bluuebunny/crossref_metadata_2025 huggingface dataset:
A molecular dynamics model for the interaction of energetic ions with SiOCH low-κ dielectric
Authors: Smirnov V. V. , Stengach A. V. , Gaynullin K. G. , Pavlovsky V. A. , Rauf S. , Ventzek P. L. G.
Published: March 2007

A molecular dynamics model is used to investigate the interaction of energetic ions with fluorocarbon passivated Si, O, C, and H (SiOCH) based low-κ dielectrics. The model includes a set of interatomic potentials required for the SiOCH–CFx interaction system, where the two- and three-body pseudopotentials have either been obtained from published literature or computed using ab initio techniques. The test structure used for the ion interaction simulations is put together through deposition of low energy SiOx+, CHy+, and H+ ions on a crystalline Si substrate. A thin fluorocarbon passivation layer is grown on the low-κ test structures by bombarding them with moderate energy CFx+ ions. Simulations of CF2+ ion interaction with the fluorocarbon passivated SiOCH samples show that the sputter yield of sample constituents (Si, O, and H) increases with ion energy and peaks at about 60°. H sputters more easily compared to other species, and the surface layer is expected to become H deficient over time. Sputtered H atoms are also generated over a broader region near the surface compared to other species. Most sputtered clusters with origin in the bulk film are ejected with energies less than 10eV and their angular and energy distributions are not sensitive to the energy or angle of the incident ion. Incident CF2+ ion breaks apart on contact with the test structure and, at high energies and near normal incidence, virtually no CF2 reflects back from the sample. Fragments of the incident ion have reflectionlike properties: peak in angular dependence function shifts towards larger angles as ion angle of incidence increases and reflected fragments are more energetic as ion angle of incidence increases. Comparison of CF2+ ion etching properties of SiOCH low-κ dielectric with SiO2 shows that more atoms are sputtered from SiOCH under identical conditions. However, as many of the sputtered atoms from the SiOCH material are light H atoms, mass etch yield from SiOCH and SiO2 is comparable. Si and O are both found to sputter more easily from SiOCH relative to SiO2. SiOCH low-κ ion etching properties are compared for several ions (CFx+, CHFy+, SiFz+, and Ar+). Results show that the etch yield from F containing ions is larger than that of Ar+, and the etch yield increases as the F content of the ion increases. Comparison of the ion etching properties of porous and nonporous low-κ dielectrics shows that, under similar conditions, the fluorocarbon passivation layer is thicker on the porous material. Due to this thicker passivation layer, mass yield from the porous dielectric material is smaller for the same ion energy.
A computationally efficient model of convective heat transfer and solidification characteristics during keyhole mode laser welding
Authors: Rai R. , Roy G. G. , DebRoy T.
Published: March 2007

Computationally efficient heat transfer models of keyhole mode laser welding ignore fluid flow in the gas, liquid, and the two phase solid-liquid regions. These models cannot be applied to high Peclet number systems where convective heat transfer affects weld pool geometry, cooling rate, and other weld attributes. Here we show that by synthesizing features of an existing model to determine keyhole shape and size with rigorous fluid flow and heat transfer calculations in the liquid and the two phase solid-liquid regions, important features of both high and low Peclet number systems can be satisfactorily simulated. The geometry of the keyhole is calculated by assuming thermal equilibrium at the gas/liquid interface and point by point heat balance at the keyhole wall. The heat transfer outside the vapor cavity is calculated by numerically solving the equations of conservation of mass, momentum, and energy. A vorticity based turbulence model is used to estimate the values of effective viscosity and effective thermal conductivity of the liquid metal in the weld pool. It is shown that the temperature profile and the weld pool shape and size depend strongly on the convective heat transfer for low thermal conductivity alloys like stainless steel. For high thermal conductivity aluminum alloys, on the other hand, convection does not play a significant role in determining the shape and size of the weld pool. The computed solidification parameters indicated that the solidification structure becomes less dendritic and coarser with the decrease in welding velocity. The results demonstrate that a numerically efficient convective heat transfer model of keyhole mode laser welding can significantly improve the current understanding of weld attributes for different materials with widely different thermal properties.
Irradiation effects on domain dynamics in ferroelectric glycine phosphite
Authors: Vanishri S. , Reddy J. N. Babu , Bhat H. L.
Published: March 2007

The effect of x-ray irradiation on polarization switching properties of a glycine phosphite (GPI) single crystal is investigated using the Merz technique. GPI undergoes a paraelectric to ferroelectric phase transition at 224.7 K. Switching studies were performed in the temperature range of 173–221 K with the applied field varying from 1.1 to 4.9 kV/cm. X-ray irradiation causes an increase in the threshold field below which switching could not occur. The activation energy for the domain wall motion is found to increase with irradiation from 0.12 eV for a pristine sample to 0.34 eV after irradiating it for 20 min. The temperature dependence of domain wall mobility shows an unusual behavior with the radiation dosage. The calculated domain wall mobility increases with the exposure time, which is contrary to expectation. This apparent increase in mobility with exposure time is attributed to the creation of a large number of radiation induced defects, identified as PO32− radicals, resulting in the loss of effective switchable volume of the sample.
Elastic anisotropy of an Fe79Si12B9 amorphous alloy thin film studied by ultrasound spectroscopy
Authors: Tarumi Ryuichi , Shibata Akira , Ogi Hirotsugu , Hirao Masahiko , Takashima Kazuki , Higo Yakichi
Published: March 2007

A complete set of elastic constants Cij of an Fe79Si12B9 amorphous alloy free standing thin film has been determined by using ultrasound spectroscopy techniques. The out-of-plane shear moduli of C44 and C55 are measured by the thickness resonance frequencies and are almost elastic isotropic in the film plane indicating that the thin film has a hexagonal-type elastic symmetry. The remaining Cij in the symmetry are determined by the modified resonance ultrasound spectroscopy coupled with a laser-Doppler interferometory. Inverse calculation showed elastic anisotropies between the in-plane and the out-of-plane directions; C33/C11=1.21 and C66/C44=1.26. Magnetic property measurements also revealed the remarkable magnetization difference between the two directions suggesting that the structural difference between the in-plane and the out-of-plane directions would be responsible for them.
Scalings and decay of fractal-generated turbulence
Authors: Hurst D. , Vassilicos J. C.
Published: March 2007

A total of 21 planar fractal grids pertaining to three different fractal families have been used in two different wind tunnels to generate turbulence. The resulting turbulent flows have been studied using hot wire anemometry. Irrespective of fractal family, the fractal-generated turbulent flows and their homogeneity, isotropy, and decay properties are strongly dependent on the fractal dimension Df≤2 of the grid, its effective mesh size Meff (which we introduce and define) and its ratio tr of largest to smallest bar thicknesses, tr=tmax∕tmin. With relatively small blockage ratios, as low as σ=25%, the fractal grids generate turbulent flows with higher turbulence intensities and Reynolds numbers than can be achieved with higher blockage ratio classical grids in similar wind tunnels and wind speeds U. The scalings and decay of the turbulence intensity u′∕U in the x direction along the tunnel’s center line are as follows (in terms of the normalized pressure drop CΔP and with similar results for v′∕U and w′∕U): (i) for fractal cross grids (Df=2), (u′∕U)2=tr2CΔPfct(x∕Meff); (ii) for fractal I grids, (u′∕U)2=tr(T∕Lmax)2CΔPfct(x∕Meff), where T is the tunnel width and Lmax is the maximum bar length on the grid; (iii) for space-filling (Df=2) fractal square grids, the turbulence intensity builds up as the turbulence is convected downstream until a distance xpeak from the grid is reached where the turbulence intensity peaks and then decays exponentially, u′2=upeak′2exp[−(x−xpeak)∕lturb], where upeak′2 increases linearly with tr, xpeak∝tminT∕Lmin (Lmin being the minimum bar length on the grid), and lturb∝λ2U∕ν (ν being the kinematic viscosity of the air and λ being the Taylor microscale); λ remains approximately constant during decay at x≫xpeak. The longitudinal and lateral integral length scales also remain approximately constant during decay at x≫xpeak.
Soft x rays from a free-electron laser resolve a single, micron-sized structure
Authors: Day Charles
Published: March 2007

An FEL’s intense beam can destroy a tiny object in femtoseconds, but not before scattered photons escape with the object’s structural plan.
Germany singles out universities for excellence
Authors: Feder Toni
Published: March 2007

In a departure from aiming for all universities to be equally good, Germany is now trying to make a select few internationally top tier.
Democrats expect to push strong science agenda in new Congress
Authors: Dawson Jim
Published: March 2007

The bipartisan spirit may be tested by confrontations on global warming and the administration’s opposition to expanded stem cell research.
The Bethe ansatz after 75 years
Authors: Batchelor Murray T.
Published: March 2007

A 1931 result that lay in obscurity for decades, Bethe’s solution to a quantum mechanical model now finds its way into everything from superconductors to string theory.
Particle acceleration in turbulent flows: A class of nonlinear stochastic models for intermittency
Authors: Bini M. , Jones W. P.
Published: March 2007

The problem of modeling the velocity and acceleration of inertial particles in turbulent flows is discussed. Particular attention is focused on the modeling of the particle Lagrangian velocity increment, especially, but not exclusively, in the case in which only the low frequencies of the carrier turbulent flow field are available. The need for suitable models arises in the simulation of particle laden flows by the means of new computational techniques such as large-eddy simulation. For this, stochastic differential equations, sde, are often introduced, though there is a lack of clarity in how such models should deal with the experimental observed far from Gaussian statistics, intermittency, and heavy tailed probability density function for particle acceleration. It is well known that Langevin-type equations are not capable of reproducing such features. It is first shown how the stochastic model for the particle Lagrangian velocity increments is far from being a Langevin equation, and it is characterized by nonlinear drift and diffusion; the statistical characteristics of this first model are shown to be in qualitative agreement with experimental findings. These results suggest an improved model for the particle dynamics based upon a more general family of nonlinear sde; the family, which is generated by a single parameter, includes both the Langevin equation and the first model as special cases. An analysis of the statistical properties of the new sde shows that the model is capable of accurately reproducing the strong deviations from Gaussianity observed in recent experiments.
Transport properties of quasi-free fermions
Authors: Aschbacher W. , Jakšić V. , Pautrat Y. , Pillet C.-A.
Published: March 2007

Using the scattering approach to the construction of nonequilibrium steady states proposed by Ruelle [J. Stat. Phys. 98, 57–75 (2000)], we study the transport properties of systems of independent electrons. We show that Landauer-Büttiker and Green-Kubo formulas hold under very general conditions.
Transition probabilities on Ga(P2,S2,andP2)+CH4 reactions
Authors: Pacheco-Sánchez J. H. , Castillo S. , Luna-García H. , Novaro O.
Published: March 2007

In a previous item the authors reported potential energy surfaces (PESs) of the Ga–CH4 reaction, where no mention was made of transition probabilities on the avoided crossings between curves. Here they tackle this problem by applying the Landau-Zener (LZ) premise to obtain transition probabilities at these crossings and they present new developments based on the LZ theory for avoided crossings by considering the insertion angle θ as the reaction parameter. The nonadiabatic transition probability depends on the angular velocity, which leads to the inertia moment. These developments are applied to the avoided crossings of three A′ PESs of the Ga(P2,S2,andP2)+CH4 interactions.
Efficient vector potential method for calculating electronic and nuclear response of infinite periodic systems to finite electric fields
Authors: Springborg Michael , Kirtman Bernard
Published: March 2007

The response of periodic systems to external electric fields is a challenging theoretical problem. The authors show how the vector potential approach yields a numerically efficient treatment of the combined electronic and nuclear response to a finite static field. Their method is based on a self-consistent reformulation of the charge flow term in the single particle Hamiltonian. Careful numerical implementation yields a treatment whose computational needs are only marginally larger than those of a conventional field-free calculation. To prove the method exemplary polymer calculations are done for a model Hamiltonian. The latter contains all essential elements of an ab initio Kohn-Sham or Hartree-Fock Hamiltonian but allows for extensive testing. The extension to three-dimensional systems is described.
New candidate emerges for a quantum spin liquid
Authors: Levi Barbara Goss
Published: March 2007

A newly synthesized mineral is perhaps the most promising material yet to realize a hypothetical state with exotic behavior.
Designer materials render objects nearly invisible to microwaves
Authors: Wilson Mark
Published: March 2007

Coordinate transformations and curved spaces, the traditional tools of general relativity, are finding applications in optical engineering.
Negative magnetoresistance in fractal Pb thin films on Si(111)
Authors: Wang Jian , Ma Xu-Cun , Qi Yun , Fu Ying-Shuang , Ji Shuai-Hua , Lu Li , Jia Jin-Feng , Xue Qi-Kun
Published: March 2007

Using a low temperature method, the authors have prepared atomically flat Pb ultrathin films on Si(111)-7×7 surface. Room temperature annealing of the films results in a percolation morphology with fractal vacancy islands where the Si substrate is exposed. The fractal film with a nominal thickness of 23 ML exhibits enhanced onset superconducting transition temperature of 7.0K and negative magnetoresistance with wide magnetoresistance terrace under perpendicular magnetic field when the film is in superconducting state. They attribute the phenomena to the coexistence of two superconducting phases in this fractal film.
Bipolar resistive switching in polycrystalline TiO2 films
Authors: Tsunoda K. , Fukuzumi Y. , Jameson J. R. , Wang Z. , Griffin P. B. , Nishi Y.
Published: March 2007

Bipolar resistive switching was found in thin polycrystalline TiO2 films formed by the thermal oxidation of sputtered Ti films. With a Ag top electrode, TiO2 film, and Pt bottom electrode, bistable resistive switching with a low operating voltage and a good uniformity was observed repeatedly without an initial electrical “forming” process. This switching phenomenon might be described as the formation and rupture of a filamentary conductive path consisting of a chain of Ag atoms. The temperature dependence of the switching voltage is discussed in terms of interstitial ionic diffusion of Ag in the TiO2 matrix.
Computer-controlled susceptometer for investigating the linear and nonlinear dielectric response
Authors: Miga S. , Dec J. , Kleemann W.
Published: March 2007

A fully automatized alternating current (ac) susceptometer is constructed for simultaneous measurements of the phase resolved complex linear and complex nonlinear ac susceptibilities of lossy and dispersive dielectric materials. This relatively simple setup allows measurements over a wide range of experimental variables, such as ac amplitudes up to 40 V, frequencies from 10−2 to 103 Hz, and temperatures from 100 to 600 K utilizing only current/voltage and analog/digital converters and a personal computer. In contrast with the commonly used analysis of the charge accumulated on a standard capacitor in series with the sample our method is based on the analysis of the current flowing directly through the sample. Absence of any capacitive voltage dividers in the measurement circuit eliminates uncontrolled phase shifts. This is why the instrument provides high quality, nonlinear susceptibility data and in particular appears as a very convenient tool for discrimination between continuous and discontinuous phase transitions when determining the sign of the real part of the third order dielectric susceptibility.
Mega-ampere submicrosecond generator GIT-32
Authors: Kovalchuk B. M. , Kharlov A. V. , Kiselev V. N. , Kumpyak E. V. , Zorin V. B. , Chupin V. V. , Morozov A. V.
Published: March 2007

The GIT-32 current generator was developed for experiments with current carrying pulsed plasma. The main parts of the generator are capacitor bank, multichannel multigap spark switches, low inductive current driving lines, and central load part. The generator consists of four identical sections, connected in parallel to one load. The capacitor bank is assembled from 32 IEK-100-0.17 (0.17μF, 40nH, 100kV) capacitors, connected in parallel. It stores ∼18kJ at 80kV charging voltage. Each two capacitors are commuted to a load by a multigap spark switch with eight parallel channels. Switches operate in ambient air at atmospheric pressure. The GIT-32 generator was tested with 10, 15, and 20nH inductive loads. At 10nH load and 80kV of charging voltage it provides 1MA of current amplitude and 490ns rise time with 0.8Ω damping resistors in discharge circuit of each capacitor and 1.34MA∕530ns without resistors. The net generator inductance without a load was optimized to be as low as 12nH, which results in extremely low self-impedance of the generator (∼0.05Ω). It ensures effective energy coupling with low impedance loads like Z pinch. The generator operates reliably without any adjustments in 40–80kV range of charging voltage. Maximum jitter (relative to a triggering pulse) at 40kV charging voltage is about 7ns and lower at higher charging voltages. Operation and handling are very simple, because no oil and no purified gases are required for the generator. The GIT-32 generator has dimensions of 3200×3200×400mm3 and total weight of about 2500kg, thus manifesting itself as a simple, robust, and cost effective apparatus.
Electrical properties of oxide heteroepitaxial p-n junctions: La1−xSrxFeO3∕SrTi0.99Nb0.01O3
Authors: Yamamoto A. , Sawa A. , Akoh H. , Kawasaki M. , Tokura Y.
Published: March 2007

Electrical properties of heteroepitaxial p-n junctions composed of correlated electron semiconductors La1−xSrxFeO3 (LSFO) (x=0.4, 0.5, 0.67, and 0.8) and a band semiconductor SrTi0.99Nb0.01O3 were analyzed. The junctions exhibited rectifying current-voltage (I-V) characteristics. The built-in potential (VbiSTO) evaluated from the capacitance-voltage characteristics under reverse bias increased from 0.9eV for x=0.4to1.2eV for x=0.8 junctions, indicating a large downward shift in chemical potential of p-type LSFO due to hole doping. Some of the junctions showed hysteretic I-V characteristics, which have been commonly observed in resistance switching devices, but others did not. LSFO junctions with higher doping (x=0.8) showed higher probability of hysteretic behavior.
Spin-polarized transport in ferromagnetic multilayered semiconductor nanostructures
Authors: Oliveira E. J. R. , da Cunha Lima A. T. , Boselli M. A. , Sipahi G. M. , Rodrigues S. C. P. , da Cunha Lima I. C.
Published: March 2007

The occurrence of inhomogeneous spin-density distribution in multilayered ferromagnetic diluted magnetic semiconductor nanostructures leads to strong dependence of the spin-polarized transport properties on these systems. The spin-dependent mobility, conductivity, and resistivity in (Ga,Mn)As∕GaAs, (Ga,Mn)N∕GaN, and (Si,Mn)∕Si multilayers are calculated as a function of temperature, scaled by the average magnetization of the diluted magnetic semiconductor layers. An increase of the resistivity near the transition temperature is obtained. The authors observed that the spin-polarized transport properties change strongly among the three materials.
In situ electrical conductivity measurement of high-pressure molten (Mg0.875,Fe0.125)2SiO4
Authors: Li Ming , Gao Chunxiao , Ma Yanzhang , Wang Duojun , Li Yanchun , Liu Jing
Published: March 2007

In situ resistance measurement of mantle mineral under high temperature and pressure has been considered an important method for studying the electrical properties and thermal states of Earth’s interior. Here the authors report the results of the electrical conductivity of molten olivine [(Mg0.875,Fe0.125)2SiO4] on diamond anvil cell with pressure at 13.2GPa and temperature at 3720K. The results indicate that the activation enthalpy of molten olivine is much less than that of solid, and its conductivity is relatively insensitive to temperature. Moreover, at the given temperature range the conductivity of molten olivine exhibits Arrhenius behavior perfectly. Compared to the results of Hawaiian tholeiite provided by Tyburczy and Waff [J. Geophys. Res. 88, 1413 (1983)] at lower pressure and temperature, the pressure effect on molten olivine conductivity is slightly weaker. This method for electrical conductivity measurement on laser-heated diamond anvil cell allows the environment simulation study of unresearched regions of Earth, no longer restricted to large cavity press.
Study of copper diffusion through a ruthenium thin film by photoemission electron microscopy
Authors: Wei Wei , Parker S. L. , Sun Y.-M. , White J. M. , Xiong Gang , Joly Alan G. , Beck Kenneth M. , Hess Wayne P.
Published: March 2007

Photoemission electron microscopy is used to study copper diffusion through a ruthenium thin film. The photoemission electron microscopy images display a large contrast between Cu and Ru due to the differences in work function, making photoemission electron microscopy an ideal methodology to study thin film diffusion in real time. Between 175 and 290°C, Cu mainly diffuses through defect sites in the thin Ru film. Uniform diffusion of Cu through the Ru film begins at approximately 300°C. The results are confirmed by x-ray photoemission spectroscopy depth profiling and scanning electron microscopy–energy dispersive x-ray spectroscopy analysis.
Prospects for electron imaging with ultrafast time resolution
Authors: Armstrong Michael R. , Reed Bryan W. , Torralva Ben R. , Browning Nigel D.
Published: March 2007

Many pivotal aspects of material science, biomechanics, and chemistry would benefit from nanometer imaging with ultrafast time resolution. Here the authors demonstrate the feasibility of short-pulse electron imaging with 10nm∕10ps spatiotemporal resolution, sufficient to characterize phenomena that propagate at the speed of sound in materials (1–10km∕s) without smearing. The authors outline resolution-degrading effects that occur at high current density followed by strategies to mitigate these effects. Finally, the authors present a model electron imaging system that achieves 10nm∕10ps spatiotemporal resolution.
Focusing quality of a split short laser pulse
Authors: Kotaki Hideyuki , Oishi Yuji , Nayuki Takuya , Fujii Takashi , Nemoto Koshichi , Nakajima Kazuhisa
Published: March 2007

For multiple laser pulse experiments, it is necessary to split a laser pulse. In order to split a short laser pulse without stretching the pulse width, the laser pulse should not pass through thick materials. For this reason, a pellicle beam splitter (BS) and/or a mirror with a hole are required as a BS for the short laser pulse. The focusing qualities of the laser pulse after passing through the pellicle BS and the mirror with a hole are the same as without the BS’s. The laser pulse quality reflected by the BSs should be considered for the laser pulse. A pellicle BS is a thin foil, so, it is weak against vibrations. One should be careful about airflows and isolation from vibration sources. The spot size of the reflected laser pulse is consistent with the size reflected by a normal mirror. The energy loss is about 10% compared with a normal mirror. A mirror with a hole is strong against external vibrations. The reflected laser pulse has a doughnut shape. The reflected laser pulse is interfered due to the shape. In order to cleanly focus the laser pulse, the inside size of the doughnut should be smaller than a half size of the outside portion of the doughnut.
Enhancement of anthracene fragmentation by circularly polarized intense femtosecond laser pulse
Authors: Murakami Masanao , Tanaka Michinori , Yatsuhashi Tomoyuki , Nakashima Nobuaki
Published: March 2007

The authors compared circularly and linearly polarized lights in the ionization and fragmentation of anthracene, using 800nm femtosecond laser pulses at intensities of 1013–1015Wcm−2. Singly and doubly charged intact molecular ions as well as numerous fragment ions were observed in the mass spectra, which were investigated as a function of laser intensity and polarization. At comparable intensities above the saturation threshold for complete ionization, the fragmentation pathways are enhanced with a circularly polarized field compared to a linearly polarized field. Resonant excitation of the molecular cation through the Au2←Bg2 transition is proposed to be the initial step to ion fragmentation. The circularly polarized field interacts with a larger fraction of the randomly oriented molecules than the linearly polarized field, and this is considered to be the reason for the enhanced fragmentation brought about by circularly polarized light.
Selectively coated photonic crystal fiber for highly sensitive fluorescence detection
Authors: Smolka Stephan , Barth Michael , Benson Oliver
Published: March 2007

The authors investigate the potential of hollow core photonic crystal fibers (HCPCFs) for highly sensitive fluorescence detection by selectively coating the central hole of a HCPCF with fluorophores. After studying the influence of reabsorption and intermolecular effects on the emission, they show that dye concentrations down to 10−9M can be detected using only nanoliter sample volumes. The results indicate that HCPCF are well suited for sensing applications involving internal emission or scattering processes, such as fluorescence based gas sensing or surface-enhanced Raman scattering, and may be integrated into compact optical devices.
Thickness dependent ion conductivity of lithium borate network glasses
Authors: Berkemeier F. , Shoar Abouzari M. , Schmitz G.
Published: March 2007

Lithium borate network glasses are possible candidates for separator membranes in all-solid-state batteries. Thin films of a Li2O-borate glass were produced by argon beam sputtering and their specific ionic conductivities were measured by impedance spectroscopy. The conductivity of as-sputtered films is about two orders of magnitude higher compared to the conductivity of bulk glasses produced from the melt. Furthermore, thin films with a thickness of 7–125nm reveal a remarkable finite size effect after annealing: with decreasing thickness the specific dc conductivity increases about three orders of magnitude.
Influence of interface layer composition on the electrical properties of epitaxial Gd2O3 thin films for high-K application
Authors: Laha Apurba , Osten H. J. , Fissel A.
Published: March 2007

The authors report on the impact of interface layer composition on electrical properties of epitaxial Gd2O3 thin films on Si(001) substrates. The electrical properties of epitaxial Gd2O3 thin films were improved significantly by controlled modification of interface layer composition. The minimum capacitance equivalent thickness estimated for Pt∕Gd2O3∕Si metal oxide semiconductor structures was as low as 0.76nm with leakage current density of 15mA∕cm2 at (Vg−VFB)=1V. The corresponding density of interface states was found to be 2.3×1012cm−2eV−1. The authors also find that a change in the interface layer composition significantly alters band alignment of Gd2O3 layer with respect to Si substrates.
Three-image diffraction enhanced imaging algorithm to extract absorption, refraction, and ultrasmall-angle scattering
Authors: Rigon Luigi , Arfelli Fulvia , Menk Ralf-Hendrik
Published: March 2007

As different methods to improve diffraction enhanced imaging are proposed, the authors introduce a simple algorithm that follows the original idea of Chapman et al. [Phys. Med. Biol. 42, 2015 (1997)], but extend it to a general object featuring absorption, refraction, and ultrasmall-angle scattering. The information relative to the three effects is decoupled, requiring only three images in input. Simulation and experiment give accurate results, provided the refraction and scattering angles are small compared to the rocking curve width. The proposed algorithm can be readily and fruitfully implemented in several applications, particularly when time and dose constraints are relevant.
Thermal instability and the growth of the InGaAs∕AlGaAs pseudomorphic high electron mobility transistor system
Authors: Pellegrino Joseph G. , Qadri Syed B. , Mahadik Nadeemullah A. , Rao Mulpuri V. , Tseng Wen F. , Thurber Robert , Gajewski Donald , Guyer Jonathan
Published: March 2007

The effects of temperature overshoot during molecular beam epitaxy growth on the transport properties of conventionally and delta-doped pseudomorphic high electron mobility transistor (pHEMT) structures have been examined. A diffuse reflectance spectroscopy (DRS)-controlled versus a thermocouple (TC)-controlled, growth scheme is compared. Several advantages of the DRS-grown pHEMTs over the TC-controlled version were observed. Modest improvements in mobility, on the order of 2%–3%, were observed in addition to a 20% reduction in carrier freeze-out for the DRS-grown pHEMTs at 77K.
Dielectric response of Sr doped CaCu3Ti4O12 ceramics
Authors: Li Wei , Schwartz Robert W. , Chen Aiping , Zhu Jinsong
Published: March 2007

Ca 1 − x Sr x Cu 3 Ti 4 O 12 (x=0, 0.1, and 0.2) ceramics were fabricated and their dielectric properties were investigated. It was found that the dielectric constant significantly decreased with the increase of Sr content at low temperature region (<250K) and remained almost unchanged at high temperature region (>250K). Three sets of relaxation peaks were observed in electric modulus plots, which were considered to be associated with grains, domain boundaries, and grain boundaries, respectively. Through the analysis of the heights and calculated activation energies of the relaxation peaks, it is strongly believed that the suppressed dielectric constant is related to the change of domain boundaries with Sr doping.
Screening the built-in electric field in 4H silicon carbide stacking faults
Authors: Juillaguet S. , Camassel J. , Albrecht M. , Chassagne T.
Published: March 2007

The authors report a detailed comparison of low temperature photoluminescence (LTPL) and cathodo luminescence (LTCL) spectra collected in the same stacking faults rich area of a 4H silicon carbide epitaxial layer. In both cases, they find that the maximum wavelength of the defect-related emission lines shifts when the excitation spot position moves across the defect zone. The shift is excitation-intensity dependent. It is very small for LTPL (4meV) but reach 20meV for LTCL. This constitutes the first experimental evidence that a screening of the quantum confined Stark effect can be achieved in 4H-SiC SF quantum wells.
Correlation between the dynamics of polar nanoregions and temperature evolution of central peaks in Pb[(Zn1∕3Nb2∕3)0.91Ti0.09]O3 ferroelectric relaxors
Authors: Ko Jae-Hyeon , Kim Do Han , Kojima Seiji
Published: March 2007

Acoustic properties of Pb[(Zn1∕3Nb2∕3)0.91Ti0.09]O3 have been investigated in a wide temperature range. The sound velocity of acoustic phonons showed a deviation from its high-temperature linear behavior at the Burns temperature TB∼730K. Upon cooling, acoustic properties exhibited significant changes at about 550K in polarization state and width of the central peak in addition to changes in frequency and damping of acoustic phonons. This finding suggests that, besides TB, another intermediate characteristic temperature exists in the evolution of relaxor dynamics, which might be related to the formation of long-lived polar nanoregions and associated local strain fields.
Relaxor behavior in the solid solution between dielectric Ba(Mg1∕3Nb2∕3)O3 and ferroelectric PbTiO3
Authors: Long Xifa , Ye Zuo-Guang
Published: March 2007

Relaxor behavior has been found in the solid solution between complex perovskite Ba(Mg1∕3Nb2∕3)O3 (BMN) and ferroelectric PbTiO3 (PT), 0.4BMN-0.6PT, prepared by solid state reactions. A strong dispersion of the maximum dielectric permittivity (ε′) appears around the temperature of Tm, which shifts towards higher temperatures with increasing frequency. The variation of Tm with frequency follows the Vogel-Fulcher relationship. The variation of 1∕ε′ with temperature above Tm deviates from the Curie-Weiss law but satisfies a Lorentz-type function. The dielectric hysteresis loops are displayed at several temperatures which become slimmer at T>Tm. The remnant polarization gradually decreases with increasing temperature and persists up to above Tm.
Temperature scaling of dynamic hysteresis in soft lead zirconate titanate bulk ceramic
Authors: Yimnirun R. , Wongmaneerung R. , Wongsaenmai S. , Ngamjarurojana A. , Ananta S. , Laosiritaworn Y.
Published: March 2007

The temperature scaling of the dynamic hysteresis was investigated in soft ferroelectric bulk ceramic. The power-law temperature scaling relations were obtained for hystersis area ⟨A⟩ and remnant polarization Pr, while the coercivity EC was found to scale linearly with temperature T. The three temperature scaling relations were also field dependent. At fixed field amplitude E0, the scaling relations take the forms of ⟨A⟩∝T−1.1024, Pr∝T−1.2322, and (EC0−EC)∝T. Furthermore, the product of Pr and EC also provides the same scaling law on the T dependence in comparison with ⟨A⟩.
Propagation of Lamb waves in one-dimensional quasiperiodic composite thin plates: A split of phonon band gap
Authors: Gao Jian , Cheng Jian-Chun , Li Baowen
Published: March 2007

The authors study numerically the propagation of Lamb waves in one-dimensional quasiperiodic composite thin plates made of tungsten (B) and silicon resin (A) arranged according to a Fibonacci sequence. It is found that the band-gap structures of Lamb waves are very different from those of bulk waves. The split of band gaps is independent of the number of layers, which is different from the quasiperiodic bulk photonic and phononic crystals. Possible applications are discussed.
Spin-orbit interaction enhanced polaron effect in two-dimensional semiconductors
Authors: Li Zhou , Ma Zhongshui , Wright A. R. , Zhang Chao
Published: March 2007

It is shown that in two-dimensional semiconductors, the electron-phonon interaction and polaron mass correction are both significantly enhanced by the Rashba spin-orbit coupling. The mass correction is positive for the upper Rashba branch and negative for the lower Rashba branch. Both Rashba branches have the same polaron binding energy, which is higher than that for systems in the absence of spin-orbit interaction. To the leading order, the correction to the binding energy is proportional to the square of the spin-orbit coupling strength.
Simultaneous filling of InAs quantum dot states from the GaAs barrier under nonresonant excitation
Authors: Rainò G. , Visimberga G. , Salhi A. , De Vittorio M. , Passaseo A. , Cingolani R. , De Giorgi M.
Published: March 2007

The authors have performed time resolved photoluminescence measurements by upconversion technique on InAs quantum dots embedded in an InGaAs∕GaAs quantum well emitting at 1.3μm at room temperature. A detailed analysis of the photoluminescence transients as a function of the excitation density and for different detection energies between the quantum dot transitions and the GaAs absorption edge shows that the intradot relaxation is slower than the direct carrier capture from the barrier states through a continuum background relaxation.
Energy harvesting with piezoelectric drum transducer
Authors: Wang Sheng , Lam Kwok Ho , Sun Cheng Liang , Kwok Kin Wing , Chan Helen Lai Wa , Guo Ming Sen , Zhao Xing-Zhong
Published: March 2007

Piezoelectric materials can convert ambient vibrations into electrical energy. In this letter, the capability of harvesting the electrical energy from mechanical vibrations in a dynamic environment through a piezoelectric drum transducer has been investigated. Under a prestress of 0.15N and a cyclic stress of 0.7N, a power of 11mW was generated at the resonance frequency of the transducer (590Hz) across an 18kΩ resistor. It is found that the energy from the transducer increases while the resonance frequency of the transducer decreases when the prestress increases. The results demonstrate the potential of the drum transducer in energy harvesting.
Formation of single crystalline ZnO nanotubes without catalysts and templates
Authors: Mensah Samuel L. , Kayastha Vijaya K. , Ivanov Ilia N. , Geohegan David B. , Yap Yoke Khin
Published: March 2007

Oxide and nitride nanotubes have gained attention for their large surface areas, wide energy band gaps, and hydrophilic natures for various innovative applications. These nanotubes were either grown by templates or multistep processes with uncontrollable crystallinity. Here the authors show that single crystal ZnO nanotubes can be directly grown on planar substrates without using catalysts and templates. These results are guided by the theory of nucleation and the vapor-solid crystal growth mechanism, which is applicable for transforming other nanowires or nanorods into nanotubular structures.
Microwave observation of the “recently found” polar OCS dimer
Authors: Minei Andrea J. , Novick Stewart E.
Published: March 2007

Recently Afshari et al. reported on the detection of a new infrared band which was assigned to the “long-anticipated polar isomer of the OCS dimer” [J. Chem. Phys. 126, 071102 (2007)]. The authors report here the microwave confirmation of their results. The lowest energy, nonpolar isomer of (OCS)2 has long been known from IR spectroscopy, while the polar form has only been deduced from qualitative beam refocusing experiments. The higher energy, polar isomer of (OCS)2 has been produced by high pressure expansion of dilute OCS in helium. A surprisingly strong microwave spectrum of Cs (OCS)2 has been observed and assigned.
Mapping the Interstellar Cloud We Live in
Authors: Schwarzschild Bertram
Published: March 2007

We often speak of the Hubble Space Telescope as a unique window on the most distant and ancient galaxies. But the HST can also tell us things we never knew about our most intimate interstellar surroundings. The 10 January Astrophysical Journal brings us the first of two articles, by Jeffrey Linsky, Seth Redfield, and colleagues at the University of Colorado, that offer a three-dimensional map of the “local interstellar cloud,” derived mostly from ultraviolet absorption spectra recorded by instruments aboard the HST. The LIC, they report, is a rather uniform, egg-shaped cloud of warm atomic hydrogen, only 20 light-years long, in which the Solar System and its surrounding heliosphere of solar wind and magnetic field sit like a tiny, offcenter yolk. (See the figure below.)
The Decreasing Arctic Ice Cover
Authors: Levi Barbara Goss
Published: March 2007

Ongoing monitoring of Earth's north polar region has been turning up numerous signs of change in its atmosphere, waters, and ice pack (see PHYSICS TODAY, November 1998, page 17). Such studies have revealed that the Arctic ice mass is shrinking, but now, it seems, the rates of decline are much more rapid than previously thought. Such changes are of concern because the polar region is expected to amplify any change in Earth's climate.
Experts Dismiss Doomsday Scenarios for RHIC
Authors: Levi Barbara Goss
Published: March 2007

The Relativistic Heavy Ion Collider (RHIC) recently completed at Brookhaven National Laboratory is expected to produce a plasma of quarks and gluons—the state in which matter is thought to have existed in the first microseconds of the Big Bang, before matter coalesced into the neutrons and protons that characterize our universe today (see PHYSICS TODAY, October 1999, page 20). Before entering any such new territory, explorers naturally speculate on what unknowns might be encountered. In this case, the speculations have included a few scenarios—recognized to be extremely remote even by those physicists who have raised them—that could possibly spell danger. Concerns over those scenarios were voiced last summer in the general press. To examine their likelihood, Brookhaven director John Marburger appointed a panel of particle physicists consisting of Wit Busza (MIT), Robert Jaffe (MIT), Jack Sandweiss (Yale University), and Frank Wilczek (Institute for Advanced Study). Their report concludes that “the candidate mechanisms for catastrophe scenarios at RHIC are firmly excluded by existing empirical evidence, compelling theoretical arguments, or both.” (www.bnl.gov/bnlweb/ rhicreport.html)
Motile Behavior of Bacteria
Authors: Berg Howard C.
Published: March 2007

Escherichia coli is a single-celled organism that lives in your gut. It is equipped with a set of rotary motors only 45 nm in diameter. Each motor drives a long, thin, helical filament that extends several cell body lengths out into the external medium. The assemblage of motor and filament is called a flagellum. The concerted motion of several flagella enables a cell to swim. A cell can move toward regions that it deems more favorable by measuring changes in the concentrations of certain chemicals in its environment (mostly nutrients), deciding whether life is getting better or worse, and then modulating the direction of rotation of its flagella. Thus, in addition to rotary engines and propellers, E. coli's standard accessories include particle counters, rate meters, and gear boxes. This microorganism is a nanotechnologist's dream. I will discuss the features that make it so, from the perspectives of several scientific disciplines: anatomy, genetics, chemistry, and physics.
Galileo Flyby Discovers Immense Lava Fountain on a Jovian Moon
Authors: Schwarzschild Bertram
Published: March 2007

By an improbable stroke of good fortune, the north-polar region of Io, the innermost of Jupiter's four Galilean moons, was putting on an extraordinary volcanic display just as NASA's Galileo spacecraft flew by to take pictures on Thanksgiving Day last year. Because its proximity to Jupiter and its orbital resonance with two neighboring moons subject it to severe tidal flexing, Io turns out to be the most volcanically active body in the Solar System. (See the cover of this issue.)
Negative Pressures and Cavitation in Liquid Helium
Authors: Maris Humphrey , Balibar Sebastien
Published: March 2007

Cavitation—the formation of bubbles—is a familiar phenomenon. Whenever a liquid is agitated violently, there is a possibility that cavitation will occur (see, for example, figure 1). In the case of boat propellers or hydraulic machines, cavitation is a problem that engineers try to avoid. In other contexts, however, cavitation can be useful—as, for example, in ultrasonic cleaning devices.
Experiments Reveal How Heat Is Mixed into Cold Dense Water in the Abyssal Ocean
Authors: Day Charles
Published: March 2007

If the world's oceans relied only on molecular diffusion and smooth laminar flow to spread the Sun's heat, they'd consist of a thin Sun-warmed layer atop a mass of icy water. But other transport mechanisms are at work. In the North Atlantic, for instance, wind-driven currents push warm water from the Caribbean to the Arctic, where it cools, sinks, and flows back southward. And throughout its journey, the seawater is swirled and agitated by continent-sized gyres and centimeter-sized turbulent eddies.
A Photon-Activated Switch Detects Single Far-Infrared Photons
Authors: Fitzgerald, Richard
Published: March 2007

The far-infrared (FIR) region of the electromagnetic spectrum, covering wavelengths between 10 μm and 1 mm, contains a wealth of information. In the laboratory, this range includes the vibrational and rotational spectra of molecules and the energy gaps of superconductors. And in astrophysics and cosmology, it is the realm of emissions from the earliest galaxies, which, due to their large redshifts, emit more than half of their energy in submillimeter radiation. Also in this spectral range is radiation from protostellar regions and planets, which are too cool to emit in the visible.
Atmospheric Infrasound
Authors: Bedard Alfred J. , Georges Thomas M.
Published: March 2007

Imagine a world in which you could hear not just nearby conversations and the noise of traffic a few blocks away, but also the sound of blasting in a quarry in the next state, the rumblings of an avalanche or volcano a thousand miles away, and the roar of a typhoon halfway around the world. Fortunately, nature has spared our senses from direct exposure to this incessant din. But our relentless quest to extend our senses has yielded instruments that can do just that—and more. Waves of infrasound, sounds at frequencies too low for us to hear, permeate the atmosphere and offer us insights into natural and human-made events on a global scale.
Radionuclide Therapy
Authors: Coursey Bert M. , Nath Ravinder
Published: March 2007

Each year in the US, about 200 000 patients receive therapy with radionuclides, most commonly in the form of sealed sources for treating gynecological and head and neck cancers and radiopharmaceuticals for treating thyroid cancer. Known as brachytherapy, this kind of treatment has attracted a resurgence of interest in the medical world, primarily because it offers a simple procedure for delivering high radiation doses to a tumor but minimal doses to the surrounding healthy tissue. Brachytherapy can provide this optimal dose distribution because radiation sources are implanted either in the tumor or very close to it. (Brachys is Greek for “near.”) This advantage is not shared by external beam therapy, in which the source of radiation is about 1 m away from the patient.
A Fresh Look at Entropy and the Second Law of Thermodynamics
Authors: Lieb Elliott H. , Yngvason Jakob
Published: March 2007

In days long gone, the second law of thermodynamics (which predated the first law) was regarded as perhaps the most perfect and unassailable law in physics. It was even supposed to have philosophical import: It has been hailed for providing a proof of the existence of God (who started the universe off in a state of low entropy, from which it is constantly degenerating); conversely, it has been rejected as being incompatible with dialectical materialism and the perfectibility of the human condition.
Chandra Probes Deeper into the Mystery of the X-Ray Background
Authors: Day Charles
Published: March 2007

One minute before midnight on 18 June 1962, an Aerobee rocket was launched from White Sands Missile Range in New Mexico. Packed into its nose cone were three Geiger counters, which Riccardo Giacconi, Herbert Gursky, Frank Paolini, and Bruno Rossi hoped would detect solar x rays fluorescing off the moon. But when the four researchers analyzed the data, they instead found something more remarkable: x rays from a point source in the constellation of Scorpio and a background signal from the sky. Thus, a 10-minute rocket flight gave birth to cosmic x-ray astronomy, and the first cosmic background radiation was discovered.