bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of earthquake recurrence times
Authors: Saichev Alexander , Sornette Didier
Published: April 2007

The statistics of recurrence times in broad areas have been reported to obey universal scaling laws, both for single homogeneous regions and when averaged over multiple regions. These unified scaling laws are characterized by intermediate power law asymptotics. On the other hand, Molchan (2005) has presented a mathematical proof that if such a universal law exists, it is necessarily an exponential, in obvious contradiction with the data. First, we generalize Molchan's argument to show that an approximate unified law can be found which is compatible with the empirical observations when incorporating the impact of the Omori‐Utsu law of earthquake triggering. We then develop the theory of the statistics of interevent times in the framework of the Epidemic‐Type Aftershock Sequence (ETAS) model of triggered seismicity and show that the empirical observations can be fully explained. Our theoretical expression well fits the empirical statistics over the whole range of recurrence times, accounting for different regimes by using only the physics of triggering quantified by the Omori‐Utsu law. The description of the statistics of recurrence times over multiple regions requires an additional subtle statistical derivation that maps the fractal geometry of earthquake epicenters onto the distribution of the average seismic rates in multiple regions. This yields a prediction in excellent agreement with the empirical data for reasonable values of the fractal dimension d ≈ 1.8, the average clustering ratio n ≈ 0.9, and the productivity exponent α ≈ 0.9 times the b value of the Gutenberg‐Richter law.
Flank instability on Mount Etna: Radon, radar interferometry, and geodetic data from the southwestern boundary of the unstable sector
Authors: Neri Marco , Guglielmino Francesco , Rust Derek
Published: April 2007

Understanding Etnean flank instability is hampered by uncertainties over its western boundary. Accordingly, we combine soil radon emission, interferometric synthetic aperture radar (InSAR), and electronic distance measurement (EDM) data to study the Ragalna fault system (RFS) on the SW flank of the volcano. Valuable synergy developed between our differing techniques, producing consistent results and serving as a model for other studies of partly obscured active faults. The RFS, limited in its surface expression, is revealed as a complex interlinked structure ∼14 km long that extends from the edifice base toward the area of summit rifting, possibly linking northeastward to the Pernicana fault system (PFS) to define the unstable sector. Short‐term deformation rates on the RFS from InSAR data reach ∼7 mm yr−1 in the satellite line of sight on the upslope segment and ∼5 mm yr−1 on the prominent central segment. Combining this with EDM data confirms the central segment of the RFS as a dextral transtensive structure, with strike‐slip and dip‐slip components of ∼3.4 and ∼3.7 mm yr−1, respectively. We measured thoron (220Rn, half‐life 56 s) as well as radon, and probably because of its limited diffusion range, this appears to be a more sensitive but previously unexploited isotope for pinpointing active near‐surface faults. Contrasting activity of the PFS and RFS reinforces proposals that the instability they bound is divided into at least three subsectors by intervening faults, while, in section, fault‐associated basal detachments also form a nested pattern. Complex temporal and spatial movement interactions are expected between these structural components of the unstable sector.
On electrodynamical measurements in the polar ionosphere with a monostatic incoherent scatter radar
Authors: Tsunoda Roland T. , Stauning Peter , Olesen Jens K.
Published: April 2007

We examine a technique that appears useful as a means of estimating the vector electric field (E) in the ionosphere with a monostatic incoherent scatter (IS) radar. The technique consists of using the altitude variation in mean Doppler velocity that is measured with an antenna beam fixed along a single line of sight. The rotation of the ion velocity vector with altitude in the E region is used to provide improved spatial and temporal resolution over other existing methods. We show that E can be extracted unambiguously from this one‐beam position measurement technique when the neutral wind (U) is negligible. When U is significant, however, neither E nor U can be determined without ambiguity. Results obtained from multipulse measurements made with the Sondrestrom IS radar are used to illustrate these concepts.
Suspended sediment concentration profiles in nonuniform flows: Is the classical perturbative approach suitable for depth‐averaged closures?
Authors: Toffolon Marco , Vignoli Gianluca
Published: April 2007

Several contributions have been proposed in the past decades with the aim to set up a reliable quantification of suspended sediment transport on the basis of depth‐averaged closure relationships for the concentration profiles. However, a definitive answer to the problem in unsteady and nonuniform conditions has not been found yet. In this paper, we compare a semianalytical solution, based on a formally correct classical perturbative approach relying on small values of characteristic parameters, with the results of a numerical model. The aim is to understand to which extent the simplified solution is able to reproduce the most important features of the complete solution. In particular, we analyze the behavior of vertical concentration profiles, depth‐averaged concentrations, and suspended sediment transport rates when temporal and spatial variations are imposed to the system. The comparison between the complete model and the perturbative approximation, pursued also with the aid of fully analytical solutions obtained in the case of constant eddy diffusivity, shows that the simplified solution is affected by some limits that are intrinsically related to the perturbative formulation itself. The main shortcoming of the perturbation approach lies in its failure to reproduce the inertia of the sedimentation process. This results in an amplification of the system response to high‐frequency external forcing, while the complete solution actually predicts a strong damping. Thus applying the perturbative model to real cases should be carefully considered when the perturbation parameter is not sufficiently small, as a rigorous application of the perturbation approach would require.
Texture development and TEM analysis of deformed CaIrO3: Implications for the D″ layer at the core‐mantle boundary
Authors: Walte Nicolas , Heidelbach Florian , Miyajima Nobuyoshi , Frost Daniel
Published: April 2007

CaIrO3 is an isostructural analog for MgSiO3 post‐perovskite (ppv), likely the dominant mineral phase in the D″ layer. At high temperature CaIrO3 undergoes a phase transition to a perovskite (pv) structure. We have performed deformation experiments of CaIrO3 pv and ppv with the deformation‐DIA apparatus. The deformed pv does not display any discernable texture even after strong coaxial shortening. In contrast, ppv displays a strong lattice preferred orientation (LPO) with the a‐axis parallel to the shear direction and the b‐axis perpendicular to the shear plane. Calculation of the seismic properties predicts a shear wave splitting with a maximum of 2.7% in the shear plane, which is qualitatively compatible with seismic measurements. Ppv displayed a high density of (010)[100] dislocations, consistent with the textural results. Thus, our experiments on CaIrO3 pv and ppv are compatible with a seismically isotropic lower mantle and anisotropic D″ layer in the Earth.
Sensitivity of U.S. surface ozone to future emissions and climate changes
Authors: Tao Zhining , Williams Allen , Huang Ho‐Chun , Caughey Michael , Liang Xin‐Zhong
Published: April 2007

The relative contributions of projected future emissions and climate changes to U.S. surface ozone concentrations are investigated focusing on California, the Midwest, the Northeast, and Texas. By 2050 regional average ozone concentrations increase by 2–15% under the IPCC SRES A1Fi (“dirty”) scenario, and decrease by 4–12% under the B1 (relatively “clean”) scenario. However, the magnitudes of ozone changes differ significantly between major metropolitan and rural areas. These ozone changes are dominated by the emissions changes in 61% area of the contiguous U.S. under the B1 scenario, but are largely determined by the projected climate changes in 46% area under the A1Fi scenario. In the ozone responses to climate changes, the biogenic emissions changes contribute strongly over the Northeast, moderately in the Midwest, and negligibly in other regions.
Impact of sintering temperature on the physical properties of the superconducting ferromagnet: RuSr2Eu1.5Ce0.5Cu2O10
Authors: Nigam R. , Pan A. V. , Dou S. X.
Published: April 2007

We report the influence of the sintering temperature on the electromagnetic behavior for the RuSr2Eu1.5Ce0.5Cu2O10 material, in which superconductivity and ferromagnetism coexist. The inadequate heat treatment results in the coexistence of the secondary Ru-1212 phase along with the dominant Ru-1222 phase. The presence of two phases leads to the magnetic superposition of the signals from both phases, which results in the observation of a small peak around 120–130K. In the pure Ru-1222 no such magnetic anomaly is observed. In addition, the impure samples exhibit a double step superconductivity transition. In the normal state, these impure samples exhibit a semiconductinglike behavior of the resistivity. In contrast, the pure Ru-1222 sample with much larger, well connected grains has a single step resistivity transition, as well as a metalliclike behavior in the normal state. Our work sends a strong message that only high quality, phase pure samples can be chosen for the investigation of the complex behavior in the Ru system.
Angular dependence of the depinning field for head-to-head domain walls at constrictions
Authors: Bedau D. , Kläui M. , Rüdiger U. , Vaz C. A. F. , Bland J. A. C. , Faini G. , Vila L. , Wernsdorfer W.
Published: April 2007

The angular dependence of the depinning field of vortex and transverse domain walls is determined experimentally for NiFe rings with nanometer size constrictions. From the angular dependence, we are able to deduce the potential landscape caused by the constriction and seen by the wall. The potential minimum for transverse walls is at the notch position so that these walls are pinned symmetrically inside the constriction. Vortex walls are pinned at a position adjacent to the constriction and this position can be determined from the angular variation of the depinning fields. Good agreement with the results of micromagnetic simulations is obtained.
“Reduced” multidimensional Vlasov simulations, with applications to electrostatic structures in space plasmas
Authors: Newman D. L. , Sen N. , Goldman M. V.
Published: April 2007

A class of two-dimensional open-boundary kinetic plasma simulations characterized as reduced Vlasov simulations have been developed and applied to the study of double layers and electron holes in Earth’s auroral downward current region. In these simulations, full Vlasov phase-space dynamics is followed only in the direction of the magnetic field or imposed current, with a limited representation of phase space employed in the perpendicular spatial dimension. These reduced methods include a dissipative moment closure for unmagnetized species and limitation of perpendicular velocity space to a ring of modes at a fixed v⊥ for weakly magnetized species. Simulations with weakly magnetized ions exhibit ion-Bernstein waves in association with electron holes and simulations with weakly magnetized or unmagnetized electrons result in electron holes elongated perpendicular to B. These results are in qualitative agreement with satellite observations and previous particle simulations.
On morphologies of gold nanoparticles grown from molecular dynamics simulation
Authors: Grochola Gregory , Russo Salvy P. , Snook Ian K.
Published: April 2007

The authors use a newly fitted gold embedded atom method potential to simulate the initial nucleation, coalescence, and kinetic growth process of vapor synthesized gold nanoparticles. Overall the population statistics obtained in this work seemed to mirror closely recent experimental HREM observations by Koga and Sugawara [Surf. Sci. 529, 23 (2003)] of inert gas synthesized nanoparticles, in the types of nanoparticles produced and qualitatively in their observance ratio. Our results strongly indicated that early stage coalescence (sintering) events and lower temperatures are the mainly responsible for the occurrence of the Dh and fcc based morphologies, while “ideal” atom by atom growth conditions produced the Ih morphology almost exclusively. These results provide a possible explanation as to why the Dh to Ih occurrence ratio increases as a function of nanoparticle size as observed by Koga and Sugawara.
Intrinsic rotation and electric field shear
Authors: Gürcan Ö. D. , Diamond P. H. , Hahm T. S. , Singh R.
Published: April 2007

A novel mechanism for the generation and amplification of intrinsic rotation at the low-mode to high-mode transition is presented. The mechanism is one where the net parallel flow is accelerated by turbulence. A preferential direction of acceleration results from the breaking of k‖→−k‖ symmetry by sheared E×B flow. It is shown that the equilibrium pressure gradient contributes a piece of the parallel Reynolds stress, which is nonzero for vanishing parallel flow, and so can accelerate the plasma, driving net intrinsic rotation. Rotation drive, transport, and fluctuation dynamics are treated self-consistently.
Strong optical emissions from two-dimensional metal photonic crystals with semiconductor multiple quantum wells
Authors: Gozu Shin-ichiro , Ueta Akio , Akahane Kouichi , Yamamoto Naokatsu , Tsuchiya Masahiro , Ohtani Naoki
Published: April 2007

We investigated two-dimensional (2D) metal photonic crystals (PhCs) with semiconductor multiple quantum wells as an active material. The 2D metal PhCs were fabricated using only a lift-off process. The lift-off process leads to a simpler fabrication process than the one for conventional PhCs and avoids any process damage induced by dry etching. The 2D metal PhCs exhibited clear photonic band effects: (1) photoluminescence (PL) peak positions changed depending on the lattice constant and (2) the PL intensity from 2D metal PhCs was higher than that without 2D metal PhCs. These effects are suitable for fabricating light-emitting diodes with a high external quantum efficiency. PL intensity depending on PL excitation power revealed slightly improved luminescence efficiency than the sample without 2D metal PhCs.
Effect of oxygen on the characteristics of radio frequency planar magnetron sputtering plasma used for aluminum oxide deposition
Authors: Kakati H. , Pal A. R. , Bailung H. , Chutia Joyanti
Published: April 2007

The characteristics of a radio frequency magnetron sputtering plasma are studied with the help of compensating Langmuir probe and optical emission spectroscopy in both reactive and nonreactive modes in argon and oxygen environment. Langmuir probe is used to measure the electron temperature, while the optical emission spectroscopy is used to get an idea about the relative concentration of metal and metal oxide formed under the different discharge conditions of argon and oxygen flow. The root mean square value of the discharge voltage and dc negative self-bias are found to be dependent on the oxygen flow rate as well as total gas pressure of the discharge. The study clearly shows the transition of the discharge from the metallic mode to the reactive mode with the increasing flow rate of oxygen, which strongly influences the sputtering. The optimum values of the control variables have been determined for the formation of aluminum oxide.
Microwave dielectric properties of carbon black filled polymers under uniaxial tension
Authors: Mdarhri A. , Brosseau C. , Carmona F.
Published: April 2007

Broadband microwave spectroscopy is commonly employed in the determination of the wave transport properties in mesostructured polymers with embedded carbon black particles. However, very little is known on how uniaxial stress affects the effective permittivity of these percolative materials. In this work we carry out a comprehensive study of the complex permittivity spectra of carbon black filled ethylene butylacrylate copolymer which is submitted to a tensile stress up to 45% over the 0.1−3 GHz frequency range and at room temperature. Permittivity measurements indicated large decreases in the real and imaginary parts of the effective permittivity of the axially elongated samples. This set of experiments illustrates that these microwave permittivity data scale as a power law in frequency, where the exponent is strongly sensitive to stress. In addition, we show that our effective permittivity measurements under stress can be explained in terms of the Gaussian molecular network model in the limit of low stress. The observed variation of the effective permittivity is consistent with that reported for related materials, i.e., carbon black filled cross-linked rubber. Our analysis points out to the importance of local properties, i.e., properties associated with specific subsystems (e.g., aggregates of carbon black particles) in understanding the effective dielectric properties of these “macroscopic homogeneous” materials. Improved reliability through real-time monitoring of strain and material parameters (permittivity and permeability) simultaneously will result in significant benefits to the community in many areas, such as aerospace, power, and automotive industries.
Mode dynamics of high power (InAl)GaN based laser diodes grown on bulk GaN substrate
Authors: Swietlik T. , Franssen G. , Czernecki R. , Leszczynski M. , Skierbiszewski C. , Grzegory I. , Suski T. , Perlin P. , Lauterbach C. , Schwarz U. T.
Published: April 2007

Time resolved scanning near-field optical microscopy was employed to study spatial and temporal dynamics of III-nitride-system-based blue light emitting laser diodes with a ridge width of 20 μm deposited on high pressure grown bulk GaN substrate. Devices were driven in a pulse regime with a current pulse length of 500 ns. Temperature effects and fluctuation in carrier concentration resulted in a complicated dynamic picture of the photon field evolution. The guided modes did not reach a stable form during a driving pulse. Due to a large antiguiding factor which is characteristic for nitride compounds, filamentation processes were clearly observable leading to the formation of up to four filaments across the ridge, each about 3 μm. Analysis of spatial and temporal evolution of the guided mode revealed strong light leakage into the highly absorptive substrate. Separation between the adjacent cavity modes equals to 0.57 nm which corresponds to the cavity length of 50 μm being very close to the thickness of GaN substrate. Detection of near-field-to-far-field evolution evidenced a considerable beam steering as a result of temperature and carrier induced refractive index changes.
Optical loss study of porous silicon and oxidized porous silicon planar waveguides
Authors: Pirasteh Parastesh , Charrier Joël , Dumeige Yannick , Haesaert Séverine , Joubert Pierre
Published: April 2007

We have studied optical losses as a function of the wavelength for planar waveguides formed from porous silicon or oxidized porous silicon. Scattered light from the surface of samples was also observed. This observation reveals the influence of porous silicon dissolution front fluctuations called waviness on propagation. After oxidation, the measured losses decreased strongly and attained a value equal to 0.5 dB/cm in the near infrared. Surface and volume scattering losses were modeled in order to determine their principal contributions to overall losses. For porous silicon waveguides obtained from a P+ silicon substrate, the losses were mainly due to absorption by the material; whereas, for oxidized porous silicon waveguides, the principal contribution depends on the used wavelength. In the visible spectrum, losses due to volume scattering were predominant while in the near infrared, surface scattering was responsible for most of the losses.
Electron irradiation induced deep centers in hydrothermally grown ZnO
Authors: Fang Z.-Q. , Claflin B. , Look D. C. , Farlow G. C.
Published: April 2007

An n-type hydrothermally grown ZnO sample becomes semi-insulating (ρ~108 Ω cm) after 1-MeV electron-irradiation. Deep traps produced by the irradiation were studied by thermally stimulated current spectroscopy. The dominant trap in the as-grown sample has an activation energy of 0.24 eV and is possibly related to LiZn acceptors. However, the electron irradiation introduces a new trap with an activation energy of 0.15 eV, and other traps of energy 0.30 and 0.80 eV, respectively. From a comparison of these results with positron annihilation experiments and density functional theory, we conclude that the 0.15-eV trap may be related to VZn.
Electrically detected and conventional magnetic resonance investigation of surface and bulk states in polyaniline thin films
Authors: Castro Fernando A. , Graeff Carlos F. O.
Published: April 2007

Electrically detected magnetic resonance (EDMR) and electron paramagnetic resonance (EPR) were used to investigate emeraldine base polyaniline films. The magnetic susceptibility presented a Curie (localized spins)—Pauli (delocalized spins) transition at 240 K, when we also observed a transition in the dependence of the g factor with temperature (T). Peak-to-peak linewidth decreases with increasing temperature, reflecting that motional narrowing limits the hyperfine and dipolar broadening in this polymer. EDMR spectra could only be observed above 250 K in accordance to EPR results. Surface and bulk transport could be separated and their analysis reflected the effect of magnetic interaction with oxygen.
Luminescence and surface properties of MgxZn1−xO thin films grown by pulsed laser deposition
Authors: Heitsch S. , Zimmermann G. , Fritsch D. , Sturm C. , Schmidt-Grund R. , Schulz C. , Hochmuth H. , Spemann D. , Benndorf G. , Rheinländer B. , Nobis Th. , Lorenz M. , Grundmann M.
Published: April 2007

We investigated the surface roughness and the luminescence properties of MgxZn1−xO thin films (0⩽x⩽0.19). The thin films were grown on a-plane sapphire substrates by pulsed laser deposition. The root-mean-square surface roughness depends on the oxygen partial pressure p(O2) applied during deposition and takes a minimal value of 0.6nm for p(O2)=1×10−3mbar. Deposition of the thin films on a ZnO buffer layer further diminishes the surface roughness. The photoluminescence maximum peak from the MgxZn1−xO thin films is due to free excitons, which are localized in potential minima at low temperatures, and shows a linear high-energy shift with increasing x. The full width at half maximum (FWHM) of the photoluminescence depends on p(O2). Samples with the smallest FWHM values were grown at p(O2)=1.6×10−2mbar. We analyze the contributions of alloy broadening and of the lateral Mg-concentration inhomogeneity to the photoluminescence FWHM. In optimized samples FWHM is limited by random alloy broadening. Scanning cathodoluminescence shows that the lateral inhomogeneity of the Mg distribution decreases with p(O2) if the samples are grown directly on the sapphire substrates. Deposition of the MgxZn1−xO thin films on a ZnO buffer layer leads to a laterally homogeneous Mg distribution in the samples.
Polarized measurements of spectrally narrowed emissions from a single crystal of a thiophene/phenylene co-oligomer
Authors: Yamao Takeshi , Ohira Takayuki , Ota Satoshi , Hotta Shu
Published: April 2007

The spectrally narrowed emissions (SNE) from a flat single crystal of a thiophene/phenylene co-oligomer were investigated in a series of polarized measurements. The SNE spectra comprise of two major lines occurring at 507 and 514 nm. Of these, the former line effectively causes the emission gain narrowing by the pulse laser excitation polarized along the c axis. The intensity of the SNE is nearly isotropic with the radial directions along the flat crystal plane paralleling the bc plane. The SNE is strongly polarized along the a axis with its emission polarization ratio of the a axis to the b axis being 16.7. These polarized SNEs can be understood by the upright configuration of the molecular long axis paralleling the a axis.
Spherical nanoindentation and deformation mechanisms in freestanding GaN films
Authors: Basu Sandip , Barsoum Michel W. , Williams Adrian D. , Moustakas T. D.
Published: April 2007

In this work, spherical nanoindentation—with nanoindenter radius of 1 or 13.5μm—was used to explore the deformation behavior of GaN freestanding films with two orientations, C (basal) and A (prismatic), grown by hydride vapor phase epitaxy. For the C plane, all the results shown herein and most of those in the literature can be explained by invoking the activation of basal slip alone. The wide distribution of pop-in stresses in the C plane is believed to depend on initial surface and/or near surface defect concentrations. By converting the nanoindentation load-displacement data to indentation stress-strain curves and comparing those for the A and C planes after the pop-ins, we conclude that basal slip is also implicated in the deformation of the A plane. The elastic moduli, determined from spherical nanoindentation, depend on the indenter size. In the C plane repeated spherical nanoindentations, to the same stress, result in reversible, hysteretic loops that are attributed to the formation of incipient kink bands and/or the to-and-fro motion of mobile dislocation walls.
He
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ion irradiation study of continuous and patterned Co∕Pd multilayers
Authors: Parekh Vishal , Smith Darren , E Chunsheng , Rantschler James , Khizroev Sakhrat , Litvinov Dmitri
Published: April 2007

Ion irradiation of continuous and patterned (Co∕Pd)n magnetic multilayer films has been studied as a mean to control magnetic anisotropy as well as to evaluate possible ion irradiation damage involved in ion-beam proximity lithography patterning. The coercivity of patterned medium was found to decrease from 11kOe for as patterned samples to 0.3kOe for samples with 800μC∕cm2 ion irradiation. Remnant squareness of the patterned samples remained essentially unchanged. As the number of bilayers increases in the sample, the effects vary, suggesting that several mechanisms of damage occur. Significantly, for typical irradiation doses used in ion-beam proximity lithography, no measurable alteration of magnetic properties was observed.
Influence of different peripheral substituents on the nonlinear optical properties of cobalt phthalocyanine core
Authors: Derkowska B. , Wojdyła M. , Bała W. , Jaworowicz K. , Karpierz M. , Grote James G. , Krupka O. , Kajzar F. , Sahraoui B.
Published: April 2007

In this article, we show how the substituting different peripheral substituents around the cobalt phthalocyanine core correlate with nonlinear optical properties. We present the results on nonlinear optical properties of solution of cobalt phthalocyanine (CoPc), cobalt phthalocyanine with DNA-CTMA surfactant complex (CoPc-DNA-CTMA), and cobalt phthalocyanine with liquid crystal (CoPc-LC) measured by degenerate four-wave mixing (DFWM) method at the 532 nm wavelength region. We found that the values of third-order nonlinear optical susceptibility (χ⟨3⟩) of CoPc-LC and CoPc-DNA-CTMA increase in comparison with the value of the third-order nonlinear optical susceptibilities of CoPc. We supposed that this is caused by increase of the charge-transfer effects and of the dipole moments of the molecule with the increase of the chain length.
Optical properties of metal nanoparticles as probed by photoemission electron microscopy
Authors: Douillard L. , Charra F. , Fiorini C. , Adam P. M. , Bachelot R. , Kostcheev S. , Lerondel G. , Lamy de la Chapelle M. , Royer P.
Published: April 2007

Interaction of light with metallic nanostructures exhibit resonances specific to eigenmodes of collective electron motion known as plasmon modes. Both these resonances and effects of electromagnetic singularities lead to local field enhancement. Photoemission electron microscopy (PEEM) was recently used to image the electrons photoemitted from these local effects. Here we report on a full PEEM spectral study of localized surface plasmons excited on nanoparticles. In particular, we show strong spectral similarities between the PEEM signal and optical extinction as a function of the incident wavelength, establishing the photoemission microscopy as a valuable tool for investigating the optical properties of metal nanoparticles.
Saturation of multiphoton absorption upon strong and ultrafast infrared laser excitation
Authors: He Guang S. , Zheng Qingdong , Baev Alexander , Prasad Paras N.
Published: April 2007

Saturation of two- and three-photon absorption has been studied in three dye-solution samples excited with 160-fs laser pulses of 775- and 1280-nm wavelengths, respectively. The effective two- and three-photon absorption coefficients measured for a given sample are not material constants, but dependent on the levels of input laser intensity. These saturation phenomena can be well interpreted by considering the depletion of ground-state population upon strong ultra-short multiphoton excitation. The approximate expressions of effective two- and three-photon absorption coefficients and their intensity dependence are proposed, based on which the experimental results can be well fitted and the saturation parameters for a given sample can be readily determined.
Effect of sample temperature on the indentation-induced phase transitions in crystalline silicon
Authors: Khayyat M. M. O. , Hasko D. G. , Chaudhri M. M.
Published: April 2007

Using four-terminal in-situ dc electrical resistance measurements and Raman spectra of residual Vickers indentations in single crystals of Si (100), it has been shown that the sample temperature, in the range of 150 to 300 K, at which a Vickers diamond indentation is made, has a strong influence on the occurrence of indentation-induced phase transitions within the plastically deformed zone around the indentation. A consistent explanation of the experimental results, based on an existing theoretical pressure-temperature phase diagram of silicon, has been provided.
Pipe flow transition threshold following localized impulsive perturbations
Authors: Mellibovsky Fernando , Meseguer Alvaro
Published: April 2007

A numerical study of the destabilizing effects of localized impulsive perturbations in pressure-driven Hagen-Poiseuille or pipe flow is presented. The numerics intend to ellucidate the intrinsic mechanisms of subcritical transition to turbulence in pipe flow by reproducing very recent experimental explorations carried out by Hof, Juel, and Mullin [Phys. Rev. Lett. 91, 244502 (2003)], concluding that the minimum amplitude of a perturbation required to cause transition scales as the inverse of the Reynolds number, i.e., O(Re−1). The numerical model simulates the experimental disturbance generator based on impulsive injection of fluid through six slits azimuthally equispaced on a perimeter around the pipe. This is accomplished by introducing a local time-dependent impulsive volume force term in the Navier-Stokes equations for the perturbation velocity field, fulfilling incompressibility constraints. A comprehensive exploration of the critical amplitudes that trigger transition as a function of the injection duration is carried out. It is concluded, in agreement with experiments, that injections lasting longer than Δtinj≃24 advective time units do not remarkably decrease the critical amplitude of transition. Threshold amplitudes for long enough injections are then computed within the range Re∊[2000,14125]. For Re≳4000 the numerical results agree with the experiments. However, for Re≲2800, neat transitions are very difficult to obtain, being impossible to provide an accurate value of the critical amplitude. The apparent disagreement with the sound regular slope of the experimental threshold is explained in terms of the differences between constant mass-flux versus pressure-driven pipe flows.
Tensile-strained GaAsN quantum dots on InP
Authors: Pohjola P. , Hakkarainen T. , Koskenvaara H. , Sopanen M. , Lipsanen H. , Sainio J.
Published: April 2007

Self-assembled quantum dots are typically fabricated from compressive-strained material systems, e.g., InAs on GaAs. In this letter, self-assembled quantum dots from tensile-strained GaAsN on InP are demonstrated. GaAsN on InP has type-I band alignment. Stranski-Krastanov growth mode is not observed, but in situ annealing of the uncapped samples results in the formation of islands. Photoluminescence spectra from the buried GaAsN show separate peaks due to a wetting layer and islands around the energies of 1.3 and 1.1eV, respectively.
Comparative analysis of resonant phonon THz quantum cascade lasers
Authors: Jirauschek Christian , Scarpa Giuseppe , Lugli Paolo , Vitiello Miriam S. , Scamarcio Gaetano
Published: April 2007

We present a comparative analysis of a set of GaAs-based THz quantum cascade lasers, based on longitudinal-optical phonon scattering depopulation, by using an ensemble Monte Carlo simulation, including both carrier-carrier and carrier-phonon scattering. The simulation shows that the parasitic injection into the states below the upper laser level limits the injection efficiency and thus the device performance at the lasing threshold. Additional detrimental effects playing an important role are identified. The simulation results are in reasonable agreement with the experimental findings.
Field structure and electron acceleration in a laser beam of a high-order Hermite-Gaussian mode
Authors: Wang P. X. , Ho Y. K. , Tang Ch. X. , Wang W.
Published: April 2007

We analyze the axial electric field intensity distribution and the phase velocity distribution of high-order Hermite-Gaussian (HG) mode laser beams. Using a three-dimensional test particle simulation, the numerical results of electrons accelerated by Hermite-Gaussian (0,0), and (3,0) mode laser beams are presented. It is established that electrons can be more favorably captured and accelerated in an odd high-order Hermite-Gaussian mode laser beam.
Model for intensity calculation in electron guns
Authors: Doyen O. , De Conto J. M. , Garnier J. P. , Lefort M. , Richard N.
Published: April 2007

The calculation of the current in an electron gun structure is one of the main investigations involved in the electron gun physics understanding. In particular, various simulation codes exist but often present some important discrepancies with experiments. Moreover, those differences cannot be reduced because of the lack of physical information in these codes. We present a simple physical three-dimensional model, valid for all kinds of gun geometries. This model presents a better precision than all the other simulation codes and models encountered and allows the real understanding of the electron gun physics. It is based only on the calculation of the Laplace electric field at the cathode, the use of the classical Child–Langmuir’s current density, and a geometrical correction to this law. Finally, the intensity versus voltage characteristic curve can be precisely described with only a few physical parameters. Indeed, we have showed that only the shape of the electric field at the cathode without beam, and a distance of an equivalent infinite planar diode gap, govern mainly the electron gun current generation.
Heun equation, Teukolsky equation, and type-D metrics
Authors: Batic D. , Schmid H.
Published: April 2007

Starting with the whole class of type-D vacuum backgrounds with cosmological constant we show that the separated Teukolsky equation for zero rest-mass fields with spins s=±2 (gravitational waves), s=±1 (electromagnetic waves), and s=±1∕2 (neutrinos) is a Heun equation in disguise.
A model for the advantage of early electron cyclotron current drive in the suppression of neoclassical tearing modes
Authors: Lazaros Avrilios , Maraschek Marc , Zohm Hartmut
Published: April 2007

An analytic model for the advantage of the early application of electron cyclotron current drive (ECCD) in the suppression of neoclassical tearing modes (NTMs) is presented. The improved performance of early ECCD is attributed to the second (smaller) saturation island width, which appears for sufficiently small (relative to the ECCD deposition width) critical island widths, in the strongly nonlinear growth rate profile. The operational range for the advantage of early ECCD is obtained, and it is shown that it is favored by broad deposition profiles. The preliminary experimental results in ASDEX Upgrade [H. Zohm et al., Nucl. Fusion 41, 197 (2001)] are consistent with the present model.
Edge-to-site reduction of Bethe-Peierls approximation for nearest neighbor exclusion cubic lattice particle systems and thermodynamic modeling of liquid silicates
Authors: Vladimirov Igor , Jak Eugene
Published: April 2007

We study an interacting particle system on the simple cubic lattice satisfying the nearest neighbor exclusion (NNE) which forbids any two nearest sites to be simultaneously occupied. Under the constraint, we develop an edge-to-site reduction of the Bethe-Peierls entropy approximation of the cluster variation method. The resulting NNE-corrected Bragg-Williams approximation is applied to statistical mechanical modeling of a liquid silicate formed by silica and a univalent network modifier, for which we derive the molar Gibbs energy of mixing and enthalpy of mixing and compare the predictions with available thermodynamic data.
Quantitative characterization of orientational order in liquid carbon tetrachloride
Authors: Rey Rossend
Published: April 2007

A simple geometrical construct is proposed for a clear-cut classification of the relative orientation between two tetrahedral molecules in terms of six orientational classes. When applied to sort out configurations from condensed phase simulations, it leads to a quantitative characterization of orientational order: A definite percentage for each class is obtained as a function of the distance between molecular centers. The basic picture that emerges, for liquid carbon tetrachloride, is that the dominant configuration for each distance is such that the number of chlorines in between both carbons diminishes with increasing separation, with a configuration here termed edge-to-face being the dominant one at contact. Regarding the range of orientational order, remnants are still noticeable at ≈20 Å, i.e., up to the fourth solvation shell. Beyond this distance the distributions are hardly distinguishable from the analytical predictions for random orientation. The analysis of the small fluctuations at such long distances shows that there are no significant differences betweeen the ranges of positional and orientational order.
Absorption spectroscopy of molecular trimers
Authors: Seibt Joachim , Dehm Volker , Würthner Frank , Engel Volker
Published: April 2007

Absorption properties of molecular trimers are studied within a model including a single monomer internal vibrational degree of freedom. Upon photoabsorption, three excited electronic states which are coupled excitonically are accessed. Band shapes resulting from different electronic coupling strengths and geometries are analyzed. It is shown that geometric information can be extracted from the band intensities. Taking data recorded for perylene bisimide aggregates as an example, the spectra for monomer, dimer, and trimer systems are compared.
Structure of liquid nitromethane: Comparison of simulation and diffraction studies
Authors: Megyes Tünde , Bálint Szabolcs , Grósz Tamás , Radnai Tamás , Bakó Imre , Almásy László
Published: April 2007

Simulation (molecular dynamics and Car-Parrinello [Phys. Rev. Lett. 55, 2471 (1985)]) and diffraction (x-ray and neutron) studies on nitromethane are compared aiming at the determination of the liquid structure. Beyond that, the capabilities of the methods to describe liquid structure are discussed. For the studied liquid, the diffraction methods are performing very well in the determination of intramolecular structure, but they do not give detailed structural information on the intermolecular structure. The good agreement between the diffraction experiments and the results of molecular dynamics simulations justifies the use of simulations for the more detailed description of the liquid structure using partial radial distribution functions and orientational correlation functions. Liquid nitromethane is described as a molecular liquid without strong intermolecular interactions such as hydrogen bonding, but with detectable orientational correlations resulting in preferential antiparallel order of the neighboring molecules.
Changes in the electronic structures and optical band gap of Ge2Sb2Te5 and N-doped Ge2Sb2Te5 during phase transition
Authors: Kim YoungKuk , Jeong K. , Cho M.-H. , Hwang Uk , Jeong H. S. , Kim Kinam
Published: April 2007

Changes in the electronic structures of Ge2Sb2Te5 (GST) and N-doped Ge2Sb2Te5 film during the phase transition from an amorphous to a crystalline phase were studied using synchrotron radiation high-resolution x-ray photoemission spectroscopy. The changes in tetrahedral and octahedral coordinated Ge 3d peaks are closely related to the changes in the chemical bonding state of GST films. The metallic Sb peak in the Sb 4d spectra of annealed GST films demonstrates that the metallic Sb atoms become segregated during thermal treatment resulting in phase separation. The incorporation of nitrogen into the GST film affects its structure and chemical bonding state, resulting in the suppression of crystallization. The incorporation of nitrogen also increases the optical band gap of the film due to the formation of a nitride.
Terahertz continuous-wave large-area traveling-wave photomixers on high-energy low-dose ion-implanted GaAs
Authors: Michael E. A. , Cámara Mayorga I. , Güsten R. , Dewald A. , Schieder R.
Published: April 2007

Nitrogen ion implantation at 3MeV and low doses into commercial semi-insulating GaAs (107Ωcm) is used to manufacture continuously operated metal-semiconductor-metal terahertz photomixer radiation sources based on a traveling-wave interdigitated-finger coplanar stripline structure. The authors systematically investigated the terahertz efficiency over the implantation dose range of 5.6×1011–1.8×1013ions∕cm2 and determined the optimum implantation dose range for operation in the frequency range 500–1000GHz, where a significant higher efficiency is obtained compared to low-temperature-grown GaAs.
Monte Carlo simulation of a film growth with reactive hydrophobic, polar, and aqueous components by a covalent bond fluctuating model
Authors: Yang Shihai , Pandey Ras B.
Published: April 2007

Using a bond fluctuating model (BFM), Monte Carlo simulations are performed to study the film growth in a mixture of reactive hydrophobic (H) and hydrophilic (P) groups in a simultaneous reactive and evaporating aqueous (A) solution on a simple three dimensional lattice. In addition to the excluded volume, short range phenomenological interactions among each constituents and kinetic functionalities are used to capture their major characteristics. The simulation involves thermodynamic equilibration via stochastic movement of each constituent by Metropolis algorithm as well as cross-linking reaction among constituents with evaporating aqueous component. The film thickness (h) and its interface width (W) are examined with a reactive aqueous solvent for a range of temperatures (T). Results are compared with a previous study [Yang et al. Macromol. Theory Simul. 15, 263 (2006)] with an effective bond fluctuation model (EBFM). Simulation data show a much slower power-law growth for h and W with BFM than that with EBFM. With BFM, growth of the film thickness can be described by h∝tγ, with a typical value γ1≈0.97 in initial time regime followed by γ2≈0.77 at T=5, for example. Growth of the interface width can also be described by a power law, W∝tβ, with β1≈0.40 initially and β2≈0.25 in later stage. Corresponding values of the exponents with EBFM are much higher, i.e., γ1≈1.84, γ2≈1.34 and β1≈1.05, β2≈0.60 at T=5. Correct restrictions on the bond length with the excluded volume used with BFM are found to have a greater effect on steady-state film thickness (hs) and the interface width (Ws) at low temperatures than that at high temperatures. The relaxation patterns of the interface width with BFM seem to change noticeably from those with EBFM. A better relaxed film with a smoother surface is thus achieved by the improved cross-linking covalent bond fluctuation model which is more realistic in capturing appropriate details of systems such as polyurethane film. The steady-state film thickness increases monotonically with the temperature possibly with two logarithmic dependences. The equilibrium interface width shows a nonmonotonic dependence: on increasing the temperature, Ws seems to increase slowly before it begins to decay Ws=4.12−1.39ln(T).
Interaction of alkali metals with perylene-3,4,9,10-tetracarboxylic-dianhydride thin films studied by IR spectroscopy
Authors: Heimer K. , Wuesten J. , Lach S. , Ziegler Ch.
Published: April 2007

In order to clarify the doping behavior of different alkali metals in perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), Fourier transform infrared spectra of PTCDA thin films doped with sodium, potassium, and cesium were measured and compared. Furthermore the vibrational properties were calculated using density-functional theory and these calculated vibrational frequencies were assigned to the experimental IR modes of the thin films.
Epitaxial silicon devices for dosimetry applications
Authors: Bruzzi M. , Bucciolini M. , Casati M. , Menichelli D. , Talamonti C. , Piemonte C. , Svensson B. G.
Published: April 2007

A straightforward improvement of the efficiency and long term stability of silicon dosimeters has been obtained with a n+-p junction surrounded by a guard-ring structure implanted on an epitaxial p-type Si layer grown on a Czochralski substrate. The sensitivity of devices made on 50-μm-thick epitaxial Si degrades by only 7% after an irradiation with 6MeV electrons up to 1.5kGy, and shows no significant further decay up to 10kGy. These results prove the enhanced radiation tolerance and stability of epitaxial diodes as compared to present state-of-the-art Si devices.
Polymer capture by electro-osmotic flow of oppositely charged nanopores
Authors: Wong C. T. A. , Muthukumar M.
Published: April 2007

The authors have addressed theoretically the hydrodynamic effect on the translocation of DNA through nanopores. They consider the cases of nanopore surface charge being opposite to the charge of the translocating polymer. The authors show that, because of the high electric field across the nanopore in DNA translocation experiments, electro-osmotic flow is able to create an absorbing region comparable to the size of the polymer around the nanopore. Within this capturing region, the velocity gradient of the fluid flow is high enough for the polymer to undergo coil-stretch transition. The stretched conformation reduces the entropic barrier of translocation. The diffusion limited translocation rate is found to be proportional to the applied voltage. In the authors’ theory, many experimental variables (electric field, surface potential, pore radius, dielectric constant, temperature, and salt concentration) appear through a single universal parameter. They have made quantitative predictions on the size of the adsorption region near the pore for the polymer and on the rate of translocation.