bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental observation of bulk band dispersions in the oxide semiconductor ZnO using soft x-ray angle-resolved photoemission spectroscopy
Authors: Kobayashi M. , Song G. S. , Kataoka T. , Sakamoto Y. , Fujimori A. , Ohkochi T. , Takeda Y. , Okane T. , Saitoh Y. , Yamagami H. , Yamahara H. , Saeki H. , Kawai T. , Tabata H.
Published: June 2009

The electronic structure of the oxide semiconductor ZnO has been investigated using soft x-ray angle-resolved photoemission spectroscopy (ARPES). The obtained band dispersions within the kx−ky planes reflect the symmetry of the Brillouin zone and show no surface-state-derived flat bands. Band dispersions along the kz direction have also been observed. The obtained band dispersions qualitatively agree with band-structure calculations except for the bandwidth. The observations provide experimental evidence that soft x-ray ARPES enables us to study the bulk band structure of semiconductors.
Electrons in quantum dots: One by one
Authors: Gustavsson S. , Leturcq R. , Ihn T. , Ensslin K. , Gossard A. C.
Published: June 2009

A quantum point contact placed close to a quantum dot can be used as a charge detector with time resolution to monitor the charge flow on the level of individual electrons. The current through the quantum point contact may take two possible values corresponding to the situation of an additional electron being on or off the quantum dot. Time traces of such two-level behavior allow to measure the average current, the tunnel rates in and out of the quantum dot, the time-dependent fluctuations of the current (noise), as well as higher-order current correlations. This high-sensitivity method to measure charge flow can also be used to detect time-resolved single-electron interference.
Photonic crystal mode terahertz lasers
Authors: Benz A. , Deutsch Ch. , Fasching G. , Unterrainer K. , Andrews A. M. , Klang P. , Hoffmann L. K. , Schrenk W. , Strasser G.
Published: June 2009

We present the design and fabrication of photonic crystal (PhC) based resonators for terahertz quantum-cascade laser with a gain maximum at 2.7 THz. The PhC provides the confinement in lateral direction, for the vertical confinement a double-metal waveguide is used. We show theoretical and experimental analyses of devices based on a central gain region, which is surrounded by a PhC mirror. The devices are lasing in the bandgaps or at high-symmetry points of the PhC, depending on the period. These concepts allow us to tune the emission of the lasers in the range of a few hundred gigahertz.
Evolution and stability of ordered SiGe islands grown on patterned Si(100) substrates
Authors: Dais C. , Mussler G. , Sigg H. , Müller E. , Solak H. H. , Grützmacher D.
Published: June 2009

SiGe quantum dots are proposed as building blocks for future Si device technology. However, in order to exploit the full potential of SiGe islands, it is necessary to control their positioning and size on a nanometer length. This is achieved by templated self-assembly, which combines substrate patterning and subsequent epitaxy. In this paper we report on the evolution of SiGe islands on patterned substrates under consideration of small template variations and postgrowth annealing. The impact of the structural variations on the optical properties of the islands is investigated by photoluminescence measurements.
Multiexciton complexes in InAs self-assembled quantum dots
Authors: Korkusinski M. , Zielinski M. , Hawrylak P.
Published: June 2009

We review our recent work on multiexciton complexes in InAs self-assembled quantum dots using a combination of effective mass, k⋅p, and atomistic sp3s∗d5 tight-binding approaches. The single-particle levels from effective mass, k⋅p, and atomistic tight-binding models are used as input into configuration-interaction calculation of multiexciton spectra. We describe the principles of the atomistic approach and apply all these computational tools to illustrate the concept of hidden symmetry as underlying principle in energy levels of multiexciton complexes, optical detection of electron spin polarization, tunneling of holes in quantum dot molecules, and tuning of multiexciton spectra with lateral electric fields for entangled photon pair generation.
Polarization fine structure and enhanced single-photon emission of self-assembled lateral InGaAs quantum dot molecules embedded in a planar microcavity
Authors: Hermannstädter C. , Witzany M. , Beirne G. J. , Schulz W.-M. , Eichfelder M. , Rossbach R. , Jetter M. , Michler P. , Wang L. , Rastelli A. , Schmidt O. G.
Published: June 2009

Single lateral InGaAs quantum dot molecules have been embedded in a planar microcavity in order to increase the luminescence extraction efficiency. Using a combination of metal-organic vapor phase and molecular beam epitaxy samples could be produced that exhibit a 30 times enhanced single-photon emission rate. We also show that the single-photon emission is fully switchable between two different molecular excitonic recombination energies by applying a lateral electric field. Furthermore, the presence of a polarization fine structure splitting of the molecular neutral excitonic states is reported which leads to two polarization split classically correlated biexciton-exciton cascades. The fine structure splitting is found to be on the order of 10 μeV.
Two-dimensional electron gases: Theory of ultrafast dynamics of electron-phonon interactions in graphene, surfaces, and quantum wells
Authors: Richter M. , Carmele A. , Butscher S. , Bücking N. , Milde F. , Kratzer P. , Scheffler M. , Knorr A.
Published: June 2009

Many-particle electron-phonon interaction effects in two-dimensional electron gases are investigated within a Born–Markov approach. We calculate the electron-phonon interaction on a microscopic level to describe relaxation processes of quantum confined electrons on ultrafast time scales. Typical examples, where two-dimensional electron gases play a role, are surfaces and two-dimensional nanostructures such as graphene and quantum wells. In graphene, we find nonequilibrium phonon generation and ultrafast cooling processes after optical excitation. Electron relaxation dynamics at the silicon (001) 2×1 surface exhibits two time scales, corresponding to intrasurface and inside bulk-scattering processes. For GaAs quantum wells, we present broad emission spectra in the terahertz range assisted by LO-phonons of the barrier material.
On the use of group theoretical and graphical techniques toward the solution of the general N-body problem
Authors: Laing W. B. , Dunn M. , Watson D. K.
Published: June 2009

Group theoretic and graphical techniques are used to derive the N-body wave function for a system of identical bosons with general interactions through first-order in a perturbation approach. This method is based on the maximal symmetry present at lowest order in a perturbation series in inverse spatial dimensions. The symmetric structure at lowest order has a point group isomorphic with the SN group, the symmetric group of N particles, and the resulting perturbation expansion of the Hamiltonian is order-by-order invariant under the permutations of the SN group. This invariance under SN imposes severe symmetry requirements on the tensor blocks needed at each order in the perturbation series. We show here that these blocks can be decomposed into a basis of binary tensors invariant under SN. This basis is small (25 terms at first order in the wave function), independent of N, and is derived using graphical techniques. This checks the N6 scaling of these terms at first order by effectively separating the N scaling problem away from the rest of the physics. The transformation of each binary tensor to the final normal coordinate basis requires the derivation of Clebsch–Gordon coefficients of SN for arbitrary N. This has been accomplished using the group theory of the symmetric group. This achievement results in an analytic solution for the wave function, exact through first order, that scales as N0, effectively circumventing intensive numerical work. This solution can be systematically improved with further analytic work by going to yet higher orders in the perturbation series.
Distinguished quantum states in a class of cosmological spacetimes and their Hadamard property
Authors: Dappiaggi Claudio , Moretti Valter , Pinamonti Nicola
Published: June 2009

In a recent paper, we proved that a large class of spacetimes, not necessarily homogeneous or isotropous and relevant at a cosmological level, possesses a preferred codimension 1 submanifold, i.e., the past cosmological horizon, on which it is possible to encode the information of a scalar field theory living in the bulk. Such bulk-to-boundary reconstruction procedure entails the identification of a preferred quasifree algebraic state for the bulk theory, enjoying remarkable properties concerning invariance under isometries (if any) of the bulk and energy positivity and reducing to well-known vacua in standard situations. In this paper, specializing to open Friedmann–Robertson–Walker models, we extend previously obtained results and we prove that the preferred state is of Hadamard form, hence the backreaction on the metric is finite and the state can be used as a starting point for renormalization procedures. Such state could play a distinguished role in the discussion of the evolution of scalar fluctuations of the metric, an analysis often performed in the development of any model describing the dynamic of an early Universe which undergoes an inflationary phase of rapid expansion in the past.
Quantum control and manipulation of donor electrons in Si-based quantum computing
Authors: Calderón M. J. , Saraiva A. , Koiller Belita , Das Sarma S.
Published: June 2009

Doped Si is a promising candidate for quantum computing due to its scalability properties, long spin coherence times, and the astonishing progress on Si technology and miniaturization in the past few decades. This proposal for a quantum computer ultimately relies on the quantum control of electrons bound to donors near a Si/barrier (e.g., SiO2) interface. We address here several important issues and define critical parameters that establish the conditions that allow the manipulation of donor electrons in Si by means of external electric and magnetic fields.
Midinfrared semiconductor optical metamaterials
Authors: Hoffman Anthony J. , Sridhar Aishwarya , Braun Phillip X. , Alekseyev Leonid , Howard Scott S. , Franz Kale J. , Cheng Liwei , Choa Fow-Sen , Sivco Deborah L. , Podolskiy Viktor A. , Narimanov Evgenii E. , Gmachl Claire
Published: June 2009

We report on a novel class of semiconductor metamaterials that employ a strongly anisotropic dielectric function to achieve negative refraction in the midinfrared region of the spectrum, ∼8.5–13 μm. We present two types of metamaterials, layered highly doped/undoped heterostructures and quantum well superlattices that are highly anisotropic. Contrary to other optical metamaterials these heterostructure systems are optically thick (up to 20 μm thick), planar, and require no additional fabrication steps beyond the initial growth. Using transmission and reflection measurements and modeling of the highly doped heterostructures, we demonstrate that these materials exhibit negative refraction. For the highly doped quantum well superlattices, we demonstrate anomalous reflection due to the strong anisotropy of the material but a determination of the sign of refraction is still difficult. This new class of semiconductor metamaterials has great potential for waveguiding and imaging applications in the long-wave infrared.
Comment on “Stokes’ first problem for an Oldroyd-B fluid in a porous half space” [Phys. Fluids 17, 023101 (2005)]
Authors: Christov C. I. , Jordan P. M.
Published: June 2009

We point out and correct a significant error in the analytical solution presented by Tan and Masuoka [Phys. Fluids 17, 023101 (2005)].
Response to “Comment on ‘Stokes’ first problem for an Oldroyd-B fluid in a porous half space’ ” [Phys. Fluids 21, 069101 (2009)]
Authors: Tan Wenchang , Masuoka Takashi
Published: June 2009

In this comment, using Fourier sine transform to the governing equation, the authors get a so-called new subsidiary equation without considering the time domain, and claim that we made a mistake at the same stage of the solution process in our previous paper [W. Tan and T. Masuoka, “Stokes’ first problem for an Oldroyd-B fluid in a porous half space,” Phys. Fluids 17, 023101 (2005)]. In this response, we would like to clarify that there is no mistake in our solution and to point out the difference between our equation and their so-called new subsidiary equation.
Decoherence effects in the intraband and interband optical transitions in InAs/GaAs quantum dots
Authors: Ferreira R. , Berthelot A. , Grange T. , Zibik E. , Cassabois G. , Wilson L.
Published: June 2009

We present a review of coherence properties of interband and intraband optical transitions in self assembled InAs/GaAs quantun dots. Indeed, recent experimental and theoretical investigations of the optical transitions in both spectral domains have allowed a better understanding of the different phenomena that affects the interaction of confined carriers with light. These studies point out the many different ways the electron-phonon interactions play a role on the optical response of quantum dots. They also stress the primary role of the close environment on the coherence characteristics of quantum dots.
ZnO grown by atomic layer deposition: A material for transparent electronics and organic heterojunctions
Authors: Guziewicz E. , Godlewski M. , Krajewski T. , Wachnicki Ł. , Szczepanik A. , Kopalko K. , Wójcik-Głodowska A. , Przeździecka E. , Paszkowicz W. , Łusakowska E. , Kruszewski P. , Huby N. , Tallarida G. , Ferrari S.
Published: June 2009

We report on zinc oxide thin films grown by atomic layer deposition at a low temperature, which is compatible with a low thermal budget required for some novel electronic devices. By selecting appropriate precursors and process parameters, we were able to obtain films with controllable electrical parameters, from heavily n-type to the resistive ones. Optimization of the growth process together with the low temperature deposition led to ZnO thin films, in which no defect-related photoluminescence bands are observed. Such films show anticorrelation between mobility and free-electron concentration, which indicates that low n electron concentration is a result of lower number of defects rather than the self-compensation effect.
Supercritical rotating flow over topography
Authors: Esler J. G. , Rump O. J. , Johnson E. R.
Published: June 2009

The flow of a one-and-a-half layer Boussinesq fluid over an obstacle of nondimensional height M, relative to the lower layer depth, is investigated in the presence of rotation, the magnitude of which is measured by a nondimensional parameter B (inverse Burger number). The supercritical regime in which the Froude number F, the ratio of the flow speed to the interfacial gravity wave speed, is significantly greater than one is considered in the shallow water (small aspect ratio) limit. The linear drag exerted by the obstacle on the flow is shown to be M2/(FF2−1)×f(B/F2−1), where f is a function specific to each obstacle. Explicit expressions are given for several common obstacle shapes, and the results are checked against nonlinear flows simulated by a shock-capturing finite volume numerical scheme. For flows within the supercritical regime (F−1⪢M2/3) the linear drag result is found to remain accurate up to (at least) M≈0.7. The success of the linear drag theory can be explained because, in the supercritical regime, strong nonlinearities are displaced to the wake regions at the flanks of the obstacle. In the presence of weak rotation and for small obstacle height the development of the nonlinear wakes is governed by the Ostrovsky–Hunter (OH) equation. Across a wide range of parameter space the wake pattern is determined by a single parameter β=3M/BF2−1. Numerical solutions of the OH equation illustrate the dependence of flow patterns and wave breaking regions on β. Results are again verified by comparison with numerical solutions of the full nonlinear rotating shallow water equations.
Lasing and polariton condensation: Two distinct transitions in GaAs microcavities with stress traps
Authors: Nelsen B. , Balili R. , Snoke D. W. , Pfeiffer L. , West K.
Published: June 2009

We have used stress to create a harmonic potential for polaritons in GaAs microcavities and have previously reported that the polaritons undergo spontaneous coherence in the trap. In this paper we present results for both trapped conditions and resonant, nontrapped conditions in the same sample. We find that the results are qualitatively different with two distinct types of transitions. At low density in the trap, the polaritons remain in the strong coupling regime while going through the threshold for onset of coherence; at higher density, there is a different threshold behavior, which occurs with weak coupling and can be identified with lasing; this transition occurs both with and without a trap. The transition at lower density can therefore be identified as a type of nonequilibrium Bose–Einstein condensation.
Buckling of empty spherical viruses under external pressure
Authors: Ru C. Q.
Published: June 2009

Motivated by structural heterogeneity and thickness nonuniformity of protein shells (such as microtubules and viral capsids), a refined elastic shell model is suggested to study the effect of transverse shear and effective bending thickness on buckling of an empty spherical viral shell under external pressure. A key feature of the model is that the transverse shear modulus of viral shells is allowed to be much lower than the in-plane shear modulus, in accordance with the weak resistance of two-dimensional protein assemblies to transverse shear. The results show that the transverse shear-induced critical pressure drop could be as big as 50%–70% for smaller-radius viral shells when the transverse shear modulus is about one order of magnitude smaller than the in-plane shear modulus, although the effect of transverse shear is negligible if the transverse shear modulus is equal to or larger than the in-plane shear modulus. These results suggest that the classical homogeneous shell model widely used in the literature would overestimate the strength of viral shells against buckling under external pressure. The refined model suggested here could extend the applicability of homogeneous elastic shell models from larger-radius viral shells to small-radius ones.
Entanglement and quantum groups
Authors: Korbicz J. K. , Wehr J. , Lewenstein M.
Published: June 2009

We describe quantum mechanical entanglement in terms of compact quantum groups. We prove an analog of positivity of partial transpose criterion and formulate a Horodecki-type theorem.
Electromagnetic fields produced by moving sources in a curved beam pipe
Authors: Goto Shin-itiro , Tucker Robin W
Published: June 2009

A new geometrical perturbation scheme is developed in order to calculate the electromagnetic fields produced by charged sources in prescribed motion moving in a nonstraight perfectly conducting beam pipe. The pipe is regarded as a perturbed infinitely long hollow right-circular cylinder. The perturbation maintains the pipe’s circular cross section while deforming its axis into a planar space curve with, in general, nonconstant curvature. Various charged source models are considered including a charged bunch and an off-axis point particle. In the ultrarelativistic limit this permits a calculation of the longitudinal wake potential in terms of powers of the product of the pipe radius and the arbitrarily varying curvature of the axial space curve. Analytic expressions to leading order are presented for beam pipes with piecewise defined constant curvature modeling pipes with straight segments linked by circular arcs of finite length. The language of differential forms is used throughout, and to illustrate the power of this formalism, a pedagogical introduction is developed by deriving the theory ab initio from Maxwell’s equations expressed intrinsically as a differential system on (Minkowski) space-time.
A stereo vision method for tracking particle flow on the weld pool surface
Authors: Zhao C. X. , Richardson I. M. , Kenjeres S. , Kleijn C. R. , Saldi Z.
Published: June 2009

The oscillation of a weld pool surface makes the fluid flow motion quite complex. Two-dimensional results cannot reflect enough information to quantitatively describe the fluid flow in the weld pool; however, there are few direct three-dimensional results available. In this paper, we describe a three-dimensional reconstruction method to measure weld pool surface features based on a single high-speed camera. A stereo adapter was added in front of the high-speed camera lens to obtain two images in the same frame from different view points at the same time. According to machine vision theory, three-dimensional parameters can be reconstructed based on two such images. In this work, three-dimensional velocity fields have been obtained using this method. Based on the calibration technique employed, the associated error is estimated to be less than 11.4%. Quantitative experimental results are useful for understanding the flow pattern, and possibly for controlling the flow of liquid in the weld pool.
Density functional theory study on Herzberg–Teller contribution in Raman scattering from 4-aminothiophenol-metal complex and metal-4-aminothiophenol-metal junction
Authors: Liu Shasha , Zhao Xiuming , Li Yuanzuo , Zhao Xiaohong , Chen Maodu
Published: June 2009

Density functional theory (DFT) and time-dependent DFT calculations have been performed to investigate the Raman scattering spectra of metal-molecule complex and metal-molecule-metal junction architectures interconnected with 4-aminothiophenol (PATP) molecule. The simulated profiles of normal Raman scattering (NRS) spectra for the two complexes (Ag2-PATP and PATP-Au2) and the two junctions (Ag2-PATP-Au2 and Au2-PATP-Ag2) are similar to each other, but exhibit obviously different Raman intensities. Due to the lager static polarizabilities of the two junctions, which directly influence the ground state chemical enhancement in NRS spectra, the calculated normal Raman intensities of them are stronger than those of two complexes by the factor of 102. We calculate preresonance Raman scattering (RRS) spectra with incident light at 1064 nm, which is much lower than the S1 electronic transition energy of complexes and junctions. Ag2-PATP-Au2 and Au2-PATP-Ag2 junctions yield higher Raman intensities than those of Ag2-PATP and PATP-Au2 complexes, especially for b2 modes. This effect is mainly attributed to charge transfer (CT) between the metal gap and the PAPT molecule which results in the occurrence of CT resonance enhancement. The calculated pre-RRS spectra strongly depend on the electronic transition state produced by new structures. With excitation at 514.5 nm, the calculated pre-RRS spectra of two complexes and two junctions are stronger than those of with excitation at 1064 nm. A charge difference densities methodology has been used to visually describe chemical enhancement mechanism of RRS spectrum. This methodology aims at visualizing intermolecular CT which provides direct evidence of the Herzberg–Teller mechanism.
The bone diagnostic instrument III: Testing mouse femora
Authors: Randall Connor , Mathews Phillip , Yurtsev Eugene , Sahar Nadder , Kohn David , Hansma Paul
Published: June 2009

Here we describe modifications that allow the bone diagnostic instrument (BDI) [P. Hansma et al., Rev. Sci. Instrum. 79, 064303 (2008); Rev. Sci. Instrum. 77, 075105 (2006)], developed to test human bone, to test the femora of mice. These modifications include reducing the effective weight of the instrument on the bone, designing and fabricating new probe assemblies to minimize damage to the small bone, developing new testing protocols that involve smaller testing forces, and fabricating a jig for securing the smaller bones for testing. With these modifications, the BDI was used to test the hypothesis that short-term running has greater benefit on the mechanical properties of the femur for young growing mice compared to older, skeletally mature mice. We measured elastic modulus, hardness, and indentation distance increase (IDI), which had previously been shown to be the best discriminators in model systems known to exhibit differences in mechanical properties at the whole bone level. In the young exercised murine femora, the IDI was significantly lower than in young control femora. Since IDI has a relation to postyield properties, these results suggest that exercise during bone development increases post yield mechanical competence. We were also able to measure effects of aging on bone properties with the BDI. There was a significant increase in the IDI, and a significant decrease in the elastic modulus and hardness between the young and old groups. Thus, with the modifications described here, the BDI can take measurements on mouse bones and obtain statistically significant results.
Multipili attachment of bacteria with helixlike pili exposed to stress
Authors: Björnham Oscar , Axner Ove
Published: June 2009

A number of biomechanical properties of various types of pili expressed by Escherichia coli, predominantly their force-versus-elongation behavior, have previously been assessed in detail on a single pilus level. In vivo, however, bacteria bind in general to host cells by a multitude of pili, which presumably provides them with adhesion properties that differs from those of single pili. Based upon the previously assessed biomechanical properties of individual pili, this work presents a theoretical analysis of the adhesion properties of multipili-attaching bacteria expressing helixlike pili exposed to an external force. Expressions for the adhesion lifetime of dual- and multipili-attaching bacteria are derived and their validity is verified by Monte Carlo simulations. It is demonstrated that the adhesion lifetime of a multipili-binding bacterium depends to a large degree on the cooperativity of the attaching pili, which, in turn, depends strongly on their internal biomechanical properties, in particular their helixlike structure and its ability to elongate, which, in turn, depends on the intrinsic properties of the bonds, e.g., their lengths and activation energies. It is shown, for example, that a decrease in the length of a layer-to-layer bond in the rod of P pili, expressed by E. coli, by 50% leads to a decrease in the adhesion lifetime of a bacterium attaching by ten pili and exposed to a force of 500 pN by three orders of magnitude. The results indicate moreover that the intrinsic properties of the rod for this particular type of pili are optimized for multipili attachment under a broad range of external forces and presumably also to its in vivo environment. For example, P pili seems to be optimized to withstand a force exposure during ∼3 s, which correspond to the time it takes for a bolus to pass a bacterium attached to the ureteral wall. Even though the results presented in this work apply quantitatively to one type of pilus, they are assumed to apply qualitatively to all helixlike pili systems expressing slip bonds.
A higher dimensional theory of electrical contact resistance
Authors: Lau Y. Y. , Tang Wilkin
Published: June 2009

The electrical contact resistance is computed for a local constriction of finite length and finite transverse dimension in a conducting current channel. Conformal mapping is used for a rectangular current channel, and an electrostatic code is used for a cylindrical current channel. The connecting bridge, which models a local electrical contact, is assumed to be made of the same conducting material as the main current channel. Very simple analytic scaling laws for the contact resistance are constructed for a wide range of geometrical aspect ratios between the main current channel and its connecting bridge, which may assume a rectangular shape (for Cartesian channel), and a cylindrical or funnel shape (for cylindrical channel). These scaling laws have been confirmed by spot checks with numerical code results. They are generalizations of the classical theory of Holm and Timsit on the contact resistance of the “a-spot,” defined as a small circular area of zero thickness through which current can flow. Potential applications and extensions of the theory are indicated.
An analytical model for transient deformation of viscoelastically coated beams: Applications to static-mode microcantilever chemical sensors
Authors: Heinrich S. M. , Wenzel M. J. , Josse F. , Dufour I.
Published: June 2009

The problem governing the transient deformation of an elastic cantilever beam with viscoelastic coating, subjected to a time-dependent coating eigenstrain, is mathematically formulated. An analytical solution for an exponential eigenstrain history, exact within the context of beam theory, is obtained in terms of the coating and base layer thicknesses, the elastic modulus of the base material, the initial coating modulus, the coating relaxation percentage (0%–100%), and the time constants of the coating’s relaxation process and its eigenstrain history. Approximate formulas, valid for thin coatings, are derived as special cases to provide insight into system behavior. Main results include (1) the time histories of the beam curvature and the coating stresses, (2) a criterion governing the response type (monotonic or “overshoot” response), and (3) simple expressions for the overshoot ratio, defined as the peak response scaled by the steady-state response, and the time at which the peak response occurs. Applications to polymer-coated microcantilever-based chemical sensors operating in the static mode are discussed.
Radiation effects on the behavior of carbon and oxygen impurities and the role of Ge in Czochralski grown Si upon annealing
Authors: Londos C. A. , Andrianakis A. , Emtsev V. , Ohyama H.
Published: June 2009

The annealing behavior of the oxygen and carbon impurities in Czochralski grown silicon (Cz-Si) was investigated in electron- and neutron-irradiated materials. The irradiated samples were subjected to isochronal anneals of up to ∼1000 °C, and the evolution of oxygen and carbon concentrations was monitored by means of infrared spectroscopy from the amplitudes of the 1106 and 605 cm−1 bands of the two impurities correspondingly. It was found that the electron irradiation does not affect the temperature of annealing of oxygen, although in the neutron-irradiated samples the oxygen band begins to decay in the spectra at a lower temperature than that in the nonirradiated samples. This behavior could be determined by supersaturation of vacancies mainly liberated from disordered regions in the latter material. This assists the oxygen aggregation process. Regarding carbon evolution, it was found that in the irradiated samples the annealing out of the 605 cm−1 band occurs at a lower temperature than that of the nonirradiated samples. Prior to the onset of decay of the 605 cm−1 band an inverse annealing stage was observed in the irradiated samples, indicating partial restoration of substitutional carbon. The general behavior was discussed with respect to the supersaturation of intrinsic defects, mainly self-interstitials. As a result, large CN(SiI)M complexes are formed. There are two processes running in parallel: the recovery of substitutional carbon from carbon-related defects and CN(SiI)M complexes and the transformation of CN(SiI)M complexes to SiC-based precipitates. Noticeably, in electron-irradiated Ge-doped Si the inverse annealing stage of substitutional carbon is suppressed. Furthermore, our results showed that the Ge doping of Cz-Si of up to 2×1020 cm−3 does not practically affect the temperature at which oxygen and carbon are completely lost in irradiated Cz-Si:Ge.
Magnetization reversal mechanism of perpendicularly exchange-coupled composite L1-FePt/CoCrPt bilayers
Authors: Xu Z. , Zhou S. M. , Ge J. J. , Du J. , Sun L.
Published: June 2009

Magnetization reversal mechanism in perpendicularly exchange-coupled composite hard/soft L10-FePt/CoCrPt bilayers with different soft layer thickness has been studied using magnetometry and magnetotransport measurements. For thin soft layers, the magnetization reversal process can be described by the rigid model. For thick soft layers, a different magnetization reversal process is observed which consists of three stages. An exchange-spring spin structure is first formed from the positive saturation magnetic field to small negative magnetic field. In the second stage, a Néel wall is driven and squeezed. In the last stage, multiple domain structure is formed laterally to reduce the dipolar interaction and the magnetization reversal is accomplished by the pinned domain wall motion in the hard layer.
Coupled magnetostructural transformations in melt-spun Ni55Mn19.6Ga25.4 ribbon: An electron spin resonance study
Authors: Rama Rao N. V. , Gopalan R. , Arout Chelvane J. , Chandrasekaran V. , Suresh K. G.
Published: June 2009

Electron spin resonance study has been carried out on melt-spun ribbon of Ni55Mn19.6Ga25.4 exhibiting coupled magnetostructural transition. The correlation of electron spin resonance, thermal and magnetic results permitted a clear distinction of various phases and their transformations. Both structural and magnetic transitions coexist in the temperature range 300≤T≤310 leading to four different magnetic phases namely paramagnetic austenite, ferromagnetic austenite, paramagnetic martensite, and ferromagnetic martensite. The sample exhibits a single paramagnetic austenite phase above 310 K while it shows a ferromagnetic martensite phase below 260 K.
Computation of potential energy surfaces with the multireference correlation consistent composite approach
Authors: Mintz Benjamin , Williams T. Gavin , Howard Levi , Wilson Angela K.
Published: June 2009

A multireference composite method that is based on the correlation consistent Composite Approach (ccCA) is introduced. The developed approach, multireference ccCA, has been utilized to compute the potential energy surfaces (PESs) of N2 and C2, which provide rigorous tests for multireference composite methods due to the large multireference character that must be correctly described as the molecules dissociate. As well, PESs provide a stringent test of a composite method because all components of the method must work in harmony for an appropriate, smooth representation across the entire surface.
Auger recombination and excited state relaxation dynamics in Hgn− (n=9–20) anion clusters
Authors: Griffin Graham B. , Ehrler Oli T. , Kammrath Aster , Young Ryan M. , Cheshnovsky Ori , Neumark Daniel M.
Published: June 2009

Using femtosecond time-resolved photoelectron imaging, electron-hole pairs are created in size-selected Hgn− anion clusters (n=9–20), and the subsequent decay dynamics are measured. These clusters eject electrons via Auger decay on time scales of 100–600 fs. There is an abrupt increase in the Auger decay time for clusters larger than Hg12−, coinciding with the onset of the transition from van der Waals to covalent bonding in mercury clusters. Our results also show evidence for subpicosecond excited state relaxation attributed to inelastic electron-electron and electron-hole scattering as well as hole-induced contraction of the cluster.
Dielectric and electro-optical studies of glycerol/ferroelectric liquid crystal mixture at room temperature
Authors: Kumar A. , Prakash J. , Choudhary A. , Biradar A. M.
Published: June 2009

The optical memory effect based on glycerol mixed deformed helix ferroelectric liquid crystal (DHFLC) material has been investigated. The observed memory effect has also been compared with the glycerol mixed ferroelectric liquid crystals (FLCs). The observance of memory effect has been verified by experimental data using dielectric spectroscopy, electro-optical, and textural studies under polarizing optical microscope. The glycerol not only helps to improve the memory effect in DHFLC material but also alters the physical parameters such as rotational viscosity, spontaneous polarization, response time, etc. too in both glycerol/DHFLC and glycerol/FLC mixtures.
Oxidation of Al doped Au clusters: A first principles study
Authors: Rajesh Chinagandham , Majumder Chiranjib
Published: June 2009

Using first principles method we report the oxidation of Al doped Au clusters. This work is divided into two parts: (i) the equilibrium structures and stability of Al doped Aun−1 clusters (n=2–7,21) and (ii) the interaction of O2 with stable clusters. The calculations are performed using the plane wave pseudopotential approach under the density functional theory and generalized gradient approximation for the exchange and correlation functional. The optimized geometries of Aun−1Al clusters indicate that the substitution of Au by Al results an early onset of three-dimensional structures from tetramer onwards. This is different from the results of transition metal doped Au clusters, where the planar conformation of Au clusters retains up to heptamer. The stability of Aun−1Al clusters has been analyzed based on the binding energy, second difference in energy, and the energy gaps between the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. Based on the energetics, the Au3Al and Au5Al clusters are found to have extraordinary stability. The oxidation mechanism of Al doped Au clusters have been studied by the interaction of O2 with Al, Au, AuAl, Au3Al, and Au20Al clusters. It is found that the oxidation of Aun−1Al clusters undergoes via dissociative mechanism, albeit significant charge transfer from Al to Au. Moreover, the O2 molecule prefers to attach at the Al site rather than at the Au site.
Theoretical study of the effect of ethynyl group on the structure and electrical properties of phenyl-thiadiazole systems as precursors of electron-conducting materials
Authors: Granadino-Roldán José M. , Garzón Andrés , García Gregorio , Peña-Ruiz Tomás , Fernández-Liencres M. Paz , Navarro Amparo , Fernández-Gómez Manuel
Published: June 2009

2,5-bis(phenylethynyl)-1,3,4-thiadiazole (PhEtTh) and 2,5-diphenyl-1,3,4-thiadiazole (PhTh) are expected to be building blocks for polymer materials that could be employed to conduct electricity due to their narrow highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gaps. In this work, a theoretical, comparative study about the effect of the ethynyl group on the planarity and electrical conductivity of this kind of systems has been carried out. Thus, several ab initio (Hartree–Fock, Moller–Plesset) and DFT (B3LYP, B3PW91, M05, M05-2X) methods and basis sets (6-31G∗, 6-31G+G∗∗, 6-311G∗∗, cc-pVDZ, cc-pVTZ) have been tested. As a result, PhEtTh showed better properties for its use as electric conducting material relative to PhTh due to its smaller HOMO-LUMO gap, as well as its enhanced trend to retain the planarity provided the reduction in steric hindrances that the ethynyl group (–C≡C–) permits. Solvent effects were also modeled for ethanol and chloroform under the conductor-like polarizable continuum model approximation. Finally, electronic transitions in gas and solution phases were predicted by using TDDFT approximation in order to compare the theoretical λmax with the experimental values reported in literature for both compounds.
Robustness of integrin signaling network
Authors: Kness Mark , Wang Ge , Zaman Muhammad H.
Published: June 2009

Integrin signaling network is responsible for regulating a wide variety of fundamental biological processes ranging from cell survival to cell death. While individual components of the network have been studied through experimental and computational methods, the network robustness and the flow of information through the network have not been characterized in a quantitative framework. Using a probability based model implemented through GRID computing, we approach the reduced signaling network and show that the network is highly robust and the final stable steady state is independent of the initial configurations. However, the path from the initial and the final state is intrinsically dependent on the state of the input nodes. Our results demonstrate a rugged funnel-like landscape for the signaling network where the final state is unique, but the paths are dependent on initial conditions.
Dependence on temperature and guanine-cytosine content of bubble length distributions in DNA
Authors: Kalosakas G. , Ares S.
Published: June 2009

We present numerical results on the temperature dependence of the distribution of bubble lengths in DNA segments of various guanine-cytosine (GC) concentrations. Base-pair openings are described by the Peyrard–Bishop–Dauxois model and the corresponding thermal equilibrium distributions of bubbles are obtained through Monte Carlo calculations for bubble sizes up to the order of a hundred base pairs. The dependence of the parameters of bubble length distribution on temperature and the GC content is investigated. We provide simple expressions which approximately describe these relations. The variation of the average bubble length is also presented. We find a temperature dependence of the exponent c that appears in the distribution of bubble lengths. If an analogous dependence exists in the loop entropy exponent of real DNA, it may be relevant to understand overstretching in force-extension experiments.
First-principles study of high-pressure structural phase transitions of magnesium
Authors: Liu Qiuxiang , Fan Changzeng , Zhang Ruijun
Published: June 2009

The structural phase transitions for the hcp, bcc, dhcp, and fcc of magnesium at hydrostatic pressures larger than about 200 GPa at zero temperature are studied by first-principles total energy calculations. The plane-wave basis pseudopotential method has been adopted, in which the generalized gradient approximation implanted in the CASTEP code is employed. By comparing the enthalpy differences of the hcp structure with other three structures under different pressures, it can be seen that when the pressure becomes higher than about 65, 130, and 190 GPa, the bcc, dhcp, and fcc structures become more stable relative to the hcp structure, respectively. Due to the lowest enthalpy value of the bcc structure above 65 GPa, it can be deduced that magnesium may transform to the bcc structure from the ground state hcp structure around 65 GPa, but no further phase transitions occur without additionally applying high temperature. In addition, the equation of state of magnesium is calculated, indicating that bcc structure is the softest phase.
Effects of electron-irradiation on electrical properties of AlGaN/GaN Schottky barrier diodes
Authors: Fang Z.-Q. , Farlow G. C. , Claflin B. , Look D. C. , Green D. S.
Published: June 2009

Effects of 1 MeV electron-irradiation at room temperature on the electrical properties of AlGaN/GaN heterostructures, including leakage currents, threshold voltages, and electron traps, have been investigated using Schottky barrier diodes (SBDs) fabricated on the AlGaN. The SBDs, before and after the irradiation with a dose of 5×1015 cm−2, were characterized by temperature dependent current-voltage and capacitance-voltage measurements and deep level transient spectroscopy. It is found that the irradiation causes (i) significant increase in leakage currents, dominated by tunneling conduction, at both reverse and low-forward biases; (ii) a clear negative shift in threshold voltage in the pinch-off region; and (iii) creation of traps Ae(∼1.1 eV) or A2(1.2 eV) and Ee(0.09 eV) in the GaN buffer and AlGaN regions. The irradiation-induced traps can be used to account for the increase in leakage currents and shift in threshold voltage. However, as compared to traps A2(1.2 eV) and E(0.13 eV) induced in thick GaN layers by electron-irradiation, the irradiation-induced traps in the AlGaN/GaN heterostructures show some changes in activation energy and electron capture behavior.
The entries of circular orthogonal ensembles
Authors: Jiang Tiefeng
Published: June 2009

Let V=(vij)n×n be a circular orthogonal ensemble. In this paper, for 1≤m≤o(n/log n), we give a bound for the tail probability of max1≤i,j≤m|vij−(1/n)yi′yj|, where Y=(y1,…,yn) is a certain n×n matrix whose entries are independent and identically distributed random variables with the standard complex normal distribution CN(0,1). In particular, this implies that, for a sequence of such matrices {Vn=(vij(n))n×n,n≥1}, as n→∞, nvij(n) converges in distribution to CN(0,1) for any i≥1,j≥1 with i≠j and nvii(n) converges in distribution to 2⋅CN(0,1) for any i≥1.
Development and assessment of a short-range meta-GGA functional
Authors: Goll Erich , Ernst Matthias , Moegle-Hofacker Franzeska , Stoll Hermann
Published: June 2009

Short-range DFT/long-range ab initio methods allow for a combination of the weak basis-set dependency of DFT with an accurate ab initio treatment of long-range effects like van der Waals interaction. In order to improve existing short-range LDA and GGA density functionals, we developed a TPSS-like short-range meta-GGA exchange-correlation functional and checked its performance in long-range CCSD(T) calculations for thermodynamical properties of the G2 set of molecules.
A pillar-array based two-dimensional photonic crystal microcavity
Authors: Xu Tao , Zhu Ning , Xu Michelle Y.-C. , Wosinski Lech , Aitchison J. Stewart , Ruda H. E.
Published: June 2009

Experimental results are presented for a high quality factor cavity based on pillar arrays. The cavity is formed from a square lattice of silicon pillars of finite height. The quality factor of the fundamental mode is about 7300 when the cavity is in air and 7800 when it is immersed in water. Since a pillar array has a large percent of connected void space and confines TM modes effectively, it has great promise for use in numerous areas including biochemical sensing and quantum cascade lasers.
Shock Hugoniot and temperature data for polystyrene obtained with quartz standard
Authors: Ozaki N. , Sano T. , Ikoma M. , Shigemori K. , Kimura T. , Miyanishi K. , Vinci T. , Ree F. H. , Azechi H. , Endo T. , Hironaka Y. , Hori Y. , Iwamoto A. , Kadono T. , Nagatomo H. , Nakai M. , Norimatsu T. , Okuchi T. , Otani K. , Sakaiya T. , Shimizu K. , Shiroshita A. , Sunahara A. , Takahashi H. , Kodama R.
Published: June 2009

Equation-of-state data, not only pressure and density but also temperature, for polystyrene (CH) are obtained up to 510 GPa. The region investigated in this work corresponds to an intermediate region, bridging a large gap between available gas-gun data below 60 GPa and laser shock data above 500 GPa. The Hugoniot parameters and shock temperature were simultaneously determined by using optical velocimeters and pyrometers as the diagnostic tools and the α-quartz as a new standard material. The CH Hugoniot obtained tends to become stiffer than a semiempirical chemical theoretical model predictions at ultrahigh pressures but is consistent with other models and available experimental data.
Modulation instability of ion acoustic waves, solitons, and their interactions in nonthermal electron-positron-ion plasmas
Authors: Zhang Jie-fang , Wang Yue-yue , Wu Lei
Published: June 2009

The propagation of ion acoustic waves in plasmas composed of ions, positrons, and nonthermally distributed electrons is investigated. By means of the reduction perturbation technique, a nonlinear Schrödinger equation is derived and the modulation instability of ion acoustic wave is analyzed, where the nonthermal parameter is found to be of significant importance. Furthermore, analytical expressions for the bright and dark solitons are obtained, and the interaction of multiple solitons is discussed.