bluuebunny/crossref_metadata_2025 huggingface dataset:
Synchronizability of network ensembles with prescribed statistical properties
Authors: Guan Shuguang , Wang Xingang , Li Kun , Wang Bing-Hong , Lai C.-H.
Published: March 2008

It has been shown that synchronizability of a network is determined by the local structure rather than the global properties. With the same global properties, networks may have very different synchronizability. In this paper, we numerically studied, through the spectral properties, the synchronizability of ensembles of networks with prescribed statistical properties. Given a degree sequence, it is found that the eigenvalues and eigenratios characterizing network synchronizability have well-defined distributions, and statistically, the networks with extremely poor synchronizability are rare. Moreover, we compared the synchronizability of three network ensembles that have the same nodes and average degree. Our work reveals that the synchronizability of a network can be significantly affected by the local pattern of connections, and the homogeneity of degree can greatly enhance network synchronizability for networks of a random nature.
Propagation of high power electromagnetic beams in overdense plasmas: Higher order paraxial theory
Authors: Sodha Mahendra Singh , Faisal Mohammad
Published: March 2008

This article presents the paraxial theory of the propagation of an initially Gaussian electromagnetic beam in an inhomogeneous plasma with an overdense region; in contrast to earlier work on penetration in overdense plasma, higher order terms (up to r4) in the expansion of the dielectric function and the eikonal have been taken into account. Three types of nonlinearities, viz., collisional, ponderomotive, and relativistic, have been considered. As expected the higher order terms do not affect the critical curves, corresponding to initial propagation without convergence or divergence. It is seen that the inclusion of higher order terms does significantly affect the dependence of the beam width on the distance of propagation. Corresponding to the case of ponderomotive nonlinearity numerical results for the dependence of beam width parameter and the axial dielectric function on the distance of propagation have been presented for specific values of the initial beam width and axial irradiance and specific spatial dependence of the electron density in the absence of the beam. Both the situations, viz., formation of bright or dark rings in the transverse irradiation pattern, have been considered. From a parametric analysis the dependence of the maximum penetration (when the axial dielectric function tends to zero) on the axial irradiance and an inhomogeneity parameter has been graphically illustrated.
Intermittency of electron density in interstellar kinetic Alfvén wave turbulence
Authors: Terry P. W. , Smith K. W.
Published: March 2008

The statistics of electron density fluctuations in kinetic Alfvén wave (KAW) turbulence is investigated in connection with the inferred Lévy statistics of pulsar signal broadening. Using analytic theory and computation, decaying KAW turbulence is shown to form coherent, intermittent current filaments in regions where local current intensity exceeds the rms value by a critical factor of a few. Nonlinear mixing is suppressed because the filament magnetic field has sufficient shear to refract away turbulent waves. While the magnetic field and density associated with the filament are as coherent as the current, they are not localized and isolated. Ampere’s law dictates that the magnetic field decays as r−1 outside the current, and KAW equipartition dictates that density has the same envelope. This structure gives the density gradient a Lévy distribution, consistent with pulsar scintillation.
The mean and turbulent flow structure of a weak hydraulic jump
Authors: Misra S. K. , Kirby J. T. , Brocchini M. , Veron F. , Thomas M. , Kambhamettu C.
Published: March 2008

The turbulent air–water interface and flow structure of a weak, turbulent hydraulic jump are analyzed in detail using particle image velocimetry measurements. The study is motivated by the need to understand the detailed dynamics of turbulence generated in steady spilling breakers and the relative importance of the reverse-flow and breaker shear layer regions with attention to their topology, mean flow, and turbulence structure. The intermittency factor derived from turbulent fluctuations of the air–water interface in the breaker region is found to fit theoretical distributions of turbulent interfaces well. A conditional averaging technique is used to calculate ensemble-averaged properties of the flow. The computed mean velocity field accurately satisfies mass conservation. A thin, curved shear layer oriented parallel to the surface is responsible for most of the turbulence production with the turbulence intensity decaying rapidly away from the toe of the breaker (location of largest surface curvature) with both increasing depth and downstream distance. The reverse-flow region, localized about the ensemble-averaged free surface, is characterized by a weak downslope mean flow and entrainment of water from below. The Reynolds shear stress is negative in the breaker shear layer, which shows that momentum diffuses upward into the shear layer from the flow underneath, and it is positive just below the mean surface indicating a downward flux of momentum from the reverse-flow region into the shear layer. The turbulence structure of the breaker shear layer resembles that of a mixing layer originating from the toe of the breaker, and the streamwise variations of the length scale and growth rate are found to be in good agreement with observed values in typical mixing layers. All evidence suggests that breaking is driven by a surface-parallel adverse pressure gradient and a streamwise flow deceleration at the toe of the breaker. Both effects force the shear layer to thicken rapidly, thereby inducing a sharp free surface curvature change at the toe.
Vortex formation in shallow flows
Authors: Rockwell Donald
Published: March 2008

Highly coherent vortex formation can occur in a variety of shallow, separated shear layers. Space-time imaging is employed to gain insight into representative physical concepts, including vortex formation due to an inherent instability, forced vortex formation due to controlled inflow, and vortex formation coupled with gravity-wave resonance.
Pressure-driven water infiltration into carbon nanotube: The effect of applied charges
Authors: Liu Ling , Qiao Yu , Chen Xi
Published: March 2008

Pressure-driven liquid infiltration into hydrophobic nanoporous solids has important applications for energy absorption. Using water infiltration into a carbon nanotube as a model system, here we show from molecular dynamics simulation that with applied charges, the effective degree of hydrophobicity can be increased, which leads to an improved adjustability of energy absorption efficiency. The attractions exerted by the charges can facilitate initial water infiltration, but they may also stick the molecules and effectively block the pathways of subsequent water entrance. Higher pressure is thus needed to infiltrate water into the tube when external charges (or electrical fields) are applied.
Short-time transport properties in dense suspensions: From neutral to charge-stabilized colloidal spheres
Authors: Banchio Adolfo J. , Nägele Gerhard
Published: March 2008

We present a detailed study of short-time dynamic properties in concentrated suspensions of charge-stabilized and of neutral colloidal spheres. The particles in many of these systems are subject to significant many-body hydrodynamic interactions. A recently developed accelerated Stokesian dynamics (ASD) simulation method is used to calculate hydrodynamic functions, wave-number-dependent collective diffusion coefficients, self-diffusion and sedimentation coefficients, and high-frequency limiting viscosities. The dynamic properties are discussed in dependence on the particle concentration and salt content. Our ASD simulation results are compared with existing theoretical predictions, notably those of the renormalized density fluctuation expansion method of Beenakker and Mazur [Physica A 126, 349 (1984)], and earlier simulation data on hard spheres. The range of applicability and the accuracy of various theoretical expressions for short-time properties are explored through comparison with the simulation data. We analyze, in particular, the validity of generalized Stokes–Einstein relations relating short-time diffusion properties to the high-frequency limiting viscosity, and we point to the distinctly different behavior of de-ionized charge-stabilized systems in comparison to hard spheres.
The unoccupied electronic structure of potassium doped copper phthalocyanine studied by near edge absorption fine structure
Authors: Molodtsova O. V. , Knupfer M. , Aristov V. Yu. , Vyalikh D. V. , Zhilin V. M. , Ossipyan Yu. A.
Published: March 2008

The unoccupied electronic structure of potassium doped copper-phthalocyanine thin films has been studied using x-ray absorption spectroscopy. The data reveal filling of the lowest unoccupied molecular orbital upon doping and related changes of the core level absorption spectra. The spectral changes can be rationalized taking into account the core level binding energies which also depend on doping.
ac electrical investigation of polysilsesquioxanic films used as humidity sensors
Authors: Castaldo A. , Quercia L. , Di Francia G. , Cassinese A. , D’Angelo P.
Published: March 2008

We tested the ac electrical properties of dielectric poly(propylmetacril heptaisobutyl-POSS)-co-(N-butylmethacrylate) thin films in order to better understand their working mechanism as relative humidity sensing devices. Samples with a 15% and 25% of cage content in weigh have been characterized as a function of temperature and frequency. The different frequency responses are reported and discussed in terms of the existing models. The temperature glass transition (Tg) has been measured by means of an electrical method on films in the same sensor configuration and correlated with the electronic transport properties. Furthermore, the interest of the material with a 15% of cage content is discussed in the contest of basic studies concerning water confinement effects at the molecular scale, i.e., the supercooling of water.
Nonadiabaticity in the iron bcc to hcp phase transformation
Authors: Johnson Donald F. , Carter Emily A.
Published: March 2008

Iron is known to undergo a pressure-induced phase transition from the ferromagnetic (FM) body-centered-cubic (bcc) α-phase to the nonmagnetic (NM) hexagonal-close-packed (hcp) ε-phase, with a large observed pressure hysteresis whose origin is still a matter of debate. Long ago, Burgers [Physica (Amsterdam) 1, 561 (1934)] proposed an adiabatic pathway for bcc to hcp transitions involving crystal shear followed by atom shuffles. However, a quantum mechanics search in six-dimensional stress-strain space reveals a much lower energy path, where the crystal smoothly shears along the entire path while the atoms shuffle only near the transition state (TS). The energy profile for this phase transition path exhibits a cusp at the TS and closely follows bcc and hcp diabatic energy wells. Both the cusp and the overlap with diabatic energy surfaces are hallmarks of nonadiabaticity, analogous to, e.g., electron transfer (ET) reactions in liquids. Fluctuations in the positions of FM bcc iron atoms near the TS induce magnetic quenching (akin to solvent fluctuations inducing ET), which then promotes NM hcp iron formation (akin to solvent reorganization after ET). We propose that the nonadiabatic nature of this transition at the atomic scale may contribute to the observed pressure hysteresis.
An XMCD study of the R(4f)–R(5d)T–(3d) interaction in R–T intermetallics
Authors: Laguna-Marco M. A. , Chaboy J. , Piquer C.
Published: March 2008

We present an x-ray magnetic circular dichroism (XMCD) study performed at the R L2 edge in RAl2 and RFe2 series. Regarding the XMCD spectra as made up of two different contributions, XMCDR and XMCDT, have allowed us to find an experimental evidence of the relationship between the XMCD signals and the R(4f)–Fe(3d) and R(4f)–R(4f) interatomic magnetic exchanges that govern the magnetic properties of the R–T intermetallic compounds. We have found that the R dependence of XMCDR and XMCDT signals remarkably mimics the modification of the nRR and the nRT molecular field coefficients, respectively. These results point out a new experimental method to study the 4f-5d-3d interactions in R–T intermetallics.
Coarse-graining the electron distribution in turbulence simulations of tokamak plasmas
Authors: Chen Yang , Parker Scott E. , Rewoldt Gregory , Ku Seung-Hoe , Park Gun-Young , Chang C-S
Published: March 2008

The coarse-graining procedure (CGP) [Y. Chen and S. E. Parker, Phys. Plasmas 14, 082301 (2007)] is implemented in the GEM code for electrons. While CGP introduces numerical dissipation in the particle-in-cell simulations, it is shown that CGP preserves physical dissipation effects such as the electron-ion collisional effects on the particle flux in ion-temperature-gradient driven turbulence. Kinetic simulation of an edge plasma is presented, which demonstrates the essential stabilizing effect of shear flow and the destabilizing effect of magnetic field perturbation in such a plasma.
Fabrication of electromechanical switch using interconnected single-walled carbon nanotubes
Authors: Chen Zhuo , Tong Lianming , Wu Zhongyun , Liu Zhongfan
Published: March 2008

We demonstrated a prototype of nanoelectromechanical switch which was fabricated with single-walled carbon nanotubes (SWCNTs). An alternating current electric field was utilized to assemble interconnected SWCNTs suspended over the focused ion beam lithography fabricated trenches on the electrode surface. By controlling the electromechanical coupling between the electrostatic and elastostatic forces acting on the SWCNTs, we realized the on and off states of the SWCNT switches. Both two-terminal and three-terminal measurements have been conducted and indicated repeatable switch effects.
Time- and space-resolved dynamics of ablation and optical breakdown induced by femtosecond laser pulses in indium phosphide
Authors: Bonse Jörn , Bachelier Guillaume , Siegel Jan , Solis Javier , Sturm Heinz
Published: March 2008

Femtosecond time-resolved microscopy has been used to analyze the structural transformation dynamics (melting, ablation, and solidification phenomena) induced by single intense 130 fs laser pulses in single-crystalline (100)-indium phosphide wafers in air on a time scale from ∼100 fs up to 8 ns. In the ablative regime close to the ablation threshold, transient surface reflectivity patterns are observed by fs microscopy on a ps to ns time scale as a consequence of the complex spatial density structure of the ablating material (dynamic Newton fringes). At higher fluences, exceeding six times the ablation threshold, optical breakdown causes another, more violent ablation regime, which reduces the energy deposition depth along with the time of significant material removal. As a consequence, ablation lasts longer in a ring-shaped region around the region of optical breakdown. This leads to the formation of a crater profile with a central protrusion. In the melting regime below the ablation threshold, the melting dynamics of indium phosphide has been quantified and subsequent superficial amorphization has been observed upon solidification on the ns time scale leading to amorphous layer thicknesses of the order of a few tens of nanometers.
Environmentally harmonized CF3I plasma for low-damage and highly selective low-k etching
Authors: Samukawa Seiji , Ichihashi Yoshinari , Ohtake Hiroto , Soda Eiichi , Saito Shuichi
Published: March 2008

Low-damage, high-rate, and highly selective low-k etching can be simultaneously satisfied using a plasma with an environmentally harmonized gas chemistry (CF3I). Such a CF3I plasma can drastically reduce the irradiation damage by ultraviolet (UV) photons during low-k etching, because the intensity of UV in CF3I plasma is much lower than that in conventional CF4 plasma. The etching selectivity of SiOCH to a photoresist can be drastically improved by using CF3I plasma because of reducing F radical generation. In addition, pulse-time-modulated CF3I plasma causes a drastic increase in the etching rate because a large amount of negative ions can be generated. These results show that CF3I plasma is a very promising candidate for low-damage and highly selective low-k etching.
The adsorption and desorption of ethanol ices from a model grain surface
Authors: Burke D. J. , Wolff A. J. , Edridge J. L. , Brown W. A.
Published: March 2008

Reflection absorption infrared spectroscopy (RAIRS) and temperature programed desorption (TPD) have been used to probe the adsorption and desorption of ethanol on highly ordered pyrolytic graphite (HOPG) at 98K. RAIR spectra for ethanol show that it forms physisorbed multilayers on the surface at 98K. Annealing multilayer ethanol ices (exposures >50L) beyond 120K gives rise to a change in morphology before crystallization within the ice occurs. TPD shows that ethanol adsorbs and desorbs molecularly on the HOPG surface and shows four different species in desorption. At low coverage, desorption of monolayer ethanol is observed and is described by first-order kinetics. With increasing coverage, a second TPD peak is observed at a lower temperature, which is assigned to an ethanol bilayer. When the coverage is further increased, a second multilayer, less strongly bound to the underlying ethanol ice film, is observed. This peak dominates the TPD spectra with increasing coverage and is characterized by fractional-order kinetics and a desorption energy of 56.3±1.7kJmol−1. At exposures exceeding 50L, formation of crystalline ethanol is also observed as a high temperature shoulder on the TPD spectrum at 160K.
Infrared–vacuum ultraviolet–pulsed field ionization–photoelectron study of CH3I+ using a high-resolution infrared laser
Authors: Xing Xi , Reed Beth , Bahng Mi-Kyung , Baek S.-J. , Wang Peng , Ng C. Y.
Published: March 2008

By using a high-resolution single mode infrared-optical parametric oscillator laser to prepare CH3I in single (J,K) rotational levels of the ν1 (symmetric C–H stretching) =1 vibrational state, we have obtained rovibrationally resolved infrared–vacuum ultraviolet–pulsed field ionization–photoelectron (IR-VUV-PFI-PE) spectra of the CH3I+(X̃E3∕22;ν1+=1;J+,P+) band, where (J,K) and (J+,P+) represent the respective rotational quantum numbers of CH3I and CH3I+. The IR-VUV-PFI-PE spectra observed for K=0 and 1 are found to have nearly identical structures. The IR-VUV-PFI-PE spectra for (J,K)=(5,0) and (7, 0) are also consistent with the previous J-selected IR-VUV-PFI-PE measurements. The analysis of these spectra indicates that the photoionization cross section of CH3I depends strongly on ∣ΔJ+∣=∣J+−J∣ but not on J and K. This observation lends strong support for the major assumption adopted for the semiempirical simulation scheme, which has been used for the simulation of the origin bands observed in VUV-PFI-PE study of polyatomic molecules. Using the state-to-state photoionization cross sections determined in this IR-VUV study, we have obtained excellent simulation of the VUV-PFI-PE origin band of CH3I+(X̃E3∕22), yielding more precise IE(CH3I)=76930.7±0.5cm−1 and ν1+=2937.8±0.2cm−1.
Probing nanoscale potential modulation by defect-induced gap states on GaAs(110) with light-modulated scanning tunneling spectroscopy
Authors: Yoshida Shoji , Kanitani Yuya , Takeuchi Osamu , Shigekawa Hidemi
Published: March 2008

We investigated charged defects on an n-GaAs(110) surface using light-modulated scanning tunneling spectroscopy. Tunneling via a single defect-induced gap state under photoillumination was observed for the isolated atomic defects. Screened Coulomb potentials induced around a charged Ga vacancy and a step edge were visualized, for the first time, with a nanometer spatial resolution. Furthermore, the charge states of the individual defects were determined on the atomic level.
Hot-electron bolometer terahertz mixers for the Herschel Space Observatory
Authors: Cherednichenko Sergey , Drakinskiy Vladimir , Berg Therese , Khosropanah Pourya , Kollberg Erik
Published: March 2008

We report on low noise terahertz mixers (1.4–1.9THz) developed for the heterodyne spectrometer onboard the Herschel Space Observatory. The mixers employ double slot antenna integrated superconducting hot-electron bolometers (HEBs) made of thin NbN films. The mixer performance was characterized in terms of detection sensitivity across the entire rf band by using a Fourier transform spectrometer (from 0.5to2.5THz, with 30GHz resolution) and also by measuring the mixer noise temperature at a limited number of discrete frequencies. The lowest mixer noise temperature recorded was 750K [double sideband (DSB)] at 1.6THz and 950K DSB at 1.9THz local oscillator (LO) frequencies. Averaged across the intermediate frequency band of 2.4–4.8GHz, the mixer noise temperature was 1100K DSB at 1.6THz and 1450K DSB at 1.9THz LO frequencies. The HEB heterodyne receiver stability has been analyzed and compared to the HEB stability in the direct detection mode. The optimal local oscillator power was determined and found to be in a 200–500nW range.
Melting and vaporization of Cu and Ni films during electron-beam heating
Authors: Sanchez Jaime A. , Pinar Mengüç M.
Published: March 2008

In this paper, we present a study of the phase change processes that take place in Cu and Ni films when they are heated with an electron-beam produced by field emission from an array of carbon nanotubes. A Monte Carlo method is adapted to solve the electron-beam Boltzmann transport equation to determine the electron distribution inside these materials. A hybrid approach is implemented to couple the two-temperature model with molecular dynamics simulations. We consider an analysis based on an order parameter and a radial distribution function to characterize the transition point at which the materials change phase. Slower electron diffusion in Ni produces more pronounced temperature gradients in both the electron system and the lattice, whereas the temperature rise throughout the Cu film is more uniform due to the faster electronic diffusion. We found that the phase change process is a combination of speed of the energy diffusion into the materials accompanied by a concentration of tensile stresses that contribute to the expansion and breakage of the films. We calculated the velocity of melting and vaporization fronts for both films and found two melting fronts for Cu converging close to the center of film with high velocity and one for Ni reaching a maximum speed and then stabilizing at a constant magnitude. We also explored the heating effects of multiple carbon nanotubes acting on these films.
Streamwise and spanwise plasma actuators for flow-induced cavity noise control
Authors: Huang Xun , Zhang Xin
Published: March 2008

Plasma actuators operating in atmospheric air can induce a body force through collisions between electrically charged particles and neutral air molecules by an externally applied electric field. The fast response and the simple structure make the plasma actuator a promising option in aerospace applications. In this work, experiments were performed with several alternative current excited plasma (streamwise and spanwise) actuators to control flow-induced noise from a cavity. It was found that the streamwise actuator induced three-dimensional variations in the shear layer. The spanwise actuator, however, has little influence on the global flow field. As a result, the streamwise actuator is more effective than the spanwise actuator in cavity noise attenuation.
Spectral properties of the two-photon absorption and emission process
Authors: Carbone R. , Fagnola F. , García J. C. , Quezada R.
Published: March 2008

The quantum Markov semigroup of the two-photon absorption and emission process has two extremal normal invariant states. Starting from an arbitrary initial state it converges toward some convex combination of these states as time goes to infinity (approach to equilibrium). We compute the exact exponential rate of this convergence showing that it depends only on the emission rates. Moreover, we show that off-diagonal matrix elements of any initial state go to zero with an exponential rate which is smaller than the exponential rate of convergence of the diagonal part. In other words quantum features of a state survive longer than the relaxation time of its classical part.
High resolution polar Kerr magnetometer for nanomagnetism and nanospintronics
Authors: Cormier M. , Ferré J. , Mougin A. , Cromières J.-P. , Klein V.
Published: March 2008

A new high resolution polar magneto-optical (MO) Kerr magnetometer, devoted to the study of nanometer sized elements with perpendicular magnetic anisotropy, is described. The unique performances of this setup in terms of sensitivity (1.2×10−15emu), stability (lateral drift ±35nm over 3h), and resolution (laser spot full width at half maximum down to 470nm) are demonstrated, and illustrated by Kerr hysteresis loop measurements on a unique ultrathin magnetic nanodot, and over small segments of ultranarrow magnetic tracks. Large scanning MO Kerr microscopy images were also obtained with the same performances.
Magnetotransport properties of CoFeB and Co∕Ru interfaces in the current-perpendicular-to-plane geometry
Authors: Ahn Chiyui , Shin Kyung-Ho , Pratt William P.
Published: March 2008

We have investigated magnetotransport properties of CoFeB and Co∕Ru interfaces by measuring the magnetoresistances of sputtered CoFeB-based exchange-biased spin valves and Co∕Ru multilayers on the first antiferromagnetic coupling peak at 4.2K in the current-perpendicular-to-plane geometry. The spin diffusion length lsf and the spin-scattering asymmetry β of Co40Fe40B20 and Co56Fe24B20 are estimated, and the interface specific resistance 2AR* and the interfacial spin-scattering anisotropy γ of Co∕Ru interfaces are constrained.
Observation of the high-Q modes inside the resonance zone of two-dimensional Bragg structures
Authors: Ginzburg N. S. , Peskov N. Yu. , Sergeev A. S. , Denisov G. G. , Kuzikov S. V. , Zaslavsky V. Yu. , Arzhannikov A. V. , Kalinin P. V. , Sinitsky S. L. , Thumm M.
Published: March 2008

Electrodynamical properties of two-dimensional Bragg resonators of planar geometry are studied in the frame of the geometrical-optical approach and in three-dimensional simulations. Results of the theoretical analysis are corroborated by data obtained in “cold” microwave measurements. Formation of the high-Q eigenmodes in the vicinity of the Bragg resonance frequency has been demonstrated.
Ab initio determination of Ehrlich–Schwoebel barriers on Cu{111}
Authors: Xiang S. K. , Huang Hanchen
Published: March 2008

Using the density-functional-theory based ab initio method, the authors determine the Ehrlich–Schwoebel barrier as a function of step thickness. The prototype material system is a Cu⟨110⟩ step of B type on a {111} surface. The calculation results show that the diffusion barrier of an adatom is 0.06eV on a flat Cu{111} surface, 0.16eV down a monolayer step, 0.39eV down a two-layer step, and 0.40eV down a multiple-layer step. The results, particularly the transition from 0.16to0.40eV, will have major impacts on the design of surface facets during synthesis of thin films and nanorods.
Finesse enhancement in silicon-on-insulator two-ring resonator system
Authors: Tobing Landobasa Y. M. , Lim Desmond C. S. , Dumon Pieter , Baets Roel , Chin Mee-Koy
Published: March 2008

We demonstrate experimentally the finesse enhancement in a pair of mutually coupled ring resonators coupled to two buses fabricated in silicon-on-insulator technology, as proposed theoretically in an earlier paper. A finesse close to 100 (or Q=30000) is obtained in a two-ring system, with the outer ring double the size of the inner ring, and an external coupling coefficient of 34%. The maximum finesse enhancement relative to the single-ring structure is 14 times, in good agreement with the theoretical prediction.
A simple yet effective multipass reflector for vibrational excitation in molecular beams
Authors: Riedel Jens , Yan Shannon , Kawamata Hiroshi , Liu Kopin
Published: March 2008

The fraction of molecules that can be vibrationally excited is often the limiting factor in many infrared laser excitation experiments, in particular, when using weak absorption bands. Reported here is a simple multipass reflector designed to overcome that obstacle. Its enhancement in pumping efficiency is demonstrated in a crossed-beam scattering experiment on the Cl+CH2D2(v1orv6=1) reactions. Compared to a double-pass arrangement, the effective laser fluence for excitation is also characterized.
Second harmonic generation in ridge Bragg reflection waveguides
Authors: Bijlani Bhavin , Abolghasem Payam , Helmy A. S.
Published: March 2008

Monolithic AlGaAs ridge Bragg reflection waveguides are used to achieve exact phase matching in a second harmonic generation experiment for a pump at 1600nm. Phase-matching bandwidth of 4.5nm, with a peak internal conversion efficiency of 205%∕Wcm2, is obtained. This conversion efficiency is comparable to numbers reported for periodically poled lithium niobate. The phase-matching peak was found to redshift with sample temperature providing tunablity at a rate of 0.25nm∕°C. Possible routes for enhancing the current efficiency levels are also discussed.
Phonon confinement and strain in CuInS2
Authors: Camus C. , Rudigier E. , Abou-Ras D. , Allsop N. A. , Unold T. , Tomm Y. , Schorr S. , Gledhill S. E. , Köhler T. , Klaer J. , Lux-Steiner M. C. , Fischer Ch.-H.
Published: March 2008

Single crystalline and polycrystalline CuInS2 samples prepared by different methods are characterized by Raman spectroscopy. The measured spectra are fitted according to the phonon confinement model. Correlation lengths were obtained, which correspond to the size of domains of perfect crystallinity. These correlation lengths are in good agreement with distances between twin defects observed by transmission electron microscopy in polycrystalline CuInS2. Additionally, the strain present in the samples was determined from the Raman spectra. A tensile strain was obtained for the polycrystalline CuInS2 thin films, which agrees well with published values for the same material.
Abnormal oxidation characteristics of SiGe∕Si-on-insulator structures depending on piled-up Ge fraction at SiO2∕SiGe interface
Authors: Tanaka Masanori , Ohka Tatsuo , Sadoh Taizoh , Miyao Masanobu
Published: March 2008

Oxidation characteristics of SiGe (Ge fraction of 0%–50%) layers grown on Si-on-insulator structures were investigated in a wide temperature range, for which different oxidation features were observed in low (<680°C), middle (700–800°C), and high (>800°C) temperature regions. In the low temperature region, enhanced oxidation dependent on the initial Ge fraction of the SiGe layers was observed. This is because the Ge fractions at the SiO2∕SiGe interfaces did not change from the initial Ge fractions in this region. In the high temperature region, the oxidation rates did not depend on the Ge fraction. This is because thick SiO2 (>200nm) formed in this region and, thus, oxidation rates were limited by the diffusion process of O atoms through the SiO2 layer. In the middle temperature region, an abnormal phenomenon was found; i.e., the oxidation rates did not depend on temperature for samples with high initial Ge fractions (>20%). Quantitative relations between the oxidation rate and fraction of Ge atoms piled up at the SiO2∕SiGe interfaces were obtained. Based on these findings, a model for temperature independent oxidation was proposed and it explains the contradicting oxidation rates, i.e. Ge enhanced oxidation and Ge retarded oxidation, in the literature.
Evaluation of domain wall motion in lead zirconate titanate ceramics by nonlinear response measurements
Authors: García J. E. , Pérez R. , Ochoa D. A. , Albareda A. , Lente M. H. , Eiras J. A.
Published: March 2008

Nonlinear response of pure and doped Pb(Zr1−xTix)O3 ceramics, with different compositions, has been analyzed in order to evaluate the domain wall motion in these materials. The study of dielectric and converse piezoelectric response shows a clear dependence of the domain wall mobility on the ferroelectric phase. Large lattice distortion in tetragonal samples produces a low mobility of the ferroelectric-ferroelastic domain walls. The influence of the type of defects on the nonlinear response has been explored. The results show that the relative increase of the domain walls mobility in donor-doped materials is greater than the decrease in acceptor-doped materials due to the pinning produced by complex defects. Rayleigh law has been used to evaluate the irreversible contribution of the domain walls movement to the nonlinear dielectric response. The analysis reveals that in presence of oxygen vacancies, the dielectric response cannot be attributed exclusively to the contribution of the irreversible domain wall motion. Furthermore, dielectric constant perpendicular to poling direction has been measured in rhombohedral donor-doped and acceptor-doped samples. The nonlinear response in the perpendicular direction is higher than in the poling direction, and this effect can be enlarged by the presence of complex defects. Dielectric nonlinear response has also been measured at different frequencies in rhombohedral donor-doped sample. A logarithmic dependence on frequency has been observed, which can be explained by the pinning effect on the ferroelectric domain walls by randomly distributed defects.
Temperature dependence surface structure of BaTiO3 thin films induced by ferroelectric phase transition
Authors: Li X. L. , Lu H. B. , Li Ming , Mai Zhenhong , Kim Hyunjung
Published: March 2008

Surface structure of BaTiO3 (BTO) films grown on SrTiO3 substrates was investigated as a function of temperature. X-ray reflectivity results show that there exists a low-density surface layer of 15Å due to surface relaxation on top of the BTO films in addition to a physical adsorbed water layer of about 5Å at 25°C. The water layer disappeared at 70°C; however, the surface layer remains its thickness until 115°C. At 140°C, the thickness of the surface layer decreases to 12Å and the electron density increases to a value near to that of the underneath BTO layer. The results of differential scanning calorimetry and the temperature dependence measurement of the lattice parameters convinced that the ferroelectric phase transition of the BTO films takes place around 135°C (∼130°C for bulk BTO). This indicates that the ferroelectric phase transition would contribute to the structural change of the surface layer due to the removal of the depolarization field. Our results indicate that the ferroelectric polarization strongly influences the surface structure of BTO films.
Energy band and vacuum level alignment at a semiconductor-molecule-metal interface
Authors: Cleri Fabrizio
Published: March 2008

We calculated the atomic structure and energy-level alignment at a representative Si-molecule-metal interface. The covalently bonded Si-molecule interface largely determines the overall band offset and the highest occupied molecular orbital position, while charge transfer across the metal-molecule interface induces localized π levels, even in the absence of covalent bonding to the metal. We propose a scheme for the vacuum level adjustment, consistent with the formation of interfacial dipoles and charge transfer to the molecular layer. The highest occupied π level of the molecule should be the main electronic state involved in the transport properties, while interface dipoles appear to be related to the interface-induced states.
Artificial pinning enhancement by multilayer nanostructures in MgB2∕Ni thin films
Authors: Takahashi Ken-ichiro , Kitaguchi Hitoshi , Doi Toshiya
Published: March 2008

We prepared MgB2∕Ni multilayer films on a polished Si (100) single crystal substrate by the growth of alternating layers of MgB2 (15nm) and Ni (∼1nm) layers. We then compared the magnetic field dependence of the critical current density Jc and the global pinning force Fp between films with and without the multilayer nanostructure. In the multilayer MgB2∕Ni film, Jc was higher in fields parallel to the substrate than in perpendicular fields, reflecting the layered nanostructure parallel to the substrate. The Jc-B curve has a plateau in parallel fields ranging between 1–5T, and Jc exceeds 1MA∕cm2, even at 6T (at 4.2K). The results indicate that the multilayer nanostructure is effective to enhance flux-pinning performance in parallel fields.
Effect of electric-field-assisted thermal annealing of poly(4-vinylphenol) film on its dielectric constant
Authors: Park Ji Hoon , Kim Eugene
Published: March 2008

We investigated the effect of annealing a polymeric dielectric film in the presence of an oscillating electric field on its dielectric constant. Films were prepared with the vertical structure of electrode/poly(4-vinylphenol)/electrode and annealed at a temperature above the glass transition temperature while applying an ac electric field ranging from 0to6V at various frequencies in the hope of altering the arrangement of polymer chains through conformational transitions influenced by the orientational polarization of the polar phenol groups. It was found that the dielectric constant increased significantly by about 30% when the annealing frequency corresponded to that at which dielectric relaxation takes place effectively.
Grain size effect of phase coexistence around morphotropic phase boundary in ferroelectric polycrystalline ceramics
Authors: Rao Wei-Feng , Wang Yu U.
Published: March 2008

Computer modeling and simulation reveals a grain size- and composition-dependent behavior of phase coexistence around the morphotropic phase boundaries in polycrystals of ferroelectric solid solutions. It shows that the width of phase coexistence composition range increases with decreasing grain sizes; phase-coexisting domain microstructures effectively reduce elastostatic, electrostatic, and domain wall energies; and grain boundaries impose internal mechanical and electric boundary conditions, which affect the phase-coexisting domain microstructures in the grains and give rise to the grain size effect of phase coexistence.
Aligning microcavity resonances in silicon photonic-crystal slabs using laser-pumped thermal tuning
Authors: Pan Jun , Huo Yijie , Yamanaka Kazuhiko , Sandhu Sunil , Scaccabarozzi Luigi , Timp Rolf , Povinelli Michelle L. , Fan Shanhui , Fejer M. M. , Harris James S.
Published: March 2008

We report the postfabrication alignment of multiple microcavity resonances in silicon photonic-crystal (PhC) slabs using laser-pumped thermal tuning. The thermal gradient resulting from a focused laser spot was used to differentially tune the resonant wavelengths of two microcavities spaced about 50μm apart. The resonant wavelengths could be brought closer together, over a tunable range of more than 5nm. A cross over in the resonant wavelengths was demonstrated, showing that two microcavities can be tuned to the identical wavelength. The results show that differential thermal tuning can be used to remove slight fabrication differences in nominally identical microcavities, relaxing the fabrication tolerances that will be required to realize coupled-resonator structures in PhCs.
Self-assembling conditions of 1O4Sr clusters in ZnTe:(Sr, O)
Authors: Elyukhin V. A.
Published: March 2008

Self-assembling (SA) conditions of 1O4Sr tetrahedral clusters in ZnTe:(Sr, O) with the Sr concentration of up to 0.02 and in the ultradilute oxygen limit are represented. The cause of SA is a decrease of the strain energy after the formation of the clusters. SA occurs at temperatures of 573, 773, and 973K if the Sr contents reach 1.23×10−4, 1.26×10−3, and 4.97×10−3, respectively. At the melting temperature of ZnTe, SA is absent at the considered Sr and oxygen concentrations. A temperature when all oxygen atoms are in clusters depends on the Sr and oxygen contents.
Room temperature photoluminescence from the amorphous Si structure generated under keV Ar-ion-induced surface rippling condition
Authors: Chini T. K. , Datta D. P. , Facsko S. , Mücklich A.
Published: March 2008

We observe room temperature (RT) visible and infrared (IR) photoluminescence (PL) bands peaked around 680 and 1020nm, respectively, from a silicon (Si) surface amorphized and patterned with ripples by 60keV Ar+ bombardment at 60° angle of ion incidence. However, the Si surface amorphized but not patterned under normal bombardment (0° angle of ion incidence) condition shows a drastic reduction in the intensity of the visible PL along with the complete suppression of IR emission. The present work demonstrates that Ar ion irradiation at rippling condition may yield a porouslike light emitting amorphous silicon (a-Si) nanostructure.
Macroscopic pattern formation preceding martensitic transformation in a ferromagnetic shape memory alloy Ni51Fe22Ga27
Authors: Murakami Y. , Shindo D. , Kainuma R. , Oikawa K. , Ishida K.
Published: March 2008

This paper reports a macroscopic pattern formation that occurs before the onset of a martensitic transformation in a ferromagnetic shape memory alloy, Ni51Fe22Ga27. Electron holography studies reveal that the magnetic flux pattern in the parent (cubic) phase changes dramatically with cooling. Interestingly, the flux pattern developed in the parent phase is inherited by the martensitic (monoclinic) phase. Observations of the diffuse electron scattering indicate that the change in the magnetic flux pattern is triggered by a lattice anomaly, which appears to produce a long-range strain field in the parent phase.
Combined nonlinear-optical electric and magnetic field response in a cadmium manganese telluride crystal
Authors: Chen Chia-Chu , Whitaker John F.
Published: March 2008

Utilizing experimental results, which demonstrate the presence of both Faraday rotation and electric-field-induced linear birefringence in a diluted-magnetic-semiconductor crystal of cadmium manganese telluride (CMT), a single probe that is capable of sensing both electric and magnetic fields independently has been developed. A higher field sensitivity and greater accuracy are observed for the CMT crystal when compared to a lithium tantalate electro-optic crystal and terbium gallium garnet magneto-optic crystal. The linear electro-optic coefficient r41 for CMT has been calculated from electric-field measurements to be 3.5±0.2pm∕V.
Determining the interfacial density of states in metal-insulator-semiconductor devices based on poly(3-hexylthiophene)
Authors: Alves N. , Taylor D. M.
Published: March 2008

Low frequency admittance measurements are used to determine the density of interface states in metal-insulator-semiconductor diodes based on the unintentionally doped, p-type semiconductor poly(3-hexylthiophene). After vacuum annealing at 90°C, interface hole trapping states are shown to be distributed in energy with their density decreasing approximately linearly from ∼20×1010to5×1010cm−2eV−1 over an energy range extending from 0.05to0.25eV above the bulk Fermi level.
Enhanced broadband near-infrared luminescence and optical amplification in Yb–Bi codoped phosphate glasses
Authors: Ruan Jian , Wu E , Zeng Heping , Zhou Shifeng , Lakshminarayana G. , Qiu Jianrong
Published: March 2008

Yb–Bi codoped phosphate glass was prepared and its properties were compared with Bi-doped phosphate glass. The broadband infrared luminescence intensity from Yb–Bi codoped glass was ∼32 times stronger than that of Bi-doped glass. The single-pass optical amplification was measured on a traditional two-wave mixing configuration. No optical amplification was observed in Bi-doped glass, while apparent broadband optical amplification between 1272 and 1336nm was observed from Yb–Bi codoped glass with 980nm laser diode excitation. The highest gain coefficient at 1272nm of Yb–Bi codoped glass reached to 2.62cm−1. Yb–Bi codoped phosphate glass is a promising material for broadband optical amplification.
Two-dimensional signatures for molecular identification
Authors: Qazi Muhammad , Vogt Thomas , Koley Goutam
Published: March 2008

Simultaneous measurements of the conductance and surface work function (SWF) changes on nanostructured graphite layers have been performed to detect several gaseous analyte molecules. It has been observed that the gradient of the SWF versus conductance response plotted for specific analyte molecules is constant irrespective of their concentration or fractional occupancy of surface adsorption sites. The SWF and conductance changes have been found to be uncorrelated for different analyte molecules, resulting in unique gradients that can be used as two-dimensional signatures for molecular identification.
Assessing plastic shear resistance of bulk metallic glasses under nanoindentation
Authors: Wang L. , Song S. X. , Nieh T. G.
Published: March 2008

Nanoindentation tests were conducted on three different bulk metallic glasses (BMGs) using both the Berkovich and cube-corner tips. Larger displacement and more pronounced pop ins were observed under cube-corner tip in all three materials. Pop-in distance and indentation depths were used to determine the plastic shear resistance of different BMGs under nanoindentation. The plastic shear resistance was found to be independent of the shape of the indenter and dependent only on material; it is linearly scaled with the shear modulus of a material.
Multilevel nonvolatile memory effects in hybrid devices containing CdSe∕ZnS nanoparticle double arrays embedded in the C60 matrices
Authors: Li Fushan , Cho Sung Hwan , Son Dong Ick , Park Kyu Ha , Kim Tae Whan
Published: March 2008

Electrical properties of nonvolatile memory devices containing core/shell CdSe∕ZnS nanoparticle double arrays embedded in the C60 layers formed by using a spin-coating technique were investigated. Transmission electron microscopy images showed that CdSe∕ZnS nanoparticles were randomly distributed in the C60 layers. Capacitance-voltage (C-V) measurements on Al∕C60/double-stacked CdSe∕ZnS nanoparticle arrays/C60∕p-Si devices showed that the flat-band voltage shift of the C-V curve related to the charge storage density was enhanced due to a stack of the CdSe∕ZnS nanoparticle layers and that the flat-band voltage shift increased with the magnitude of applied bias voltage due to the variations of the charged electron density in the stacked CdSe∕ZnS nanoparticle double arrays.
Real-time thermal imaging of catastrophic optical damage in red-emitting high-power diode lasers
Authors: Ziegler Mathias , Tomm Jens W. , Elsaesser Thomas , Matthiesen Clemens , Bou Sanayeh Marwan , Brick Peter
Published: March 2008

The dynamics of the catastrophic optical damage process under continuous wave operation is analyzed in red-emitting high-power diode lasers by means of combined thermal and optical near-field (NF) imaging with cameras. The catastrophic process is revealed as extremely fast (Δt⩽2.3ms) and spatially confined. It is connected with a pronounced impulsive temperature change. Its coincidence with the most intense NF filament is indicative of the critical nature of thermal runaway in the catastrophic process.
High-mobility, low-power, and fast-switching organic field-effect transistors with ionic liquids
Authors: Ono S. , Seki S. , Hirahara R. , Tominari Y. , Takeya J.
Published: March 2008

We report high-mobility rubrene single-crystal field-effect transistors with ionic-liquid (IL) electrolytes used for gate dielectric layers. As the result of fast ionic diffusion to form electric double layers, their capacitances remain more than 1μF∕cm2 even at 0.1MHz. With high carrier mobility of 1.2cm2∕Vs in the rubrene crystal, pronounced current amplification is achieved at the gate voltage of only 0.2V, which is two orders of magnitude smaller than that necessary for organic thin-film transistors with dielectric gate insulators. The results demonstrate that the IL/organic semiconductor interfaces are suited to realize low-power and fast-switching field-effect transistors without sacrificing carrier mobility in forming the solid/liquid interfaces.