bluuebunny/crossref_metadata_2025 huggingface dataset:
Femtosecond Raman spectra of cis-stilbene and trans-stilbene with isotopomers in solution
Authors: Dobryakov A. L. , Ioffe I. , Granovsky A. A. , Ernsting N. P. , Kovalenko S. A.
Published: December 2012

Femtosecond stimulated Raman spectra of trans-stilbene (D0), its isotopomers D2, D10, D12, 13C2 and of cis-stilbene in hexane are measured in the ground (S0) and excited (S1) electronic states. The ground 13C2 and excited D12 spectra are presented for the first time; the excited cis-spectra differ substantially from previously published ones. S1 Raman bands of trans-stilbene are 20 cm−1 wide corresponding to ∼1 ps vibrational dephasing. For cis-stilbene the bands are broadened to 40 cm−1 reflecting a short excited-state lifetime of 0.3 ps, in agreement with transient absorption data. From a dynamic shift of the 1569 cm−1 band, pump-induced intramolecular cooling is estimated to be less than 20 K. Many S1 Raman lines are detected for the first time. Vibrational spectra are calculated at MP2/cc-pVTZ (for S0) and XMCQDPT2/cc-pVTZ (for S1) levels of theory. Experimental and computational results can be used for a re-evalution of Rice-Ramsberger-Kassel-Marcus (RRKM) predictions for this famous photoisomeration reaction.
On the elastic, elastic-plastic properties of Au nanowires in the range of diameter 1-200 nm
Authors: Deb Nath S. K. , Kim Sung-Gaun
Published: December 2012

In the present study, we obtain Young's modulus and yield strength of 〈100〉 Au nanowire in the range of diameters 1-30 nm by tension and bending tests using molecular dynamics simulations. Double clamped Au nanowire is bended applying a point load at its middle span using cylindrical indenter by the atomistic approach. The superiority of the present bending technique is highlighted by analyzing the distribution of Von Misses stress of the present bending Au nanowire by 3D finite element modeling. First, Young's modulus and yield strength of Au nanowires are determined using classical theory of continuum mechanics. Then the obtained Young's modulus and yield strength of Au nanowires are corrected using 3D finite element modeling based on inverse technique [Deb Nath et al. Appl. Phys. A 103(2), 493 (2011) and Tohmyoh et al. Appl. Phys. A 103(2), 285 (2011)]. Effects of anisotropy on the tension and bending stiffness, tension and bending strength of Au nanowires are also discussed with graphs. Effects of temperature on the tension and bending stiffness, tension and bending strength of Au nanowires are discussed. Effects of vertical displacement of the indenter on the mid span of double clamped Au nanowires on the bending stiffness and strength during molecular dynamics simulation are discussed. Besides, the obtained Young's modulus and yield strength of Au nanowires by Wu et al. [Nature Mater. 4, 525 (2005)] in the range of diameters 40 to 200 nm using the theory of classical continuum mechanics are corrected using the 3D finite element modeling based on inverse technique [Deb Nath et al. Appl. Phys. A 103(2), 493 (2011) and Tohmyoh et al. Appl. Phys. A 103(2), 285 (2011)].
Anomalous resistive switching phenomenon
Authors: Mojarad Shahin A. , Goss J. P. , Kwa Kelvin S. K. , Petrov Peter K. , Zou Bin , Alford Neil , O'Neill Anthony
Published: December 2012

Resistive switching was observed in Pt/SrTiO3/Pt capacitor devices. The switching depends on both the amplitude and polarity of the applied voltage and cannot be described as either bipolar or unipolar resistive switching. We term this behavior antipolar due to the opposite polarity of the set voltage relative to the previous reset voltage. A model based on electron injection by tunneling at interfaces and a Poole-Frenkel mechanism through the bulk is proposed. This model is quantified by use of a simple mathematical equation to simulate the experimental results.
Mechanisms of dissipation of an oscillating quartz tuning fork immersed in He II at high pressures
Authors: Gritsenko I. A. , Zadorozhko A. A. , Sheshin G. A.
Published: December 2012

The dissipative processes that occur with immersing a vibrating tuning fork in superfluid helium are investigated. The tuning forks resonance width Δf of frequencies from 32 to 97 kHz was measured in the temperature range from 0.2 to 2.5 K and He II pressure from SVP to 24.9 atm. Some of the tuning forks were in the original can (closed tuning fork), and for some tuning forks the can was either completely or partially removed (opened fork). We found that for the open tuning forks two dissipation mechanisms are clearly revealed in the temperature dependence of Δf, namely, acoustic radiation and scattering of ballistic thermal excitations at low temperatures, and viscous friction at high temperatures. At low temperatures (below ∼ 0.8 K) acoustic dissipation dominates, and the model of quadrupole oscillator for a tuning fork can be applied. We found that acoustic radiation for closed tuning forks is less effective and appears at lower temperatures. The first experimental data on dissipative processes in the quartz tuning fork–He II system at increased liquid pressures are obtained. It is shown that, for high frequency tuning forks the resonance bandwidth decreases with increasing pressure, i.e., with increasing wavelength of sound λ, according to the law λ−5. At low frequencies and low temperatures, with increasing mean free path of thermal excitations the resonance bandwidth is well described by the model of ballistic scattering.
Resistive and magnetoresistive properties of compacted CrO2 powders with different types of intergranular dielectric layers
Authors: Dalakova N. V. , Belevtsev B. I. , Beliayev E. Yu. , Bludov A. N. , Pashchenko V. N. , Osmolovsky M. G. , Osmolovskaya O. M.
Published: December 2012

Resistive, magnetoresistive and magnetic properties of four types of pressed CrO2 powders synthesized from chromic anhydride by the hydrothermal method were investigated. The new synthesis method allowed controlling the thickness of dielectric shells. The powders consisted of rounded particles (≈120 nm in dia.) or acicular crystals (≈22.9 nm in dia. and 302 nm in length). The particles were covered with a surface dielectric shell of different thickness and type (e.g. β-CrOOH oxyhydroxide or chromium oxide Cr2O3). The influence of the properties and the thickness of intergranular dielectric layers as well as the shape of CrO2 particles on the tunneling resistance and magnetoresistance (MR) of the pressed powders was studied. It was found that at low temperatures all the investigated samples displayed a nonmetallic temperature dependence of resistance and a giant negative MR. The maximal values of MR at T ≈ 5 K were found to be approx. 37% in relatively low magnetic fields (0.5 T). The MR decreased rapidly with increasing temperature (down to approx. 1% in 1 T at T ≈ 200 K). At low temperatures the powders with acicular particles exhibited a new type of MR hysteresis and nonmonotonous dependence of MR with increasing magnetic field. A nonmonotonous temperature dependence Hp(T), where Hp is the field in which the resistance is maximal, mismatch between the values of Hp and coercive force Hc, and the anisotropy of MR as a function of mutual orientation of transport current and magnetic field were observed.
Photon generation in ferromagnetic point contacts
Authors: Kadigrobov A. M. , Shekhter R. I. , Jonson M.
Published: December 2012

We show theoretically that a significant spin accumulation can occur in electric point contacts between two ferromagnetic electrodes with different magnetizations. Under appropriate conditions an inverse population of spin-split electronic levels results in stimulated emission of photons in the presence of a resonant electromagnetic field. The intensity of the emitted radiation can be several orders of magnitude higher than in typical semiconductor laser materials for two reasons. (1) The density of conduction electrons in a metal point contact is much larger than in semiconductors. (2) The strength of the coupling between the electron spins and the electromagnetic field that is responsible for the radiative spin-flip transitions is set by the magnetic exchange energy and can therefore be very large, as suggested by Kadigrobov et al. [Europhys. Lett. 67, 948 (2004)].
Polarization bremsstrahlung study of the surface of xenon clusters: The pseudocrystalline state
Authors: Gnatchenko E. V. , Nechay A. N. , Tkachenko A. A. , Samovarov V. N.
Published: December 2012

The half width of the polarization bremsstrahlung profile is measured as a function of the number W(N) of atoms in free clusters of xenon. The clusters are excited by electrons with energies of 0.7 and 0.3 keV, which preferentially probe the core and surface, respectively, of the clusters. It turns out the W(N) curves are entirely different for electrons with energies of 0.7 and 0.3 keV. An analysis of this difference indicates that the crystalline fcc core of clusters with sizes N ≈ 2000/8000 atoms/cluster (R ≈ 30–50 Å) is covered with a noncrystalline shell. This layer is examined in terms of the possible formation of glassy and pseudocrystalline states. For the first time a pseudocrystalline state is found in inert gas clusters. In this state the system is structurally unstable and constantly fluctuates between the liquid and solid phases.
Magnetotransport studies of SiGe-based p-type heterostructures: Problems with the determination of effective mass
Authors: Berkutov I. B. , Andrievskii V. V. , Komnik Yu. F. , Kolesnichenko Yu. A. , Morris R. J. H. , Leadley D. R. , Mironov O. A.
Published: December 2012

The use of Shubnikov-de Haas oscillations for determining effective mass is illustrated by a study of the magnetotransport properties of the two-dimensional hole gas in Si1−xGex (x = 0.13, 0.36, 0.95, 0.98) quantum wells. For some samples the data cannot be fitted to standard theoretical curves in which the scattering of charge carriers is described by the conventional Dingle factor. The reasons for the discrepancies between the experiment the theory are: (i) the effect of spin splitting on the amplitude of the peak in the SdH oscillations; (ii) extra broadening of the Landau levels attributable to an inhomogeneous distribution of the carrier concentration; (iii) the coexistence of short and long-range scattering potentials; and, (iv) population of the second energy level in the quantum well. Ways of calculating the effective hole masses m* for all these cases are presented and values of m* are found for the heterostructures studied here.
Analysis of cathode geometry to minimize cathode erosion in direct current microplasma jet
Authors: Causa Federica , Ghezzi Francesco , Dellasega David , Caniello Roberto , Grosso Giovanni
Published: December 2012

Microplasma jets are now widely used for deposition, etching, and materials processing. The present study focuses on the investigation of the influence of cathode geometry on deposition quality, for microplasma jet deposition systems in low vacuum. The interest here is understanding the influence of hydrogen on sputtering and/or evaporation of the electrodes. Samples obtained with two cathode geometries with tapered and rectangular cross-sections have been investigated experimentally by scanning electron microscopy and energy dispersion X-ray spectroscopy. Samples obtained with a tapered-geometry cathode present heavy contamination, demonstrating cathode erosion, while samples obtained with a rectangular-cross-section cathode are free from contamination. These experimental characteristics were explained by modelling results showing a larger radial component of the electric field at the cathode inner wall of the tapered cathode. As a result, ion acceleration is larger, explaining the observed cathode erosion in this case. Results from the present investigation also show that the ratio of radial to axial field components is larger for the rectangular geometry case, thus, qualitatively explaining the presence of micro-hollow cathode discharge over a wide range of currents observed in this case. In the light of the above findings, the rectangular cathode geometry is considered to be more effective to achieve cleaner deposition.
Graphene/Li-ion battery
Authors: Kheirabadi Narjes , Shafiekhani Azizollah
Published: December 2012

Density function theory calculations were carried out to clarify storage states of Lithium (Li) ions in graphene clusters. The adsorption energy, spin polarization, charge distribution, electronic gap, surface curvature, and dipole momentum were calculated for each cluster. Li-ion adsorbed graphene, doped by one Li atom is spin polarized, so there would be different gaps for different spin polarization in electrons. Calculation results demonstrated that a smaller cluster between each two larger clusters is preferable, because it could improve graphene Li-ion batteries; consequently, the most proper graphene anode structure has been proposed.
Cyclic impulsive compression loading along the radial and tangential wood directions causes localized fatigue
Authors: Salmi Ari , Montonen Risto , Salminen Lauri I. , Suuronen Jussi-Petteri , Serimaa Ritva , Hæggström Edward
Published: December 2012

We report for the first time on the existence of a localized reduction in elasticity caused by repeated compression impaction applied along the tangential wood direction. Previous research indicates that localized strain profiles are generated by such cyclic impacting on wood along its radial direction. This finding is significant for the paper/board-making industry where wood is exposed to cyclic unipolar compression during grinding. However, the effect of the impacting direction, with respect to the orientation of the annual rings, on the localization phenomenon is unknown. In addition, the shape of the developing fatigue layer is unclear. We revisit the localization phenomenon with a focus on tangential impacting. We employed ultrasonics and x-ray tomography to quantify the induced fatigue. An interlacing technique increased the precision of the ultrasonic stiffness depth profiling technique. We studied both radial and tangential wood annual ring geometries. We used ultrasound to quantify the drop in shear modulus resulting from impacting. Both radial and tangential geometries featured strain localization, but the shape of the fatigued layer was different in the radial and tangential wood geometries (steeper profile in the radial geometry). The fatiguing reduced the shear modulus. These results tell us about the ratio of the number of hits that need to be delivered along the radial and tangential direction of the ground wood, respectively. This insight may translate into a design pattern for an energy saving grind stone surface.
Specific features of relaxation of magnons in an easy-plane antiferromagnet in the framework of the sigma-model
Authors: Butrim V. I. , Ivanov B. A.
Published: December 2012

It is shown that for the anisotropic sigma-model with an easy-plane anisotropy abnormal behavior of damping of quasi-particles (magnons) takes place. For activationless elementary excitations with linear dispersion at small wave vectors, the damping rate is proportional to the forth power of frequency (and not quadratic, as for standard Goldstone quasiparticles, e.g., for a purely isotropic sigma-model), which can be referred to as “super-Goldstone” damping behavior. As usual, damping for the second branch of quasi-particles, having finite activation, has a finite value for small wave vectors, but it also exhibits a peculiarity, namely, at low temperatures it has a power dependence on the temperature, and does not contain the standard exponentially small temperature multiplier.
Germano identity-based subgrid-scale modeling: A brief survey of variations on a fertile theme
Authors: Meneveau Charles
Published: December 2012

It has now been over 20 years since the introduction of the Germano identity. Mostly, the identity has been applied to closures for the subgrid-scale fluxes required in large eddy simulations in the bulk of turbulent flows. However, the basic ideas underlying the Germano identity can be applied in various other contexts. In recent years a number of such generalizations have been developed, and several of these are surveyed in this paper. The survey is based on an interpretation of the Germano identity stating that the sum of resolved and modeled contributions to basic quantities of intrinsic physical interest must be independent of filter scale. The focus of this survey is on the conceptual bases of the various generalizations and their common features, as a way of pointing to possible further extensions.
Biomimetic flow control based on morphological features of living creatures
Authors: Choi Haecheon , Park Hyungmin , Sagong Woong , Lee Sang-im
Published: December 2012

Despite the long history of biomimetics (or biomimetic engineering), a scientific discipline of implementing nature-inspired ideas to engineering systems for their performance enhancement, successful developments have been made only recently, especially in the field of flow control. In the present paper, we discuss flow controls based on the biomimetic approach, paying special attention to surface morphology of living creatures, to develop novel concepts or devices for drag reduction and aerodynamic performance enhancement. We consider two types of flow control devices: (1) devices attached or added to wing surfaces for high aerodynamic performance and (2) smart surfaces for low skin friction. Several examples of successful biomimetic flow controls are presented and discussed in this paper. Further issues like the difference in the operating environments (e.g., the Reynolds number) between the biological and engineering systems are discussed. Finally, guidelines for effective integration of engineering and biology are suggested.
Energy concentration by spherical converging shocks generated in a shock tube
Authors: Kjellander Malte , Tillmark Nils , Apazidis Nicholas
Published: December 2012

Spherical converging shock waves are produced in a conventional shock tube with a circular cross-section. Initially, plane shocks are transformed into the shape of a spherical cap by means of a smoothly convergent cross-section. The wall shape in the transformation section is designed to gradually change the form of the shock wave until it approaches a spherical shape. Thereafter, the shock enters a conical section where it converges towards the apex of the cone. Numerical calculations with the axisymmetric Euler equations show that the spherical form is only slightly dependent on the initial Mach number of the plane shock within the range 1.5 < MS < 5.5, and is preserved to a close vicinity of the focal point. The test gas is heated to very high temperatures as a result of shock convergence and emits a bright light pulse at the tip of the test section. The light radiation is collected by optical fibers mounted at the tip of the convergence chamber and investigated by photometric and spectroscopic measurements. Experiments are performed with argon and nitrogen and with different initial Mach numbers. The radiation of the shock-heated argon closely resembles blackbody radiation. Fits to the experimental data result in apparent blackbody temperatures in argon of up to ∼27 000 K, some 250 ns after the focusing instant. The initial Mach number in these spectrometric runs is MS = 3.9, indicating an efficient amplification of the shock wave strength.
Numerical investigation of wavy and spiral Taylor-Görtler vortices in medium spherical gaps
Authors: Yuan Li
Published: December 2012

The spherical Couette flow between two concentric spheres with only the inner sphere rotating is simulated by solving the 3D incompressible Navier-Stokes equations with a fifth order upwind compact finite difference method. Two moderate clearance ratios, β = (R2 − R1)/R1 = 0.14 and 0.18, respectively, are chosen for comparison with previous experimental and numerical results. First, the spiral Taylor-Görtler (TG) vortex flow at Re = 1110 for β = 0.14 [W. M. Sha and K. Nakabayashi, “On the structure and formation of spiral Taylor-Görtler vortices in spherical Couette flow,” J. Fluid Mech. 431, 323–345 (2001)10.1017/S0022112000003128] is found to develop traveling waves at Re = 1800. A wavy TG vortex flow formed at low Re numbers can return to steady TG vortex as Re number is increased to a critical value Re = 6600, thus confirming the occurrence of a reverse Hopf bifurcation from limit cycle to fixed point [K. Nakabayashi, W. M. Sha, and Y. Tsuchida, “Relaminarization phenomena and external-disturbance effects in spherical Couette flow,” J. Fluid Mech. 534, 327–350 (2005)]. Second, multiple supercritical flows for β = 0.18 [M. Wimmer, “Experiments on a viscous fluid flow between concentric rotating spheres,” J. Fluid Mech. 78, 317–335 (1976)10.1017/S0022112076002462] are simulated for a wide range of Re numbers from the first instability (Re ≈ 655) up to the proximity of transition to turbulence (Re ≈ 8000). The simulation confirms Wimmer's experimental observation that a periodic 2-vortex flow coexists with the steady 0- and 1-vortex flows in certain low Re range. There is also a reverse Hopf bifurcation for this periodic wavy 2-vortex flow at Re = 2270. As Re number is further increased, the steady 0- and 2-vortex flows begin to form spiral waves in the secondary flow region for Re ⩾ 6500, while the 1-vortex flow has similar spiral disturbances for Re = 8000. Multiple higher modes with different numbers of spiral waves can be generated by using different wavenumbers in the imposed perturbation. Detailed description of these multiple higher modes is given in terms of rotational frequency, wavenumber, and spatial structure.
Experimental study on side force alleviation of conical forebody with a fluttering flag
Authors: Zhang Weiwei , Liu Xiaobo , Zhai Jian , Ye Zhengyin
Published: December 2012

Slender bodies of revolution placed at high angles of attack will produce a steady asymmetric vortex pair. This behavior is determined by the spatial instability of the flow. The asymmetric vortex pair is very sensitive to small disturbances near the tip. This paper presents a simple side force alleviation method without a power unit for slender bodies at high angles of attack. The method uses the self-excited oscillation (flutter) of a small cantilever flag as the excitation source on the cone tip. An experimental study is performed to investigate the characteristics of the asymmetric side forces of a 20° cone-cylinder slender body with and without a small flutter flag. Pressure distribution at eight cross sections of the cone forebody is measured at different free-stream velocities and different angles of attack. The process demonstrates that the excitation provided by the fluttering flag can change the asymmetric bistable flow state to a well symmetric one at high angles of attack, and that the corresponding side force and yaw moment will significantly decrease. The flutter strength of the flag will be enhanced by increasing the free-stream velocity. The effectiveness of the side force alleviation method will also be increased.
Ice polyamorphism in the minimal Mercedes-Benz model of water
Authors: Cartwright Julyan H. E. , Piro Oreste , Sánchez Pedro A. , Sintes Tomás
Published: December 2012

We investigate ice polyamorphism in the context of the two-dimensional Mercedes-Benz model of water. We find a first-order phase transition between a crystalline phase and a high-density amorphous phase. Furthermore, we find a reversible transformation between two amorphous structures of high and low density; however, we find this to be a continuous and not an abrupt transition, as the low-density amorphous phase does not show structural stability. We discuss the origin of this behavior and its implications with regard to the minimal generic modeling of polyamorphism.
Gaussian attenuation hybrid scheme applied to the Ernzerhof-Perdew exchange hole model (Gau-PBEh)
Authors: Song Jong-Won , Yamashita Koichi , Hirao Kimihiko
Published: December 2012

Recently, we developed a Gaussian attenuation (Gau) scheme for solid-state bandgap calculation that uses a two-electron Gaussian function operator to include short-range Hartree-Fock exchange and combined it with the long-range Perdew-Burke-Ernzerhof (PBE) exchange correlation functional (Gau-PBE). Here, we apply the Ernzerhof-Perdew exchange hole (EP) model of PBE (PBEh) as a long-range density functional theory (DFT) exchange part to the Gau scheme (Gau-PBEh). We found that applying the EP model to the Gau scheme improves atomization energies and solid-state lattice constants and that the exact exchange included using the Gau scheme plays a critical role in simultaneously reproducing solid-state bandgaps and barrier heights. In addition, Gau-PBEh takes nearly the same computation time for bandgap calculations as Gau-PBE, implying less than 60% of the time taken in Heyd-Scuseria-Ernzerhof hybrid DFT functional calculations.
The influence of internal rotational barriers and temperature on static and dynamic properties of bulk atactic polystyrene
Authors: Xie Shi-Jie , Qian Hu-Jun , Lu Zhong-Yuan
Published: December 2012

We present molecular dynamics simulations of a chemically realistic model as well as a quasi-freely rotating chain model for bulk atactic polystyrene in a temperature range from 240 to 500 K to characterize the role of temperature and internal rotational barriers on static and dynamics properties of bulk polystyrene. We demonstrate that on different length scales, the change of structure shows different behavior upon cooling, and the internal rotational barriers play a similar role as temperature in this respect. We also show that larger plateau value of particle mean-squared displacement does not comply with the cage size predicted by the mode-coupling theory. It can be attributed to large steric hindrance between styrene units in the system. When the temperature is decreased, dynamic heterogeneity of conformational transition is found to become increasingly important for the conformational relaxation. Moreover, we have established a relation among the cage effect, the dynamic heterogeneity, and the conformational relaxation on the time scale of α- and β-relaxations.
Nonlinear dynamic heat capacity of a bead-spring polymeric glass former
Authors: Brown Jonathan R. , McCoy John D.
Published: December 2012

Nonlinear dynamics of a simple bead-spring glass-forming polymer were studied with molecular dynamics simulations. The energy response to sinusoidal variations in the temperature was tracked in order to evaluate the dynamic heat capacity. The amplitude dependence of the response is the focus of the current paper where pronounced nonlinear behavior is observed for large amplitudes in the temperature “driving force.” We generalize the usual linear response analysis to the nonlinear regime so that higher order terms in the Fourier series of the energy response can be compactly analyzed. This is done by grouping all Fourier terms contributing to entropy generation into a “loss” contribution and the remainder yields the “storage” term. Finally, the bead-spring system is mapped onto three simpler models. First is a potential energy inspired “trap” model consisting of interconnected potential energy meta-basins and barriers. Second is the Tool-Narayanaswamy-Moynihan (TNM) model. Third is a version of the TNM model with a temperature dependent heat capacity. Qualitatively similar nonlinear behaviors are observed in all cases.
Effect of the counter cation on the third order nonlinearity in anionic Au dithiolene complexes
Authors: Iliopoulos K. , El-Ghayoury A. , Derkowska B. , Ranganathan A. , Batail P. , Gindre D. , Sahraoui B.
Published: December 2012

In this work, we present the third order nonlinear optical investigation of two gold complexes, which differ by the nature of the counter cations. The impact of the different design in the architecture through a set of hydrogen bonds in the case of Au-Mel of the systems on the nonlinearity has been studied by means of the Z-scan setup under 532 nm, 30 ps laser excitation, allowing for the determination of the nonlinear absorption and refraction of the samples. Significant modification of the nonlinear optical response between the two metal complexes has been found suggesting a clear effect of the counter cation.
Electric field induced needle-pulsed arc discharge carbon nanotube production apparatus: Circuitry and mechanical design
Authors: Kia Kaveh Kazemi , Bonabi Fahimeh
Published: December 2012

A simple and low cost apparatus is reported to produce multiwall carbon nanotubes and carbon nano-onions by a low power short pulsed arc discharge reactor. The electric circuitry and the mechanical design details and a micro-filtering assembly are described. The pulsed-plasma is generated and applied between two graphite electrodes. The pulse width is 0.3 μs. A strong dc electric field is established along side the electrodes. The repetitive discharges occur in less than 1 mm distance between a sharp tip graphite rod as anode, and a tubular graphite as cathode. A hydrocarbon vapor, as carbon source, is introduced through the graphite nozzle in the cathode assembly. The pressure of the chamber is controlled by a vacuum pump. A magnetic field, perpendicular to the plasma path, is provided. The results show that the synergetic use of a pulsed-current and a dc power supply enables us to synthesize carbon nanoparticles with short pulsed plasma. The simplicity and inexpensiveness of this plan is noticeable. Pulsed nature of plasma provides some extra degrees of freedom that make the production more controllable. Effects of some design parameters such as electric field, pulse frequency, and cathode shape are discussed. The products are examined using scanning probe microscopy techniques.
Polarization of ions under geometric confinement in the strong-interacting regime: A two-ring model study
Authors: Chen Yiing-Rei , Ng Sio-Kit , Lee Chi-Lun
Published: December 2012

We study the polarization and electrostatic interactions of an ionic system under geometric confinement in the strong-interacting regime. The geometric confinement is introduced via a simple two-ring model, where ions of each species are confined on a ring, respectively. The observed polarization curve exhibits staircase transitions in the low-temperature regime, due to the crossover between energy local minima. We examine the criterion for the validity of the linear response theory and introduce a simple two-state picture that illustrates the signatures of the crossover phenomena.
Reaction dynamics with the multi-layer multi-configurational time-dependent Hartree approach: H + CH4 → H2 + CH3 rate constants for different potentials
Authors: Welsch Ralph , Manthe Uwe
Published: December 2012

The multi-layer extension of the multi-configurational time-dependent Hartree (MCTDH) approach is applied to the investigation of elementary bimolecular chemical reactions. Cumulative reaction probabilities and thermal rate constants of the H + CH4 → H2 + CH3 reaction are calculated using flux correlation functions and the quantum transition state concept. Different coordinate systems and potential energy surfaces (PESs) are studied. The convergence properties of different layerings are investigated and the efficiency of multi-layer MCTDH approach is compared to the standard MCTDH approach. It is found that the multi-layer approach can decrease the numerical effort by more than an order of magnitude. The increased efficiency resulting from the multi-layer MCTDH approach is crucial for quantum dynamical calculations on recent global H + CH4 → H2 + CH3 PESs, e.g., the ZBB3-PES [Z. Xie, J. M. Bowman, and X. Zhang, J. Chem. Phys. 125, 133120 (2006)10.1063/1.2238871] based on permutational invariant polynomials, which are numerically more demanding than earlier PESs. The results indicate that an accurate description of all transition state frequencies is important to obtain accurate thermal rate constants.
Correction for dynamic bias error in transmission measurements of void fraction
Authors: Andersson P. , Sundén E. Andersson , Svärd S. Jacobsson , Sjöstrand H.
Published: December 2012

Dynamic bias errors occur in transmission measurements, such as X-ray, gamma, or neutron radiography or tomography. This is observed when the properties of the object are not stationary in time and its average properties are assessed. The nonlinear measurement response to changes in transmission within the time scale of the measurement implies a bias, which can be difficult to correct for. A typical example is the tomographic or radiographic mapping of void content in dynamic two-phase flow systems. In this work, the dynamic bias error is described and a method to make a first-order correction is derived. A prerequisite for this method is variance estimates of the system dynamics, which can be obtained using high-speed, time-resolved data acquisition. However, in the absence of such acquisition, a priori knowledge might be used to substitute the time resolved data. Using synthetic data, a void fraction measurement case study has been simulated to demonstrate the performance of the suggested method. The transmission length of the radiation in the object under study and the type of fluctuation of the void fraction have been varied. Significant decreases in the dynamic bias error were achieved to the expense of marginal decreases in precision.
Alfvén waves in dusty plasmas with plasma particles described by anisotropic kappa distributions
Authors: Galvão R. A. , Ziebell L. F. , Gaelzer R. , de Juli M. C.
Published: December 2012

We utilize a kinetic description to study the dispersion relation of Alfvén waves propagating parallelly to the ambient magnetic field in a dusty plasma, taking into account the fluctuation of the charge of the dust particles, which is due to inelastic collisions with electrons and ions. We consider a plasma in which the velocity distribution functions of the plasma particles are modelled as anisotropic kappa distributions, study the dispersion relation for several combinations of the parameters κ∥ and κ⊥, and emphasize the effect of the anisotropy of the distributions on the mode coupling which occurs in a dusty plasma, between waves in the branch of circularly polarized waves and waves in the whistler branch.
Evidence for ultra-fast heating in intense-laser irradiated reduced-mass targets
Authors: Neumayer P. , Aurand B. , Costa Fraga R. A. , Ecker B. , Grisenti R. E. , Gumberidze A. , Hochhaus D. C. , Kalinin A. , Kaluza M. C. , Kühl T. , Polz J. , Reuschl R. , Stöhlker T. , Winters D. , Winters N. , Yin Z.
Published: December 2012

We report on an experiment irradiating individual argon droplets of 20 μm diameter with laser pulses of several Joule energy at intensities of 1019 W/cm2. K-shell emission spectroscopy was employed to determine the hot electron energy fraction and the time-integrated charge-state distribution. Spectral fitting indicates that bulk temperatures up to 160 eV are reached. Modelling of the hot-electron relaxation and generation of K-shell emission with collisional hot-electron stopping only is incompatible with the experimental results, and the data suggest an additional ultra-fast (sub-ps) heating contribution. For example, including resistive heating in the modelling yields a much better agreement with the observed final bulk temperature and qualitatively reproduces the observed charge state distribution.
Investigation of post-discharge processes in nitrogen at low pressure
Authors: Pejovic Momcilo M. , Nesic Nikola T. , Pejovic Milic M. , Brajovic Dragan V. , Denic Ivana V.
Published: December 2012

The processes which are a consequence of neutral active particles presence in post-discharge nitrogen-filled tube at 13.3 mbar pressure have been analyzed. The analysis has been performed based on the experimental data of electrical breakdown time delay as a function of afterglow period τ. The most significant parameters such as applied voltage, discharge current, time, and exposure to radiation have been varied. It has been shown that the increase in applied voltage and discharge time, as well as exposure to UV radiation, leads to the decrease of the mean value of electrical breakdown time delay t¯d. This decrease occurs for τ>70 ms, when N(S4) atoms play a dominant role in breakdown initiation. The increase in discharge current leads to the decrease of t¯d values for τ≤70 ms, when positive ions dominantly induce breakdown. The most important reactions which lead to formation of positive ions and neutral active particles are also presented in the paper.
Nonlocal theory of electromagnetic wave decay into two electromagnetic waves in a rippled density plasma channel
Authors: Sati Priti , Tripathi V. K.
Published: December 2012

Parametric decay of a large amplitude electromagnetic wave into two electromagnetic modes in a rippled density plasma channel is investigated. The channel is taken to possess step density profile besides a density ripple of axial wave vector. The density ripple accounts for the momentum mismatch between the interacting waves and facilitates nonlinear coupling. For a given pump wave frequency, the requisite ripple wave number varies only a little w.r.t. the frequency of the low frequency decay wave. The radial localization of electromagnetic wave reduces the growth rate of the parametric instability. The growth rate decreases with the frequency of low frequency electromagnetic wave.
Analytic formulae for centrifugal effects on turbulent transport of trace impurities in tokamak plasmas
Authors: Angioni C. , Casson F. J. , Veth C. , Peeters A. G.
Published: December 2012

A local gyrokinetic description of the centrifugal effects on impurity transport in tokamak plasmas is presented, which extends previous models with the inclusion of the gradient of the background toroidal angular velocity in the equilibrium distribution. The equations are implemented in a gyrokinetic code. An analytical model is derived and formulae are proposed which allow the calculation of centrifugal effects on impurity transport in the limit where centrifugal effects are large only for heavy impurities in trace concentration, but are negligible for the bulk plasma. The analytic formulae are shown to be in quantitative agreement with the numerical results and are proposed for complementing present transport models.
Bipolar snapback in junctionless transistors for capacitorless dynamic random access memory
Authors: Singh Parihar Mukta , Ghosh Dipankar , Alastair Armstrong G. , Kranti Abhinav
Published: December 2012

In this work, we analyze the snapback effect and extract the effective bipolar current gain in junctionless nanotransistors. The optimal electron and hole concentrations required to trigger and sustain bipolar snapback in junctionless transistors have been evaluated. The occurrence of snapback at lower drain bias (≅ 2 V) in junctionless devices in comparison to conventional inversion mode transistors demonstrates the enormous potential for static power reduction in capacitorless dynamic random access memories. High values (40–70) of effective bipolar current gain achieved in optimally designed junctionless transistors can be utilized to improve the sensing margin for dynamic memories.
Coalescence of liquid drops: Different models versus experiment
Authors: Sprittles J. E. , Shikhmurzaev Y. D.
Published: December 2012

The process of coalescence of two identical liquid drops is simulated numerically in the framework of two essentially different mathematical models, and the results are compared with experimental data on the very early stages of the coalescence process reported recently. The first model tested is the “conventional” one, where it is assumed that coalescence as the formation of a single body of fluid occurs by an instant appearance of a liquid bridge smoothly connecting the two drops, and the subsequent process is the evolution of this single body of fluid driven by capillary forces. The second model under investigation considers coalescence as a process where a section of the free surface becomes trapped between the bulk phases as the drops are pressed against each other, and it is the gradual disappearance of this “internal interface” that leads to the formation of a single body of fluid and the conventional model taking over. Using the full numerical solution of the problem in the framework of each of the two models, we show that the recently reported electrical measurements probing the very early stages of the process are better described by the interface formation/disappearance model. New theory-guided experiments are suggested that would help to further elucidate the details of the coalescence phenomenon. As a by-product of our research, the range of validity of different “scaling laws” advanced as approximate solutions to the problem formulated using the conventional model is established.
Raman spectrum method for characterization of pull-in voltages of graphene capacitive shunt switches
Authors: Li Peng , You Zheng , Cui Tianhong
Published: December 2012

An approach using Raman spectrum method is reported to measure pull-in voltages of graphene capacitive shunt switches. When the bias excesses the pull-in voltage, the Raman spectrum's intensity largely decreases. Two factors that contribute to the intensity reduction are investigated. Moreover, by monitoring the frequency shift of G peak and 2D band, we are able to detect the pull-in voltage and measure the strain change in graphene beams during switching.
Performance and stability of solution-based cadmium sulfide thin film transistors: Role of CdS cluster size and film composition
Authors: Salas-Villasenor A. L. , Mejia I. , Sotelo-Lerma M. , Gnade B. E , Quevedo-Lopez M. A.
Published: December 2012

Improved carrier mobility and threshold voltage (VT) stability in cadmium sulfide (CdS) thin film transistors (TFTs) were studied and attributed to larger grain clusters in thicker CdS films rather than individual crystallite size. Non-zero VT shifts (∼200 mV) in thicker films are attributed to the presence of cadmium hydroxide [Cd(OH)2] at the dielectric/CdS interface resulting from the chemical bath deposition process used to deposit the CdS films. VT and mobility analyses indicate that clusters of CdS grains have a larger impact on TFT performance and stability than the presence of impurities in the bulk of the CdS. TFTs using this fabrication method achieved mobilities of ∼22 cm2/Vs with VT of 7 V and ΔVT of <200 mV after testing. The maximum processing temperature is 100 °C which makes this process compatible with flexible substrates.
Observations of non-collective x-ray scattering in warm dense carbon plasma
Authors: Lihua Bao , Jiyan Zhang , Xiaoding Zhang , Yang Zhao , Yongkun Ding
Published: December 2012

An experiment for observing the spectrally resolved non-collective x-ray scattering in warm dense carbon plasma is presented in this paper. The experiment used Ta M-band x-rays to heat a foamed carbon cylinder sample isochorically and measured the scattering spectrum with a HOPG crystal spectrometer. The spectrum was compared with the calculation results using a Born-Mermin-approximation model. The best fitting was found at an electron temperature of Te=34 eV and an electron density of ne=1.6×1023cm−3.
Real time x-ray studies during nanostructure formation on silicon via low energy ion beam irradiation using ultrathin iron films
Authors: El-Atwani Osman , Suslova Anastassiya , DeMasi Alexander , Gonderman Sean , Fowler Justin , El-Atwani Mohamad , Ludwig Karl , Paul Allain Jean
Published: December 2012

Real time grazing incidence small angle x-ray scattering and x-ray fluorescence (XRF) are used to elucidate nanodot formation on silicon surfaces during low energy ion beam irradiation of ultrathin iron-coated silicon substrates. Four surface modification stages were identified: (1) surface roughening due to film erosion, (2) surface smoothing and silicon-iron mixing, (3) structure formation, and (4) structure smoothing. The results conclude that 2.5 × 1015 iron atoms in a 50 nm depth triggers surface nanopatterning with a correlated nanodots distance of 25 nm. Moreover, there is a wide window in time where the surface can have correlated nanostructures even after the removal of all the iron atoms from the sample as confirmed by XRF and ex-situ x-ray photoelectron spectroscopy (XPS). In addition, in-situ XPS results indicated silicide formation, which plays a role in the structure formation mechanism.
The impact of oxygen on the morphology of gas-phase prepared Au nanoparticles
Authors: Pohl D. , Surrey A. , Schultz L. , Rellinghaus B.
Published: December 2012

We present an easy procedure for the synthesis of single crystalline gold nanoparticles with a mean diameter of 4 nm using a DC-sputtering in an argon-oxygen gas mixture. Morphology population statistics have been determined to quantify the influence of oxygen. It is found that the particles undergo a structural transition from predominantly icosahedral to single crystalline particles with increasing amount of oxygen. Aberration-corrected high-resolution transmission electron microscopy investigation proves that likewise prepared single crystalline nanoparticles are defect and oxygen free. In contrast, the icosahedral particles prepared with pure argon show the presence of edge dislocations pointing to an energetic disfavoring already at these relatively small particle sizes. This morphology control of clean and uncovered Au nanoparticles provides a high application potential, e.g., for studying the influence of the particle morphology on plasmonic and catalytic properties.
Superradiant Raman laser magnetometer
Authors: Weiner Joshua M. , Cox Kevin C. , Bohnet Justin G. , Chen Zilong , Thompson James K.
Published: December 2012

We demonstrate a proof-of-principle magnetometer that relies on the active oscillation of a cold atom Raman laser to continuously map a field-sensitive atomic phase onto the phase of the radiated light. We demonstrate wideband sensitivity during continuous active oscillation, as well as narrowband sensitivity in passive Ramsey-like mode with translation of the narrowband detection in frequency using spin-echo techniques. The sensor operates with a sensitivity of 190 pT/Hz at 1 kHz and effective sensing volume of 2×10−3 mm3. Fundamental quantum limits on the magnetic field sensitivity of an ideal detector are also considered.
High sensitivity and fast response and recovery times in a ZnO nanorod array/p-Si self-powered ultraviolet detector
Authors: Hassan J. J. , Mahdi M. A. , Kasim S. J. , Ahmed Naser M. , Abu Hassan H. , Hassan Z.
Published: December 2012

High quality, vertically aligned ZnO nanorods were grown on a silicon substrate, using microwave-assisted chemical bath deposition with poly (vinyl alcohol)-Zn(OH)2 nanocomposites as seed layer. The structure and surface morphology of the prepared ZnO nanorod arrays were characterized using X-ray diffraction and scanning electron microscopy. The optical properties were assessed using photoluminescence measurements; the results showed a high-intensity UV peak, and a lower intensity, broader visible peak. Upon exposure to 395 nm light at a zero-bias voltage, the UV detector showed a high sensitivity of 8000% and fast response and recovery times of 25 and 22 ms, respectively.
Note: FixSol solvation model and FIXPVA2 tessellation scheme
Authors: Thellamurege Nandun M. , Li Hui
Published: December 2012

The FixSol conductorlike solvation model and the fixed points with variable areas (FIXPVA2) surface tessellation scheme are described. In the FixSol model, the singularity of the surface charge interaction is avoided by introducing a soft-core potential. The use of soft-core potential allows for the use of the FIXPVA2 scheme to partition the molecular surface into tesserae with desirable areas. The FixSol model is used together with biological force field to perform molecular dynamics simulation for proteins and DNA molecules in solution.