bluuebunny/crossref_metadata_2025 huggingface dataset:
Elastic-plastic shock wave profiles in oriented single crystals of cyclotrimethylene trinitramine (RDX) at 2.25GPa
Authors: Hooks Daniel E. , Ramos Kyle J. , Martinez A. Richard
Published: August 2006

Plate impact experiments were performed on oriented single crystals of the energetic material cyclotrimethylene trinitramine (RDX). The experiments were performed to determine the anisotropic dynamic yield point for the RDX crystal, as well as to provide data for continuum modeling efforts. Impact was on the (111), (210), and (100) planes to access 3, 2, and 0 slip systems, respectively. Velocity history profiles were measured using Doppler interferometry. Impacts on the (210) plane resulted in nominally conventional results, with distinct elastic and plastic waves, stress relaxation, elastic precursor decay, and increasing wave separation with propagation distance. Velocity profiles from impacts on the (111) plane had no discernable precursor, although an inflection seen in the thicker samples might be the nearly overdriven elastic wave. Wave arrival times signaled a slower elastic wave speed in the (111) profiles. Several unexpected features were observed in the elastic precursor of the profiles from impacts on the (100) plane. Up to three distinct step features were resolved in these profiles in the region of the elastic precursor; these features are not understood. In preparing samples for these experiments, it was noted that the (100) crystal slabs were exceptionally brittle. Wave speeds determined from the shock experiments were consistent with both pulse-echo wave speed measurements and wave speeds calculated from the measured elastic tensor. The elastic limit, as indicated by the peak of the leading wave, was relatively isotropic.
Experiments on the three-dimensional incompressible Richtmyer-Meshkov instability
Authors: Chapman P. R. , Jacobs J. W.
Published: August 2006

The three-dimensional (3D) Richtmyer-Meshkov instability of incompressible, miscible liquids with a 3D single-mode initial perturbation is investigated. This study uses the apparatus of the earlier experiments of Niederhaus and Jacobs [J. Fluid Mech. 485, 243 (2003)] in which the instability is generated by impulsively accelerating a tank containing the two liquids. However, the present investigation uses a tank with square cross section allowing the generation of a square-mode 3D initial perturbation by lateral oscillation along the tank’s diagonal. Amplitude measurements of the 3D instability are found to be effectively collapsed by the dimensionless scaling used in the two-dimensional (2D) study and to be in good agreement with linear stability theory up until a dimensionless time kν0t≈1, later than is found for the 2D flow. Late-time 3D amplitude measurements show faster growth than 2D as is predicted by popular bubble models. However, late-time growth rate measurements are found to deviate from model predictions at the latest times showing a constant growth rate instead of the 1∕t dependence given by the models. The constant late-time growth rate is the result of the observed vorticity distribution which takes the form of an array of upward and downward traveling vortex rings. This fundamental difference between existing models and observation indicates that bubble models may not be suitable for predicting the behavior of the low Atwood number instability which is vortex dominated.
Lattice site location and annealing behavior of implanted Ca and Sr in GaN
Authors: De Vries B. , Vantomme A. , Wahl U. , Correia J. G. , Araújo J. P. , Lojkowski W. , Kolesnikov D. ,   ISOLDE Collaboration
Published: August 2006

We report on the lattice location of ion-implanted Ca and Sr in thin films of single-crystalline wurtzite GaN. Using the emission channeling technique the angular distributions of β− particles emitted by the radioactive isotopes 45Ca (t1/2=163.8 d) and 89Sr (t1/2=50.53 d) were monitored with a position-sensitive detector following 60 keV room-temperature implantation. Our experiments give direct evidence that ∼90% of Ca and >60% of Sr atoms were occupying substitutional Ga sites with root mean square displacements of the order of 0.15–0.30 Å, i.e., larger than the expected thermal vibration amplitude of 0.074 Å. Annealing the Ca implanted samples at 1100–1350 °C in high-pressure N2 atmosphere resulted in a better incorporation into the substitutional Ga site. The Sr implanted sample showed a small decrease in rms displacements for vacuum annealing up to 900 °C, while the substitutional fraction remained nearly constant. The annealing behavior of the rms displacements can explain why annealing temperatures above 1100 °C are needed to achieve electrical and optical activations, despite the fact that the majority of the acceptors are already located on Ga sites immediately after ion implantation.
Exciton migration in organic thin films
Authors: Zhou Y. C. , Wu Y. , Ma L. L. , Zhou J. , Ding X. M. , Hou X. Y.
Published: August 2006

Limitations of the analytical method for calculating the exciton distribution in organic thin films, attributed to the improper boundary conditions when the organic film approaches the exciton diffusion length, were analyzed by comparison with an exciton random walk simulation. The random walk simulation results are in better agreement with in situ photoluminescence measurements than predictions based on the one-dimensional (1D) diffusion equation, especially for thin films (<15nm). The three-dimensional exciton diffusion length in tris(8-hydroxyquinoline) aluminum is determined to be 26nm, equivalent to 15nm upon projection to 1D. The result is not sensitive to the molecular size, a parameter arbitrarily set in the simulation. In addition, the exciton distribution in operating organic light emitting devices was also simulated.
The ultrafast excitation processes in femtosecond laser-induced damage in dielectric omnidirectional reflectors
Authors: Jia T. Q. , Sun H. Y. , Li X. X. , Feng D. H. , Li C. B. , Xu S. Z. , Li R. X. , Xu Z. Z. , Kuroda H.
Published: August 2006

A pump and probe system is developed, where the probe pulse duration τ is less than 60fs while the pump pulse is stretched up to 150–670fs. The time-resolved excitation processes and damage mechanisms in the omnidirectional reflectors SiO2∕TiO2 and ZnS∕MgF2 are studied. It is found that as the pump pulse energy is higher than the threshold value, the reflectivity of the probe pulse decreases rapidly during the former half, rather than around the peak of the pump pulse. A coupled dynamic model based on the avalanche ionization (AI) theory is used to study the excitation processes in the sample and its inverse influences on the pump pulse. The results indicate that as pulse duration is longer than 150fs, photoionization (PI) and AI both play important roles in the generation of conduction band electrons (CBEs); the CBE density generated via AI is higher than that via PI by a factor of 102–104. The theory explains well the experimental results about the ultrafast excitation processes and the threshold fluences.
Fabrication of GaN-based nanoscale device structures utilizing focused ion beam induced Pt deposition
Authors: Motayed Abhishek , Davydov Albert V. , Vaudin Mark D. , Levin Igor , Melngailis John , Mohammad S. N.
Published: August 2006

In this work we have demonstrated nanoscale GaN device structures made from individual GaN nanowires and electrical contacts utilizing focused ion beam (FIB) induced Pt deposition. These GaN nanowires were grown by direct reaction of Ga vapor with NH3 and had diameters ranging from 100nmto250nm and lengths up to 200μm. As-grown nanowires were dispersed on SiO2 coated p++ Si substrate. A 30keV Ga+ ion beam was used to dissociate (trimethyl)methylcyclopentadienyl-platinum precursor for depositing Pt contacts to GaN nanowires. FIB-deposited Pt contacts to GaN nanowires showed nonlinear I-V characteristics, which turned linear after annealing at 500°C for 30s in argon. Resistivity of a GaN nanowire measured using a four terminal contact geometry fabricated by depositing Pt with a FIB was in the range of 5×10−3Ωcm. Temperature dependent resistivity measurement of the GaN nanowire revealed semiconducting behavior with a weak temperature dependence of the resistivity. In this study both Ohmic and Schottky contacts to GaN nanowires have been realized with FIB-deposited Pt contacts. Barrier height and ideality factor have been extracted for the metal-GaN nanowire Schottky junctions, which had low reverse breakdown voltage and large ideality factor of 18. Modulation of the current through the nanowire was achieved by applying a bias to the Si substrate acting as a backgate. n-type depletion mode behavior was observed in the GaN nanotransistor, which was consistent with the impurity related background concentrations expected in this type of growth method. The heat generation during FIB deposition and the ion damage appeared to cause noticeable swelling of the nanowires under the Pt contacts. Electron beam induced Pt deposition was also used to fabricate electrical contacts to the nanowires.
Mixing effects in postdischarge modeling of electric discharge oxygen-iodine laser experiments
Authors: Palla Andrew D. , Carroll David L. , Verdeyen Joseph T. , Solomon Wayne C.
Published: August 2006

In an electric discharge oxygen-iodine laser, laser action at 1315nm on the I(P1∕22)→I(P3∕22) transition of atomic iodine is obtained by a near resonant energy transfer from O2(aΔ1) which is produced using a low-pressure electric discharge. The discharge production of atomic oxygen, ozone, and other excited species adds higher levels of complexity to the postdischarge kinetics which are not encountered in a classic purely chemical O2(aΔ1) generation system. Mixing effects are also present. In this paper we present postdischarge modeling results obtained using a modified version of the BLAZE-II gas laser code. A 28 species, 105 reaction chemical kinetic reaction set for the postdischarge kinetics is presented. Calculations were performed to ascertain the impact of a two stream mixing mechanism on the numerical model and to study gain as a function of reactant mass flow rates. The calculations were compared with experimental data. Agreement with experimental data was improved with the addition of new kinetics and the mixing mechanism.
Large polaron tunneling and anomalous dielectric response in complex layered systems
Authors: De Sukanta , Dey Arup , De S. K.
Published: August 2006

Electrical conductivity and dielectric spectra of nanocomposites of conducting polyaniline and layered vanadyl phosphate VOPO4∙2H2O are studied at low temperature. Electrons and protons contribute to electrical conduction having the maximum conductivity of the order ∼10−5Scm−1. A discontinuity in electrical conductivity has been observed at about 210K. Conductivity with higher activation energy is dominated by protons. Smaller activation energy at lower temperature may be due to major electron conduction. Large polaron tunneling contributes to the electrical conduction process. The dielectric spectra exhibit anomalous frequency dispersion. Two frequency exponents in dielectric spectra are distinguished by considering intercluster and intracluster displacement of charge carriers.
Hydrodynamic equations and collective modes in a system comprising a porous medium and a He3–He4 superfluid mixture
Authors: Kekutia Sh. E. , Chkhaidze N. D.
Published: August 2006

The dynamical equations are obtained for a system comprising a porous material and a He3–He4 superfluid mixture. The elastic coefficients that appear in the equations are expressed in terms of physically measurable quantities. The equations obtained describe all of the bulk modes that propagate in a porous medium filled with a He3–He4 superfluid mixture. The propagation of acoustic modes is investigated for an aerogel filled with a He3–He4 superfluid mixture. The velocities of the longitudinal modes propagating in that system are calculated, and the relations between oscillatory quantities are established.
Magnetic and electronic phase separation driven by structural clustering in La0.7(Ca1−ySry)0.3MnO3 thin films
Authors: Prokhorov V. G. , Komashko V. A. , Kaminsky G. G. , Lee Y. P. , Hyun Y. H. , Yu K. K. , Park J. S. , Svetchnikov V. L.
Published: August 2006

The structural, magnetic and transport properties of La0.7(Ca1−ySry)0.3MnO3 films deposited on a LaAlO3 (001) single-crystalline substrate by rf-magnetron sputtering using “soft” (or powder) targets are investigated. It is found that at 0.3⩽y⩽0.5 both rhombohedral (R¯3c) and orthorhombic (Pnma) crystal phases are coexistent at room temperature, forming a nanoclustered microstructure. The clustered films manifest the two-stage magnetic and electronic transitions typical for phase-separated systems. It is shown that for 0.5⩾y⩾0 a nonlinear (almost parabolic) field-dependent magnetoresistance is typical at room temperature, while for 0.65⩽y⩽1.0 it transforms to a linear behavior. The magnetotransport properties of the films are explained within the framework of a field-dependent activation energy model. The magnetic phase diagram for the La0.7(Ca1−ySry)0.3MnO3 thin-film system is presented.
Influence of the elastic subsystem on phase transitions in ferromagnets with exchange and single-ion anisotropies
Authors: Freedman Yu. A. , Klevets Ph. N. , Matyunin D. A.
Published: August 2006

The influence of planar and bulk elastic interactions on the phase states of an ultrathin ferromagnetic film with anisotropic exchange interaction is investigated for different relationships among the material constants. It is shown that when the elastic interactions, with competing exchange and single-ion anisotropies, and the magnetic dipole interaction are taken into account, a cascade of phase transitions appears. Furthermore, taking the “planar” elastic interaction into account leads to realization of an additional phase, with an easy axis in the film plane. This state is absent in the case of a bulk elastic subsystem.
On the mean-field theory of a two-sublattice antiferromagnet
Authors: Zvyagin A. A. , Skorobagat’ko G. A.
Published: August 2006

Numerical solutions of the mean-field theory of a two-sublattice antiferromagnet with a strong crystalline electric field are studied. The validity of the solutions is checked with the help of known analytical results for the special case of the lowest doublet (S=1∕2). For larger values of the lowest multiplet (S>1∕2) the temperature and magnetic field dependencies of the antiferromagnetic order parameter and the total magnetization are calculated and their features are analyzed. We predict new re-entrant phase transitions between Néel and paramagnetic phases for S⩾1.
Influence of oxygen nonstoichiometry on the crystal structure and magnetic properties of the cation-deficient manganites Pr0.9MnOx (2.85<x<2.90)
Authors: Mantytskaya O. S. , Kolesova I. M. , Troyanchuk I. O. , Szymczak H. , Sirenko V. A. , Eremenko V. V.
Published: August 2006

X-ray diffraction and magnetic studies of the Pr0.9MnOx system are carried out for different oxygen concentrations in the interval 2.85<x<2.90. It is found that an antiferromagnet-ferromagnet phase transition occurs with increasing oxygen concentration in the solid solutions of the series Pr0.9MnOx. The crystal structure and magnetic properties of the most highly reduced sample Pr0.9MnO2.85 are close to those of the stoichiometric manganite PrMnO3. Pr0.9MnO2.90 is a ferromagnetic insulator with Tc=120K. In this sample a transition from an orbitally ordered phase to an orbitally disordered phase occurs over a wide range of temperatures. The properties of the samples are explained by the hypothesis that a portion of the praseodymium ions (up to 5%) can be substituted by trivalent manganese ions without forming oxygen vacancies. Upon oxidation of the samples, vacancies of the praseodymium cations appear, while the oxygen skeleton is preserved.
Weak localization and charge-carrier interaction effects in a two-dimensional hole gas in a germanium quantum well in a SiGe∕Ge∕SiGe heterostructure
Authors: Berkutov I. B. , Komnik Yu. F. , Andrievskii V. V. , Mironov O. A. , Myronov M. , Leadley D. R.
Published: August 2006

The weak localization and interaction effects for charge carriers in a two-dimensional hole gas in a pure germanium quantum well in a SiGe∕Ge∕SiGe heterostructure with a hole density of 5.68×1011cm−2 and mobility of 4.68×104cm2V−1s−1 are investigated. The resistance measurements were made at temperatures from 46mKto10K and magnetic fields up to 15T. The magnetic-field dependence of the resistivity exhibits Shubnikov-de Haas oscillations and quantum Hall effect steps. At very low magnetic fields (B<0.1T) a weak localization effect for holes is revealed, which makes for a negative magnetoresistance and growth of the resistance with decreasing temperature (at T<2K). The manifestation of the interaction effect is observed and analyzed over a wide range of temperatures and magnetic fields. With increasing temperature the manifestation of the interaction-induced quantum correction passes from the diffusive regime to an intermediate and then to the ballistic regime. In all regions the behavior of the interaction quantum correction is in good agreement with the modern theoretical predictions.
Green polyfluorene-conducting polymer interfaces: Energy level alignment and device performance
Authors: Koch Norbert , Elschner Andreas , Johnson Robert L.
Published: August 2006

Photoemission studies on interfaces between an electroluminescent green polyfluorene (GPF) and two conducting polymers comprising ethylenedioxythiophene and sulfonate moieties are reported. Despite the chemical similarity of both conducting polymers, the hole injection barriers to GPF were found to differ by 0.35eV and the interface dipoles by up to −0.4eV. Polymer light emitting devices fabricated with the two conducting polymers exhibit nearly identical current-voltage characteristics, but the luminance-voltage characteristic is enhanced by a factor of 20 by the presence of near-surface poly(styrenesulfonate). We attribute the enhancement to the electron- blocking behavior of the sulfonate moieties at the GPF interface and the concomitant modification of the device built-in voltage.
Thermal expansion and polyamorphism of N2–C60 solutions
Authors: Manzhelii V. G. , Dolbin A. V. , Esel’son V. B. , Gavrilko V. G. , Gadd G. E. , Moricca S. , Cassidy D. , Sundqvist B.
Published: August 2006

The linear coefficients α(T) of N2–C60 solutions with 9.9% and 100% of the C60 lattice thermal expansion interstitials filled with N2 are investigated in the interval 2.2–24K. The dependence α(T) has a hysteresis suggesting co-existence of two types of orientational glasses in these solutions. The features of the glasses are compared. The characteristic times of phase transformations in the solutions and reorientation of C60 molecules are estimated.
On the possibility of observing the conventional quantum Hall effect in graphene
Authors: Gaididei Yu. B. , Loktev V. M.
Published: August 2006

It is shown that the conventional quantum Hall effect can be observed in graphene at high magnetic fields as a result of intrasublattice electron transport in its two-sublattice structure.
Surface modifications of Cu(In,Ga)S2 thin film solar cell absorbers by KCN and H2O2∕H2SO4 treatments
Authors: Weinhardt L. , Fuchs O. , Groß D. , Umbach E. , Heske C. , Dhere N. G. , Kadam A. A. , Kulkarni S. S.
Published: August 2006

KCN etching of the CuxS surface layer formed during the production process of Cu(In,Ga)S2 thin film solar cell absorbers as well as subsequent H2O2∕H2SO4 etching of the Cu(In,Ga)S2 surface have been investigated using x-ray photoelectron spectroscopy, x-ray excited Auger electron spectroscopy, and x-ray emission spectroscopy. We find that the KCN etching removes the CuxS layer—being identified as Cu2S—and that there is K deposited during this step, which is removed by the subsequent H2O2∕H2SO4 oxidation treatment. When a CdS buffer layer is deposited on the absorber directly after KCN etching, a K compound (KCO3) is observed at the CdS surface.
Laser scanning microscopy of guided vortex flow in microstructured high-Tc films
Authors: Lukashenko A. , Ustinov A. V. , Zhuravel A. P. , Hollmann E. , Wördenweber R.
Published: August 2006

We report the visualization of guidance of vortices by artificial microholes (antidots) in superconducting thin films using a low-temperature laser scanning microscope. Previously, guided motion of vortices via tilted rows of antidots in YBa2Cu3O7 films was detected indirectly by using resistive Hall-type measurements. Here we prove that vortices are steered between antidots into a priori chosen direction by imaging of resistive photoresponse with a spatial resolution down to about 1μm. We observe predominant paths for vortex motion. Vortices are nucleated and annihilated at antidots, i.e., antidots define starting and ending points of predominant vortex paths. Depending on the misorientation angle between rows of antidots and the current-driven direction of vortex motion, different channels dominate in antidot-guided vortex motion. Our experimental results can be explained by the n-channel model. Finally, we present direct measurements of the local critical currents. This technique can be used as a quantitative method for the analysis of vortex motion in micropatterned thin films.
Utilizing lateral current spreading in multijunction solar cells: An alternative approach to detecting mechanical defects
Authors: Zimmermann C. G.
Published: August 2006

Lateral current transport in forward biased triple junction solar cells is analyzed. A model based on distributed sheet resistances in combination with ideal p-n junctions is developed. Experimentally, the spatial distribution of the electroluminescence radiation is recorded and used as a tool to deduce the current densities in the individual junctions. In a specific test case, a cell which contains both area with and without externally injected current, the current spreading across the boundary between the two cell areas is reproduced accurately. Subsequently the impact of a mechanical defect on the current distributions in the individual junctions is addressed through this model. It is demonstrated that the carrier recombination at the surface of the crack acts as a current sink with a long range impact on the current densities. In addition a characteristic splitting of the current densities in top cell and middle cell is observed which implies that mechanical defects can be easily identified in a cell by their electroluminescence signature. The dependence of this phenomenon on the semiconductor properties in the basic cell structure is identified.
Switching in C60-fullerene based field effect transistors
Authors: Matt G. J. , Singh Th. B. , Sariciftci N. S. , Montaigne Ramil A. , Sitter H.
Published: August 2006

We are reporting on the electrical properties of a bottom gate C60-fullerene based n-channel organic field effect transistor. The C60 thin film was epitaxially grown using hot wall epitaxy on top of an organic dielectric divinyltetramethyldisiloxane-bis(benzocyclobutene). The device performance depends on the growth parameters during the C60 film growth. Optimization of the growth parameters leads to a C60 film of a low total number of traps, and the drain-source current is increased by two orders in magnitude. We propose that the high current-densities are caused by space charge limited currents beside the gate induced space charge.
Polarization-sensitive optical phenomena in thick semiconducting nanowires
Authors: Ruda H. E. , Shik A.
Published: August 2006

For semiconducting nanowires with the dielectric constant ε differing from that of their environment, the distribution of ac electric fields, created in a nanowire by an external lightwave and emitted outside by an effective dipole in a nanowire, is found. The results are used for calculating the spectra and polarization properties of optical absorption, luminescence, and photoconductivity in such nanowires. For relatively thick nanowires, with diameter a comparable to the light wavelength, all polarization characteristics exhibit strong oscillations with the light frequency ω and even change the sign over some intervals of ωa∕c. Some of these phenomena have already been observed experimentally. In a system of randomly oriented nanowires, they result in a polarization memory where polarization of photoluminescence is determined by the polarization of the exciting light, by a factor strongly dependent on the frequencies of both the exciting and emitted light.
Characteristic fragment size distributions in dynamic fragmentation
Authors: Zhou Fenghua , Molinari Jean-François , Ramesh K. T.
Published: August 2006

The one-dimensional fragmentation of a dynamically expanding ring (Mott’s problem) is studied numerically to obtain the fragment signatures under different strain rates. An empirical formula is proposed to calculate an average fragment size. Rayleigh distribution is found to describe the statistical properties of the fragment populations.
Influence of dielectric core and embedding medium on the local field enhancement for gold nanoshells
Authors: Jian Zhu , Caili Zhang
Published: August 2006

The dependence of the local field enhancement in gold nanoshells was investigated as a function of the core and embedding medium dielectric constant. Numerical calculations based on quasistatic theory indicated two local field factor peaks. If the core dielectric constant is a fixed value and the dielectric constant of surrounding medium is increased from 1.0 to 6.0, the shorter wavelength peak redshifts and decreases nonlinearly, whereas the longer wavelength peak redshifts, increases first and then decreases. On the contrary, if the surrounding dielectric constant is a fixed value and the core dielectric constant is increased from 1.0 to 6.0, the shorter wavelength peak redshifts and increases nonlinearly, whereas the longer wavelength peak redshifts and decreases nonlinearly.
Temperature dependence of electroluminescence from silicon p-i-n light-emitting diodes
Authors: Li Cheng , Lai Hongkai , Chen Songyan , Suemasu T. , Hasegawa F.
Published: August 2006

The temperature dependence of electroluminescence from silicon p-i-n light-emitting diodes with a layer of β-FeSi2 particles inserted in intrinsic silicon was investigated. Anomalous blueshift of the peak energy and enhanced electroluminescence intensity of the silicon band-edge emission were observed at temperatures from 50to200K. The electroluminescence intensity was enhanced due to longer diffusion paths of the injected electrons at elevated temperature, as well as thermal escape of the electrons from the β-FeSi2 particles. The low peak energy compared to that from bulk silicon at low temperature is due to the bound electron-hole pairs induced by the strain potential at the interface between silicon and β-FeSi2 particles. The blueshift of the peak is ascribed to the transition of bound electron-hole pairs into free excitons at elevated temperature. Room temperature electroluminescence from such a silicon light-emitting diode can be obtained at a low current density of 0.3A∕cm2.
Dynamics of multiple plumes in laser ablation: Modeling of the shielding effect
Authors: Zinovik Igor , Povitsky Alex
Published: August 2006

The scattering and absorption of laser radiation by previously ablated plumes in laser ablation (known as the shielding effect) dramatically affect the efficiency of laser ablation process. The ablated plumes consisting of water vapor, droplets, and particles are modeled as a gas-particle equilibrium mixture by solution of the Euler equations combined with the transport equation for the ratio of heat capacities. Shielding effect on the overall ablated mass by multiple plumes is studied for a wide range of concentration of particles in vaporized plumes, various laser repetition rates, scattering, and absorption of laser energy. The shielding phenomenon is studied for short sequences of discrete plumes to focus on the shielding effect of individual plumes. The results of numerical modeling were compared to experimental results of laser-induced water explosive vaporization. Ablation rate was calculated for a single ablated plume and for the sequence of six laser pulses at the repetition rates of 0.33 and 1MHz at which gas dynamics interactions between plumes are strong but plumes have not yet form a continuous jet. A single ablated plume has an initial semispherical shape which transforms into mushroomlike cloud with a thin stem and a ring vortex as it was observed in experiments with water and cornea ablation. For the plume with a given ablated mass, the longer ejection of plume with smaller density produces the plume with smaller shielding capacity. For multiple laser pulses, the velocity of ejected mixture increases from the center of the target to its periphery because of the shielding effect. The ablated mass of the current plume depends on the attenuation of the incident laser beam energy caused by the propagation of laser beam through previously ablated plumes. In the case of laser energy absorption, the ablation rate per pulse exceeds 2–2.5 times the rate obtained for the laser energy scattering.
Thermoelectric properties of doped titanium disulfides
Authors: Abbott Edward E. , Kolis Joseph W. , Lowhorn Nathan D. , Sams William , Rao Apparao , Tritt Terry M.
Published: August 2006

We report herein the doping of titanium disulfide (TiS2) with the pnictides (Pn): P, As, and Sb. The incorporation of these pnictides into titanium disulfide (TiS2−xPnx) is performed at extremely low concentrations (x∼0.2%). The effects on the electronic transport of titanium disulfide by doping with arsenic is quite profound, reducing the resistivity and thermopower to 0.2mΩcm and −35μV∕K at 300K, respectively, from 1.8mΩcm and −170μV∕K at 300K for the parent compound TiS2. For a wide range of thermopower values we find that the thermopower (α) of these doped titanium disulfides is linearly related to the infrared reflectivity minimum and can be correlated by the experimentally determined proportionality of λ=−0.0457α, where λ is the wavelength of the minimum.
Time-dependent density functional study of the electronic potential energy curves and excitation spectrum of the oxygen molecule
Authors: Guan Jingang , Wang Fan , Ziegler Tom , Cox Hazel
Published: August 2006

Orbital energies, ionization potentials, molecular constants, potential energy curves, and the excitation spectrum of O2 are calculated using time-dependent density functional theory (TDDFT) with Tamm-Dancoff approximation (TDA). The calculated negative highest occupied molecular orbital energy (−εHOMO) is compared with the energy difference ionization potential for five exchange correlation functionals consisting of the local density approximation (LDAxc), gradient corrected Becke exchange plus Perdew correlation (B88X+P86C), gradient regulated asymptotic correction (GRAC), statistical average of orbital potentials (SAOP), and van Leeuwen and Baerends asymptotically correct potential (LB94). The potential energy curves calculated using TDDFT with the TDA at internuclear distances from 1.0to1.8Å are divided into three groups according to the electron configurations. The 1πu41πg2 electron configuration gives rise to the XΣg−3, aΔg1, and bΣg+1 states; the 1πu31πg3 electron configuration gives rise to the cΣu−1, CΔu3, and AΣu+3 states; and the BΣu−3, AΔu1, and fΣu+1 states are determined by the mixing of two or more electron configurations. The excitation spectrum of the oxygen molecule, calculated with the aforementioned exchange correlation functionals, shows that the results are quite sensitive to the choice of functional. The LDAxc and the B88X+P86C functionals produce similar spectroscopic patterns with a single strongly absorbing band positioned at 19.82 and 19.72eV, respectively, while the asymptotically corrected exchange correlation functionals of the SAOP and the LB94 varieties yield similar excitation spectra where the computed strongly absorbing band is located at 16.09 and 16.42eV, respectively. However, all of the exchange correlation functionals yield only one strongly absorbing band (oscillator strength greater than 0.1) in the energy interval of 0–20eV, which is assigned to a XΣg−3 to Σu−3 transition. Furthermore, the oxygen molecule has a rich spectrum in the energy range of 14–20eV and no spin allowed absorption bands are predicted to be observed in the range of 0–6eV.
Quantum macrostates, equivalence of ensembles, and an H-theorem
Authors: De Roeck Wojciech , Maes Christian , Netočný Karel
Published: August 2006

Before the thermodynamic limit, macroscopic averages need not commute for a quantum system. As a consequence, aspects of macroscopic fluctuations or of constrained equilibrium require a careful analysis, when dealing with several observables. We propose an implementation of ideas that go back to John von Neumann’s writing about the macroscopic measurement. We apply our scheme to the relation between macroscopic autonomy and an H-theorem, and to the problem of equivalence of ensembles. In particular, we show how the latter is related to the asymptotic equipartition theorem. The main point of departure is an expression of a law of large numbers for a sequence of states that start to concentrate, as the size of the system gets larger, on the macroscopic values for the different macroscopic observables. Deviations from that law are governed by the entropy.
Electronic effect of kink in the edge dislocation in bcc iron: A first-principles study
Authors: Chen Li-qun , Wang Chong-yu , Yu Tao
Published: August 2006

Using the self-consistent discrete variational method based on density functional theory, we have investigated the electronic effect of kinks in the ⟨100⟩{010} and 1∕2⟨111⟩{011} edge dislocations (EDs) in bcc iron. Our calculations indicate that the kink formation energy is closely related to the type of ED. The EDs containing kinks are less stable than the corresponding EDs without kinks. Furthermore, we have calculated the structural energy, the interatomic energy, the local density of states, and the charge-density difference for these systems. We find that the bonds between the atoms along the slip direction in the kinked EDs are strengthened compared to those in the corresponding EDs. In addition, the calculations of the structural energy and the local density of states show that the atoms on the kink are more active than those in the corresponding ED. These results lead us to conclude that the kinks provide the structural and energetic preparation for the dislocation motion.
Space charge effects in photoemission with a low repetition, high intensity femtosecond laser source
Authors: Passlack S. , Mathias S. , Andreyev O. , Mittnacht D. , Aeschlimann M. , Bauer M.
Published: August 2006

In this paper, we present experimental results on the effect of space charging in photoelectron spectroscopy from a surface using a pulsed and intense femtosecond light source. We particularly focus on a quantitative evaluation of the induced spectral broadening. Our results are compared with analytic calculations based on energy conservation considerations as well as with experimental results from measurements using picosecond pulses for the excitation process. As a measure of space charge effects, we monitored the angular and energy distributions of the photoemission from the occupied Shockley surface state of Cu(111) as a function of the total number N of the photoemitted electrons per laser pulse. Our results show that spectral distortions exist for the entire laser fluence regime probed. The energetic broadening of the surface state peak can be fitted with remarkable accuracy by a N dependence, in agreement with the theoretical predictions and different from the experimental picosecond results, where a dominating linear dependence has been reported. In addition to a pure energetic broadening of the photoemission spectra, we also identify modifications in the angular distribution of the photoemitted electrons due to space charge effects.
Charge trapping properties at silicon nitride/silicon oxide interface studied by variable-temperature electrostatic force microscopy
Authors: Tzeng S.-D. , Gwo S.
Published: August 2006

Charge trapping properties of electrons and holes in ultrathin nitride-oxide-silicon (NOS) structures were quantitatively determined by variable-temperature electrostatic force microscopy (EFM). From charge retention characteristics obtained at temperatures between 250 and 370°C and assuming that the dominant charge decay mechanism is thermal emission followed by oxide tunneling, we find that there are considerable deep trap centers at the nitride-oxide interface. For electron, the interface trap energy and density were determined to be about 1.52eV and 1.46×1012cm−2, respectively. For hole, these are about 1.01eV and 1.08×1012cm−2, respectively. In addition, the capture cross section of electron can be extracted to be 4.8×10−16cm2. The qualitative and quantitative determination of charge trapping properties and possible charge decay mechanism reported in this work can be very useful for the characterization of oxide-nitride-silicon based charge storage devices.
Inelastic deformation behavior of La0.6Sr0.4FeO3 perovskite
Authors: Orlovskaya Nina , Anderson Harlan , Brodnikovskyy Mykola , Lugovy Mykola , Reece Michael John
Published: August 2006

Inelastic deformation behavior of the rhombohedral La0.6Sr0.4FeO3 perovskite is reported. Such behavior is very similar to the ferroelastic deformation of LaCoO3 or lead zirconate titanate perovskites, except that no twin structure was found to exist in La0.6Sr0.4FeO3. The possible mechanisms responsible for such nonlinear deformation behavior are discussed.
Nonlinear transformation of electromagnetic wave in time-varying plasma medium: Longitudinal propagation
Authors: Trifkovic Zoran M. , Stanic Bozidar V.
Published: August 2006

The nonlinear transformation of a linearly polarized source plane electromagnetic wave, propagating along an external static magnetic field in free space for t<0, due to the sudden creation of homogeneous plasma at t=0 and sudden switching off the external magnetic field after an elapsed time interval τ, is studied. This transformation has been analyzed by using the second order perturbation theory in radio approximation. For t>τ, the stationary (rectification and space-varying) modes, the electron and electromagnetic plasma oscillations and traveling electron and electromagnetic plasma waves have been created. Among the large variety of nonlinear longitudinal electric modes the appearance of rectification mode was observed. The efficiency of excitation of the longitudinal electric rectification mode has been studied for different values of source wave angular frequency, electron cyclotron angular frequency, and duration time τ of external static magnetic field.
Photoswitching in polymer-dispersed liquid crystals
Authors: Cupelli D. , De Filpo G. , Chidichimo G. , Nicoletta F. P.
Published: August 2006

While dealing with photonics, we find that user-controllable electro-optical devices are of great importance. In this paper, the optical control of the light scattering properties of polymer-dispersed liquid crystals has been achieved by dissolving a photoconductive material in the polymer matrix. In the dark, an electric field, lower than the threshold one, is applied across the film, which, consequently, appears opaque. After irradiation with an adequate light beam, the change in the electrical properties of the polymer matrix enhances the effective field across the liquid crystal droplets, making the film transparent. The device can be switched off by turning off either the light source or the electric field. The effects of light intensity and photoconductor concentration on the film phototunability are also discussed.
Parity nonconservation contribution to the nuclear magnetic resonance shielding constants of chiral molecules: A four-component relativistic study
Authors: Bast Radovan , Schwerdtfeger Peter , Saue Trond
Published: August 2006

A systematic four-component relativistic study of the parity nonconservation (PNC) contribution to the (isotropic) NMR shielding constants of chiral molecules is presented for the P enantiomers of the series H2X2 (X=O17,S33,Se77,Te125,Po209). The PNC contributions are obtained within a linear response approach at the Hartree-Fock level. A careful design of the basis sets is necessary. The four-component relativistic results based on the Dirac-Coulomb Hamiltonian are compared with the nonrelativistic Lévy-Leblond results and those obtained by the spin-free modified Dirac Hamiltonian. The calculations confirm the nonrelativistic scaling law Z2.4 of the PNC contribution with respect to nuclear charge Z. However, the calculations also show that the overall scaling is significantly modified by relativistic effects. The scalar relativistic effect scales as Z4.7 for the selected set of molecules, whereas the spin-orbit effect, of opposite sign, scales better than Z6 and completely dominates the PNC contribution for the heaviest elements. This opens up the intriguing possibility of the experimental observation of PNC effects on NMR parameters of molecules containing heavy atoms. The presented formalism is expected to be valuable in assisting the search for suitable candidate molecules.
Strain-induced polarization in wurtzite III-nitride semipolar layers
Authors: Romanov A. E. , Baker T. J. , Nakamura S. , Speck J. S. ,   ERATO/JST UCSB Group
Published: August 2006

This paper presents growth orientation dependence of the piezoelectric polarization of InxGa1−xN and AlyGa1−yN layers lattice matched to GaN. This topic has become relevant with the advent of growing nitride based devices on semipolar planes [A. Chakraborty et al., Jpn. J. Appl. Phys., Part 2 44, L945 (2005)]. The calculations demonstrate that for strained InxGa1−xN and AlyGa1−yN layers lattice matched to GaN, the piezoelectric polarization becomes zero for nonpolar orientations and also at another point ≈45° tilted from the c plane. The zero crossover has only a very small dependence on the In or Al content of the ternary alloy layer. With the addition of spontaneous polarization, the angle at which the total polarization equals zero increases slightly for InxGa1−xN, but the exact value depends on the In content. For AlyGa1−yN mismatched layers the effect of spontaneous polarization becomes important by increasing the crossover point to ∼70° from c-axis oriented films. These calculations were performed using the most recent and convincing values for the piezoelectric and elasticity constants, and applying Vegard’s law to estimate the constants in the ternary InxGa1−xN and AlyGa1−yN layers.
Dielectric scaling of a zero-Schottky-barrier, 5nm gate, carbon nanotube transistor with source/drain underlaps
Authors: Alam Khairul , Lake Roger K.
Published: August 2006

The effect of gate dielectric on coaxially gated, Schottky-barrier, carbon nanotube field-effect transistors with source and drain underlaps is investigated. For 2nm thick dielectrics, the substitution of SiO2 with ZrO2 has little effect on the on-current and the subthreshold slope. The principal effect is a change in the intrinsic and parasitic gate capacitances which affects the delay time, cut-off frequency, and Coulomb blockade of the ambipolar leakage current. Using a relatively low-K gate dielectric (as opposed to a high-K gate dielectric) increases the speed performance by reducing parasitic components of the gate capacitance. For a 50nm long, 1.5nm diameter, zero-Schottky-barrier carbon nanotube (CNT) with a 5nm gate and a 2nm SiO2 dielectric, we obtain a delay time of 31fs, a cutoff frequency of 8.9THz, an inverse subthreshold slope S=66mV∕dec, and an on-off current ratio of 8×105 with VDD=0.4V. Oxide thickness dependence of the on-off current ratio, inverse subthreshold slope, and intrinsic cut-off frequency is also investigated.
The role of barrier layer on Ohmic performance of Ti∕Al-based contact metallizations on AlGaN∕GaN heterostructures
Authors: Mohammed Fitih M. , Wang Liang , Adesida Ilesanmi , Piner Eddie
Published: August 2006

Ohmic performance and surface morphology of Ti∕Al∕metal∕Au schemes, where metal is Ti, Mo, Pt, Ir, Ni, Ta, or Nb, have been studied to identify the role of the barrier layer on contact formation and reliability of multilayered metallizations. Results show a strong dependence of contact resistance and specific contact resistivity, as well as surface morphology, on the type of metal layer used. Annealing temperature dependent Ohmic performance was observed for the different schemes. The existence of distinct reaction domains with discernable compositional contrast is evidenced on the surface of annealed contacts. The observed variations in Ohmic performance and surface morphology did not have any correlation with the physical properties of the elemental metal layer. This suggests that contrary to the generally accepted opinion of the layer acting as a mere barrier layer, which serves to prevent indiffusion of Au and outdiffusion of Al, results from the present study provided insights on the active role this layer plays on the characteristics of metallization schemes. Although thermal annealing optimization for any of the Ti∕Al∕metal∕Au scheme can enable low-resistance contact formation, however, surface morphology and reliability (e.g., lateral encroachment) considerations limit the practical usage of the schemes to a few such as Ti∕Al∕Mo∕Au and Ti∕Al∕Pt∕Au.
Stress control structures for microelectromechanical systems using structural mechanics approach
Authors: Krupenkin T. , Lifton V. A. , Tran A. , Vuillemin J. , Carr D. W.
Published: August 2006

A method of stress control in microelectromechanical systems (MEMS) devices is presented that consists of creating counterbalancing structures to position stressed layers at the neutral plane of the device, eliminating the bending momentum acting on the device. Upon metallization, many MEMS elements such as silicon membranes show substantial bow under the stress developed as a result of the difference in the thermal expansion coefficients of a metal and silicon. The proposed membranes with the counterbalancing structures remain flat in the entire test temperature range (25–150°C). The method gives material-independent solution to stress-induced curvature problems in a variety of ultrathin devices.
Metal printing with modified polymer bonding lithography
Authors: Yu Xinhong , Yu Shunyang , Wang Zhe , Ma Dongge , Han Yanchun
Published: August 2006

A simple and versatile metal thin film patterning method is introduced, using the adhesion between polymer and metal thin film as the driving force. Solvent vapor treatment is used to increase the adhesive ability between polymer and metal film in two reverse processes. After selective transfer printing, metal patterns on polymer film or flat elastomeric stamp can be fabricated. Multilayer metal patterns can also be fabricated with this method through multiple printing. Finally, this method was used to fabricate polymer field-effect transistors. The discussions on the effects of solvent on the transistor performance have been given.
First-principles calculations on MgO: Phonon theory versus mean-field potential approach
Authors: Wang Y. , Liu Z.-K. , Chen L.-Q. , Burakovsky L. , Ahuja R.
Published: August 2006

Various thermodynamic properties of MgO were studied using both ab initio phonon theory and the mean-field potential (MFP) approach. They include thermal pressure, thermal expansion, the 300K equation of state, and the shock Hugoniot. It is found that the results of ab initio phonon theory and the MFP approach agree with each other, except that ab initio phonon theory gave a poor description of the thermal pressure when temperature became relatively high.