bluuebunny/crossref_metadata_2025 huggingface dataset:
Femtosecond laser and swift-ion damage in lithium niobate: A comparative analysis
Authors: García-Navarro A. , Agulló-López F. , Olivares J. , Lamela J. , Jaque F.
Published: May 2008

Relevant damage features associated with femtosecond pulse laser and swift-ion irradiations on LiNbO3 crystals are comparatively discussed. Experiments described in this paper include irradiations with repetitive femtosecond-laser pulses (800 nm, 130 fs) and irradiation with O, F, Si, and Cl ions at energies in the range of 0.2–1 MeV/amu where electronic stopping power is dominant. Data are semiquantitatively discussed by using a two-step phenomenological scheme. The first step corresponds to massive electronic excitation either by photons (primarily three-photon absorption) or ions (via ion-electron collisions) leading to a dense electron-hole plasma. The second step involves the relaxation of the stored excitation energy causing bond breaking and defect generation. It is described at a phenomenological level within a unified thermal spike scheme previously developed to account for damage by swift ions. A key common feature for the two irradiation sources is a well-defined intrinsic threshold in the deposited energy density Uth required to initiate observable damage in a pristine crystal: Uth≈1.3×104−2×104J/cm3 for amorphization in the case of ions and Uth≈7×104 J/cm3 for ablation in the case of laser pulses. The morphology of the heavily damaged regions (ion-induced tracks and laser-induced craters) generated above threshold and its evolution with the deposited energy are also comparatively discussed. The data show that damage in both types of experiments is cumulative and increases on successive irradiations. As a consequence, a certain incubation energy density has to be delivered either by the ions or laser photons in order to start observable damage under subthreshold conditions. The parallelism between the effects of laser pulses and ion impacts is well appreciated when they are described in terms of the ratio between the deposited energy density and the corresponding threshold value.
Confinement effects on decay rate of surface electron states over liquid helium
Authors: Sokolov S. S. , Monarkha Yu. P. , Villas-Bôas J. M. , Studart Nelson
Published: May 2008

The decay rate of excited states of surface electrons in liquid helium, trapped in a quantum dot system, is evaluated, taking into account the process of spontaneous radiation of two ripplons with short wavelength. It is found that the values of the decay rate in later process are rather higher than those for the one-ripplon process previously calculated. The upper-bound limit lifetime of excited states of surface electrons in a quantum dot is found to be τ<10−6–10−7s.
Electronic and structural investigations of gold clusters doped with copper: Aun−1Cu− (n=13–19)
Authors: Zorriasatein Shahab , Joshi Kavita , Kanhere D. G.
Published: May 2008

We have obtained the ground state and the equilibrium geometries of Aun− and Aun−1Cu− in the size range of n=13–19. We have used first principles density functional theory within plane wave and Gaussian basis set methods. For each of the cluster we have obtained at least 100 distinct isomers. The anions of gold clusters undergo two structural transformations, the first one from flat cage to hollow cage and the second one from hollow cage to pyramidal structure. The Cu doped clusters do not show any flat cage structures as the ground state. The copper doped systems evolve from a general 3D structure to hollow cage with Cu trapped inside the cage at n=16 and then to pyramidal structure at n=19. The introduction of copper atom enhances the binding energy per atom as compared to gold cluster anions.
Effects of flow shear and Alfvén waves on two-dimensional magnetohydrodynamic turbulence
Authors: Douglas Jamie , Kim Eun-jin , Thyagaraja A.
Published: May 2008

The suppression of turbulent transport by large scale mean shear flows and uniform magnetic fields is investigated in two-dimensional magnetohydrodynamic turbulence driven by a small-scale forcing with finite correlation time. By numerical integration the turbulent magnetic diffusivity DT is shown to be significantly quenched, with a scaling DT∝B−2Ω0−5∕4, which is much more severe than in the case of a short or delta correlated forcing typified by white noise, studied in E. Kim and B. Dubrulle [Phys. Plasmas 8, 813 (2001)]. Here B and Ω0 are magnetic field strength and flow shear rate, respectively. The forcing with finite correlation time also leads to much stronger suppression of momentum transport through the cancellation of the Reynolds stress by the Maxwell stress with a positive small value of turbulent viscosity, νT>0. While fluctuating kinetic and magnetic energies are unaffected by the magnetic field just as in the case of a delta correlated forcing, they are much more severely quenched by flow shear than in that of a delta correlated forcing. Underlying physical mechanisms for the reduction of turbulent transport and turbulence level by flow shear and magnetic field are discussed.
Subdiffusive dynamics of a liquid crystal in the isotropic phase
Authors: De Gaetani Luca , Prampolini Giacomo , Tani Alessandro
Published: May 2008

The isotropic phase dynamics of a system of 4-n-hexyl-4′-cyano-biphenyl (6CB) molecules has been studied by molecular dynamics computer simulations. We have explored the range of 275–330K keeping the system isotropic, although supercooled under its nematic transition temperature. The weak rototranslational coupling allowed us to separately evaluate translational (TDOF) and orientational degrees of freedom (ODOF). Evidences of subdiffusive dynamics, more apparent at the lowest temperatures, are found in translational and orientational dynamics. Mean square displacement as well as self-intermediate center of mass and rotational scattering functions show a plateau, also visible in the orientational correlation function. According to the mode coupling theory (MCT), this plateau is the signature of the β-relaxation regime. Three-time intermediate scattering functions reveal that the plateau is related to a homogeneous dynamics, more extended in time for the orientational degrees of freedom (up to 1ns). The time-temperature superposition principle and the factorization property predicted by the idealized version of MCT hold, again for both kinds of dynamics. The temperature dependence of diffusion coefficient and orientational relaxation time is well described by a power law. Critical temperatures Tc are 244±6 and 258±6K, respectively, the latter is some 10K below the corresponding experimental values. The different values of Tc we obtained indicate that ODOF freezes earlier than TDOF. This appears due to the strongly anisotropic environment that surrounds a 6CB molecule, even in the isotropic phase. The lifetime of these “cages,” estimated by time dependent conditional probability functions, is strongly temperature dependent, ranging from some hundreds of picoseconds at 320K to a few nanoseconds at 275K.
Increased conductivity of a hole transport layer due to oxidation by a molecular nanomagnet
Authors: Cheylan S. , Puigdollers J. , Bolink H. J. , Coronado E. , Voz C. , Alcubilla R. , Badenes G.
Published: May 2008

Thin film transistors based on polyarylamine poly(N,N′-diphenyl-N,N′bis(4-hexylphenyl)-[1,1′biphenyl]-4,4′-diamine (pTPD) were fabricated using spin coating in order to measure the mobility of pTPD upon oxidation. Partially oxidized pTPD with a molecular magnetic cluster showed an increase in mobility of over two orders of magnitude. A transition in the mobility of pTPD upon doping could also be observed by the presence of a maximum obtained for a given oxidant ratio and subsequent decrease for a higher ratio. Such result agrees well with a previously reported model based on the combined effect of dipolar broadening of the density of states and transport manifold filling.
Measurement and noise performance of nano-superconducting-quantum-interference devices fabricated by focused ion beam
Authors: Hao L. , Macfarlane J. C. , Gallop J. C. , Cox D. , Beyer J. , Drung D. , Schurig T.
Published: May 2008

Science and industry demand ever more sensitive measurements on ever smaller systems, as exemplified by spintronics, nanoelectromechanical system, and spin-based quantum information processing, where single electronic spin detection poses a grand challenge. Superconducting quantum interference devices (SQUIDs) have yet to be effectively applied to nanoscale measurements. Here, we show that a simple bilayer deposition route, combining photolithography with focused ion beam patterning, produces high performance nanoscale SQUIDs. We present results of noise measurements on these nanoSQUIDs which correspond to a magnetic flux sensitivity of around 0.2μΦ0∕Hz1∕2. This represents one of the lowest noise values achieved for a SQUID device operating above 1K.
Calculation of resistive magnetohydrodynamics and two-fluid tearing modes by example of reversed-field-pinch-like plasma
Authors: Svidzinski V. A. , Li H.
Published: May 2008

An algorithm suitable for numerical solution of linear eigenmode problems in resistive magnetohydrodynamics (MHD) and two-fluid MHD models without prior approximations is presented. For these plasma models, sets of equations suitable for numerical solution are derived and the details of the algorithm of this solution are given. The algorithm is general and is suitable for solution of boundary (eigenmode) problems for different plasma configurations. It is most effective, however, in one-dimensional models since the grid size has to be sufficiently small in order to resolve the tearing layer together with the scale of the size of the plasma. The technique is applied for solving for tearing eigenmodes in reversed field pinch (RFP) -like plasma in plane geometry. Results of resistive MHD and two-fluid models are compared in this case, showing that the two-fluid effects on tearing modes in RFPs are sizable.
Low leakage current and enhanced ferroelectric properties of Ti and Zn codoped BiFeO3 thin film
Authors: Hu G. D. , Fan S. H. , Yang C. H. , Wu W. B.
Published: May 2008

Bi Fe O 3 (BFO), Ti(2%)-doped BFO (BFTO), Zn(2%)-doped BFO (BFZO), as well as Ti (1%) and Zn (1%) codoped BFO (BFTZO) films were deposited on Pt∕Ti∕SiO2∕Si substrates by using a metal organic decomposition process. Well saturated P-E hysteresis loops can be obtained in BFZO and BFTZO films due to their lower leakage currents compared to those of BFO and BFTO films. In comparison with BFZO film, BFTZO film exhibits a much larger remanent polarization (Pr∼84μC∕cm2), a lower coercive field (2Ec∼570kV∕cm), as well as stronger charge-retaining ability and fatigue resistance. These phenomena can be explained based on the formation of the defect complexes between the acceptors and oxygen vacancies in the films.
Carbon nanotube motors driven by carbon nanotube
Authors: Takagi Yoshiteru , Uda Tsuyoshi , Ohno Takahisa
Published: May 2008

We propose a new type of carbon nanotube (CNT) motor composed of a single-wall CNT (SWCNT) and a double-wall CNT (DWCNT), that are in mechanical contact. The rotational motion of our CNT motor is controllable by the translational motion of the SWCNT along the axis of the DWCNT. From molecular dynamics simulations, we show how our CNT motor can be driven in a controlled manner.
Conformational study of jet-cooled L-phenylglycine
Authors: Kim Hyung Min , Han Kyu Young , Park Juyeon , Kim Seong Keun , Kim Zee Hwan
Published: May 2008

We investigated the conformational structures of L-phenylglycine in the gas phase by photoionization and double resonance spectroscopy techniques as well as high-level ab initio calculations. The UV-UV and IR-UV double resonance spectroscopy suggested that there exists only one conformer that has a free OH band for the carboxyl group. Rotational contour analysis combined with ab initio calculation indicated that the conformer we detected by resonant two-photon ionization was not one of those found by Sanz et al. in their microwave spectroscopic study [Chem. Eur. J. 12, 2564 (2006)]. Different methods of vaporization along with different expansion and cooling conditions and different detection methods are believed to be the culprit for such intriguing discrepancy. The identical hydrogen bonding structure of our phenylglycine conformer with the most abundant conformer of glycine found in helium droplets and their nearly identical OH frequencies suggest that the skeletal structure of glycine is not significantly altered by phenyl substitution.
Theoretical study of the dynamics of the H+CH4 and H+C2H6 reactions using a specific-reaction-parameter semiempirical Hamiltonian
Authors: Layfield Joshua P. , Owens Matthew D. , Troya Diego
Published: May 2008

We present a theoretical study of the reactions of hydrogen atoms with methane and ethane molecules and isotopomers. High-accuracy electronic-structure calculations have been carried out to characterize representative regions of the potential-energy surface (PES) of various reaction pathways, including H abstraction and H exchange. These ab initio calculations have been subsequently employed to derive an improved set of parameters for the modified symmetrically-orthogonalized intermediate neglect of differential overlap (MSINDO) semiempirical Hamiltonian, which are specific to the H+alkane family of reactions. The specific-reaction-parameter (SRP) Hamiltonian has then been used to perform a quasiclassical-trajectory study of both the H+CH4 and H+C2H6 reactions. The calculated values of dynamics properties of the H+CH4→H2+CH3 reaction and isotopologues, including alkyl product speed distributions, diatomic product internal-state distributions, and cross sections, are generally in good agreement with experiment and with the results provided by the ZBB3 PES [Z. Xie et al., J. Chem. Phys. 125, 133120 (2006)]. The results of trajectories propagated with the SRP Hamiltonian for the H+C2H6→H2+C2H5 reaction also agree with experiment. The level of agreement between the results calculated with the SRP Hamiltonian and experiment in both the H+methane and H+ethane reactions indicates that semiempirical Hamiltonians can be improved for not only a specific reaction but also a family of reactions.
Call for experimental test of a revised mathematical form for empirical field emission current-voltage characteristics
Authors: Forbes Richard G.
Published: May 2008

There are now good theoretical reasons to believe that experimental current-voltage (i-V) relationships for cold field electron emission (CFE) should approximately obey the empirical law i=CVκexp(−B∕V), where B, C, and κ are effectively constant, and κ≠2. Since −dlni∕d(1∕V)=(V2∕i)di∕dV=B+κV, κ for a given emission situation could be experimentally established from an appropriate data plot if sufficiently accurate current-voltage measurements were available. Knowledge of experimental κ values could lead to greater physical understanding of CFE and variations as between materials. It is strongly urged that the feasibility of measurements of this kind be carefully explored.
Topology of magnetic-field induced electron current density in the cubane molecule
Authors: Pelloni Stefano , Lazzeretti Paolo
Published: May 2008

A spatial model of the electronic current density induced in the cubane molecule by applying an external magnetic-field has been constructed employing quantum mechanical methods at the Hartree–Fock level of accuracy. The topological features of the current density vector field are described via a stagnation graph that shows the isolated points and the lines at which the current vanishes. Shielding density maps based on the differential Biot–Savart law, along with a collection of current density maps, explain magnetic shielding at hydrogen and carbon nuclei, and virtual shielding at ring and cage centers.
Local hardness equalization: Exploiting the ambiguity
Authors: Ayers Paul W. , Parr Robert G.
Published: May 2008

In the density-functional theory of chemical reactivity, the local hardness is known to be an ambiguous concept. The mathematical structure associated with this problematic situation is elaborated and three common definitions for the local hardness are critically examined: the frontier local hardness [S. K. Ghosh, Chem. Phys. Lett. 172, 77 (1990)], the total local hardness [S. K. Ghosh and M. Berkowitz, J. Chem. Phys. 83, 2976 (1985)], and the unconstrained local hardness [P. W. Ayers and R. G. Parr, J. Am. Chem. Soc. 122, 2010 (2000)]. The frontier local hardness has particularly nice properties: (a) it has smaller norm than most, if not all, other choices of the local hardness and (b) it is “unbiased” in an information-theoretic sense. For the ground electronic state of a molecular system, the frontier local hardness is equal to the global hardness. For an electronic system in its ground state, both the chemical potential and the frontier local hardness are equalized. The frontier local hardness equalization principle provides a computational approach for designing reagents with desirable chemical reactivity profiles.
Ab initio based polarizable force field parametrization
Authors: Masia Marco
Published: May 2008

Experimental and simulation studies of anion-water systems have pointed out the importance of molecular polarization for many phenomena ranging from hydrogen-bond dynamics to water interfaces structure. The study of such systems at molecular level is usually made with classical molecular dynamics simulations. Structural and dynamical features are deeply influenced by molecular and ionic polarizability, which parametrization in classical force field has been an object of long-standing efforts. Although when classical models are compared to ab initio calculations at condensed phase, it is found that the water dipole moments are underestimated by ∼30%, while the anion shows an overpolarization at short distances. A model for chloride-water polarizable interaction is parametrized here, making use of Car–Parrinello simulations at condensed phase. The results hint to an innovative approach in polarizable force fields development, based on ab initio simulations, which do not suffer for the mentioned drawbacks. The method is general and can be applied to the modeling of different systems ranging from biomolecular to solid state simulations.
Bundling dynamics of single walled carbon nanotubes in aqueous suspensions
Authors: Eda Goki , Fanchini Giovanni , Kanwal Alokik , Chhowalla Manish
Published: May 2008

A simple optical method based on absorption of monochromatic light to investigate the dynamics of single walled carbon nanotube (SWCNT) suspensions is described. The well dispersed suspensions display a complex behavior, exhibiting peaks due to resonant scattering from SWCNT bundles with increasing diameters as a function of time. The results indicate that the bundling of SWCNTs initiates almost immediately after termination of sonication (after ∼0.1 h) and continues to increase up to a critical time (∼10 h), above which precipitation according to the Stokes relationship occurs. The absorbance behavior can be explained by the depletion of the effective medium as well as the Mie scattering from growth of bundles. A semiquantitative analysis of the experimental data based on the Mie theory of light scattering from cylindrical particles allows the extraction of diameters at the nucleation and growth of SWCNT bundles. The bundling dynamics have been correlated with the electrical properties such as the sheet resistance and transistor characteristics of the SWCNT thin films. Our work is a useful step toward reproducible solution processed electronics because it provides a simple method to monitor the quality of SWCNT suspensions in real time and correlate it to device characteristics.
Freestanding diffractive optical elements as light extractors for burning plasma experiments
Authors: Stutman D. , Caravelli G. , Finkenthal M. , Wright G. , Whyte D. , Moldovan N. , Kaita R. , Roquemore L.
Published: May 2008

Optical diagnostics will be critical for the operation and performance assessment of burning plasma experiments, such as ITER. At the same time, extracting light for these diagnostics with reflective mirrors becomes difficult in the burning plasma environment due to the deleterious effects of the prolonged exposure on plasma and nuclear radiations. As an alternative, we explore the possibility to use freestanding diffractive optical elements, such as transmission gratings and zone plates, as light extractors. Since in diffractive systems, light is deflected by periodic slits rather than by a surface, these may withstand plasma exposure with less degradation of their optical properties. To investigate this possibility, we developed freestanding transmission gratings for the visible range and exposed them to conditions resembling (or even exceeding) those expected for the ITER “first mirrors.” The results of this study indicate that the gratings can withstand high heat fluxes and plasma and energetic radiation bombardment. Additionally, in contrast to the reflective elements, the extraction efficiency of diffractive elements may even improve with plasma exposure, which is possibly due to the shaping and thinning of the grating bars by plasma erosion. Moreover, in tightly collimated configurations, even very thin gratings can be used to extract light from hot fusion plasmas, as demonstrated by our tests of an extreme ultraviolet extractor at the National Spherical Torus Experiment.
Terahertz characterization of multi-walled carbon nanotube films
Authors: Wu Ziran , Wang Lu , Peng Yitian , Young Abram , Seraphin Supapan , Xin Hao
Published: May 2008

Multi-walled carbon nanotube films are characterized using terahertz time-domain spectroscopy. Both transmission and reflection experiments are performed in order to measure both the complex refractive index and the wave impedance. This method allows simultaneous extraction of both the permittivity (ε=ε′−iε″) and permeability (μ=μ′−iμ″) without any assumptions. Experimental results are obtained from 50 to 370 GHz and compared well with the microwave data (8–50 GHz) of the same sample measured using a vector network analyzer. The measured complex permittivity can be fitted with a Drude–Lorentz model in the 8–370 GHz frequency range.
Theory of laser enhancement of ultracold reactions: The fermion-boson population transfer by adiabatic passage of Li6+Li6Li7(Tr=1mK)→Li6Li6+Li7(Tp=1mK)
Authors: Li Xuan , Parker Gregory A.
Published: May 2008

We present a new theory of population transfer by adiabatic passage. This theory relates laser catalysis to adiabatic passage, enhancing chemical reactions with the freedom to choose the translational energies of the reactants and products separately. The process, A+BC↔ℏωpABC*(v)↔ℏωsAB+C, involves two laser fields that are slowly varying so the process is adiabatic, and sufficiently intense so the population of the intermediate bound complex (ABC) is minimized. We apply this theory to the collinear exchange reaction Li6+Li27(Tr)↔ℏωp(Li6Li7Li7)*↔ℏωsLi6Li7(Tp)+Li7. We show that at translational energies Tp=Tr=1mK with a narrow energy bandwidth of δE=0.01mK, we can obtain nearly total (⩾98%) population transfer from the reactant to the product states. This can be done with a pump laser and a Stokes laser in an “intuitive” sequence (tp<ts) at a low intensity (Ip⩽600MW∕cm2) and a “coincident” sequence (tp=ts) at a higher intensity.
The compact capacitor bank CQ-1.5 employed in magnetically driven isentropic compression and high velocity flyer plate experiments
Authors: Wang Guiji , Sun Chengwei , Tan Fuli , Zhao Jianheng , Zhang Ning , Liu Cangli , Mo Jianjun , Wang Ganghua , Wang Xiaosong
Published: May 2008

Based on the low inductance capacitor, the parallel-plate transmission line, and the explosive network closing switch, a compact pulsed power generator CQ-1.5 has been developed at the Institute of Fluid Physics and is capable to deliver a current of peak of 1.5MA within rise time of 500–570ns into a 2–3nH inductive load. The work is motivated to do isentropic compression experiments (ICEs) on metals up to 30–50GPa and to launch flyer plates at velocities over 8km∕s. The experiments were conducted with the diagnostics of both Doppler pin system and velocity interferometer system for any reflectors, and the measured free surface velocity histories of ICE samples were treated with a backward integration code. The results show that the isentropes of Cu and Al samples under 35GPa are close to their Hugoniots within a deviation of 3%. The LY12 aluminum flyer plates were accelerated to a velocity over 8.96km∕s.
Spin-dependent density of states in Ga1−xMnxAs probed by tunneling spectroscopy
Authors: Saito H. , Yamamoto A. , Yuasa S. , Ando K.
Published: May 2008

We conducted spin-dependent tunneling spectroscopy measurements in Fe∕ZnSe∕Ga1−xMnxAs magnetic tunnel diodes that had a metal-insulator-semiconductor structure. The tunneling magnetoresistance signal in differential conductance spectra (dI∕dV−V) was found to disappear for bias voltages (V) higher than 350meV for a bias direction where electrons tunnel into Ga1−xMnxAs. This indicates that there is exchange splitting in the density of states of Ga1−xMnxAs up to 350meV above the Fermi energy.
Nanoscale thickness double-gated field effect silicon sensors for sensitive pH detection in fluid
Authors: Elibol Oguz H. , Reddy Bobby , Bashir Rashid
Published: May 2008

In this work, we report on the optimization of a double-gate silicon-on-insulator field effect device operation to maximize pH sensitivity. The operating point can be fine tuned by independently biasing the fluid and the back gate of the device. Choosing the bias points such that device is nearly depleted results in an exponential current response—in our case, 0.70decade per unit change in pH. This value is comparable to results obtained with devices that have been further scaled in width, reported at the forefront of the field, and close to the ideal value of 1decade∕pH. By using a thin active area, sensitivity is increased due to increased coupling between the two conducting surfaces of the devices.
Accelerating dislocations to transonic and supersonic speeds in anisotropic metals
Authors: Tsuzuki Helio , Branicio Paulo S. , Rino José Pedro
Published: May 2008

The dynamics of stress-accelerated dislocations in copper is investigated using molecular dynamics simulations. The structure and motion of dissociated edge dislocations are analyzed using the common neighborhood parameter and local stresses. Dislocations are accelerated by high shear stresses and reach stable velocities in the two transonic regimes. Supersonic dislocations are generated but are transient, as they require high stresses, which trigger nucleation of multiple dislocation dipoles. A velocity plateau in the first transonic regime indicates a radiation-free state in agreement with theoretical predictions.
Interfacial reactions between indium tin oxide and triphenylamine tetramer layers induced by photoirradiation
Authors: Satoh Toshikazu , Fujikawa Hisayoshi , Yamamoto Ichiro , Murasaki Takanori , Kato Yoshifumi
Published: May 2008

The effects of photoirradiation on the interfacial chemical reactions between indium tin oxide (ITO) films and layers of triphenylamine tetramer (TPTE) were investigated by using in situ x-ray photoelectron spectroscopy (XPS). Thin TPTE layers deposited onto sputter-deposited ITO films were irradiated with violet light-emitting diodes (peak wavelength: 380 nm). Shifts in the peak positions of spectral components that originated in the organic layer toward the higher binding-energy side were observed in the XPS profiles during the early stages of irradiation. No further peak shifts were observed after additional irradiation. An increase in the ratio of the organic component in the O 1s spectra was also observed during the photoirradiation. The ratio of the organic component increased in proportion to the cube root of the irradiation time. These results suggest that photoirradiation induces an increase in the height of the carrier injection barrier at the interface between TPTE and ITO in the early stages of the irradiation, possibly due to the rapid diffusion controlled formation and growth of an oxidized TPTE layer, which is considered to act as a high resistance layer.
Experimental evidence of an incomplete thermalization of the energy in an x-ray microcalorimeter with a Ta∕Au absorber
Authors: Perinati E. , Barbera M. , Varisco S. , Silver E. , Beeman J. , Pigot C.
Published: May 2008

We have conducted an experimental test at our XACT facility using an x-ray microcalorimeter with Ta∕Au absorber and neutron transmutation doped germanium thermal sensor. The test was aimed at measuring the percentage of energy effectively thermalized after absorption of x-ray photons in superconducting tantalum. Moreover, in general, possible formation of long living quasiparticles implies that by using a superconducting absorber, a fraction of the deposited energy could not be thermalized on the useful time scale of the thermal sensor. To investigate this scenario, we exploited an absorber made of gold, where no energy trapping is expected, with a small piece of superconducting tantalum attached on top. We obtained evidence that the thermalization of photons absorbed in tantalum is delayed by energy trapping from quasiparticles. We compare the experimental results with numerical simulations and derive a value for the intrinsic lifetime of quasiparticles.
Unfolding core asymmetries with x-ray emission images in symmetry diagnostic experiments
Authors: Huang T. X. , Ding Y. K. , Zheng Z. J. , Miao W. Y. , Cao Z. R. , Jiang S. E. , Liu S. Y. , Liu Z. L.
Published: May 2008

A novel inversion technique is proposed to unfold core asymmetries at the source with x-ray emission images, which were obtained from imploded surrogate capsules in symmetry diagnostic experiments. The axisymmetrical core emission can be expanded as a Fourier series, with Legendre polynomials and spherical Bessel functions as bases concerned with polar angle and radius, respectively. A least-squares estimator is employed to obtain the unknown coefficients from its two-dimensional image data. The unfolded Legendre coefficients can be further used to test modeling of drive asymmetries in hohlraums. This technique is also demonstrated with a proof-of-principle experiment performed on the Shenguang II laser facility [L. Zunqi et al., Chin. J. Lasers B10, 6 (2001)].
Electromagnetic three-dimensional reconstruction of targets from free space experimental data
Authors: Geffrin J.-M. , Chaumet P. C. , Eyraud C. , Belkebir K. , Sabouroux P.
Published: May 2008

This paper deals with the problem of reconstructing the relative permittivity of three-dimensional targets using experimental scattered fields. The fields concerned were measured in an anechoic chamber on the surface of a sphere surrounding the target. The inverse scattering problem is reformulated as an optimization problem that is iteratively solved thanks to a conjugate gradient method and by using the coupled dipoles method as a forward problem solver. The measurement technique and the inversion procedure are briefly described with the inversion results. This work demonstrates the reliability of the experiments and the efficiency of the proposed inverse scattering scheme.
Enhancement of organic magnetoresistance by electrical conditioning
Authors: Niedermeier U. , Vieth M. , Pätzold R. , Sarfert W. , von Seggern H.
Published: May 2008

We demonstrate that electrical conditioning can be used as an efficient method to enhance the organic magnetoresistance effect in organic light emitting diodes. Depending on duration and intensity of the conditioning process the absolute value of the magnetoresistance effect can be increased from ∼1% to values exceeding 15% at 40mT in devices based on poly(paraphenylene vinylene). Qualitatively, the increase in magnetoresistance can be correlated with a decrease in luminance during the conditioning process. From this we conclude that device degradation mechanisms are responsible for the enhancement of organic magnetoresistance.
UV-induced protonation of molecules adsorbed on ice surfaces at low temperature
Authors: Moon Eui-Seong , Lee Chang-Woo , Kim Joon-Ki , Park Seong-Chan , Kang Heon
Published: May 2008

UV irradiation of ice films adsorbed with methylamine molecules induces protonation of the adsorbate molecules at low temperature (50–130K). The observation indicates that long-lived protonic defects are created in the ice film by UV light, and they transfer protons to the adsorbate molecules via tunneling mechanism at low temperature. The methylammonium ion formed by proton transfer remains to be stable at the ice surface. It is suggested that this solid-phase protonation might play a significant role in the production of molecular ions in interstellar clouds.
Valence band offset of ZnO∕4H-SiC heterojunction measured by x-ray photoelectron spectroscopy
Authors: Fan H. B. , Sun G. S. , Yang S. Y. , Zhang P. F. , Zhang R. Q. , Wei H. Y. , Jiao C. M. , Liu X. L. , Chen Y. H. , Zhu Q. S. , Wang Z. G.
Published: May 2008

The valence band offset (VBO) of the wurtzite ZnO∕4H-SiC heterojunction is directly determined to be 1.61±0.23eV by x-ray photoelectron spectroscopy. The conduction band offset is deduced to be 1.50±0.23eV from the known VBO value, which indicates a type-II band alignment for this heterojunction. The experimental VBO value is confirmed and in good agreement with the calculated value based on the transitive property of heterojunctions between ZnO, SiC, and GaN.
Self-assembly of well-aligned 3C-SiC ripples by focused ion beam
Authors: Zhang Jiaming , Wei Qiangmin , Lian Jie , Jiang Weilin , Weber William J. , Ewing Rodney C.
Published: May 2008

Well-aligned ripple structures on the surface of a single crystal of 3C-SiC were created by focused ion beam bombardment, and the resulting morphology and topography were characterized using in situ focused ion beam/scanning electron microscopy, as well as ex situ atomic force microscopy. The ripple structure formed as a result of ion sputtering beyond a critical incident angle (∼50°), and its characteristic wavelength varied from 158to296nm with changes in the incident angle and ion beam flux. The geometry, ordering, and homogeneity of the ripples can be well controlled by varying the ion beam incident angle and beam current, as required for the fabrication of nanostructures that use SiC for optical and electronic applications.
Quantum interference effects and magnetic scattering in the electrical resistivity of Ni nanocrystallites in TiN matrix
Authors: Khatua P. , Nath T. K. , Banerjee Mitali , Majumdar A. K.
Published: May 2008

The electrical resistivity, ρ of self-assembled, epitaxial nickel nanocrystallites in titanium nitride (TiN) matrix, was measured in fields from 0to4T between 4.2 and 300K. At lower temperatures, ρ(T) conclusively shows −T dependence below the observed minima at Tmin unlike −lnT in two dimensional multilayers. In this three dimensional system, the origin of the minima lays in quantum interference effects. The ρ(T) data well above Tmin, interpreted in terms of electron-phonon and electron-magnon scatterings, show that the temperature dependence of the composite Ni∕TiN sample mainly comes from that of Ni and the residual resistivity from TiN.
Effects of well thickness on the spectral properties of In0.5Ga0.5As∕GaAs∕Al0.2Ga0.8As quantum dots-in-a-well infrared photodetectors
Authors: Jolley G. , Fu L. , Tan H. H. , Jagadish C.
Published: May 2008

We report on the effects of the quantum well (QW) thickness on the spectral response and other characteristics of In0.5Ga0.5As∕GaAs∕Al0.2Ga0.8As quantum dots-in-a-well infrared photodetectors grown by low-pressure metal-organic chemical vapor deposition. The main device properties are observed to have a strong dependence on the QW parameters.
A structural, magnetic, and Mössbauer spectral study of the DyCo4−xFexB compounds, with x=–3
Authors: Mayot Hervé , Isnard Olivier , Grandjean Fernande , Long Gary J.
Published: May 2008

The DyCo4−xFexB compounds, with x=0, 1, 1.5, 2, 2.5, and 3, have been investigated by x-ray diffraction, magnetic measurements, and iron-57 Mössbauer spectroscopy. The substitution of cobalt by iron induces both an anisotropic increase in the unit-cell volume and a large increase in the Curie temperature. The 4.2K magnetization decreases with increasing iron content. The Mössbauer spectra of the DyCo4−xFexB compounds with x=1, 1.5, and 2 reveal rather small iron hyperfine fields of approximately 16T and large quadrupole interactions of +1.0 and −1.0mm∕s, for the 6i and 2c sites, respectively. The relative areas of the 6i and 2c spectral components indicate a strong preferential substitution of iron on the 2c site. In DyCo3FeB approximately 70% of the iron occupies the 2c site; a strong increase in the a lattice parameter and in the Curie temperature is observed between DyCo4B and DyCo3FeB and smaller increases are observed for x>1. The compensation temperature of the DyCo4−xFexB compounds decreases from 350to270K between x=0 and 3, respectively, as a result of the iron induced increase in the transition metal magnetization.
Robust single-parameter quantized charge pumping
Authors: Kaestner B. , Kashcheyevs V. , Hein G. , Pierz K. , Siegner U. , Schumacher H. W.
Published: May 2008

This paper investigates a scheme for quantized charge pumping based on single-parameter modulation. The device was realized in an AlGaAs–GaAs gated nanowire. We find a remarkable robustness of the quantized regime against variations in the driving signal, which increases with applied rf power. This feature, together with its simple configuration, makes this device a potential module for a scalable source of quantized current.
Scanning x-ray diffraction with 200nm spatial resolution
Authors: Hanke M. , Dubslaff M. , Schmidbauer M. , Boeck T. , Schöder S. , Burghammer M. , Riekel C. , Patommel J. , Schroer C. G.
Published: May 2008

We report a scanning technique, which combines high resolution x-ray diffraction (in reciprocal space) along with a submicrometer spatial resolution (in real space). SiGe∕Si(001) Stranski–Krastanow islands served here as a well investigated model system to check the limits of the developed method. A set of refractive silicon x-ray lenses focused the x-ray beam size down to a diameter of 200nm (full width at half maximum), which enables scanning of individual micrometer-sized and even smaller islands. By illuminating diverse {111} island side facets, crystal truncation rods of different orientations were independently excited and thus became distinguishable in the scattering patterns. The reassembling of these locally resolved diffraction patterns coincides with both an integral measurement and a corresponding scattering simulation.
Capacitive extensometry for transient strain analysis of dielectric elastomer actuators
Authors: Keplinger Christoph , Kaltenbrunner Martin , Arnold Nikita , Bauer Siegfried
Published: May 2008

Dielectric elastomer actuators (DEAs) are promising structural units for artificial muscles and robotic elements. Understanding the safe and failure mode regimes of such DEAs is essential for controlling the actuator. We develop an electrical characterization technique for obtaining information on the transient strain in the actuator and analyze the behavior of the actuator in safe and failure operation regimes, in particular in the pull-in instability mode. Additionally, the technique allows the strain-dependent measurement of the electrode resistance. The current measurement based technique can be also applied for actuator control with feedback loops.
Temperature insensitive linewidth enhancement factor of p-type doped InAs∕GaAs quantum-dot lasers emitting at 1.3μm
Authors: Cong D.-Y. , Martinez A. , Merghem K. , Ramdane A. , Provost J.-G. , Fischer M. , Krestnikov I. , Kovsh A.
Published: May 2008

The temperature dependence of microwave properties—relaxation frequency and Henry factor—of undoped and p-type doped ten InAs∕GaAs quantum-dot layer lasers is reported in the 20–80°C range. It is shown that the linewidth enhancement factor of the p-type doped devices is temperature insensitive while that of the undoped lasers shows a strong dependence for temperatures above 40°C.
High efficiency and low efficiency roll off in white phosphorescent organic light-emitting diodes by managing host structures
Authors: Yook Kyoung Soo , Lee Jun Yeob
Published: May 2008

High efficiency phosphorescent white organic light-emitting diodes with little efficiency roll off were developed by managing the charge transport properties of the host materials. The emitting layers were stacked at a sequence of red/blue/green from the hole transport layer side and charge injection could be easily controlled by changing the host materials in the red and green emitting layers. A high quantum efficiency of 13% was obtained at a luminance of 1000cd∕m2. In addition, the quantum efficiency of the white device at low luminance was maintained up to 10000cd∕m2 without any decrease.
Investigations of half and full select disturb rates in a toggle magnetic random access memory
Authors: Robertazzi R. P. , Worledge D. C. , Nowak J.
Published: May 2008

Immunity to half select disturbs and self-activated toggle errors were measured for toggle magnetic memory cells at high switching speeds (200ps rise time pulses, 100ns in duration). Measurements of the immunity of cells to word line or bit line half selects indicate lifetimes for magnetic reversal of at least 1037 disturbs can be expected for properly designed cells. The lifetime for error free toggling has also been measured. The data indicate that error free toggle switching of a cell can be expected to be greater than 1018 toggles for optimized write fields.
The role of herbivory and fouling on the invasive green alga Caulerpa filiformis in temperate Australian waters
Authors: Cummings D. O. , Williamson J. E.
Published: May 2008

The green macroalga Caulerpa filiformis is rapidly dominating algal assemblages in shallow subtidal regions along the New South Wales (NSW) coast. Given that invasive species are known for their competitive superiority, the interactions between C. filiformis and herbivores and fouling species was investigated and were compared with those co-occurring native species. Extensive field surveys were carried out to assess purported damage from herbivores, and co-occurrence of C. filiformis with herbivorous fish and invertebrate species. Signs of herbivory on C. filiformis were visible, and several common herbivorous invertebrates and fish co-occurred with the alga. Laboratory and field feeding assays with fresh algae indicated that generalist invertebrate herbivores such as Turbo torquatus did not consume C. filiformis, whereas herbivorous fish did consume C. filiformis but not in preference to other palatable algae. C. filiformis was fouled at similar concentrations and with similar epiphytic species to other co-occurring algae. Thus the ability of C. filiformis to deter herbivory and fouling by using biotic deterrents (both chemical and structural) is limited and unlikely to be the major factor driving its successful invasion into NSW habitats.
Small hypervascular hepatocellular carcinoma: limited value of portal and delayed phases on dynamic magnetic resonance imaging
Authors: Yu J. S. , Lee J. H. , Chung J. J. , Kim J. H. , Kim K. W.
Published: May 2008

Background: Characterization of small nodules in the cirrhotic liver is always challenging in clinical practice. In the differential diagnosis of small hypervascular lesions, it has been reported that portal venous or delayed hypointensity is a useful sign to characterize hepatocellular carcinomas (HCCs) during dynamic magnetic resonance (MR) imaging. However, few studies have assessed the diagnostic value of this sign.

Purpose: To determine the diagnostic value of portal-phase (PP) and delayed-phase (DP) images for the diagnosis of small hypervascular HCCs during intravenous (IV) contrast-enhanced dynamic MR imaging of cirrhotic liver.

Material and Methods: A total of 69 small (6–20 mm) hypervascular HCCs in 53 cirrhotic patients were subjected to a retrospective analysis of the signal intensities (hypo-, iso-, or hyperintense) and rim enhancement on PP and 5-min DP images from three-phased dynamic MR imaging according to the pre-contrast T1- and T2-weighted imaging features. After exclusion of 33 subcapsular wedge-shaped pseudolesions and three hemangiomas by typical imaging features, 74 centrally located small hypervascular benign or pseudolesions were used as a control group for comparative analyses.

Results: The sensitivities of PP hypointensity, DP hypointensity, and rim enhancement in the diagnosis were 11%, 29%, and 18%, respectively, for 6–10-mm hypervascular HCCs, and 42%, 63%, and 58%, respectively, for 16–20-mm lesions. After exclusion of the 48 lesions showing T2-weighted hyperintensity (HCCs, n=39; benign lesions, n=9), the overall sensitivity for diagnosis of small hypervascular HCCs decreased (8.3%, 25.0%, and 8.3%, respectively).

Conclusion: Although DP provides a better sensitivity than PP, both PP and DP have very limited diagnostic value for diagnosis of small hypervascular HCCs during dynamic MR imaging of the cirrhotic liver.
Position and Role of the BK Channel α Subunit S0 Helix Inferred from Disulfide Crosslinking
Authors: Liu Guoxia , Zakharov Sergey I. , Yang Lin , Deng Shi-Xian , Landry Donald W. , Karlin Arthur , Marx Steven O.
Published: May 2008

The position and role of the unique N-terminal transmembrane (TM) helix, S0, in large-conductance, voltage- and calcium-activated potassium (BK) channels are undetermined. From the extents of intra-subunit, endogenous disulfide bond formation between cysteines substituted for the residues just outside the membrane domain, we infer that the extracellular flank of S0 is surrounded on three sides by the extracellular flanks of TM helices S1 and S2 and the four-residue extracellular loop between S3 and S4. Eight different double cysteine–substituted alphas, each with one cysteine in the S0 flank and one in the S3–S4 loop, were at least 90% disulfide cross-linked. Two of these alphas formed channels in which 90% cross-linking had no effect on the V50 or on the activation and deactivation rate constants. This implies that the extracellular ends of S0, S3, and S4 are close in the resting state and move in concert during voltage sensor activation. The association of S0 with the gating charge bearing S3 and S4 could contribute to the considerably larger electrostatic energy required to activate the BK channel compared with typical voltage-gated potassium channels with six TM helices.