bluuebunny/crossref_metadata_2025 huggingface dataset:
Performance analysis of cooperative amplify‐and‐forward orthogonal frequency division multiplexing systems with power amplifier non‐linearity
Authors: Fernandes C. Alexandre R. , da Costa Daniel B. , de Almeida André L. F.
Published: October 2014

Orthogonal frequency division multiplexing (OFDM) is considered as a key technology for future wireless communication systems. One of the main disadvantages of OFDM systems is the high peak‐to‐average power ratio (PAPR) of the transmitted signals. When a non‐linear power amplifier (PA) is present, the high PAPR may cause the introduction of non‐linear distortions. On the other hand, cooperative communications have emerged as a promising technology for wireless networks. A theoretical analysis of an amplify‐and‐forward OFDM system taking into consideration the non‐linear distortions introduced by a PA is done. More specifically, approximate analytical expressions for the outage probability and average symbol error rate (SER) are derived and studied under several special situations. The performance analysis sheds light on the influence of PA non‐linearity on the outage probability and the average SER of an OFDM system. Particularly, it is shown that the non‐linear PA has a more significant effect over the average SER for medium and high signal‐to‐noise ratio (SNR) values and for small‐order constellations. The analytical results are validated by means of computer simulations for different system configurations and SNR levels.
Performance analysis of two‐way decode‐and‐forward relaying in the presence of co‐channel interferences
Authors: Hataminia Sajad , Vahidian Saeed , Mohammadi Mohammadali , Ahmadian‐Attari Mahmoud
Published: October 2014

This study investigates the performance of two‐way decode‐and‐forward (DF) relaying networks, considering transmissions over independent but not necessarily identically distributed (i.n.i.d.) Rayleigh fading channels, in the presence of multiple co‐channel interferers at both the relay and end‐source nodes. Both asymmetrical and symmetrical cases, of whether the channels from source terminals to the relay are identically distributed or not, are considered. Specifically, closed‐form expressions for the cumulative distribution function of the equivalent signal‐to‐interference‐plus‐noise ratio (SINR) in different cases are derived, based on which the exact symbol error probability (SEP) and the systems’ achievable rate are derived and analysed. Based on the analytic results, the authors study the impacts of system parameters, such as interference power and number of interferers on the performance of the system. Furthermore, the system behavior at high signal‐to‐noise ratio (SNR) values is studied via deriving the asymptotic SEP. The results of this study are attested through Monte Carlo simulations.
Adaptive CLEAN algorithm for millimetre wave synthetic aperture imaging radiometer in near field
Authors: Chen Jianfei , Li Yuehua , Wang Jianqiao , Li Yuanjiang , Zhang Yilong
Published: October 2014

High‐resolution millimetre wave images of contiguous targets often suffer from the influence of sidelobe artefacts, partial correlation between targets and so on. Owing to the characteristics of near‐field synthetic aperture imaging radiometers [such as the changing point spread function (PSF) and slender sideline], the existing CLEAN algorithms are unsuitable for near‐field synthetic aperture imaging. This study is devoted to establishing a novel CLEAN algorithm (named adaptive CLEAN) to clean the reconstructed millimetre wave images accurately. First, the characteristics of near‐field synthetic aperture imaging are studied. Then, the adaptive CLEAN algorithm is established based on these characteristics. In this study, the authors amend the amplitude of the targets and select the matching PSF for them according to their azimuths. Unlike other CLEAN algorithms, the parameters of this adaptive CLEAN algorithm are calculated by a formula, which is concluded from a lot of simulation experiments. Finally, the effectiveness of the proposed adaptive CLEAN algorithm is tested by several simulation experiments, and the superiority is also demonstrated by comparing it with the existing CLEAN algorithm. The results demonstrate that the proposed method is an efficient, feasible algorithm for cleaning the reconstructed images, irrespective of the point or contiguous targets.
Low energy isomers of (H2O)25 from a hierarchical method based on Monte Carlo temperature basin paving and molecular tailoring approaches benchmarked by MP2 calculations
Authors: Sahu Nityananda , Gadre Shridhar R. , Rakshit Avijit , Bandyopadhyay Pradipta , Miliordos Evangelos , Xantheas Sotiris S.
Published: October 2014

We report new global minimum candidate structures for the (H2O)25 cluster that are lower in energy than the ones reported previously and correspond to hydrogen bonded networks with 42 hydrogen bonds and an interior, fully coordinated water molecule. These were obtained as a result of a hierarchical approach based on initial Monte Carlo Temperature Basin Paving sampling of the cluster's Potential Energy Surface with the Effective Fragment Potential, subsequent geometry optimization using the Molecular Tailoring Approach with the fragments treated at the second order Møller-Plesset (MP2) perturbation (MTA-MP2) and final refinement of the entire cluster at the MP2 level of theory. The MTA-MP2 optimized cluster geometries, constructed from the fragments, were found to be within <0.5 kcal/mol from the minimum geometries obtained from the MP2 optimization of the entire (H2O)25 cluster. In addition, the grafting of the MTA-MP2 energies yields electronic energies that are within <0.3 kcal/mol from the MP2 energies of the entire cluster while preserving their energy rank order. Finally, the MTA-MP2 approach was found to reproduce the MP2 harmonic vibrational frequencies, constructed from the fragments, quite accurately when compared to the MP2 ones of the entire cluster in both the HOH bending and the OH stretching regions of the spectra.
Lateral migration of a viscoelastic drop in a Newtonian fluid in a shear flow near a wall
Authors: Mukherjee Swarnajay , Sarkar Kausik
Published: October 2014

Wall induced lateral migration of a viscoelastic (FENE-MCR) drop in a Newtonian fluid is investigated. Just like a Newtonian drop, a viscoelastic drop reaches a quasi-steady state where the lateral velocity only depends on the instantaneous distance from the wall. The drop migration velocity and the deformation scale inversely with the square and the cube of the distance from the wall, respectively. The migration velocity varies non-monotonically with increasing viscoelasticity (increasing Deborah number); initially increasing and then decreasing. An analytical explanation has been given of the effects by computing the migration velocity as arising from an image stresslet field due to the drop. The semi-analytical expression matches well with the simulated migration velocity away from the wall. It contains a viscoelastic stresslet component apart from those arising from interfacial tension and viscosity ratio. The migration dynamics is a result of the competition between the viscous (interfacial tension and viscosity ratio) and the viscoelastic effects. The viscoelastic stresslet contribution towards the migration velocity steadily increases. But the interfacial stresslet—arising purely from the drop shape—first increases and then decreases with rising Deborah number causing the migration velocity to be non-monotonic. The geometric effect of the interfacial stresslet is caused by a corresponding nonmonotonic variation of the drop inclination. High viscosity ratio is briefly considered to show that the drop viscoelasticity could stabilize a drop against breakup, and the increase in migration velocity due to viscoelasticity is larger compared to the viscosity-matched case.
Arginine-phosphate salt bridges between histones and DNA: Intermolecular actuators that control nucleosome architecture
Authors: Yusufaly Tahir I. , Li Yun , Singh Gautam , Olson Wilma K.
Published: October 2014

Structural bioinformatics and van der Waals density functional theory are combined to investigate the mechanochemical impact of a major class of histone-DNA interactions, namely, the formation of salt bridges between arginine residues in histones and phosphate groups on the DNA backbone. Principal component analysis reveals that the configurational fluctuations of the sugar-phosphate backbone display sequence-specific directionality and variability, and clustering of nucleosome crystal structures identifies two major salt-bridge configurations: a monodentate form in which the arginine end-group guanidinium only forms one hydrogen bond with the phosphate, and a bidentate form in which it forms two. Density functional theory calculations highlight that the combination of sequence, denticity, and salt-bridge positioning enables the histones to apply a tunable mechanochemical stress to the DNA via precise and specific activation of backbone deformations. The results suggest that selection for specific placements of van der Waals contacts, with high-precision control of the spatial distribution of intermolecular forces, may serve as an underlying evolutionary design principle for the structure and function of nucleosomes, a conjecture that is corroborated by previous experimental studies.
Theory of optical transitions in conjugated polymers. II. Real systems
Authors: Marcus Max , Tozer Oliver Robert , Barford William
Published: October 2014

The theory of optical transitions developed in Barford and Marcus [“Theory of optical transitions in conjugated polymers. I. Ideal systems,” J. Chem. Phys. 141, 164101 (2014)] for linear, ordered polymer chains is extended in this paper to model conformationally disordered systems. Our key result is that in the Born-Oppenheimer regime the emission intensities are proportional to S(1)/⟨IPR⟩, where S(1) is the Huang-Rhys parameter for a monomer. ⟨IPR⟩ is the average inverse participation ratio for the emitting species, i.e., local exciton ground states (LEGSs). Since the spatial coherence of LEGSs determines the spatial extent of chromophores, the significance of this result is that it directly relates experimental observables to chromophore sizes (where ⟨IPR⟩ is half the mean chromophore size in monomer units). This result is independent of the chromophore shape, because of the Born-Oppenheimer factorization of the many body wavefunction. We verify this prediction by density matrix renormalization group (DMRG) calculations of the Frenkel-Holstein model in the adiabatic limit for both linear, disordered chains and for coiled, ordered chains. We also model optical spectra for poly(p-phenylene) and poly(p-phenylene-vinylene) oligomers and polymers. For oligomers, we solve the fully quantized Frenkel-Holstein model via the DMRG method. For polymers, we use the much simpler method of solving the one-particle Frenkel model and employ the Born-Oppenheimer expressions relating the effective Franck-Condon factor of a chromophore to its inverse participation ratio. We show that increased disorder decreases chromophore sizes and increases the inhomogeneous broadening, but has a non-monotonic effect on transition energies. We also show that as planarizing the polymer chain increases the exciton band width, it causes the chromophore sizes to increase, the transition energies to decrease, and the broadening to decrease. Finally, we show that the absorption spectra are more broadened than the emission spectra and that the broadening of the absorption spectra increases as the chains become more coiled. This is primarily because absorption occurs to both LEGSs and quasi-extended exciton states (QEESs), and QEES acquire increased intensity as chromophores bend, while emission only occurs from LEGSs.
Deviational simulation of phonon transport in graphene ribbons with ab initio scattering
Authors: Landon Colin D. , Hadjiconstantinou Nicolas G.
Published: October 2014

We present a deviational Monte Carlo method for solving the Boltzmann-Peierls equation with ab initio 3-phonon scattering, for temporally and spatially dependent thermal transport problems in arbitrary geometries. Phonon dispersion relations and transition rates for graphene are obtained from density functional theory calculations. The ab initio scattering operator is simulated by an energy-conserving stochastic algorithm embedded within a deviational, low-variance Monte Carlo formulation. The deviational formulation ensures that simulations are computationally feasible for arbitrarily small temperature differences, while the stochastic treatment of the scattering operator is both efficient and exhibits no timestep error. The proposed method, in which geometry and phonon-boundary scattering are explicitly treated, is extensively validated by comparison to analytical results, previous numerical solutions and experiments. It is subsequently used to generate solutions for heat transport in graphene ribbons of various geometries and evaluate the validity of some common approximations found in the literature. Our results show that modeling transport in long ribbons of finite width using the homogeneous Boltzmann equation and approximating phonon-boundary scattering using an additional homogeneous scattering rate introduces an error on the order of 10% at room temperature, with the maximum deviation reaching 30% in the middle of the transition regime.
Two-component multi-configurational second-order perturbation theory with Kramers restricted complete active space self-consistent field reference function and spin-orbit relativistic effective core potential
Authors: Kim Inkoo , Lee Yoon Sup
Published: October 2014

We report the formulation and implementation of KRCASPT2, a two-component multi-configurational second-order perturbation theory based on Kramers restricted complete active space self-consistent field (KRCASSCF) reference function, in the framework of the spin-orbit relativistic effective core potential. The zeroth-order Hamiltonian is defined as the sum of nondiagonal one-electron operators with generalized two-component Fock matrix elements as scalar factors. The Kramers symmetry within the zeroth-order Hamiltonian is maintained via the use of a state-averaged density, allowing a consistent treatment of degenerate states. The explicit expressions are derived for the matrix elements of the zeroth-order Hamiltonian as well as for the perturbation vector. The use of a fully variational reference function and nondiagonal operators in relativistic multi-configurational perturbation theory is reported for the first time. A series of initial calculations are performed on the ionization potential and excitation energies of the atoms of the 6p-block; the results display a significant improvement over those from KRCASSCF, showing a closer agreement with experimental results. Accurate atomic properties of the superheavy elements of the 7p-block are also presented, and the electronic structures of the low-lying excited states are compared with those of their lighter homologues.
Superconducting quantum interference device microsusceptometer balanced over a wide bandwidth for nuclear magnetic resonance applications
Authors: Vinante A. , Mezzena R. , Falferi P.
Published: October 2014

Superconducting Quantum Interference Device (SQUID) microsusceptometers have been widely used to study magnetic properties of materials at microscale. As intrinsically balanced devices, they could also be exploited for direct SQUID-detection of nuclear magnetic resonance (NMR) from micron sized samples, or for SQUID readout of mechanically detected NMR from submicron sized samples. Here, we demonstrate a double balancing technique that enables achievement of very low residual imbalance of a SQUID microsusceptometer over a wide bandwidth. In particular, we can generate ac magnetic fields within the SQUID loop as large as 1 mT, for frequencies ranging from dc up to a few MHz. As an application, we demonstrate direct detection of NMR from 1H spins in a glycerol droplet placed directly on top of the 20 μm SQUID loops.
Enstrophy and passive scalar transport near the turbulent/non-turbulent interface in a turbulent planar jet flow
Authors: Watanabe T. , Sakai Y. , Nagata K. , Ito Y. , Hayase T.
Published: October 2014

The enstrophy (ω2/2) and passive scalar (ϕ) transport near the turbulent/non-turbulent (T/NT) interface is investigated using direct numerical simulation of a planar jet with passive scalar transport. To take into account the interface movement, we derive the transport equations for the enstrophy and the scalar in a local coordinate system moving with the T/NT interface. The characteristics of the T/NT interface are analyzed for three interface orientations. The cross-streamwise edge and the leading edge face the cross-streamwise and streamwise directions, respectively, and the trailing edge is opposite to the leading edge. The propagation velocity of the T/NT interface is derived from the enstrophy transport equation in the local coordinate system. The T/NT interface propagates toward the non-turbulent region on average at the cross-streamwise and leading edges, whereas the trailing edge frequently propagates into the turbulent region. The conditional average of the enstrophy transport equation in the local coordinate system shows that viscous diffusion transports, toward the non-turbulent region, enstrophy, that is advected from the turbulent core region or is produced slightly inside the T/NT interface. Viscous diffusion contributes greatly to the enstrophy growth in the region very close to the T/NT interface. The transport equation for the scalar ϕ in the local coordinate system is used to analyze the scalar transport near the T/NT interface. The conditional average of the advection term shows that ϕ in the non-turbulent region is frequently transported into the turbulent region across the cross-streamwise and leading edges by interface propagation toward the non-turbulent region. In contrast, ϕ in the turbulent region is frequently transported into the non-turbulent region across the trailing edge. The conditional averages of the advection and molecular diffusion terms show that both the interface propagation and the molecular diffusion contribute to the scalar transport across the T/NT interface.
Tomographic analysis of the nonthermal x-ray bursts during disruption instability in the T-10 tokamak
Authors: Savrukhin P. V. , Ermolaeva A. I. , Shestakov E. A. , Khramenkov A. V.
Published: October 2014

Non-thermal x-ray radiation (Eγ up to 150 keV) is measured in the T-10 tokamaks during disruption instability using two sets of CdTe detectors (10 vertical and 7 horizontal view detectors). Special narrow cupper tubes collimators with lead screening and CdTe detectors integrated with amplifiers inside metallic containers provides enhanced spatial resolution of the system (r ∼ 3 cm) and assures protection from the parasitic hard x-ray (Eγ up to 1.5 MeV) and electromagnetic loads during disruption. Spatial localization of the nonthermal x-ray emissivity is reconstructed using tomographic Cormack technique with SVD matrix inversion. Analysis indicated appearance of an intensive non-thermal x-ray bursts during initial stage of the disruptions at high density. The bursts are characterized by repetitive spikes (2–3 kHz) of the x-ray emissivity from the plasma core area. Analysis indicated that the spikes can be connected with acceleration of the non-thermal electrons in enhanced longitudinal electric fields induced during energy quench at the disruption instability.
A doubly curved elliptical crystal spectrometer for the study of localized x-ray absorption in hot plasmas
Authors: Cahill Adam D. , Hoyt Cad L. , Pikuz Sergei A. , Shelkovenko Tania , Hammer David A.
Published: October 2014

X-ray absorption spectroscopy is a powerful tool for the diagnosis of plasmas over a wide range of both temperature and density. However, such a measurement is often limited to probing plasmas with temperatures well below that of the x-ray source in order to avoid object plasma emission lines from obscuring important features of the absorption spectrum. This has excluded many plasmas from being investigated by this technique. We have developed an x-ray spectrometer that provides the ability to record absorption spectra from higher temperature plasmas than the usual approach allows without the risk of data contamination by line radiation emitted by the plasma under study. This is accomplished using a doubly curved mica crystal which is bent both elliptically and cylindrically. We present here the foundational work in the design and development of this spectrometer along with initial results obtained with an aluminum x-pinch as the object plasma.
Coupled-oscillator theory of dispersion and Casimir-Polder interactions
Authors: Berman P. R. , Ford G. W. , Milonni P. W.
Published: October 2014

We address the question of the applicability of the argument theorem (of complex variable theory) to the calculation of two distinct energies: (i) the first-order dispersion interaction energy of two separated oscillators, when one of the oscillators is excited initially and (ii) the Casimir-Polder interaction of a ground-state quantum oscillator near a perfectly conducting plane. We show that the argument theorem can be used to obtain the generally accepted equation for the first-order dispersion interaction energy, which is oscillatory and varies as the inverse power of the separation r of the oscillators for separations much greater than an optical wavelength. However, for such separations, the interaction energy cannot be transformed into an integral over the positive imaginary axis. If the argument theorem is used incorrectly to relate the interaction energy to an integral over the positive imaginary axis, the interaction energy is non-oscillatory and varies as r−4, a result found by several authors. Rather remarkably, this incorrect expression for the dispersion energy actually corresponds to the nonperturbative Casimir-Polder energy for a ground-state quantum oscillator near a perfectly conducting wall, as we show using the so-called “remarkable formula” for the free energy of an oscillator coupled to a heat bath [G. W. Ford, J. T. Lewis, and R. F. O’Connell, Phys. Rev. Lett. 55, 2273 (1985)]. A derivation of that formula from basic results of statistical mechanics and the independent oscillator model of a heat bath is presented.
The role of material flexibility on the drying transition of water between hydrophobic objects: A thermodynamic analysis
Authors: Altabet Y. Elia , Debenedetti Pablo G.
Published: October 2014

Liquid water confined between hydrophobic objects of sufficient size becomes metastable with respect to its vapor at separations smaller than a critical drying distance. Macroscopic thermodynamic arguments predicting this distance have been restricted to the limit of perfectly rigid confining materials. However, no material is perfectly rigid and it is of interest to account for this fact in the thermodynamic analysis. We present a theory that combines the current macroscopic theory with the thermodynamics of elasticity to derive an expression for the critical drying distance for liquids confined between flexible materials. The resulting expression is the sum of the well-known drying distance for perfectly rigid confining materials and a new term that accounts for flexibility. Thermodynamic arguments show that this new term is necessarily positive, meaning that flexibility increases the critical drying distance. To study the expected magnitude and scaling behavior of the flexible term, we consider the specific case of water and present an example of drying between thin square elastic plates that are simply supported along two opposite edges and free at the remaining two. We find that the flexible term can be the same order of magnitude or greater than the rigid solution for materials of biological interest at ambient conditions. In addition, we find that when the rigid solution scales with the characteristic size of the immersed objects, the flexible term is independent of size and vice versa. Thus, the scaling behavior of the overall drying distance will depend on the relative weights of the rigid and flexible contributions.
Phase diagram of power law and Lennard-Jones systems: Crystal phases
Authors: Travesset Alex
Published: October 2014

An extensive characterization of the low temperature phase diagram of particles interacting with power law or Lennard-Jones potentials is provided from Lattice Dynamical Theory. For power law systems, only two lattice structures are stable for certain values of the exponent (or softness) (A15, body centered cube (bcc)) and two more (face centered cubic (fcc), hexagonal close packed (hcp)) are always stable. Among them, only the fcc and bcc are equilibrium states. For Lennard-Jones systems, the equilibrium states are either hcp or fcc, with a coexistence curve in pressure and temperature that shows reentrant behavior. The hcp solid never coexists with the liquid. In all cases analyzed, for both power law and Lennard-Jones potentials, the fcc crystal has higher entropy than the hcp. The role of anharmonic terms is thoroughly analyzed and a general thermodynamic integration to account for them is proposed.
Polymer adsorption on reconstructed Au(001): A statistical description of P3HT by scanning tunneling microscopy and coarse-grained Monte Carlo simulations
Authors: Förster S. , Kohl E. , Ivanov M. , Gross J. , Widdra W. , Janke W.
Published: October 2014

We report on a combined theoretical and experimental characterization of isolated Poly(3-hexylthiophene) (P3HT) chains weakly adsorbed on a reconstructed Au(001) surface. The local chain conformations of in situ deposited P3HT molecules were investigated by means of scanning tunneling microscopy. For comparison, Monte Carlo simulations of the system were performed up to a maximum chain length of 60 monomer units. The dependence of the end-to-end distance and the radius of gyration on the polymer chain length shows a good agreement between experiment and Monte Carlo simulations using simple updates for short chains.
Density functional and theoretical study of the temperature and pressure dependency of the plasmon energy of solids
Authors: Attarian Shandiz M. , Gauvin R.
Published: October 2014

The temperature and pressure dependency of the volume plasmon energy of solids was investigated by density functional theory calculations. The volume change of crystal is the major factor responsible for the variation of valence electron density and plasmon energy in the free electron model. Hence, to introduce the effect of temperature and pressure for the density functional theory calculations of plasmon energy, the temperature and pressure dependency of lattice parameter was used. Also, by combination of the free electron model and the equation of state based on the pseudo-spinodal approach, the temperature and pressure dependency of the plasmon energy was modeled. The suggested model is in good agreement with the results of density functional theory calculations and available experimental data for elements with the free electron behavior.
Sapphire screws and strength test on them at liquid nitrogen temperature
Authors: Hirose Eiichi , Sakakibara Yusuke , Igarashi Yukihiko , Ishii Takashi
Published: October 2014

We fabricated several sapphire screw threads and performed a strength test on them at the liquid nitrogen temperature of 77 K. The screw threads were subjected to and withstood a 3000 N load. To the best of our knowledge, this is the first strength test of sapphire screw threads at a cryogenic temperature. The result suggests a new way of connecting sapphire components. Although sapphire is already used in many applications, the result may provide a new way to use the material as a structural element in even more applications.
Dynamic properties of micro-particles in ultrasonic transportation using phase-controllable standing waves
Authors: Jia Kun , Mei Deqing , Meng Jianxin , Yang Keji
Published: October 2014

Ultrasonic manipulation has become an attractive method for surface-sensitive objects in micro-technology. Related phenomena, such as radiation force, multiple scattering, and acoustic streaming, have been widely studied. However, in current studies, the behavior of micro-particles in potential force fields is always analyzed in a quasi-static manner. We developed a dynamic model of a dilute micro-particle in the commonly used two-dimensional ultrasonic manipulation system to provide a systemic and quantitative analysis of the transient properties of particle movement. In this model, the acoustic streaming and hydrodynamic forces, omitted in previous work, were both considered. The trajectory of a spherical silica particle with different initial conditions was derived by numerically solving the established nonlinear differential integral equation system, which was then validated experimentally. The envelope of the experimental data on the x-axis showed good agreement with the theoretical calculation, and the greater influence on the y-axis of the deviation between the actual sound field and the ideal distribution employed in our dynamic model could account for the differences in displacement in that direction. Finally, the influence of particle size on its movement and the effect of acoustic streaming on calculating the hydrodynamic forces for an isolated particle with motion relative to the fluid were analyzed theoretically. It was found that the ultrasonic manipulation system will translate from an under-damped system to an over-damped system with a decrease in particle size and the micro-scale acoustic streaming velocity was negligible when calculating the hydrodynamic forces on the particle in the ultrasonic manipulation system.
Travelling fronts of the CO oxidation on Pd(111) with coverage-dependent diffusion
Authors: Cisternas Jaime , Karpitschka Stefan , Wehner Stefan
Published: October 2014

In this work, we study a surface reaction on Pd(111) crystals under ultra-high-vacuum conditions that can be modeled by two coupled reaction-diffusion equations. In the bistable regime, the reaction exhibits travelling fronts that can be observed experimentally using photo electron emission microscopy. The spatial profile of the fronts reveals a coverage-dependent diffusivity for one of the species. We propose a method to solve the nonlinear eigenvalue problem and compute the direction and the speed of the fronts based on a geometrical construction in phase-space. This method successfully captures the dependence of the speed on control parameters and diffusivities.
Polymer segregation under confinement: Free energy calculations and segregation dynamics simulations
Authors: Polson James M. , Montgomery Logan G.
Published: October 2014

Monte Carlo simulations are used to study the behavior of two polymers under confinement in a cylindrical tube. Each polymer is modeled as a chain of hard spheres. We measure the free energy of the system, F, as a function of the distance between the centers of mass of the polymers, λ, and examine the effects on the free energy functions of varying the channel diameter D and length L, as well as the polymer length N and bending rigidity κ. For infinitely long cylinders, F is a maximum at λ = 0, and decreases with λ until the polymers are no longer in contact. For flexible chains (κ = 0), the polymers overlap along the cylinder for low λ, while above some critical value of λ they are longitudinally compressed and non-overlapping while still in contact. We find that the free energy barrier height, ΔF ≡ F(0) − F(∞), scales as ΔF/kBT ∼ ND−1.93 ± 0.01, for N ⩽ 200 and D ⩽ 9σ, where σ is the monomer diameter. In addition, the overlap free energy appears to scale as F/kBT = Nf(λ/N; D) for sufficiently large N, where f is a function parameterized by the cylinder diameter D. For channels of finite length, the free energy barrier height increases with increasing confinement aspect ratio L/D at fixed volume fraction ϕ, and it decreases with increasing ϕ at fixed L/D. Increasing the polymer bending rigidity κ monotonically reduces the overlap free energy. For strongly confined systems, where the chain persistence length P satisfies D ≪ P, F varies linearly with λ with a slope that scales as F′(λ) ∼ −kBTD−βP−α, where β ≈ 2 and α ≈ 0.37 for N = 200 chains. These exponent values deviate slightly from those predicted using a simple model, possibly due to insufficiently satisfying the conditions defining the Odijk regime. Finally, we use Monte Carlo dynamics simulations to examine polymer segregation dynamics for fully flexible chains and observe segregation rates that decrease with decreasing entropic force magnitude, f ≡ |dF/dλ|. For both infinite-length and finite-length channels, the polymers are not conformationally relaxed at later times during segregation.
Graphene folding on flat substrates
Authors: Chen Xiaoming , Zhang Liuyang , Zhao Yadong , Wang Xianqiao , Ke Changhong
Published: October 2014

We present a combined experimental-theoretical study of graphene folding on flat substrates. The structure and deformation of the folded graphene sheet are experimentally characterized by atomic force microscopy. The local graphene folding behaviors are interpreted based on nonlinear continuum mechanics modeling and molecular dynamics simulations. Our study on self-folding of a trilayer graphene sheet reports a bending stiffness of about 6.57 eV, which is about four times the reported values for monolayer graphene. Our results reveal that an intriguing free sliding phenomenon occurs at the interlayer van der Waals interfaces during the graphene folding process. This work demonstrates that it is a plausible venue to quantify the bending stiffness of graphene based on its self-folding conformation on flat substrates. The findings reported in this work are useful to a better understanding of the mechanical properties of graphene and in the pursuit of its applications.
Full spectrum of the two-photon and the two-mode quantum Rabi models
Authors: Dossa Anselme F. , Avossevou Gabriel Y. H.
Published: October 2014

This paper is concerned with the rigorous analytical determination of the spectrum of the two-photon and the two-mode quantum Rabi models. To reach this goal, we exploit the hidden symmetries in these models by means of the unitary and similarity transformations in addition to the Bargmann-Fock space description. In each case, the purely quantum mechanical problem of the Rabi model studied is reduced to solutions for differential equations. This eventually gives a third-order differential equation for each of these models, which is reduced to a second-order differential equation by additional transformations. The analytical expressions of the wave functions describing the energy levels are obtained in terms of the confluent hypergeometric functions.
Dynamical realizations of $\mathcal {N}=1$N=1 l-conformal Galilei superalgebra
Authors: Masterov Ivan
Published: October 2014

Dynamical systems which are invariant under \documentclass[12pt]{minimal}\begin{document}$\mathcal {N}=1$\end{document}N=1 supersymmetric extension of the l-conformal Galilei algebra are constructed. These include a free \documentclass[12pt]{minimal}\begin{document}$\mathcal {N}=1$\end{document}N=1 superparticle which is governed by higher derivative equations of motion and an \documentclass[12pt]{minimal}\begin{document}$\mathcal {N}=1$\end{document}N=1 supersymmetric Pais-Uhlenbeck oscillator for a particular choice of its frequencies. A Niederer-like transformation which links the models is proposed.
Superconductivity and metallic behavior in PbxCyOδ structures prepared by focused electron beam induced deposition
Authors: Winhold M. , Weirich P. M. , Schwalb C. H. , Huth M.
Published: October 2014

Focused electron beam induced deposition as a direct-write approach possesses great potential to meet the demands for superconducting nanostructure fabrication especially regarding its 3D patterning capabilities combined with the high resolution in the nanometer regime. So far, however, it was not possible to fabricate superconducting structures with this technique. In this work, we present a lead-based superconductor prepared by focused electron beam induced deposition by dissociation of the precursor tetraethyllead. The as-grown structures exhibit metallic behavior and a minimum resistivity in the normal state of ρ = 16 μΩcm at T = 9 K followed by a superconducting transition at Tc = 7.2 K.
Dual-wavelength quantum cascade laser for trace gas spectroscopy
Authors: Jágerská J. , Jouy P. , Hugi A. , Tuzson B. , Looser H. , Mangold M. , Beck M. , Emmenegger L. , Faist J.
Published: October 2014

We demonstrate a sequentially operating dual-wavelength quantum cascade laser with electrically separated laser sections, emitting single-mode at 5.25 and 6.25 μm. Based on a single waveguide ridge, this laser represents a considerable asset to optical sensing and trace gas spectroscopy, as it allows probing multiple gas species with spectrally distant absorption features using conventional optical setups without any beam combining optics. The laser capability was demonstrated in simultaneous NO and NO2 detection, reaching sub-ppb detection limits and selectivity comparable to conventional high-end spectroscopic systems.
Effect of grain size on the melting point of confined thin aluminum films
Authors: Wejrzanowski Tomasz , Lewandowska Malgorzata , Sikorski Krzysztof , Kurzydlowski Krzysztof J.
Published: October 2014

The melting of aluminum thin film was studied by a molecular dynamics (MD) simulation technique. The effect of the grain size and type of confinement was investigated for aluminum film with a constant thickness of 4 nm. The results show that coherent intercrystalline interface suppress the transition of solid aluminum into liquid, while free-surface gives melting point depression. The mechanism of melting of polycrystalline aluminum thin film was investigated. It was found that melting starts at grain boundaries and propagates to grain interiors. The melting point was calculated from the Lindemann index criterion, taking into account only atoms near to grain boundaries. This made it possible to extend melting point calculations to bigger grains, which require a long time (in the MD scale) to be fully molten. The results show that 4 nm thick film of aluminum melts at a temperature lower than the melting point of bulk aluminum (933 K) only when the grain size is reduced to 6 nm.
Faceting control in core-shell GaN micropillars using selective epitaxy
Authors: Krylyuk Sergiy , Debnath Ratan , Yoon Heayoung P. , King Matthew R. , Ha Jong-Yoon , Wen Baomei , Motayed Abhishek , Davydov Albert V.
Published: October 2014

We report on the fabrication of large-area, vertically aligned GaN epitaxial core-shell micropillar arrays. The two-step process consists of inductively coupled plasma (ICP) etching of lithographically patterned GaN-on-Si substrate to produce an array of micropillars followed by selective growth of GaN shells over these pillars using Hydride Vapor Phase Epitaxy (HVPE). The most significant aspect of the study is the demonstration of the sidewall facet control in the shells, ranging from {11̄01} semi-polar to {11̄00} non-polar planes, by employing a post-ICP chemical etch and by tuning the HVPE growth temperature. Room-temperature photoluminescence, cathodoluminescence, and Raman scattering measurements reveal substantial reduction of parasitic yellow luminescence as well as strain-relaxation in the core-shell structures. In addition, X-ray diffraction indicates improved crystal quality after the shell formation. This study demonstrates the feasibility of selective epitaxy on micro-/nano- engineered templates for realizing high-quality GaN-on-Si devices.
Chemiresistive gas sensors employing solution-processed metal oxide quantum dot films
Authors: Liu Huan , Xu Songman , Li Min , Shao Gang , Song Huaibing , Zhang Wenkai , Wei Wendian , He Mingze , Gao Liang , Song Haisheng , Tang Jiang
Published: October 2014

We report low-temperature chemiresistive gas sensors based on tin oxide colloidal quantum dots (CQDs), in which the benefits of CQDs such as extremely small crystal size, solution-processability, and tunable surface activity are exploited to enhance the gas-sensing effect. The sensor fabrication is simply employing spin-coating followed by a solid-state ligand exchange treatment at room temperature in air ambient. The optimal gas sensor exhibited rapid and significant decrease in resistance upon H2S gas exposure when operated at 70 °C, and it was fully recoverable upon gas release. We observed a power law correlation between the sensor response and H2S gas concentration, and the sensing mechanism was discussed using the completely depletion model with a flat band diagram.
Interfacial stress transfer in a graphene nanosheet toughened hydroxyapatite composite
Authors: Zhang L. , Zhang X. G. , Chen Y. , Su J. N. , Liu W. W. , Zhang T. H. , Qi F. , Wang Y. G.
Published: October 2014

In recent years, graphene has emerged as potential reinforcing nanofiller in the composites for structural engineering due to its extraordinary high elastic modulus and mechanical strength. As recognized, the transfer of stress from a low modulus matrix to a high-modulus reinforcing graphene and the interfacial behavior at a graphene-matrix interface is the fundamental issue in these composites. In the case of graphene nanosheet (GNS) reinforced hydroxyapatite (HA) composite, this research presented analytical models and simulated that the number of graphene layers of GNSs has little effect on the maximum axial stress (∼0.35 GPa) and the maximum shear stress (∼0.14 GPa) at a GNS-HA interface, and the energy dissipation by GNS pull-out decreases with increasing the number of graphene layers due to weak bonding between them. Also, GNS-HA interfacial delamination and/or GNS rupture were also indentified to be the two key failure mechanisms. The computed results are expected to facilitate a better understanding of the interfacial behavior at a GNS-ceramic interface and to achieve tough ceramics reinforced with GNSs.
Nanoparticle size determination using optical microscopes
Authors: Attota Ravikiran , Kavuri Premsagar Purushotham , Kang Hyeonggon , Kasica Richard , Chen Lei
Published: October 2014

We present a simple method for size determination of nanoparticles using conventional optical microscopes. The method, called through-focus scanning optical microscopy, makes use of the four-dimensional optical information collected at different focus positions. Low partial coherence illumination combined with analysis of through-focus optical content enables nanoparticle size determination with nanometer scale sensitivity. We experimentally demonstrate this using fabricated Si nanodots and spherical gold nanoparticles. The method is economical, as no hardware modifications to conventional optical microscopes are needed. In addition, the method also has high throughput and potential for soft nanoparticle size determination without distortion.
Ultra-sharp oscillatory magneto-resistance in spatially confined La0.3Pr0.4Ca0.3MnO3 epitaxial thin films
Authors: Alagoz H. S. , Jeon J. , Boos R. , Ahangharnejhad R. H. , Chow K. H. , Jung J.
Published: October 2014

Our investigations of magneto-transport properties of La0.3Pr0.4Ca0.3MnO3 manganite thin films of reduced dimensions revealed dramatic changes in R(θ), the dependence of resistivity on the angle between the magnetic field direction and the current direction, and consequently in the anisotropic magneto-resistance. A regular oscillatory  sin2θ form of R(θ) is replaced by a very sharp rectangular-shaped ones when the dimensions of the system become comparable to the size of the intrinsic electronic domains. We discuss possible mechanisms that could be responsible for these changes.
Long range emission enhancement and anisotropy in coupled quantum dots induced by aligned gold nanoantenna
Authors: Tripathi L. N. , Praveena M. , Valson Pranay , Basu J. K.
Published: October 2014

Quantum dot arrays have been projected as the material of choice for next generation displays and photodetectors. Extensive ongoing research aims at improving optical and electrical efficiencies of such devices. We report experimental results on non-local long range emission intensity enhancement and anisotropy in quantum dot assemblies induced by isolated and partially aligned gold nanoantennas. Spatially resolved photoluminescence clearly demonstrate that the effect is maximum, when the longitudinal surface plasmon resonance of the nanoantenna is resonant with the emission maxima of the quantum dots. We estimated the decay length of this enhancement to be ∼2.6 μm, which is considerably larger than the range of near field interaction of metal nanoantenna. Numerical simulations qualitatively capture the near field behavior of the nanorods but fail to match the experimentally observed non-local effects. We have suggested how strong interactions of quantum dots in the close packed assemblies, mediated by the nanoantennas, could lead to such observed behavior.
Wide bandgap engineering of (AlGa)2O3 films
Authors: Zhang Fabi , Saito Katsuhiko , Tanaka Tooru , Nishio Mitsuhiro , Arita Makoto , Guo Qixin
Published: October 2014

Bandgap tunable (AlGa)2O3 films were deposited on sapphire substrates by pulsed laser deposition (PLD). The deposited films are of high transmittance as measured by spectrophotometer. The Al content in films is almost the same as that in targets. The measurement of bandgap energies by examining the onset of inelastic energy loss in core-level atomic spectra using X-ray photoelectron spectroscopy is proved to be valid for determining the bandgap of (AlGa)2O3 films as it is in good agreement with the bandgap values from transmittance spectra. The measured bandgap of (AlGa)2O3 films increases continuously with the Al content covering the whole Al content range from about 5 to 7 eV, indicating PLD is a promising growth technology for growing bandgap tunable (AlGa)2O3 films.
Dynamic competition between island growth and coalescence in metal-on-insulator deposition
Authors: Lü B. , Elofsson V. , Münger E. P. , Sarakinos K.
Published: October 2014

The morphology of thin metal films and nanostructures synthesized from the vapor phase on insulating substrates is strongly influenced by the coalescence of islands. Here, we derive analytically the quantitative criterion for coalescence suppression by combining atomistic nucleation theory and a classical model of coalescence. Growth simulations show that using this criterion, a coalescence-free growth regime can be reached in which morphological evolution is solely determined by island nucleation, growth, and impingement. Experimental validation for the ability to control the rate of coalescence using this criterion and navigate between different growth regimes is provided by in situ monitoring of Ag deposition on SiO2. Our findings pave the way for creating thin films and nanostructures that exhibit a wide range of morphologies and physical attributes in a knowledge-based manner.