrella-like structures of AlB7− and AlB8−
Authors: Galeev Timur R. , Romanescu Constantin , Li Wei-Li , Wang Lai-Sheng , Boldyrev Alexander I.
Published: September 2011

The structures and the electronic properties of two aluminum-doped boron clusters, AlB7− and AlB8−, were investigated using photoelectron spectroscopy and ab initio calculations. The photoelectron spectra of AlB7− and AlB8− are both broad, suggesting significant geometry changes between the ground states of the anions and the neutrals. Unbiased global minimum searches were carried out and the calculated vertical electron detachment energies were used to compare with the experimental data. We found that the Al atom does not simply replace a B atom in the parent B8− and B9− planar clusters in AlB7− and AlB8−. Instead, the global minima of the two doped-clusters are of umbrella shapes, featuring an Al atom interacting ionically with a hexagonal and heptagonal pyramidal B7 (C6v) and B8 (C7v) fragment, respectively. These unique umbrella-type structures are understood on the basis of the special stability of the quasi-planar B73− and planar B82− molecular wheels derived from double aromaticity.
Optical properties of homeotropically aligned flexoelectric nematic layers deformed by direct current electric field
Authors: Buczkowska M. , Derfel G.
Published: September 2011

Optical properties of homeotropically aligned flexoelectric nematic layers deformed by dc electric fields were analyzed by calculating the phase difference ΔΦ between ordinary and extraordinary light rays passing through a layer placed between crossed polarizers. A phase difference arises when the bias voltage U exceeds the threshold value UT. Close to the threshold, ΔΦ increases linearly with U. The possibility of using this effect to detect the flexoelectric properties of the nematic was explored by simulated experiments. The method considered in this paper is based on measurement of the slope of the linear section of the voltage dependence of ΔΦ, which depends on the sum of the flexoelectric coefficients and which differs from a well-known theoretical slope that ignores flexoelectric properties. The difference between the slopes reveals the flexoelectric properties of the nematic. The effectiveness of the proposed method is discussed.
Permeability spectra of yttrium iron garnet and its granular composite materials under dc magnetic field
Authors: Tsutaoka Takanori , Kasagi Teruhiro , Hatakeyama Kenichi
Published: September 2011

Relative complex permeability spectra (μr=μr′-iμr′′) and the dc magnetic field effect on them for a yttrium iron garnet (YIG) and its granular composite materials have been studied to evaluate the negative permeability characteristics. In the sintered YIG, two distinct peaks corresponding to the domain wall and the gyromagnetic spin resonance were observed in the imaginary part μr′′ under zero magnetic field; the real part of complex permeability μr′ shows a small negative value in a certain frequency range. The Lorentz type magnetic resonance with the negative permeability dispersion was observed under dc magnetic field. Permeability spectra were evaluated by the numerical fitting of actual measurement data to a resonance formula using six parameters (resonance frequencies, static susceptibilities, and damping factors of the domain wall motion and the gyromagnetic spin rotation). The dc magnetic field suppresses the domain wall contribution and the spin component becomes dominant. In the YIG granular composite material, the permeability dispersion frequency shifts to higher frequency region due to demagnetizing field; the spin component becomes dominant. Negative permeability spectra were also observed in the high content YIG composites under the dc field. The negative permeability spectra of YIG composite materials can also be applied to the left-handed material as well as the sintered YIG.
Deformation behaviors of an armchair boron-nitride nanotube under axial tensile strains
Authors: Liao Ming-Liang , Wang Yao-Chun , Ju Shin-Pon , Lien Ting-Wei , Huang Li-Fan
Published: September 2011

Deformation behaviors of an (8,8) boron-nitride nanotube (BNNT) under axial tensile strains were investigated via molecular dynamics (MD) simulations. The Tersoff potential was employed in the simulations with potential parameters determined by fitting the MD simulations results to those obtained from density functional theory calculations for BNNTs with the aid of the force-matching method. Variations in the axial stress, bond lengths, bond angles, radial buckling, and slip vectors with tensile strain were all examined. The axial, the radial, and tangential components of the slip vector were employed to monitor the local elongation, the local necking, and the local twisting deformations, respectively, near the tensile failure of the BNNT. From this study, it was noted that the BNNT started to fail at the failure strain of 26.7%. The components of the slip vector grew abruptly and rapidly after the failure strain, especially for the axial component. This implies that the local elongation dominates the tensile failure of the BNNT. With further axial tensile strains, subsequent bond breaking was found in the BNNT and finally resulted in a chain-like failure mode before complete breaking of the BNNT. No apparent yielding was noticed before the tensile failure of the BNNT.
Barrier height and tunneling aspects in (110) CrO2 with its natural barrier
Authors: Pathak M. , Mazumdar D. , Karthik V. , Zhang X. , Chetry K. B. , Keshavarz S. , LeClair P. , Gupta A.
Published: September 2011

We have investigated (110) CrO2/natural barrier/Co magnetic tunnel junctions for their barrier and magneto-transport properties. A negative tunnel magnetoresistance (TMR) of over 5% was observed in micro-fabricated devices at 4.2 K, which is comparable to TMR values obtained with (100) CrO2. Both transport and cross-sectional transmission electron microscopy analysis reveal a natural barrier thickness 3.5 ± 0.5 nm. However, we obtain a low effective barrier height of 0.4 eV from transport measurements. The inelastic electron tunneling spectroscopy showed significant bias dependence with peak positions showing vibrational modes, which deviate from stoichiometric Cr2O3. We conclude that the transport characteristics are controlled by defects within the natural barrier, consistent with recent theoretical reports.
Three-dimensional endoscopic photoacoustic imaging based on multielement linear transducer array
Authors: Yuan Yi , Yang Sihua , Xing Da
Published: September 2011

An implementation system of three-dimensional endoscopic photoacoustic imaging is presented. The developed endoscopic photoacoustic detector integrates a multielement linear transducer array, a reflective device, a Plexiglass tube, and ultrasonic coupling medium. To match with the acoustic impendence of Plexiglass tube, a glycerin solution with 45% volume percentage was used as the ultrasonic coupling medium. This ultrasonic coupling medium can decrease photoacoustic pressure transmission loss during the progress of photoacoustic signal propagation. The capability of the system for three-dimensional imaging was verified with chicken breast tissue. Furthermore, pig normal rectal tissue and mouse breast tumor tissue in an ex vivo cavity model were imaged by the system. The reconstructed three-dimensional photoacoustic image presented the structural information of normal and lesion tissue. The experimental results demonstrate the multielement-based endoscopic photoacoustic imaging system with inside-out laser exciting mode has the ability of reconstructing three-dimensional images of biology tissue.
Application of the double relaxation oscillation superconducting quantum interference device sensor to micro-tesla 1H nuclear magnetic resonance experiments
Authors: Kang Chan Seok , Kim Kiwoong , Lee Seong-Joo , Hwang Seong-min , Kim Jin-Mok , Yu Kwon Kyu , Kwon Hyukchan , Lee Sang Kil , Lee Yong-Ho
Published: September 2011

We developed an ultra-low field (ULF)-nuclear magnetic resonance (NMR) measurement system capable of working with a measurement field (Bm) of several micro-tesla and performed basic NMR studies with a double relaxation oscillation superconducting quantum interference device (DROS) instead of conventional dc-SQUIDs. DROS is a SQUID sensor utilizing a relaxation oscillation between a dc-SQUID and a relaxation circuit; the new unit consists of an inductor and a resistor, and is connected in parallel with the SQUID. DROS has a 10 times larger flux-to-voltage transfer coefficient (∼mV/ϕ0) than that of the dc-SQUID, and this large transfer coefficient enables the acquisition of the SQUID signal with a simple flux-locked-loop (FLL) circuit using room temperature pre-amplifiers. The DROS second-order gradiometer showed average field noise of 9.2 μϕ0/√Hz in a magnetically shielded room (MSR). In addition, a current limiter formed of a Josephson junction array was put in a flux-transformer of DROS to prevent excessive currents that can be generated from the high pre-polarization field (Bp). Using this system, we measured an 1H NMR signal in water under 2.8 μT Bm field and reconstructed a one-dimensional MR image from the 1H NMR signal under a gradient field BG of 4.09 nT/mm. In addition, we confirmed that the ULF-NMR system can measure the NMR signal in the presence of metal without any distortion by measuring the NMR signal of a sample wrapped with metal. Lastly, we have measured the scalar J-coupling of trimethylphosphate and were able to confirm a clear doublet NMR signal with the coupling strength J3[P,H] = 10.4 ± 0.8 Hz. Finally, because the existing ULF-NMR/MRI studies were almost all performed with dc-SQUID based systems, we constructed a dc-SQUID-based ULF-NMR system in addition to the DROS based system and compared the characteristics of the two different systems by operating the two systems under identical experimental conditions.
Preparation of superior lubricious amorphous carbon films co-doped by silicon and aluminum
Authors: Liu Xiaoqiang , Hao Junying , Yang Jun , Zheng Jianyun , Liang Yongmin , Liu Weimin
Published: September 2011

Silicon (Si) and aluminum (Al) co-doped amorphous carbon films ((Si, Al)–C:H) were deposited on Si and stainless steel substrates by radio frequency (13.56 MHz) magnetron sputtering. The Al and Si were found to jointly regulate the hybridized carbon bonds. Mechanical properties of the films were detected by nano-indention and scratch tests. The nano-indention results revealed that all the samples exhibited good elastic recovery rate, among which the highest one was beyond 84%. Besides co-regulating the hybridizations of carbon, the co-doped Si and Al also had a common regulation on the mechanical and tribological properties. Especially, the film containing 1.6 at. % of Si and 0.9 at. % of Al showed a super-low friction (< 0.01) and a superior wear resistance in ambient air.
Kinetics of the 3C-6H polytypic transition in 3C-SiC single crystals: A diffuse X-ray scattering study
Authors: Dompoint D. , Boulle A. , Galben-Sandulache I. , Chaussende D. , Hoa L. T. M. , Ouisse T. , Eyidi D. , Demenet J. L. , Beaufort M. F. , Rabier J.
Published: September 2011

In this work, the kinetics of the 3C-6H polytypic transition in 3C-SiC single crystals are studied in details by means of diffuse x-ray scattering (DXS) coupled with numerical simulations and transmission electron microscopy and optical birefringence microscopy. Upon high-temperature annealing, spatially correlated stacking faults (SFs), lying in the {111} planes, are generated within the crystal and tend to form bands of partially transformed SiC. It is shown that the numerical simulation of the DXS curves allows to unambiguously deduce the transformation level within these bands, as well as the volume fraction corresponding to these bands. Increasing annealing time results (1) in the growth of the partially transformed regions by the glide of the partial dislocations bounding the SFs and (2) in the generation of new SFs within the crystal by means of a double-cross slip motion. The kinetics of each of these mechanisms are presented and discussed with respect to the annealing temperature, the initial SF density and crystalline quality.
Intermittency and geometrical statistics of three-dimensional homogeneous magnetohydrodynamic turbulence: A wavelet viewpoint
Authors: Yoshimatsu Katsunori , Schneider Kai , Okamoto Naoya , Kawahara Yasuhiro , Farge Marie
Published: September 2011

Scale-dependent and geometrical statistics of three-dimensional incompressible homogeneous magnetohydrodynamic turbulence without mean magnetic field are examined by means of the orthogonal wavelet decomposition. The flow is computed by direct numerical simulation with a Fourier spectral method at resolution 5123 and a unit magnetic Prandtl number. Scale-dependent second and higher order statistics of the velocity and magnetic fields allow to quantify their intermittency in terms of spatial fluctuations of the energy spectra, the flatness, and the probability distribution functions at different scales. Different scale-dependent relative helicities, e.g., kinetic, cross, and magnetic relative helicities, yield geometrical information on alignment between the different scale-dependent fields. At each scale, the alignment between the velocity and magnetic field is found to be more pronounced than the other alignments considered here, i.e., the scale-dependent alignment between the velocity and vorticity, the scale-dependent alignment between the magnetic field and its vector potential, and the scale-dependent alignment between the magnetic field and the current density. Finally, statistical scale-dependent analyses of both Eulerian and Lagrangian accelerations and the corresponding time-derivatives of the magnetic field are performed. It is found that the Lagrangian acceleration does not exhibit substantially stronger intermittency compared to the Eulerian acceleration, in contrast to hydrodynamic turbulence where the Lagrangian acceleration shows much stronger intermittency than the Eulerian acceleration. The Eulerian time-derivative of the magnetic field is more intermittent than the Lagrangian time-derivative of the magnetic field.
Enhanced sampling of particular degrees of freedom in molecular systems based on adiabatic decoupling and temperature or force scaling
Authors: Kunz Anna-Pitschna E. , Liu Haiyan , van Gunsteren Wilfred F.
Published: September 2011

A method to enhance sampling of a small subset of Nh particular degrees of freedom of a system of Nh + Nl degrees of freedom is presented. It makes use of adiabatically decoupling these degrees of freedom by increasing their mass followed by either increasing their temperature or reducing their interaction or the force acting on them. The appropriate statistical-mechanical expressions for use of these methods in simulation studies are derived. As long as the subset of mass-increased degrees of freedom is small compared to the total number of degrees of freedom of the system, sampling of this subset of degrees of freedom can be much enhanced at the cost of a slight perturbation of the configurational distribution. This is illustrated for a test system of 1000 SPC, simple point charge, water molecules at 300 K and a density of 997 kg m−3. Various fractions Nh/(Nh + Nl) of water molecules were adiabatically decoupled to different degrees. The size of the diffusion coefficient of these decoupled water molecules was used as a measure for how much the sampling was enhanced and the average potential energy per water molecule was used as a measure of how much the configurational distribution of the system gets distorted. A variety of parameter values was investigated and it was found that for Nh/(Nh + Nl) ⩽ 0.1 the diffusion of the Nh molecules could be enhanced by factors up to 35 depending on the method, the ratio Nh/(Nh + Nl), the extent of adiabatic decoupling, and the temperature or force scaling factors, at the cost of a slight perturbation of the configurational distribution.
Relativistic Jahn-Teller effects in the photoelectron spectra of tetrahedral P4, As4, Sb4, and Bi4
Authors: Opalka Daniel , Poluyanov Leonid V. , Domcke Wolfgang
Published: September 2011

The group-V tetrahedral cluster cations \documentclass[12pt]{minimal}\begin{document}${\rm P}_{\text{4}}^{+}$\end{document}P4+, \documentclass[12pt]{minimal}\begin{document}${\rm As}_{\text{4}}^{+}$\end{document} As 4+, \documentclass[12pt]{minimal}\begin{document}${\rm Sb}_{\text{4}}^{+}$\end{document} Sb 4+, and \documentclass[12pt]{minimal}\begin{document}${\rm Bi}_{\text{4}}^{+}$\end{document} Bi 4+ are known to exhibit exceptionally strong Jahn-Teller (JT) effects of electrostatic origin in their 2E ground states and 2T2 excited states. It has been predicted that there exist, in addition, JT couplings of relativistic origin (arising from the spin-orbit (SO) operator) in 2E and 2T2 states of tetrahedral systems, which should become relevant for the heavier elements. In the present work, the JT and SO couplings in the group-V tetramer cations have been analyzed with ab initio relativistic electronic structure calculations. The vibronic line spectra and the band shapes of the photoelectron spectra were simulated with time-dependent quantum wave-packet methods. The results provide insight into the interplay of electrostatic and relativistic JT couplings and SO splittings in the complex photoelectron spectra of these systems.
Laser wake-field acceleration in pre-formed plasma channel created by pre-pulse pedestal of terawatt laser pulse
Authors: Sanyasi Rao Bobbili , Chakera Juzer Ali , Naik Prasad Anant , Kumar Mukund , Gupta Parshotam Dass
Published: September 2011

The role of nanosecond duration pre-pulse pedestal (Amplified Spontaneous Emission (ASE) pre-pulse) in the propagation of 45 fs, 4 TW Ti:Sapphire laser pulse through a helium gas jet target has been investigated. We observed that the pre-pulse pedestal of about 1 ns duration and intensity 3 × 1012 W/cm2 creates pre-formed plasma with optical guiding channel like structure in the gas-jet at density around 3 × 1019 cm−3. Guiding of the 45 fs laser pulse (IL = 3 × 1018 W/cm2) in the pre-formed plasma channel, over a distance much longer than the Rayleigh length was also observed. The guiding of the laser pulse resulted in the generation of high energy electron beam by laser wake-field acceleration of self-injected electrons. The accelerated electron beam was quasi-monoenergetic with peak energy up to 50 MeV, low divergence in the range of 3–6 mrad, and bunch charge up to 100 pC.
Minority charge carrier lifetime mapping of crystalline silicon wafers by time-resolved photoluminescence imaging
Authors: Kiliani David , Micard Gabriel , Steuer Benjamin , Raabe Bernd , Herguth Axel , Hahn Giso
Published: September 2011

A camera-based method to record spatially and time-resolved photoluminescence images of crystalline silicon wafers was developed. The camera signal is modulated by a rotating shutter wheel, allowing for a wide range of camera types to be used for the measurement and easy integration into existing photoluminescence setups. The temporal resolution is sufficient to record the decay curve of photoexcited charge carriers in surface-passivated silicon wafers. A transient measurement of minority carrier lifetimes down to less than 10 μs can be obtained for each pixel individually, without the need for any external calibration.
Electron correlation via frozen Gaussian dynamics
Authors: Elliott Peter , Maitra Neepa T.
Published: September 2011

We investigate the accuracy and efficiency of the semiclassical frozen Gaussian method in describing electron dynamics in real time. Model systems of two soft-Coulomb-interacting electrons are used to study correlated dynamics under non-perturbative electric fields, as well as the excitation spectrum. The results show that a recently proposed method that combines exact-exchange with semiclassical correlation to propagate the one-body density-matrix holds promise for electron dynamics in many situations that either wavefunction or density-functional methods have difficulty describing. The results also however point out challenges in such a method that need to be addressed before it can become widely applicable.
Effect of driving frequency on excitation of turbulence in a kinetic plasma
Authors: Parashar T. N. , Servidio S. , Shay M. A. , Breech B. , Matthaeus W. H.
Published: September 2011

The effect of driving frequency on the efficiency of turbulence generation through magnetic forcing is studied using kinetic hybrid simulations with fully kinetic ions and fluid electrons. The efficiency of driving is quantified by examining the energy input into magnetic field as well as the thermal energy for various driving frequencies. The driving is efficient in exciting turbulence and heating the plasma when the time period of the driving is larger than the nonlinear time of the system. For driving at faster time scales, the energy input is weak and the steady state energy is much lower. The heating of the plasma is correlated with intermittent properties of the magnetic field, which are manifested as non-Gaussian statistics. Implications for turbulence in solar corona are discussed.
High-quality proton bunch from laser interaction with a gas-filled cone target
Authors: Wang H. Y. , Lin C. , Zheng F. L. , Lu Y. R. , Guo Z. Y. , He X. T. , Chen J. E. , Yan X. Q.
Published: September 2011

Generation of high-energy proton bunch from interaction of an intense short circularly polarized (CP) laser pulse with a gas-filled cone target (GCT) is investigated using two-dimensional particle-in-cell simulation. The GCT target consists of a hollow cone filled with near-critical gas-plasma and a thin foil attached to the tip of the cone. It is observed that as the laser pulse propagates in the gas-plasma, the nonlinear focusing will result in an enhancement of the laser pulse intensity. It is shown that a large number of energetic electrons are generated from the gas-plasma and accelerated by the self-focused laser pulse. The energetic electrons then transports through the foil, forming a backside sheath field which is stronger than that produced by a simple planar target. A quasi-monoenergetic proton beam with maximum energy of 181 MeV is produced from this GCT target irradiated by a CP laser pulse at an intensity of 2.6 × 1020 W/cm2, which is nearly three times higher compared to simple planar target (67 MeV).
Analysis of saturation phenomena in Cerenkov free-electron lasers with a planar waveguide
Authors: Fares Hesham , Yamada Minoru
Published: September 2011

In this paper, the operation of the stimulated emission in Cerenkov free-electron laser (CFEL) is studied on the basis of the modulations of electron velocity and density by the electromagnetic (EM) field. The influence of the electron relaxation, due to mutual electrons collisions, on the electron dynamics is taken into account. We investigate the growth characteristics of Cerenkov laser operating in the small-signal and saturation regimes. In the saturation regime, the effect of velocity reduction of the electron beam on the gain dynamics is demonstrated. We also show that our results match with those of other well-known treatments in the small-signal gain limit.
A highly sensitive evaluation method for the determination of different current conduction mechanisms through dielectric layers
Authors: Murakami Katsuhisa , Rommel Mathias , Yanev Vasil , Erlbacher Tobias , Bauer Anton J. , Frey Lothar
Published: September 2011

Current conduction mechanisms through a metal-oxide semiconductor capacitor with a 9.6 nm thick SiO2 dielectric layer are characterized via Fowler-Nordheim (FN) and Poole-Frenkel (PF) plots, as well as through the analysis of the power exponent parameter α = d(log I)/d(log V). It is shown that the evaluation by means of α is much more sensitive in the accurate identification of different current conduction mechanisms. If FN tunneling and PF conduction occur simultaneously, evaluation using the α-voltage (α-V) plot actually allows one to determine the fraction of each conduction mechanism quantitatively. Even a slight current saturation due to minority carrier depletion, which cannot be detected through the evaluation of the current-voltage characteristics using FN or PF plots, can be detected by the α-V plot.
Quadratically convergent algorithm for orbital optimization in the orbital-optimized coupled-cluster doubles method and in orbital-optimized second-order Møller-Plesset perturbation theory
Authors: Bozkaya Uğur , Turney Justin M. , Yamaguchi Yukio , Schaefer Henry F. , Sherrill C. David
Published: September 2011

Using a Lagrangian-based approach, we present a more elegant derivation of the equations necessary for the variational optimization of the molecular orbitals (MOs) for the coupled-cluster doubles (CCD) method and second-order Møller-Plesset perturbation theory (MP2). These orbital-optimized theories are referred to as OO-CCD and OO-MP2 (or simply “OD” and “OMP2” for short), respectively. We also present an improved algorithm for orbital optimization in these methods. Explicit equations for response density matrices, the MO gradient, and the MO Hessian are reported both in spin-orbital and closed-shell spin-adapted forms. The Newton-Raphson algorithm is used for the optimization procedure using the MO gradient and Hessian. Further, orbital stability analyses are also carried out at correlated levels. The OD and OMP2 approaches are compared with the standard MP2, CCD, CCSD, and CCSD(T) methods. All these methods are applied to H2O, three diatomics, and the ${\rm O}_4^+$O4+ molecule. Results demonstrate that the CCSD and OD methods give nearly identical results for H2O and diatomics; however, in symmetry-breaking problems as exemplified by ${\rm O}_4^+$O4+, the OD method provides better results for vibrational frequencies. The OD method has further advantages over CCSD: its analytic gradients are easier to compute since there is no need to solve the coupled-perturbed equations for the orbital response, the computation of one-electron properties are easier because there is no response contribution to the particle density matrices, the variational optimized orbitals can be readily extended to allow inactive orbitals, it avoids spurious second-order poles in its response function, and its transition dipole moments are gauge invariant. The OMP2 has these same advantages over canonical MP2, making it promising for excited state properties via linear response theory. The quadratically convergent orbital-optimization procedure converges quickly for OMP2, and provides molecular properties that are somewhat different than those of MP2 for most of the test cases considered (although they are similar for H2O). Bond lengths are somewhat longer, and vibrational frequencies somewhat smaller, for OMP2 compared to MP2. In the difficult case of ${\rm O}_4^+$O4+, results for several vibrational frequencies are significantly improved in going from MP2 to OMP2.
Organic position sensitive photodetectors based on lateral donor-acceptor concentration gradients
Authors: Cabanillas–Gonzalez Juan , Peña–Rodríguez Ovidio , Suarez Lopez Inma , Schmidt Malte , Alonso M. Isabel , Goñi Alejandro R. , Campoy–Quiles Mariano
Published: September 2011

We report on an organic photodiode configuration based on a donor/acceptor bilayer with opposed complementary thickness gradients. We rely on the strong dependence of the optical field on thickness to achieve spatially dependent spectral responses, a proof-of-concept for position sensitive detection. Ratiometric photocurrent values at two wavelengths allow for position determination independently of the light intensity with a spatial resolution below 600 μm.
InGaN light-emitting diodes with indium-tin-oxide photonic crystal current-spreading layer
Authors: Li K. H. , Choi H. W.
Published: September 2011

Photonic crystal patterns on the indium tin oxide layer of an InGaN/GaN light-emitting diode are fabricated via nanosphere lithography in combination with dry etching. The silica spheres acting as an etch mask are self-assembled into a hexagonal closed-packed monolayer array. After etching, the photonic crystal (PhC) pattern is formed across the indium-tin-oxide (ITO) films so that the semiconductor layers are left intact and thus free of etch damages. Despite slight degradation to the electrical properties, the ITO-PhC light-emitting diodes (LEDs) exhibit enhancements of their optical emission power by as much as 64% over an as-grown LED. The optical performances and mechanisms of the photonic crystal LEDs are investigated with the aid of rigorous coupled wave analysis and finite-difference time-domain simulations.
Temperature dependent upconversion luminescence of Yb/Er codoped NaYF4 nanocrystals
Authors: Wu Keyue , Cui Jingbiao , Kong Xiangxing , Wang Yunjun
Published: September 2011

The upconversion luminescence of Yb/Er codoped NaYF4 nanocrystals was investigated at different temperatures from 295 to 10 K. The temperature dependent luminescence intensity of the nanocrystals showed very different behaviors as compared to their bulk powders. The intensity of the green emissions initially increased and then decreased as the measurement temperature was decreased. The experimental data were explained in terms of enhanced relaxation processes in the doped nanocrystals, which may need to be taken into account in many potential applications such as thermal sensing and biosensors.
Study of defects in an electroresistive Au/La2/3Sr1/3MnO3/SrTiO3(001) heterostructure by positron annihilation
Authors: Ferragut R. , Dupaquier A. , Brivio S. , Bertacco R. , Egger W.
Published: September 2011

Defects in an ultrathin Au/La2/3Sr1/3MnO3/SrTiO3 (Au/LSMO/STO) heterostructure displaying electroresistive behavior were studied using variable energy positron annihilation spectroscopy. Vacancy-like defects were found to be the dominant positron traps in the LSMO and STO thin perovskite oxides with a number density >1017 cm−3 and 2 × 1017 cm−3 in the STO substrate. High defect density was revealed by strong positron trapping at the Au/LSMO interface. Oxygen deficiency in LSMO would be the main source of these traps. Besides, a low density of sub-nano voids of ∼6 Å was found in the substrate and in the thin LSMO/STO films.
Melting and mechanical properties of polymer grafted lipid bilayer membranes
Authors: Thakkar Foram M. , Ayappa K. G.
Published: September 2011

The influence of polymer grafting on the phase behavior and elastic properties of two tail lipid bilayers have been investigated using dissipative particle dynamics simulations. For the range of polymer lengths studied, the Lc to Lα transition temperature is not significantly affected for grafting fractions, Gf between 0.16 and 0.25. A decrease in the transition temperature is observed at a relatively high grafting fraction, Gf = 0.36. At low temperatures, a small increase in the area per head group, ah, at high Gf leads to an increase in the chain tilt, inducing order in the bilayer and the solvent. The onset of the phase transition occurs with the nucleation of small patches of thinned membrane which grow and form continuous domains as the temperature increases. This region is the co-existence region between the Lβ(thick) and the Lα(thin) phases. The simulation results for the membrane area expansion as a function of the grafting density conform extremely well to the scalings predicted by self-consistent mean field theories. We find that the bending modulus shows a small decrease for short polymers (number of beads, Np = 10) and low Gf, where the influence of polymer is reduced when compared to the effect of the increased ah. For longer polymers (Np > 15), the bending modulus increases monotonically with increase in grafted polymer. Using the results from mean field theory, we partition the contributions to the bending modulus from the membrane and the polymer and show that the dominant contribution to the increased bending modulus arises from the grafted polymer.
Effects of strain, d-band filling, and oxidation state on the bulk electronic structure of cubic 3d perovskites
Authors: Akhade Sneha A. , Kitchin John R.
Published: September 2011

The properties of the d-band structure of the transition metal atom in cubic LaBO3 and SrBO3 perovskites (where B = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) and their dependence on strain, d-band filling, and oxidation state were investigated using density functional theory calculations and atom-projected density of states. The strain dependence of the d-band width is shown to depend systematically on the size of the B atom. We show that the transition metal d-band width and center are linearly correlated with each other in agreement with a rectangular band model. A simple matrix element formalism based on the solid state table can readily predict the strain dependence of the d-band width.
Aqueous electrolytes confined within functionalized silica nanopores
Authors: Videla Pablo E. , Sala Jonàs , Martí Jordi , Guàrdia Elvira , Laria Daniel
Published: September 2011

Molecular dynamics simulations have been carried out to investigate structural and dynamical characteristics of NaCl aqueous solutions confined within silica nanopores in contact with a “bulk-like” reservoir. Two types of pores, with diameters intermediate between 20 Å and 37.5 Å, were investigated: The first one corresponded to hydrophobic cavities, in which the prevailing wall-solution interactions were of the Lennard-Jones type. In addition, we also examined the behavior of solutions trapped within hydrophilic cavities, in which a set of unsaturated O-sites at the wall were transformed in polar silanol Si–OH groups. In all cases, the overall concentrations of the trapped electrolytes exhibited important reductions that, in the case of the narrowest pores, attained 50% of the bulk value. Local concentrations within the pores also showed important fluctuations. In hydrophobic cavities, the close vicinity of the pore wall was coated exclusively by the solvent, whereas in hydrophilic pores, selective adsorption of Na+ ions was also observed. Mass and charge transport were also investigated. Individual diffusion coefficients did not present large modifications from what is perceived in the bulk; contrasting, the electrical conductivity exhibited important reductions. The qualitative differences are rationalized in terms of simple geometrical considerations.
A molecular Debye-Hückel theory and its applications to electrolyte solutions
Authors: Xiao Tiejun , Song Xueyu
Published: September 2011

In this report, a molecular Debye-Hückel theory for ionic fluids is developed. Starting from the macroscopic Maxwell equations for bulk systems, the dispersion relation leads to a generalized Debye-Hückel theory which is related to the dressed ion theory in the static case. Due to the multi-pole structure of dielectric function of ionic fluids, the electric potential around a single ion has a multi-Yukawa form. Given the dielectric function, the multi-Yukawa potential can be determined from our molecular Debye-Hückel theory, hence, the electrostatic contributions to thermodynamic properties of ionic fluids can be obtained. Applications to binary as well as multi-component primitive models of electrolyte solutions demonstrated the accuracy of our approach. More importantly, for electrolyte solution models with soft short-ranged interactions, it is shown that the traditional perturbation theory can be extended to ionic fluids successfully just as the perturbation theory has been successfully used for short-ranged systems.
Dependence of exchange coupling direction on cooling-field strength
Authors: Son Jangyup , Lee Soogil , Lee Sangho , Kim Sanghoon , Hong Jongill
Published: September 2011

We studied the dependence of exchange coupling on cooling-field strength in an exchange-biased spin valve with a synthetic antiferromagnetic layer by experiment and theory. Our theory calculates magnetic anisotropy energies in each magnetic layer composing the spin valve during the field-cooling process, finds the minimum state of total energy, and explains how the magnetizations in the layers interact with one another during field-cooling under various cooling-field strengths. Calculations based on the theory well match results of the experimental measurements. Our observation shows that one has to carefully choose the cooling-field strength optimal for designing exchange-biased spin devices having a synthetic antiferromagnetic layer; otherwise the exchange coupling direction can significantly deviate from the cooling-field direction, which impairs performance.
Relativistic Bernstein waves in a degenerate plasma
Authors: Ali Muddasir , Hussain Azhar , Murtaza G.
Published: September 2011

Bernstein mode for a relativistic degenerate electron plasma is investigated. Using relativistic Vlasov-Maxwell equations, a general expression for the conductivity tensor is derived and then employing Fermi-Dirac distribution function a generalized dispersion relation for the Bernstein mode is obtained. Two limiting cases, i.e., non-relativistic and ultra-relativistic are discussed. The dispersion relations obtained are also graphically presented for some specific values of the parameters depicting how the propagation characteristics of Bernstein waves as well as the Upper Hybrid oscillations are modified with the increase in plasma number density.
Areal density evolution of isolated surface perturbations at the onset of x-ray ablation Richtmyer-Meshkov growth
Authors: Loomis E. N. , Braun D. , Batha S. H. , Sorce C. , Landen O. L.
Published: September 2011

Isolated defects on inertial confinement fusion ignition capsules are a concern as defects taller than a few hundred nanometers are calculated to form jets of high-Z material, which enter the main fuel. If this mixing of high-Z material is not controlled, a serious degradation in thermonuclear burn can occur. A path towards controlling the growth of defects on the outer surface of plastic capsules is currently under development, but requires accurate predictions of defect evolution driven by the early time ablative Richtmyer-Meshkov (RM) effect. The chief uncertainty is the Equation of State (EOS) for polystyrene and its effect on ablative RM. We report on measurements of the growth of isolated defects made at the onset of ablative RM oscillations driven by x-ray ablation to differentiate between EOS models used in design calculations. Experiments at the OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] used on-axis area backlighting radiography and x-ray framing cameras to determine bump areal densities at discrete times. Bumps 12 and 14 μm tall and 33 μm FWHM were found to grow to 2 × their initial areal density by 3 ns after the start of the drive laser pulse. Shock speed measurements established target conditions resulting from the ablation process. The tabular LEOS 5310 [D. Young and E. Corey, J. Appl. Phys. 78, 3748 (1995)] model shows good agreement with measured shock speeds and bump growth whereas the QEOS model [R. More et al., Phys. Fluids 31, 3059 (1988)] over predicts shock speed and under predicts bump growth by 6×. Differences in ablative RM behavior were also found for x-ray ablation compared to laser ablation, which result in an overestimation (or non-existence) of oscillation frequency for x-ray ablation as predicted by theory.
Micro axial tomography: A miniaturized, versatile stage device to overcome resolution anisotropy in fluorescence light microscopy
Authors: Staier Florian , Eipel Heinz , Matula Petr , Evsikov Alexei V. , Kozubek Michal , Cremer Christoph , Hausmann Michael
Published: September 2011

With the development of novel fluorescence techniques, high resolution light microscopy has become a challenging technique for investigations of the three-dimensional (3D) micro-cosmos in cells and sub-cellular components. So far, all fluorescence microscopes applied for 3D imaging in biosciences show a spatially anisotropic point spread function resulting in an anisotropic optical resolution or point localization precision. To overcome this shortcoming, micro axial tomography was suggested which allows object tilting on the microscopic stage and leads to an improvement in localization precision and spatial resolution. Here, we present a miniaturized device which can be implemented in a motor driven microscope stage. The footprint of this device corresponds to a standard microscope slide. A special glass fiber can manually be adjusted in the object space of the microscope lens. A stepwise fiber rotation can be controlled by a miniaturized stepping motor incorporated into the device. By means of a special mounting device, test particles were fixed onto glass fibers, optically localized with high precision, and automatically rotated to obtain views from different perspective angles under which distances of corresponding pairs of objects were determined. From these angle dependent distance values, the real 3D distance was calculated with a precision in the ten nanometer range (corresponding here to an optical resolution of 10–30 nm) using standard microscopic equipment. As a proof of concept, the spindle apparatus of a mature mouse oocyte was imaged during metaphase II meiotic arrest under different perspectives. Only very few images registered under different rotation angles are sufficient for full 3D reconstruction. The results indicate the principal advantage of the micro axial tomography approach for many microscopic setups therein and also those of improved resolutions as obtained by high precision localization determination.
Distortion in the thermal noise spectrum and quality factor of nanomechanical devices due to finite frequency resolution with applications to the atomic force microscope
Authors: Sader John E. , Sanelli Julian , Hughes Barry D. , Monty Jason P. , Bieske Evan J.
Published: September 2011

The thermal noise spectrum of nanomechanical devices is commonly used to characterize their mechanical properties and energy dissipation. This spectrum is measured from finite time series of Brownian motion of the device, which is windowed and Fourier transformed. Here, we present a theoretical and experimental investigation of the effect of such finite sampling on the measured device quality factor. We prove that if no spectral window is used, the thermal noise spectrum retains its original Lorentzian distribution but with a reduced quality factor, indicating an apparent enhancement in energy dissipation. A simple analytical formula is derived connecting the true and measured quality factors – this enables extraction of the true device quality factor from measured data. Common windows used to reduce spectral leakage are found to distort the (true) Lorentzian shape, potentially making fitting problematic. These findings are expected to be of particular importance for devices with high quality factors, where spectral resolution can be limited in practice. Comparison and validation using measurements on atomic force microscope cantilevers are presented.
Reactive molecular dynamics simulation of early stage of dry oxidation of Si (100) surface
Authors: Pamungkas Mauludi Ariesto , Joe Minwoong , Kim Byung-Hyun , Lee Kwang-Ryeol
Published: September 2011

Initial stage of oxidation of Si (100) surface by O2 molecules was investigated in atomic scale by molecular dynamics (MD) simulation at 300 K and 1200 K without external constraint on the oxygen molecules. A reactive force field was used for the simulation to handle charge variation as well as breaking and forming of the chemical bonds associated with the oxidation reaction. Results of the present simulation are in good agreement with previous first principle calculations and experimental observations: the oxygen molecules spontaneously dissociated on the Si (100) surface and reacted with Si first layer without energy barrier. The simulation also exhibited that the reacted oxygen preferentially located in the back bonds of the surface dimer. Consecutive oxidation simulation with 300 O2 molecules showed that the diffusion of oxygen atom into the subsurface of clean Si surface can occur during very short time of the present oxidation simulation. The present MD simulation also revealed that the oxidation at 300 K results in more stoichiometric oxide layer than that at 1200 K.
Cerenkov and cyclotron Cerenkov instabilities in a dielectric loaded parallel plate waveguide sheet electron beam system
Authors: Zhao Ding , Ding Yaogen
Published: September 2011

A dielectric loaded parallel plate waveguide sheet electron beam system can be taken as a reliable model for the practical dielectric loaded rectangular waveguide sheet beam system that has a transverse cross section with a large width to height ratio. By using kinetic theory, the dispersion equations for Cerenkov and cyclotron Cerenkov instabilities in the parallel plate waveguide sheet beam system have been obtained rigorously. The dependences of the growth rate of both instabilities on the electric and structural parameters have also been investigated in detail through numerical calculations. It is worthwhile to point out that adopting an electron beam with transverse velocity can evidently improve the growth rate of Cerenkov instability, which seems like the case of cyclotron Cerenkov instability.