bluuebunny/crossref_metadata_2025 huggingface dataset:
Stability of the shape of a translating viscoelastic drop at low Reynolds number
Authors: Wu H. , Haj-Hariri H. , Borhan A.
Published: November 2012

We examine the stability of the shape of a viscoelastic drop translating in an immiscible, otherwise quiescent Newtonian fluid at low Reynolds number. Non-Newtonian stresses in the drop phase are characterized by the finitely extensible nonlinear elastic-Chilcott-Rallison model. A boundary integral method is used to numerically examine the time-evolution of initially perturbed drop shapes over a range of dimensionless parameters. For sufficiently small capillary numbers, the drop achieves an oblate spheroidal steady shape. For large initial deformations or capillary numbers, however, the drop deforms continuously and eventually breaks up through either the formation of an elongated tail or the development of a re-entrant cavity at its trailing end. These mechanisms of drop breakup are qualitatively similar to those reported earlier for Newtonian drops [C. J. Koh and L. G. Leal, Phys. Fluids A 1, 1309 (1989)10.1063/1.857359; C. Pozrikidis, J. Fluid Mech. 210, 1 (1990)10.1017/S0022112090001203; H. A. Stone, Annu. Rev. Fluid Mech. 26, 65 (1994)10.1146/annurev.fl.26.010194.000433]. Compared to the case of a Newtonian drop, drop phase elasticity is found to have a stabilizing (destabilizing) effect for initially oblate (prolate) shape perturbations, due to the development of polymeric stresses caused by deformation of polymer chains in alignment with streamlines of the flow. Polymer viscosity has a strong influence on the stability of the shape of drops, whereas polymer relaxation time and extensibility have relatively weak influences.
The energy transfer between the ports of an implemented gyrator using LM13700 operational transconductance amplifier
Authors: Tatai Ildiko , Zaharie Ioan
Published: November 2012

In this paper a gyrator implementation using a LM13700 operational transconductance amplifier is analyzed. It was first verified under PSpice simulation and experimentally the antireciprocity of this gyrator, i.e., its properties. This type of gyrator can be used for controlling the energy transfer from one port to the other by modifying the bias currents of the operational transconductance amplifier.
Superconducting quantum interference devices based set-up for probing current noise and correlations in three-terminal devices
Authors: Pfeffer A. H. , Kaviraj B. , Coupiac O. , Lefloch F.
Published: November 2012

We have implemented a new experimental set-up for precise measurements of current fluctuations in three-terminal devices. The system operates at very low temperatures (30 mK) and is equipped with three superconducting quantum interference devices (SQUIDs) as low noise current amplifiers. A SQUID input coil is connected to each terminal of a sample allowing the acquisition of time-dependent current everywhere in the circuit. From these traces, we can measure the current mean value, the noise, and cross-correlations between different branches of a device. In this paper, we present calibration results of noise and cross-correlations obtained using low impedance macroscopic resistors. From these results, we can extract the noise level of the set-up and show that there are no intrinsic correlations due to the measurement scheme. We also studied noise and correlations as a function of a dc current and estimated the electronic temperature of various macroscopic resistors.
Incorporating configurational-bias Monte Carlo into the Wang-Landau algorithm for continuous molecular systems
Authors: Maerzke Katie A. , Gai Lili , Cummings Peter T. , McCabe Clare
Published: November 2012

Configurational-bias Monte Carlo has been incorporated into the Wang-Landau method. Although the Wang-Landau algorithm enables the calculation of the complete density of states, its applicability to continuous molecular systems has been limited to simple models. With the inclusion of more advanced sampling techniques, such as configurational-bias, the Wang-Landau method can be used to simulate complex chemical systems. The accuracy and efficiency of the method is assessed using as a test case systems of linear alkanes represented by a united-atom model. With strict convergence criteria, the density of states derived from the Wang-Landau algorithm yields the correct heat capacity when compared to conventional Boltzmann sampling simulations.
Origin-independent calculation of quadrupole intensities in X-ray spectroscopy
Authors: Bernadotte Stephan , Atkins Andrew J. , Jacob Christoph R.
Published: November 2012

For electronic excitations in the ultraviolet and visible range of the electromagnetic spectrum, the intensities are usually calculated within the dipole approximation, which assumes that the oscillating electric field is constant over the length scale of the transition. For the short wavelengths used in hard X-ray spectroscopy, the dipole approximation may not be adequate. In particular, for metal K-edge X-ray absorption spectroscopy (XAS), it becomes necessary to include higher-order contributions. In quantum-chemical approaches to X-ray spectroscopy, these so-called quadrupole intensities have so far been calculated by including contributions depending on the square of the electric-quadrupole and magnetic-dipole transition moments. However, the resulting quadrupole intensities depend on the choice of the origin of the coordinate system. Here, we show that for obtaining an origin-independent theory, one has to include all contributions that are of the same order in the wave vector consistently. This leads to two additional contributions depending on products of the electric-dipole and electric-octupole and of the electric-dipole and magnetic-quadrupole transition moments, respectively. We have implemented such an origin-independent calculation of quadrupole intensities in XAS within time-dependent density-functional theory, and demonstrate its usefulness for the calculation of metal and ligand K-edge XAS spectra of transition metal complexes.
Using an Au interlayer to enhance electron field emission properties of ultrananocrystalline diamond films
Authors: Chen Huang-Chin , Jothiramalingam Sankaran Kamatchi , Lo Shen-Chuan , Lin Li-Jiaun , Tai Nyan-Hwa , Lee Chi-Young , Lin I-Nan
Published: November 2012

We observe that an Au interlayer markedly enhances the electrical field emission (EFE) properties of ultrananocrystalline diamond (UNCD) films on Si substrates. The EFE properties of UNCD/Au/Si films can be turned on at a lower field and attain a higher current density than in UNCD films grown on Si substrates without an Au interlayer. Transmission electron microscopy reveals that the Au interlayer induces the formation of SiC clusters, preventing the formation of a resistive amorphous carbon layer that nucleates the diamond clusters. This improves the diamond-to-substrate interfacial conductivity. Moreover, there is an abundant nano-graphite phase, which is presumably induced by the coalescence of nano-sized diamond clusters. The percolation of the nano-graphite clusters helps transport electrons, improving the conductivity of the UNCD films. We believe that the simultaneous increase in the conductivity of the UNCD-to-Si interface and the bulk of the UNCD films is the main factor enhancing electrical conductivity and EFE properties of the films.
Physical properties of a two-component system at the Fermi and Sharvin length scales
Authors: Armstrong Jason N. , Gande Eric M. , Vinti John W. , Hua Susan Z. , Deep Chopra Harsh
Published: November 2012

Previously, we have reported the measurement of various physical properties at the Fermi and Sharvin length scales in pure elements (1-component systems). In the present study, the evolution of physical properties is mapped in a 2-component system at these length scales, using Au-Ag alloys. These alloys are well known to have complete solubility in each other at all compositions in the bulk and an ideal system to vary the surface energy of the alloy simply by changing the alloy composition. At sample sizes where surface effects dominate (less than ∼2–3 nm), varying the alloy composition is found to cause dramatic changes in force required to rupture the bonds (strength) as well as atomic cohesion (modulus) and can be directly attributed to segregation of higher surface energy Au from the lower surface energy Ag. In other words, the Au-Ag system with complete solubility in the bulk exhibits segregation at these length scales. This breakdown of bulk solubility rules for alloying (the so-called Hume-Rothery rules) even in near-ideal solid solutions has consequences for future atomic-scale devices.
A double output pulsed high current thyratron driver
Authors: Reghu T. , Kumar Manoj , Verma Abrat , Mandloi Vagesh , Kukreja L. M. , Shrivastava P.
Published: November 2012

The design and development of a double output pulsed high current driver for high power multi-grid thyratron is reported. The driver generates a 100 A current pulse of 2 μs duration with a compliance voltage of 1000 V for driving grid-1. A voltage pulse of 1200 V and 1.2 μs duration, superimposed with −150 V bias has been generated for driving grid-2. A delay of 1 μs between the two drive pulses is achieved with the use of a simple circuit. The rate of rise of voltage better than 10 kV/μs and jitter of ±3 ns is achieved for grid-2 pulse. This driver module has been successfully used in a 50 kV, CX1575C thyratron switched pulsed power supply to drive a multi-joule transversely excited atmospheric CO2 laser at 100 Hz. The grid driver module can also be used for driving any high current thyratrons with minor external changes.
A “saddle-node” bifurcation scenario for birth or destruction of a Smale–Williams solenoid
Authors: Isaeva Olga B. , Kuznetsov Sergey P. , Sataev Igor R.
Published: November 2012

Formation or destruction of hyperbolic chaotic attractor under parameter variation is considered with an example represented by Smale–Williams solenoid in stroboscopic Poincaré map of two alternately excited non-autonomous van der Pol oscillators. The transition occupies a narrow but finite parameter interval and progresses in such way that periodic orbits constituting a “skeleton” of the attractor undergo saddle-node bifurcation events involving partner orbits from the attractor and from a non-attracting invariant set, which forms together with its stable manifold a basin boundary of the attractor.
Ultra-high-frequency piecewise-linear chaos using delayed feedback loops
Authors: Cohen Seth D. , Rontani Damien , Gauthier Daniel J.
Published: November 2012

We report on an ultra-high-frequency (>1 GHz), piecewise-linear chaotic system designed from low-cost, commercially available electronic components. The system is composed of two electronic time-delayed feedback loops: A primary analog loop with a variable gain that produces multi-mode oscillations centered around 2 GHz and a secondary loop that switches the variable gain between two different values by means of a digital-like signal. We demonstrate experimentally and numerically that such an approach allows for the simultaneous generation of analog and digital chaos, where the digital chaos can be used to partition the system's attractor, forming the foundation for a symbolic dynamics with potential applications in noise-resilient communications and radar.
The asymptotic behavior of the order parameter for the infinite-N Kuramoto model
Authors: Mirollo Renato E.
Published: November 2012

The Kuramoto model, first proposed in 1975, consists of a population of sinusoidally coupled oscillators with random natural frequencies. It has served as an idealized model for coupled oscillator systems in physics, chemistry, and biology. This paper addresses a long-standing problem about the infinite-N Kuramoto model, which is to describe the asymptotic behavior of the order parameter for this system. For coupling below a critical level, Kuramoto predicted that the order parameter would decay to 0. We use Fourier transform methods to prove that for general initial conditions, this decay is not exponential; in fact, exponential decay to 0 can only occur if the initial condition satisfies a fairly strong regularity condition that we describe. Our theorem is a partial converse to the recent results of Ott and Antonsen, who proved that for a special class of initial conditions, the order parameter does converge exponentially to its limiting value. Consequently, our result shows that the Ott–Antonsen ansatz does not completely capture all the possible asymptotic behavior in the full Kuramoto system.
Breaking a chaotic direct sequence spread spectrum communication system using interacting multiple model-unscented Kalman filter
Authors: Lu Gan , Bo Xiong
Published: November 2012

In this paper, a new method to break chaotic direct sequence spread spectrum (CD3S) communication systems is proposed. Here, the CD3S communication system transmitting different information symbols is considered as a combination of two subsystems which are driven by two different chaotic dynamic models, respectively. At every single time moment, the CD3S signal can be regarded as generated by the subsystem corresponding to the information symbol transmitted. Then, based on the multiple model form of CD3S signals, an interacting multiple model unscented Kalman filter with model switching detection mechanism is exploited to track the CD3S signals. The l2-norm of tracking errors is used to choose the model which best matches the intercepted signals. Thus, the information symbols are recovered indirectly. Compared with the existing methods, the proposed algorithm can: (1) reduce the influence of a low spreading factor; (2) calculate the spreading factor using the length of time intervals between model switching; and (3) be more effective under scenarios of low signal-to-noise ratio or multipath fading. Simulation results verify the superiority of the proposed method.
Emergence of scaling associated with complex branched wave structures in optical medium
Authors: Ni Xuan , Lai Ying-Cheng , Wang Wen-Xu
Published: November 2012

Branched wave structures, an unconventional wave propagation pattern, can arise in random media. Experimental evidence has accumulated, revealing the occurrence of these waves in systems ranging from microwave and optical systems to solid-state devices. Experiments have also established the universal feature that the wave-intensity statistics deviate from Gaussian and typically possess a long-tail distribution, implying the existence of spatially localized regions with extraordinarily high intensity concentration (“hot” spots). Despite previous efforts, the origin of branched wave pattern is currently an issue of debate. Recently, we proposed a “minimal” model of wave propagation and scattering in optical media, taking into account the essential physics for generating robust branched flows: (1) a finite-size medium for linear wave propagation and (2) random scatterers whose refractive indices deviate continuously from that of the background medium. Here we provide extensive numerical evidence and a comprehensive analytic treatment of the scaling behavior to establish that branched wave patterns can emerge as a general phenomenon in wide parameter regime in between the weak-scattering limit and Anderson localization. The basic physical mechanisms to form branched waves are breakup of waves by a single scatterer and constructive interference of broken waves from multiple scatterers. Despite simplicity of our model, analysis of the scattering field naturally yields an algebraic (power-law) statistic in the high wave-intensity distribution, indicating that our model is able to capture the generic physical origin of these special wave patterns. The insights so obtained can be used to better understand the origin of complex extreme wave patterns, whose occurrences can have significant impact on the performance of the underlying physical systems or devices.
Spin-dependent electron transport in graphene junctions in the presence of Rashba spin-orbit interaction
Authors: Esmaeilzadeh Mahdi , Ahmadi Somaieh
Published: November 2012

In this paper, we study the spin-dependent electron transport properties of graphene n-p-n junction in the presence of Rashba spin-orbit interaction using transfer matrix method. It is found that for a graphene n-p-n junction, the spin-resolved electron conductance depends on the strength of Rashba spin-orbit interaction and the built-in potential of graphene junctions. For an appropriate value of Rashba strength, perfect electron spin-inversion with high conductance can take place when the graphene junction is biased with sufficient built-in potential. In this case, the graphene junction can be used as a spin-inverter which is an important device in spintronic. Finally, the spin-dependent conductance for a graphene n-n-n junction is investigated and compared with that of n-p-n junction.
In-vivo visualization of melanoma tumor microvessels and blood flow velocity changes accompanying tumor growth
Authors: Ishida Hiroki , Hachiga Tadashi , Andoh Tsugunobu , Akiguchi Shunsuke
Published: November 2012

We demonstrate that using micro multipoint laser Doppler velocimetry (μ-MLDV) for noninvasive in-vivo imaging of blood vessels is useful for diagnosing malignant melanomas by comparison with visual diagnosis by dermoscopy. The blood flow velocity in microvessels varied during growth of melanomas transplanted in mouse ears. Mouse ears were observed by μ-MLDV up to 16 days after transplantation. The blood flow velocity in the tumor increased with increasing time and reached maximum of 4.5 mm/s at 9 days, which is more than twice that prior to transplantation. After 12 days, when the lesion had grown to an area of 6.6 mm2, we observed the formation of new blood vessels in the tumor. Finally, when the lesion had an area of 18 mm2 after 16 days, the flow velocity in the tumor decreased to approximately 3.2 mm/s.
Nano-particle magnetism with a dispersion of particle sizes
Authors: El-Hilo M.
Published: November 2012

In this paper, different definitions of the distribution functions that can be used to account for the magnetization of dispersed nano-particle systems are discussed. The volume and number weighted distributions are both found to be equally valid for the representation of distribution functions in nanoparticle magnetic systems. This study also shows that the room temperature magnetization curve M(H) of a superparamagnetic system is sensitive to the particle size distribution parameters and, that for a non-interacting system, these parameters can be unambiguously determined. Furthermore, the temperature variation of the initial susceptibility χ(T) for a dispersion of particle sizes is also examined using both the exact and the critical approaches. The critical approach is found to be a reasonable and valid approximation, since the discrepancy in the calculated χ(T) curves between the exact and the approximated form of the function exp(−tm/τ) was found to be very small. For a dispersion of anisotropy energies, both approaches adequately describe the unblocking process of the particle magnetic moments within the system when the temperature is increased. In this study, the distribution of the critical transition temperatures that can be obtained from the temperature variation of the initial susceptibility is also examined. For different definitions of the distribution functions, the retrieved distributions from the experimental data are bound to be different. Furthermore, the calculated temperature variation of the initial susceptibility is found to be sensitive to the constant value of the frequency factor f0 used in the calculations. The discrepancy in calculating the χ(T) curve using an improper value of f0 is larger than that discrepancy arising from the step-like approximation of exp(−tm/τ). Thus, the f0 value has to be calculated using the physical parameters of the system and not just taken as a constant value between 109 and 1011 Hz.
Complex band structures and evanescent Bloch waves in two-dimensional finite phononic plate
Authors: Oudich Mourad , Badreddine Assouar M.
Published: November 2012

The complex band structure of a two-dimensional (2D) phononic crystal based on solid or air cylindrical inclusions in solid finite thickness plate is investigated. The plane wave expansion method is developed with a specific extension to calculate the evanescent Bloch modes propagating in the phononic plate. It is shown that, in the opposite of 2D infinite phononic crystals, the complex band structures of Lamb modes exhibits a coupling between in-plane and out-of plane Lamb modes. The polarization and the behavior of the evanescent waves is analyzed and discussed as function of the plate thickness to figure out the physics behind the opening of phononic band gap for the different Lamb modes.
Hot phonons contribution to Joule heating in single-walled carbon nanotubes
Authors: Gautreau Pierre , Ragab Tarek , Basaran Cemal
Published: November 2012

The influence of hot phonons on the electron-phonon scattering rate and Joule heating is studied via an ensemble Monte Carlo (EMC) simulation with a step by step update of the phonon occupation number to account for the generation of hot phonons. The hot phonon contribution to Joule heating appears to be a function of the applied electric force field at room temperature, while it becomes independent of the applied electric force field for higher temperatures. The influence of hot phonons on Joule heating is more pronounced around room temperature and diminishes for higher temperatures. The results of the ensemble Monte Carlo simulation at very high temperatures (around 1800 K and above) suggest that the presence of non-equilibrium phonons may reduce the Joule heating of single-walled carbon nanotubes (SWCNTs).
Compositional dependence of the 1.8 μm emission properties of Tm3+ ions in silicate glass
Authors: Wang Xin , Fan Sijun , Li Kefeng , Zhang Lei , Wang Shikai , Hu Lili
Published: November 2012

The compositional dependence of the 1.8 μm emission properties of Tm3+ ion-doped lead silicate glasses is investigated. Judd–Ofelt parameters are calculated and their variation with different glass modifier ions is obtained. The Judd–Ofelt parameters increase with decreased modifier ionic radius. A large spontaneous emission probability and a large emission cross-section are found to be related with the strength of the modifier ion. Fluorescence spectra are analyzed using rate equations and compared with recorded data. The results are very close, indicating the reliability of this method. Non-radiative probability is deduced by fitting the fluorescence decay curve; it becomes smaller with increased ionic field strength. Energy transfer processes are studied using the extended overlap integral method.
Amplifying mirrors for terahertz plasmons
Authors: Sydoruk O. , Syms R. R. A. , Solymar L.
Published: November 2012

Semiconductor plasmons have long held out a promise for terahertz generation, but competitive plasmonic mechanisms have yet to be found. Here, we introduce amplifying terahertz mirrors: planar interfaces for two-dimensional electron channels that amplify plasmons in the presence of electron drift. In contrast to existing formulations, we develop a rigorous mode matching technique that takes the complete mode spectrum into account. Mirrors are characterized by plasmon reflection and transmission coefficients whose values can increase with drift. Amplitude and power coefficients are determined, and conditions are found for their values to exceed unity. Resonators based on different combinations of amplifying mirrors are investigated, and an asymmetric configuration (consisting of two different electron channels confined between conducting planes) whose roundtrip gain can exceed unity is identified. The unusual conditions needed for oscillation are examined in detail and the general advantages of asymmetric arrangements are highlighted. Finally, the potential of mode matching as a universal tool for plasmonics is discussed.
Reduced hierarchy equations of motion approach with Drude plus Brownian spectral distribution: Probing electron transfer processes by means of two-dimensional correlation spectroscopy
Authors: Tanimura Yoshitaka
Published: November 2012

We theoretically investigate an electron transfer (ET) process in a dissipative environment by means of two-dimensional (2D) correlation spectroscopy. We extend the reduced hierarchy equations of motion approach to include both overdamped Drude and underdamped Brownian modes. While the overdamped mode describes the inhomogeneity of a system in the slow modulation limit, the underdamped mode expresses the primary vibrational mode coupled with the electronic states. We outline a procedure for calculating 2D correlation spectrum that incorporates the ET processes. The present approach has the capability of dealing with system-bath coherence under an external perturbation, which is important to calculate nonlinear response functions for non-Markovian noise. The calculated 2D spectrum exhibits the effects of the ET processes through the presence of ET transition peaks along the Ω1 axis, as well as the decay of echo signals.
Interpreting picosecond acoustics in the case of low interface stiffness
Authors: Hohensee Gregory T. , Hsieh Wen-Pin , Losego Mark D. , Cahill David G.
Published: November 2012

Analysis of data acquired in time-domain thermoreflectance (TDTR) experiments requires accurate measurements of the thickness of the metal film optical transducer that absorbs energy from the pump optical pulse and provides a temperature dependent reflectivity that is interrogated by the probe optical pulse. This thickness measurement is typically accomplished using picosecond acoustics. The presence of contaminants and native oxides at the interface between the sample and transducer often produce a picosecond acoustics signal that is difficult to interpret. We describe heuristics for addressing this common difficulty in interpreting picosecond acoustic data. The use of these heuristics can reduce the propagation of uncertainties and improve the accuracy of TDTR measurements of thermal transport properties.
A full optically operated magnetometer array: An experimental study
Authors: IJsselsteijn R. , Kielpinski M. , Woetzel S. , Scholtes T. , Kessler E. , Stolz R. , Schultze V. , Meyer H.-G.
Published: November 2012

We show the operation of an optically pumped magnetometer array in a 50 μT magnetic field. The various components for a fully optical and non-magnetic detector unit were constructed and evaluated, from which a prototype unit was assembled with fiber coupled electronics. In this unit the magnetometers were operated using the intensity modulated method and heated with an off-resonant laser. Calculations on the temperature distribution were used to design the magnetometer array. Different magnetometers in such a detector unit were characterized and showed identical performance. Without applying noise reduction schemes, the obtained magnetic field resolution is a factor 2.5 above the shot noise level down to frequencies of about 7 Hz.
A tunable low-energy photon source for high-resolution angle-resolved photoemission spectroscopy
Authors: Harter John W. , King Philip D. C. , Monkman Eric J. , Shai Daniel E. , Nie Yuefeng , Uchida Masaki , Burganov Bulat , Chatterjee Shouvik , Shen Kyle M.
Published: November 2012

We describe a tunable low-energy photon source consisting of a laser-driven xenon plasma lamp coupled to a Czerny-Turner monochromator. The combined tunability, brightness, and narrow spectral bandwidth make this light source useful in laboratory-based high-resolution photoemission spectroscopy experiments. The source supplies photons with energies up to ∼7 eV, delivering under typical conditions >1012 ph/s within a 10 meV spectral bandwidth, which is comparable to helium plasma lamps and many synchrotron beamlines. We first describe the lamp and monochromator system and then characterize its output, with attention to those parameters which are of interest for photoemission experiments. Finally, we present angle-resolved photoemission spectroscopy data using the light source and compare its performance to a conventional helium plasma lamp.
On the role of stacking faults on dislocation generation and dislocation cluster formation in multicrystalline silicon
Authors: Kivambe Maulid M. , Ervik Torunn , Ryningen Birgit , Stokkan Gaute
Published: November 2012

The microstructure of highly dislocated stacking fault regions (dislocation density >106 cm−2) in industrial cast multicrystalline silicon has been investigated by light microscopy, scanning electron microscopy, and transmission electron microscopy. Our observations indicate that stacking faults form strong barriers to lattice dislocation movement and to the formation of sub grain boundaries. Stepped and curved stacking fault edges appear to generate dislocations. The observations suggest that stacking faults play an important role in the plasticity as well as in the formation of the microstructure of dislocations in multicrystalline silicon.
Zeeman shift of two-dimensional optical signals of Mg-porphyrin dimers with circularly polarized beams
Authors: Rodriguez Justo J. , Mukamel Shaul
Published: November 2012

Two-dimensional (2D) electronic spectra of Mg-porphyrin dimers with circularly polarized laser pulses are simulated for two molecular geometries. The Zeeman shift of the non-chiral 2D spectra induced by a magnetic field highlights spectral regions that are dominated by exciton states with large total angular momentum, and are particularly sensitive to the geometry. These 2D shifts show much higher sensitivity to the molecular array than the absorption spectrum shifts. For different molecular geometries, only the intensity of the 2D spectrum peaks changes while the form of the Zeeman shifts in the 2D frequency space is modified.
Quantum origins of molecular recognition and olfaction in drosophila
Authors: Bittner Eric R. , Madalan Adrian , Czader Arkadiusz , Roman Gregg
Published: November 2012

The standard model for molecular recognition of an odorant is that receptor sites discriminate by molecular geometry as evidenced that two chiral molecules may smell very differently. However, recent studies of isotopically labeled olfactants indicate that there may be a molecular vibration-sensing component to olfactory reception, specifically in the spectral region around 2300 cm−1. Here, we present a donor-bridge-acceptor model for olfaction which attempts to explain this effect. Our model, based upon accurate quantum chemical calculations of the olfactant (bridge) in its neutral and ionized states, posits that internal modes of the olfactant are excited impulsively during hole transfer from a donor to acceptor site on the receptor, specifically those modes that are resonant with the tunneling gap. By projecting the impulsive force onto the internal modes, we can determine which modes are excited at a given value of the donor-acceptor tunneling gap. Only those modes resonant with the tunneling gap and are impulsively excited will give a significant contribution to the inelastic transfer rate. Using acetophenone as a test case, our model and experiments on D. melanogaster suggest that isotopomers of a given olfactant give rise to different odorant qualities. These results support the notion that inelastic scattering effects may play a role in discriminating between isotopomers but that this is not a general spectroscopic effect.
Growth and interfacial properties of epitaxial CaCuO2 thin films
Authors: Fuchs D. , Müller P. , Sleem A. , Schneider R. , Gerthsen D. , Löhneysen H. v.
Published: November 2012

Epitaxial growth and interfacial properties of CaCuO2 films deposited on various perovskite related substrates by pulsed laser ablation were investigated. Highly c-axis oriented films can be stabilized on SrTiO3, LaAlO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, and NdGaO3 single crystalline substrates. However, the interface region between film and substrate is often found to be strongly defective. CaCuO2 films grown on (001)- and (110)-oriented NdGaO3 show the best crystallinity with lattice parameters nearly the same as for bulk CaCuO2 and a mosaic spread of about 0.1°. Regions where the interfacial growth is nearly perfect, dominantly show an A-site termination of NdGaO3, i.e., a layer stacking sequence of GaO2/NdO/CuO2/Ca. Resistivity measurements on the films show thermally activated behavior between 300 K and 170 K, i.e., ln R ∼ TA/T, where the activation temperature TA amounts to 116 K, and Mott variable range hopping, i.e., ln R ∼ T−1/4, below about 120 K. With decreasing film thickness, the resistivity increases remarkably, indicating that the interfacial conductivity is more insulating than metallic in character as expected from hole-doping of CaCuO2 due to apical oxygen at the NdO/CuO2 interface. Measurements of the magnetization do not show any indications for two-dimensional superconductivity at the interface. The experimental results suggest that the conductivity is rather caused by particulates of cupric oxide found in a distance of about 10 nm from the interface in the upper part of the film.
Magnetic and transport characteristics of long-period [(LaMnO3)n/(SrMnO3)n]m (n ≥ 3) superlattices
Authors: Wang Geming , Du Ruifang , Wu Di , Li Aidong
Published: November 2012

Artificial long-period superlattices of antiferromagnetic LaMnO3 and SrMnO3 ([(LaMnO3)n/(SrMnO3)n]m with n ≥ 3) were deposited on SrTiO3 (001) substrates by pulsed laser depositon monitored in situ by reflective high-energy electron diffraction. X-ray diffraction and atomic force microscopy studies reveal the sharp interfaces and smooth surface in the superlattices. Magnetic and transport characteristics are observed to depend strongly on the period of the superlattice. An interfacial ferromagnetic phase is found to coexist with the two antiferromagnetic components. The observed ferromagnetic magnetization is ascribed to Mn3+-O-Mn4+ double exchange across the LaMnO3/SrMnO3 interface. The spin-glass-like behaviors, the large magnetoresistance, and the evolution of the transport mechanism with increasing period in the superlattices are discussed in terms of spin frustration due to the competition between the ferromagnetic and antiferromagnetic interactions.
Visible photoluminescence from MgAl2O4 spinel with cation disorder and oxygen vacancy
Authors: Sawai Shigeto , Uchino Takashi
Published: November 2012

Magnesium aluminate spinel, MgAl2O4, is a structurally and compositionally interesting crystal since it can exhibit significant deviations from the stoichiometric composition because of the ability of the lattice to tolerate substantial cation disorder. We show that MgAl2O4 spinels with excess Al2O3 non-stoichiometry can accommodate a large amount of positively charged oxygen vacancies (F+ centers) when they are heated under vacuum at ∼1900 °C using a high-frequency induction heating unit with a graphite crucible. Thus the obtained spinels show a bluish white photoluminescence (PL) with a PL quantum yield of ∼20%, which is attributed to excitation and recombination processes of the F+ center. The PL signal is characterized by a broad spectral band peaking at ∼2.7 eV (∼460 nm) and decays in a nonexponential manner on the time scale of several tens of microseconds. The F+ center is supposed to interact with nearby positively and negatively charged defects derived from cation disorder in the spinel structure, forming clusters of various defect centers.
Measuring the structure of thin soft matter films under confinement: A surface-force type apparatus for neutron reflection, based on a flexible membrane approach
Authors: de Vos Wiebe M. , Mears Laura L. E. , Richardson Robert M. , Cosgrove Terence , Dalgliesh Robert M. , Prescott Stuart W.
Published: November 2012

A unique surface force type apparatus that allows the investigation of a confined thin film using neutron reflection is described. The central feature of the setup consists of a solid substrate (silicon) and a flexible polymer membrane (Melinex®). We show that inflation of the membrane against the solid surface provides close and even contact between the interfaces over a large surface area. Both heavy water and air can be completely squeezed out from between the flexible film and the solid substrate, leaving them in molecular contact. The strength of confinement is controlled by the pressure used to inflate the membrane. Dust provides a small problem for this approach as it can get trapped between membrane and substrate to prevent a small part of the membrane from making good contact with the substrate. This results in the measured neutron reflectivity containing a small component of an unwanted reflection, between 10% and 20% at low confining pressures (1 bar) and between 1% and 5% at high confining pressures (5 bar). However, we show that this extra signal does not prevent good and clear information on the structure of thin films being extracted from the neutron reflectivity. The effects of confinement are illustrated with data from a poly(vinyl pyrollidone) gel layer in water, a polyelectrolyte multilayer in water, and with data from a stack of supported lipid-bilayers swollen with D2O vapor. The data demonstrates the potential of this apparatus to provide information on the structure of thin films under confinement for a known confining pressure.
Multi-pole multi-zero frequency-independent phase-shifter
Authors: Bitar M. A. , Gallo A. , Volpe F. A.
Published: November 2012

A multi-pole, multi-zero design allowed realizing a “true” phase-shifter (not time-delayer) of flat frequency-response over more than three decades (30 Hz–100 kHz), which can be extended to higher frequencies or broader bands thanks to a modular design. Frequency-dependent optimization of a single resistance made also the gain flat to within few percents. The frequency-independent phase-shifter presented can find application in any experiment in which an action needs to be taken (e.g., a measurement needs to be performed) at a fixed phase-delay relative to an event, regardless of how rapidly the system rotates or oscillates.
Empirical laws of particle extraction from single-grid source of bipolar ion-electron flow
Authors: Dudin S. V. , Rafalskyi D. V.
Published: November 2012

The present research is devoted to the problem of extraction grid choice for a single-grid source of bipolar ion-electron flow. The paper contains detailed reference information on ion and electron extraction characteristics of 10 different grids with broad range of parameters: aperture width (0.09-0.6 mm), grid transparency (0.19-0.51), thickness (0.036-0.5 mm), and with different aperture geometry. The grids with square, circular, and slit apertures were made with different technologies: laser cutting, welding, weaving, and electrolytic erosion. The general regularities of the ion and electron extraction from the single-grid source are experimentally researched for the cases of dc and RF extraction grid biasing. A conclusion has been made that the maximum extracted ion current at low ion energy (0-200 eV) does not significantly vary for all the grids and does not exceed half of the primary ion current from plasma multiplied by the optical grid transparency. The low-energy limit of efficient ion extraction has been discovered which cannot be overcome by the aperture narrowing. A conclusion is made that the RF extraction mode is superior for all the researched grids since it is characterized by higher extracted ion current at any acceleration voltage for any grid with much more simple and smooth extraction curves behavior in comparison to the dc case as well as absence of arcing, jumps, and hysteresis of the measured curves at any RF voltages. The unique ability of the RF biased single-grid source of simultaneous ion/electron emission has been studied. The measured maximal attainable ion beam current compensation ratio is always sufficiently higher than 1 and typically varies in the range 2–6. The results obtained in the present paper demonstrate prospective of the single-grid source in space thruster applications and in modern technologies, particularly for ion beam processing of wide bandgap semiconductor devices such as GaN and SiC transistors due to inherent precise beam neutralization.
Source effects in analyzer-based X-ray phase contrast imaging with conventional sources
Authors: Hönnicke M. G. , Manica J. , Mazzaro I. , Cusatis C. , Huang X.-R.
Published: November 2012

Several recent papers have shown the implementation of analyzer based X-ray phase contrast imaging (ABI) with conventional X-ray sources. The high flux is always a requirement to make the technique useful for bio-medical applications. Here, we present and discuss three important parameters, which need to be taken into account, when searching for the high flux ABI: anisotropic magnification, double image, and source size spread due to intrinsic dispersive diffraction by asymmetrically cut crystals. These parameters, if not well optimized, may cause important features in the acquired images which can mislead the interpretation. A few ways to minimize these effects are implemented and discussed, including some experimental results.
Effect of pressure on the anomalous response functions of a confined water monolayer at low temperature
Authors: Mazza Marco G. , Stokely Kevin , Stanley H. Eugene , Franzese Giancarlo
Published: November 2012

We study a coarse-grained model for a water monolayer that cannot crystallize due to the presence of confining interfaces, such as protein powders or inorganic surfaces. Using both Monte Carlo simulations and mean field calculations, we calculate three response functions: the isobaric specific heat CP, the isothermal compressibility KT, and the isobaric thermal expansivity αP. At low temperature T, we find two distinct maxima in CP, KT, and |αP|, all converging toward a liquid-liquid critical point (LLCP) with increasing pressure P. We show that the maximum in CP at higher T is due to the fluctuations of hydrogen (H) bond formation and that the second maximum at lower T is due to the cooperativity among the H bonds. We discuss a similar effect in KT and |αP|. If this cooperativity were not taken into account, both the lower-T maximum and the LLCP would disappear. However, comparison with recent experiments on water hydrating protein powders provides evidence for the existence of the lower-T maximum, supporting the hypothesized LLCP at positive P and finite T. The model also predicts that when P moves closer to the critical P the CP maxima move closer in T until they merge at the LLCP. Considering that other scenarios for water are thermodynamically possible, we discuss how an experimental measurement of the changing separation in T between the two maxima of CP as P increases could determine the best scenario for describing water.
Evaluation of the pressure tensor and surface tension for molecular fluids with discontinuous potentials using the volume perturbation method
Authors: Jiménez-Serratos Guadalupe , Vega Carlos , Gil-Villegas Alejandro
Published: November 2012

In this article we apply the volume-perturbation method to systems of particles interacting via discontinuous potentials. We have found that an accurate Monte Carlo simulation protocol can be used in order to study properties of very general non-spherical systems with discontinuous potentials, such as chain molecules and spherocylinders with square-well interactions, and chain molecules with square-well and square-shoulder interactions. From the simulation results obtained for these systems we verify that: (1) the method reproduces the pressure as used in NPT simulations; (2) discontinuous infinite repulsive interactions give asymmetric contributions to the pressure when compression and expansion movements are used; however for finite interactions these contributions are symmetric; and (3) the pressure contributions preserve the additivity of the potential interactions. Density profiles and surface tension for subcritical conditions are accurately predicted.
Shock Hugoniot calculations of polymers using quantum mechanics and molecular dynamics
Authors: Chantawansri Tanya L. , Sirk Timothy W. , Byrd Edward F. C. , Andzelm Jan W. , Rice Betsy M.
Published: November 2012

Using quantum mechanics (QM) and classical force-field based molecular dynamics (FF), we have calculated the principle shock Hugoniot curves for numerous amorphous polymers including poly[methyl methacrylate] (PMMA), poly[styrene], polycarbonate, as well as both the amorphous and crystalline forms of poly[ethylene]. In the FF calculations, we considered a non-reactive force field (i.e., polymer consistent FF). The QM calculations were performed with density functional theory (DFT) using dispersion corrected atom centered pseudopotentials. Overall, results obtained by DFT show much better agreement with available experimental data than classical force fields. In particular, DFT calculated Hugoniot curves for PMMA up to 74 GPa are in very good agreement with experimental data, where a preliminary study of chain fracture and association was also performed. Structure analysis calculations of the radius of gyration and carbon-carbon radial distribution function were also carried out to elucidate contraction of the polymer chains with increasing pressure.
Excitonic splitting and coherent electronic energy transfer in the gas-phase benzoic acid dimer
Authors: Ottiger Philipp , Leutwyler Samuel
Published: November 2012

The benzoic acid dimer, (BZA)2, is a paradigmatic symmetric hydrogen bonded dimer with two strong antiparallel hydrogen bonds. The excitonic S1/S2 state splitting and coherent electronic energy transfer within supersonically cooled (BZA)2 and its 13C−, d1 −, d2 −, and 13C/d1 − isotopomers have been investigated by mass-resolved two-color resonant two-photon ionization spectroscopy. The (BZA)2-(h − h) and (BZA)2-(d − d) dimers are C2h symmetric, hence only the S2 ← S0 transition can be observed, the S1 ← S0 transition being strictly electric-dipole forbidden. A single 12C/13C or H/D isotopic substitution reduces the symmetry of the dimer to Cs, so that the isotopic heterodimers (BZA)2 − 13C, (BZA)2 −(h − d), (BZA)2 −(h13C−d), and (BZA)2 −(h − d13C) show both S1 ← S0 and S2 ← S0 bands. The S1/S2 exciton splitting inferred is Δexc = 0.94 ± 0.1 cm−1. This is the smallest splitting observed so far for any H-bonded gas-phase dimer. Additional isotope-dependent contributions to the splittings, Δiso, arise from the change of the zero-point vibrational energy upon electronic excitation and range from Δiso = 3.3 cm−1 upon 12C/13C substitution to 14.8 cm−1 for carboxy H/D substitution. The degree of excitonic localization/delocalization can be sensitively measured via the relative intensities of the S1 ← S0 and S2 ← S0 origin bands; near-complete localization is observed even for a single 12C/13C substitution. The S1/ S2 energy gap of (BZA)2 is \documentclass[12pt]{minimal}\begin{document}$\Delta ^{exc}_{calc} = 11$\end{document}Δcalcexc=11 cm−1 when calculated by the approximate second-order perturbation theory (CC2) method. Upon correction for vibronic quenching, this decreases to \documentclass[12pt]{minimal}\begin{document}$\Delta ^{exc}_{vibron}=2.1$\end{document}Δvibronexc=2.1 cm−1 [P. Ottiger et al., J. Chem. Phys. 136, 174308 (2012)]10.1063/1.4705119, in good agreement with the observed Δexc = 0.94 cm−1. The observed excitonic splittings can be converted to exciton hopping times τexc. For the (BZA)2−(h − h) homodimer τexc = 18 ps, which is nearly 40 times shorter than the double proton transfer time of (BZA)2 in its excited state [Kalkman et al., ChemPhysChem 9, 1788 (2008)]10.1002/cphc.200800214. Thus, the electronic energy transfer is much faster than the proton-transfer in \documentclass[12pt]{minimal}\begin{document}${\rm (BZA)}^{\ast}_2$\end{document}( BZA )2*.