bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular phosphorus ion source for semiconductor technology
Authors: Gushenets V. I. , Bugaev A. S. , Oks E. M. , Hershcovitch A. , Kulevoy T. V.
Published: February 2012

This paper presents results on the generation of molecular phosphorus ion beams in a hot filament ion source. Solid red phosphorous is evaporated mainly as tetra-atomic molecules up to a temperature of 800°C. Thus, one of the main conditions for producing maximum P4+ fraction in the beam is to keep the temperature of the phosphorous oven, the steam line and the discharge chamber walls no greater than 800°C. The prior version of our ion source was equipped with a discharge chamber cooling system. The modified source ensured a P4+ ion beam current greater than 30% of the total beam current.
Loss tangent imaging: Theory and simulations of repulsive-mode tapping atomic force microscopy
Authors: Proksch Roger , Yablon Dalia G.
Published: February 2012

An expression for loss tangent measurement of a surface in amplitude modulation atomic force microscopy is derived using only the cantilever phase and the normalized cantilever amplitude. This provides a direct measurement of substrate compositional information that only requires tuning of the cantilever resonance to provide quantitative information. Furthermore, the loss tangent expression incorporates both the lost and stored energy into one term that represents a fundamental interpretation of the phase signal in amplitude modulation imaging. Numerical solutions of a cantilever tip interacting with a simple Voigt modeled surface agree with the derived loss tangent to within a few percent.
A fresh look at dense hydrogen under pressure. I. An introduction to the problem, and an index probing equalization of H–H distances
Authors: Labet Vanessa , Gonzalez-Morelos Paulina , Hoffmann Roald , Ashcroft N. W.
Published: February 2012

In the first of a series of four papers on hydrogen under pressure, and its transitions from an initiating molecular state, we begin by defining carefully the problem, and setting the distance scale of interactions of protons and electrons in molecular aggregates of the first of the elements. Following a review of the experimental situation, in particular the phase diagram of hydrogen, in as much as it is known, and the behavior of its vibrons and rotons, we move onto the setting up of a numerical laboratory for probing the underlying physics and chemistry of interactions in hydrogen as the pressure increases. The laboratory consists of the preferred static structures emerging from calculations on the system in the range of 1 atm to 500 GPa, those of Pickard and Needs. The intermolecular (inter-pair) H···H separations naturally decrease with increasing pressure, first rapidly so, then more slowly. The intramolecular (intra-pair) H–H distances vary over a much smaller scale (0.05 Å) as the pressure increases, first decreasing, then increasing, and finally decreasing. We define an equalization function to gauge the approach to equality of the first neighbor and shortest next neighbor H (proton) separations in this numerical laboratory. And we find that metallization is likely to occur before bond equalization.
A fresh look at dense hydrogen under pressure. III. Two competing effects and the resulting intra-molecular H-H separation in solid hydrogen under pressure
Authors: Labet Vanessa , Hoffmann Roald , Ashcroft N. W.
Published: February 2012

A preliminary discussion of the general problem of localization of wave functions, and the way it is approached in theoretical condensed matter physics (Wannier functions) and theoretical chemistry (localized or fragment orbitals) is followed by an application of the ideas of Paper II in this series to the structures of hydrogen as they evolve under increasing pressure. The idea that emerges is that of simultaneously operative physical (reduction of available space by an increasingly stiff wall of neighboring molecules) and chemical (depopulation of the σg bonding molecular orbital of H2, and population of the antibonding σu* MO) factors. The two effects work in the same direction of reducing the intermolecular separation as the pressure increases, but compete, working in opposite directions, in their effect on the intramolecular (nearest neighbor, intra-pair) distance. We examine the population of σg and σu* MOs in our numerical laboratory, as well as the total electron transfer (small), and polarization (moderate, where allowed by symmetry) of the component H2 molecules. From a molecular model of two interacting H2 molecules we find a linear relationship between the electron transfer from σg to σu* of a hydrogen molecular fragment and the intramolecular H-H separation, and that, in turn, allows us to estimate the expected bond lengths in H2 under pressure if the first effect (that of simple confinement) was absent. In essence, the intramolecular H-H separations under pressure are much shorter than they would be, were there no physical/confinement effect. We then use this knowledge to understand how the separate E and PV terms contribute to hydrogen phase changes with increasing pressure.
A fresh look at dense hydrogen under pressure. IV. Two structural models on the road from paired to monatomic hydrogen, via a possible non-crystalline phase
Authors: Labet Vanessa , Hoffmann Roald , Ashcroft N. W.
Published: February 2012

In this paper, we examine the transition from a molecular to monatomic solid in hydrogen over a wide pressure range. This is achieved by setting up two models in which a single parameter δ allows the evolution from a molecular structure to a monatomic one of high coordination. Both models are based on a cubic Bravais lattice with eight atoms in the unit cell; one belongs to space group $Pa\bar 3$Pa3¯, the other to space group $R\bar 3m$R3¯m. In $Pa\bar 3$Pa3¯ one moves from effective 1-coordination, a molecule, to a simple cubic 6-coordinated structure but through a very special point (the golden mean is involved) of 7-coordination. In $R\bar 3m$R3¯m, the evolution is from 1 to 4 and then to 3 to 6-coordinate. If one studies the enthalpy as a function of pressure as these two structures evolve (δ increases), one sees the expected stabilization of minima with increased coordination (moving from 1 to 6 to 7 in the $Pa\bar 3$Pa3¯ structure, for instance). Interestingly, at some specific pressures, there are in both structures relatively large regions of phase space where the enthalpy remains roughly the same. Although the structures studied are always higher in enthalpy than the computationally best structures for solid hydrogen – those emerging from the Pickard and Needs or McMahon and Ceperley numerical laboratories – this result is suggestive of the possibility of a microscopically non-crystalline or “soft” phase of hydrogen at elevated pressures, one in which there is a substantial range of roughly equi-enthalpic geometries available to the system. A scaling argument for potential dynamic stabilization of such a phase is presented.
Multiplexing single-beam coherent anti-stokes Raman spectroscopy with heterodyne detection
Authors: Wipfler Alexander , Buckup Tiago , Motzkus Marcus
Published: February 2012

Multiplex detection of single beam coherent anti-Stokes Raman spectra is achieved by phase shaping of a narrow gate probe. The spectrally resolved signal is amplified via a local oscillator derived from the same single beam and is disentangled via double quadrature spectral interferometry. Spectral resolution and spectral range are defined by the width and position of the phase gate, which can be easily set by a femtosecond pulse shaper. Such phase-only multiplexed single beam coherent anti-Stokes Raman spectroscopy is demonstrated for simple molecules and is combined with Raman line reconstruction methods to obtain dispersion free Raman resonances.
The physics of aerobreakup. II. Viscous liquids
Authors: Theofanous T. G. , Mitkin V. V. , Ng C. L. , Chang C-H. , Deng X. , Sushchikh S.
Published: February 2012

We extend the work of Theofanous and Li [“On the physics of aerobreakup,” Phys. Fluids 20, 052103 (2008)] on aerobreakup physics of water-like, low viscosity liquid drops, to Newtonian liquids of any viscosity. The scope includes the full range of aerodynamics from near incompressible to high Mach number flows. The key physics of Rayleigh–Taylor piercing (RTP, first criticality) and of shear-induced entrainment (SIE, second and terminal criticality) are verified and quantified by new viscosity- and capillarity-based scalings for fluids of any viscosity. The relevance and predictive power of linear stability analysis of the Rayleigh–Taylor and Kelvin–Helmholtz problems (both including viscosity) is demonstrated for the RTP and the SIE regimes, respectively. The advanced stages of breakup and of the resulting particle-clouds are observed and clear definition and quantification of breakup times are offered.
Unraveling rotation-vibration mixing in highly fluxional molecules using diffusion Monte Carlo: Applications to ${\rm H}_3^+$H3+ and H3O+
Authors: Petit Andrew S. , Wellen Bethany A. , McCoy Anne B.
Published: February 2012

A thorough examination of the use of fixed-node diffusion Monte Carlo for the study of rotation-vibration mixing in systems that undergo large amplitude vibrational motions is reported. Using \documentclass[12pt]{minimal}\begin{document}${\rm H}_3^+$\end{document}H3+ as a model system, the overall accuracy of the method is tested by comparing the results of these calculations with those from converged variational calculations. The effects of the presence of a large amplitude inversion mode on rotation-vibration mixing are considered by comparing the \documentclass[12pt]{minimal}\begin{document}${\rm H}_3^+$\end{document}H3+ results with those for H3O+. Finally, analysis of the results of the fixed-node diffusion Monte Carlo calculations performed in different nodal regions is found to provide clear indications of when some of the methodology's underlying assumptions are breaking down as well as provide physical insights into the form of the rotation-vibration coupling that is most likely responsible.
Atomic structure of a Σ3 [110]/(111) grain boundary in CeO2
Authors: Feng B. , Hojo H. , Mizoguchi T. , Ohta H. , Findlay S. D. , Sato Y. , Shibata N. , Yamamoto T. , Ikuhara Y.
Published: February 2012

The atomic structure of a Σ3 [110]/(111) grain boundary in CeO2 was studied by scanning transmission electron microscopy, electron energy loss spectroscopy, and the first-principles calculations. It was revealed that this grain boundary does not promote the formation of oxygen vacancies and keeps oxygen stoichiometry, which is different from that of Σ5 CeO2 grain boundary studied previously [H. Hojo, T. Mizoguchi, H. Ohta, S. D. Findlay, N. Shibata, T. Yamamoto, and Y. Ikuhara, Nano Lett. 10, 4668 (2010)]. It was found that the difference in grain boundary oxygen stoichiometry is correlated with the grain boundary atomic structure.
Transition between turbulent magnetically driven flow states in a Rayleigh-Bénard cell
Authors: Grants I. , Gerbeth G.
Published: February 2012

Transition between turbulent flow regimes is studied experimentally in a cylinder of liquid mercury heated from below under the influence of a rotating magnetic field. The latter creates a rotating flow which almost completely suppresses the temperature fluctuation near horizontal boundaries at a much lower angular velocity than a simple mechanical rotation. Our experiment confirms that this effect persists in the deep turbulent range to Grashof numbers as high as about 109. An intermediate range is observed for Gr > 2 × 108 with the temperature fluctuation suppressed in the core but near the sidewall. This is explained by turbulent friction replacing the Coriolis force as the leading retarding force. The linear instability of a simplified model is studied numerically. The model considers a base flow consisting of a uniform rotation and a formally independent uniform meridional flow in a cylinder with an adverse vertical temperature gradient. The model shows that the bulk meridional flow being itself much slower than the rotation is able to delay the Rayleigh-Bénard instability.
Binary scattering model for Lennard-Jones potential: Transport coefficients and collision integrals for non-equilibrium gas flow simulations
Authors: Venkattraman Ayyaswamy , Alexeenko Alina A.
Published: February 2012

A Lennard-Jones (LJ) binary interaction model for dilute gases is obtained by representing the exact scattering angle as a polynomial expansion in non-dimensional collision variables. Rigorous theoretical verification of the model is performed by comparison with exact values of diffusion and viscosity cross sections and related collision integrals. The collision quantities given by the polynomial approximation model agree within 3.5% with those of the exact LJ scattering. The proposed model is compared in detail with the generalized soft sphere (GSS) model which is the closest in terms of fidelity among existing direct simulation Monte Carlo collision models. The GSS model's performance for the collision integral used in the first approximate of viscosity coefficient is comparable to the proposed model for most reduced temperatures. However, other collision integrals deviate significantly, even at moderate reduced temperatures. The high fidelity of the proposed model at low reduced temperatures enables non-equilibrium simulations of gases with deep LJ potential well such as metallic vapors. The model is based on the scattering angle as opposed to viscosity or diffusion coefficients and provides a direct link to molecular dynamics simulations.
B80 and B101–103 clusters: Remarkable stability of the core-shell structures established by validated density functionals
Authors: Li Fengyu , Jin Peng , Jiang De-en , Wang Lu , Zhang Shengbai B. , Zhao Jijun , Chen Zhongfang
Published: February 2012

Prompted by the very recent claim that the volleyball-shaped B80 fullerene [X. Wang, Phys. Rev. B 82, 153409 (2010)10.1103/PhysRevB.82.153409] is lower in energy than the B80 buckyball [N. G. Szwacki, A. Sadrzadeh, and B. I. Yakobson, Phys. Rev. Lett. 98, 166804 (2007)10.1103/PhysRevLett.98.166804] and core-shell structure [J. Zhao, L. Wang, F. Li, and Z. Chen, J. Phys. Chem. A 114, 9969 (2010)10.1021/jp1018873], and inspired by the most recent finding of another core-shell isomer as the lowest energy B80 isomer [S. De, A. Willand, M. Amsler, P. Pochet, L. Genovese, and S. Goedecher, Phys. Rev. Lett. 106, 225502 (2011)10.1103/PhysRevLett.106.225502], we carefully evaluated the performance of the density functional methods in the energetics of boron clusters and confirmed that the core-shell construction (stuffed fullerene) is thermodynamically the most favorable structural pattern for B80. Our global minimum search showed that both B101 and B103 also prefer a core-shell structure and that B103 can reach the complete core-shell configuration. We called for great attention to the theoretical community when using density functionals to investigate boron-related nanomaterials.
Dirac equation for generalized Pöschl-Teller scalar and vector potentials and a Coulomb tensor interaction by Nikiforov-Uvarov method
Authors: Hassanabadi H. , Maghsoodi E. , Zarrinkamar S. , Rahimov H.
Published: February 2012

Approximate analytical solutions of spin and pseudospin symmetry limits of Dirac equation are reported for the generalized Pöschl-Teller scalar and vector potentials and a Coulomb tensor interaction by Nikiforov-Uvarov method. On the contrary to the cumbersome numerical procedures, the analytical approach followed here can be followed even by the undergraduate students.
Scattering resonances in slow NH3–He collisions
Authors: Gubbels Koos B. , Meerakker Sebastiaan Y. T. van de , Groenenboom Gerrit C. , Meijer Gerard , Avoird Ad van der
Published: February 2012

We theoretically study slow collisions of NH3 molecules with He atoms, where we focus in particular on the observation of scattering resonances. We calculate state-to-state integral and differential cross sections for collision energies ranging from 10−4 cm−1 to 130 cm−1, using fully converged quantum close-coupling calculations. To describe the interaction between the NH3 molecules and the He atoms, we present a four-dimensional potential energy surface, based on an accurate fit of 4180 ab initio points. Prior to collision, we consider the ammonia molecules to be in their antisymmetric umbrella state with angular momentum j = 1 and projection k = 1, which is a suitable state for Stark deceleration. We find pronounced shape and Feshbach resonances, especially for inelastic collisions into the symmetric umbrella state with j = k = 1. We analyze the observed resonant structures in detail by looking at scattering wavefunctions, phase shifts, and lifetimes. Finally, we discuss the prospects for observing the predicted scattering resonances in future crossed molecular beam experiments with a Stark-decelerated NH3 beam.
Molecular geometry of OC⋅⋅⋅AgI determined by broadband rotational spectroscopy and ab initio calculations
Authors: Stephens Susanna L. , Mizukami Wataru , Tew David P. , Walker Nicholas R. , Legon Anthony C.
Published: February 2012

Pure rotational spectra of the ground vibrational states of six isotopologues of OC⋅⋅⋅AgI have been measured by chirped-pulse Fourier transform microwave spectroscopy. The spectra are assigned to determine the rotational constant, B0, centrifugal distortion constant, DJ, and nuclear quadrupole coupling constant of the iodine atom, χaa(I). The complex is linear. Isotopic substitutions at the silver, carbon, and oxygen atoms allow bond lengths to be established by the r0, rs, and $r_m^{{\rm (1)}}$rm(1)methods of structure determination. The length of the C−O bond, r(CO), in the r0 geometry for OC⋅⋅⋅AgI is 0.008 Å shorter than that found in the free CO molecule. The length of the Ag−I bond, r(AgI), is 0.013 Å shorter than in free AgI. χaa(I) is determined to be −769.84(22) MHz for OC⋅⋅⋅107AgI implying an ionic character of 0.66 for the metal halide bond. Attachment of carbon monoxide to the isolated AgI molecule results in an increase of the ionic character of AgI of 0.12. The molecular structure and spectroscopic parameters determined from the experimental data are presented alongside the results of calculations at the explicitly correlated coupled-cluster singles, doubles and perturbative triples level. Vibrational frequencies, the electric dipole moment, the nuclear quadrupole coupling constant, and the dissociation energy of the molecule have been calculated.
Photoelectron spectroscopy of 1-nitropropane and 1-nitrobutane anions
Authors: Adams Christopher L. , Knurr Benjamin J. , Weber J. Mathias
Published: February 2012

We present low-energy velocity map photoelectron imaging results for bare and Ar-solvated 1-nitropropane and 1-nitrobutane anions. We report the adiabatic electron affinity of 1-nitropropane as (223 ± 6) meV and that of 1-nitrobutane as (240 ± 6 meV). The vertical detachment energies of these two species are found to be (0.92 ± 0.05) and (0.88 ± 0.05) eV, respectively. The photoelectron spectra are discussed in the framework of Franck-Condon simulations based on density functional theory. We observe unusual resonances in the photoelectron spectra of both ions under study, whose kinetic energy is independent of the photon energy of the detaching radiation. We discuss possible origins of these resonances as rescattering phenomena, consistent with the experimental photoelectron angular distributions.
Far-from-equilibrium processes without net thermal exchange via energy sorting
Authors: Vilar Jose M. G. , Rubi J. Miguel
Published: February 2012

Many important processes at the microscale require far-from-equilibrium conditions to occur, as in the functioning of mesoscopic bioreactors, nanoscopic rotors, and nanoscale mass conveyors. Achieving such conditions, however, is typically based on energy inputs that strongly affect the thermal properties of the environment and the controllability of the system itself. Here, we present a general class of far-from-equilibrium processes that suppress the net thermal exchange with the environment by maintaining the Maxwell-Boltzmann velocity distribution intact. This new phenomenon, referred to as ghost equilibrium, results from the statistical cancellation of superheated and subcooled nonequilibrated degrees of freedom that are autonomously generated through a microscale energy sorting process. We provide general conditions to observe this phenomenon and study its implications for manipulating energy at the microscale. The results are applied explicitly to two mechanistically different cases, an ensemble of rotational dipoles and a gas of trapped particles, which encompass a great variety of common situations involving both rotational and translational degrees of freedom.
Electronic states of cyclophanes with small bridges
Authors: Dodziuk H. , Vetokhina V. , Hopf H. , Luboradzki R. , Gaweł P. , Waluk J.
Published: February 2012

Electronic absorption and magnetic circular dichroism were recorded for five cyclophanes with ethano bridges: [2.2]paracyclophane, (1,2,4)[2.2.2]cyclophane, (1,2,4;1,2,5)[2.2.2]cyclophane, (1,2,3,4,5,6)(1,2,3,4,5,6)cyclophane, and trans-[2.2]metacyclophane. Spectral and structural analyses were based on geometry optimization and calculations of transition energies, carried out using density functional theory methods. The assignments have been proposed for several electronic transitions observed in the region below 52 000 cm−1. The observation of transitions which should be forbidden in the high D2h symmetry [2.2]paracyclophane suggests a twisted ground state structure of D2 symmetry, although the former structure with large amplitude vibrations at room temperature cannot be excluded. The PBE0 functional turned out to appropriately reproduce the inter-ring distances and electronic transition energies.
Temperature dependent phonon Raman scattering of highly a-axis oriented CoFe2O4 inverse spinel ferromagnetic films grown by pulsed laser deposition
Authors: Liao Y. Y. , Li Y. W. , Hu Z. G. , Chu J. H.
Published: February 2012

Lattice vibrations of highly a-axis oriented CoFe2O4 (CFO) films have been investigated by Raman scattering in the temperature range of 80-873 K. The five phonon modes T1g(2), T1g(3), Eg, A1g(1), A1g(2), and their evolutions can be uniquely distinguished. It was found that an electron transfer between Co2+ and Fe3+ cations occurs in octahedral sites at about 173 K. The structure disorder in the CFO films appears with increasing the temperature, which indicates the cation migration between tetrahedral and octahedral sites. The phenomena suggest the structural transformation trend from inverse spinel to normal spinel at the elevated temperatures.
Wavelet decomposition of forced turbulence: Applicability of the iterative Donoho-Johnstone threshold
Authors: Lord Jesse W. , Rast Mark P. , Mckinlay Christopher , Clyne John , Mininni Pablo D.
Published: February 2012

We examine the decomposition of forced Taylor-Green and Arn'old-Beltrami-Childress (ABC) flows into coherent and incoherent components using an orthonormal wavelet decomposition. We ask whether wavelet coefficient thresholding based on the Donoho-Johnstone criterion can extract a coherent vortex signal while leaving behind Gaussian random noise. We find that no threshold yields a strictly Gaussian incoherent component, and that the most Gaussian incoherent flow is found for data compression lower than that achieved with the fully iterated Donoho-Johnstone threshold. Moreover, even at such low compression, the incoherent component shows clear signs of large-scale spatial correlations that are signatures of the forcings used to drive the flows.
An optical analysis tool for avoiding dust formation in very-high frequency hydrogen diluted silane plasmas at low substrate temperatures
Authors: de Jong M. M. , de Koning J. , Rath J. K. , Schropp R. E. I.
Published: February 2012

Control of the formation of dust particles in a silane deposition plasma is very important for avoiding electrical shunts in devices, such as thin film silicon solar cells. In this work we present a noninvasive in situ method for identification of the plasma regime, based on optical emission spectroscopy (OES), which can be applied to silane/hydrogen plasmas at low substrate temperatures. By monitoring the OES spectra as a function of the position perpendicular to the plasma electrodes we developed a method to identify the transition of a plasma from the dust free to a dusty regime, which was confirmed by TEM images of layers deposited in both regimes. Using this technique we mapped this transition as a function of applied forward very-high frequency (VHF) power and hydrogen dilution at different substrate temperatures. The advantage of this technique is that the experiment is insensitive to optical transmission loss at the viewport due to deposition of silicon films. As the transition from the dust free to the dusty regime is substrate temperature dependent and the transition from amorphous to nanocrystalline growth mainly depends on hydrogen dilution, a limited parameter window has been defined in which dust-free amorphous silicon can be deposited at low substrate temperatures. A single simple OES technique can be used for in situ monitoring of amorphous to nanocrystalline transition as well as the onset of the dusty regime in a thin film silicon cell fabrication process.
Electronic structure of ternary CdxZn1−xO (0 ≤ x ≤ 0.075) alloys
Authors: Hung-Chun Lai Henry , Kuznetsov Vladimir L. , Egdell Russell G. , Edwards Peter P.
Published: February 2012

Narrowing of the electronic bandgap of ZnO by doping with cadmium opens up further potential uses of the material for photocatalytic applications. However, the mechanism of this important bandgap modification is still unclear. This letter demonstrates that for doped materials, the Fermi level resides within the bandgap in the bulk but lies above the conduction band minimum at the surface of the material, thus producing downward band bending and electron accumulation in the near surface region.
A generalized gradient approximation for exchange derived from the model potential of van Leeuwen and Baerends
Authors: Gaiduk Alex P. , Staroverov Viktor N.
Published: February 2012

The common way to obtain energies from Kohn-Sham exchange potentials is by using the Levy-Perdew virial relation. For potentials that are not functional derivatives (i.e., nearly all model exchange potentials in existence), this approach leads to energy expressions that lack translational and rotational invariance. We propose a method for constructing potential-based energy functionals that are free from these artifacts. It relies on the same line-integration technique that gives rise to the Levy-Perdew relation, but uses density scaling instead of coordinate scaling. The method is applicable to any exchange or correlation potential that depends on the density explicitly, and correctly recovers the parent energy functional from a functional derivative. To illustrate our approach we develop a properly invariant generalized gradient approximation for exchange starting from the model potential of van Leeuwen and Baerends.
Magnetic-field effects in illuminated tetracene field-effect transistors
Authors: Saragi Tobat P. I. , Reichert Thomas
Published: February 2012

We report on magnetic-field effects in illuminated tetracene field-effect transistors. A decrease in the photocurrent is observed when an external magnetic field is applied. In this case, the resistance of the conducting channel increases by up to 0.75%, which leads to positive magnetoresistance. This can be attributed to: (1) A magnetic field-induced decrease in the interaction rate between triplet excitons and trapped holes. (2) The coupling between the singlet and the triplet manifold of states is decreased in a magnetic field and, hence, the singlet exciton fission and triplet exciton fusion rate constants are reduced.
Pt on graphene monolayers supported on a Ni(111) substrate: Relativistic density-functional calculations
Authors: Błoński Piotr , Hafner Jürgen
Published: February 2012

The structural, energetic, and magnetic properties of Pt atoms and dimers adsorbed on a Ni-supported graphene layer have been investigated using density-functional calculations, including the influence of dispersion forces and of spin-orbit coupling. Dispersion forces are found to be essential to stabilize a chemisorbed graphene layer on the Ni(111) surface. The presence of the Ni-substrate leads not only to a stronger interaction of Pt atoms and dimers with graphene but also to a locally increased binding between graphene and the substrate and a complex reconstruction of the adlayer. The stronger binding of the dimer also stabilizes a flat adsorption geometry in contrast to the upright geometry on a free-standing graphene layer. These effects are further enhanced by dispersion corrections. Isolated Pt adatoms and flat dimers are found to be non-magnetic, while an upright Pt dimer has strongly anisotropic spin and orbital moments. For the clean C/Ni(111) system, we calculate an in-plane magnetic anisotropy, which is also conserved in the presence of isolated Pt adatoms. Surprisingly, upright Pt-dimers induce a re-orientation of the easy magnetic axis to a direction perpendicular to the surface, in analogy to Pt2 on a free-standing graphene layer and to the axial anisotropy of a gas-phase Pt2 dimer.
Survival of interacting Brownian particles in crowded one-dimensional environment
Authors: Ryabov Artem , Chvosta Petr
Published: February 2012

We investigate a diffusive motion of a system of interacting Brownian particles in quasi-one-dimensional micropores. In particular, we consider a semi-infinite 1D geometry with a partially absorbing boundary and the hard-core inter-particle interaction. Due to the absorbing boundary the number of particles in the pore gradually decreases. We present the exact analytical solution of the problem. Our procedure merely requires the knowledge of the corresponding single-particle problem. First, we calculate the simultaneous probability density of having still a definite number (N − k) of surviving particles at definite coordinates. Focusing on an arbitrary tagged particle, we derive the exact probability density of its coordinate. Second, we present a complete probabilistic description of the emerging escape process. The survival probabilities for the individual particles are calculated, the first and the second moments of the exit times are discussed. Generally speaking, although the original inter-particle interaction possesses a point-like character, it induces entropic repulsive forces which, e.g., push the leftmost (rightmost) particle towards (opposite) the absorbing boundary thereby accelerating (decelerating) its escape. More importantly, as compared to the reference problem for the non-interacting particles, the interaction changes the dynamical exponents which characterize the long-time asymptotic dynamics. Interesting new insights emerge after we interpret our model in terms of (a) diffusion of a single particle in a N-dimensional space, and (b) order statistics defined on a system of N-independent, identically distributed random variables.
A snapshot of electrified nanodroplets undergoing Coulomb fission
Authors: Arscott Steve , Descatoire Cédric , Buchaillot Lionel , Ashcroft Alison E.
Published: February 2012

We investigate the size distribution of electrically charged nanodroplets using atomic force microscopy (AFM). The droplets were generated using nano- and micro-scale silicon tips. A brief voltage pulse results in a “snapshot” of charged nanodroplets on a Cr surface. AFM of the traces left by the nanodroplets revealed that certain droplet diameters are favored suggesting droplet fission due to Rayleigh instability at nanometer length scales. The most occurring droplet diameters are 85.9(4.1) nm and 167.1 nm (9.7 nm) for nano- and micro-scale tips, respectively.
Numerical approaches to determine the interface tension of curved interfaces from free energy calculations
Authors: Tröster A. , Oettel M. , Block B. , Virnau P. , Binder K.
Published: February 2012

A recently proposed method to obtain the surface free energy σ(R) of spherical droplets and bubbles of fluids, using a thermodynamic analysis of two-phase coexistence in finite boxes at fixed total density, is reconsidered and extended. Building on a comprehensive review of the basic thermodynamic theory, it is shown that from this analysis one can extract both the equimolar radius Re as well as the radius Rs of the surface of tension. Hence the free energy barrier that needs to be overcome in nucleation events where critical droplets and bubbles are formed can be reliably estimated for the range of radii that is of physical interest. It is found that the conventional theory of nucleation, where the interface tension of planar liquid–vapor interfaces is used to predict nucleation barriers, leads to a significant overestimation, and this failure is particularly large for bubbles. Furthermore, different routes to estimate the effective radius-dependent Tolman length δ(Rs) from simulations in the canonical ensemble are discussed. Thus we obtain an instructive exemplification of the basic quantities and relations of the thermodynamic theory of metastable droplets/bubbles using simulations. However, the simulation results for δ(Rs) employing a truncated Lennard–Jones system suffer to some extent from unexplained finite size effects, while no such finite size effects are found in corresponding density functional calculations. The numerical results are compatible with the expectation that δ(Rs → ∞) is slightly negative and of the order of one tenth of a Lennard–Jones diameter, but much larger systems need to be simulated to allow more precise estimates of δ(Rs → ∞).
Controlling domain walls velocities in ferromagnetic ring-shaped nanowires
Authors: Negoita M. , Hayward T. J. , Allwood D. A.
Published: February 2012

We demonstrate a method by which domain walls (DWs) in planar magnetic ring-shaped nanowires can be propagated controllably at arbitrarily low velocities by confining them to geometrically defined energy minima. Using this technique, we propagate domain walls around a ring-shaped nanowire at velocities as small as 0.6 mm/s, low enough to allow ultra-cold atoms to be transported in magnetic “traps” formed by the domain wall’s stray field. We also show how the frequency of an external applied rotating field can be used to determine the domain walls’ velocity and that the thermally activated depinning of the walls from defects ultimately limits the precision to which their motion can be controlled.
Understanding ultrafast carrier dynamics in single quasi-one-dimensional Si nanowires
Authors: Seo M. A. , Dayeh S. A. , Upadhya P. C. , Martinez J. A. , Swartzentruber B. S. , Picraux S. T. , Taylor A. J. , Prasankumar R. P.
Published: February 2012

We use femtosecond optical pump-probe spectroscopy to study ultrafast carrier dynamics in single quasi-one-dimensional silicon nanowires. By isolating individual nanowires, we avoid complications resulting from the broad size and alignment distribution in nanowire ensembles, allowing us to directly probe ultrafast carrier dynamics. Spatially-resolved experiments demonstrate the influence of surface-mediated mechanisms on carrier dynamics in a single NW, while polarization-resolved experiments reveal a clear anisotropy in carrier lifetimes measured parallel and perpendicular to the long axis of the NW, due to density-dependent Auger recombination. These results suggest the possibility of tailoring carrier relaxation in a single nanowire for a desired application.
Clouds of particles in a periodic shear flow
Authors: Metzger Bloen , Butler Jason E.
Published: February 2012

We have investigated the time evolution of a cloud of non-Brownian particles subjected to a periodic shear flow in an otherwise pure liquid at low Reynolds number. This experiment illustrates the irreversible nature of particulate systems submitted to a shear. When repeating the cycles of shear, we have found that clouds of particles progressively disperse in the flow direction until reaching a threshold critical volume fraction that depends upon the strain amplitude; this critical volume fraction coincides with measurements of the threshold for reversibility found from experiments on homogeneous suspensions in periodic shear. Two distinct patterns, including a “galaxy-like” shape, are observed for the evolution of the clouds and the transition between the patterns is identified using a simple scaling analysis. Movies are available with the online version of the paper.
Chalcogenide glass surface passivation of a GaAs bipolar transistor for unique avalanche terahertz emitters and picosecond switches
Authors: Vainshtein Sergey , Javadyan Valeri , Duan Guoyong , Tsendin Konstantin , Hovhannisyan Rafael , Kostamovaara Juha
Published: February 2012

The ultra-narrow “collapsing” field domains discovered recently in avalanching GaAs bipolar junction transistor provide a physical basis for designing unique THz emitters and superfast switches. Reliability in devices operating near their volume breakdown voltage requires decisive suppression of premature surface breakdown. We demonstrate here complete, durable surface breakdown suppression through simple deposition of a massive chalcogenide glass layer on the mesa surface by means of a negative charge formed at the interface.
Trapping and collection of uniform size droplets for nanoparticle synthesis
Authors: Doh I. , Erdem E. Y. , Pisano A. P.
Published: February 2012

This paper presents a simple and fast method for droplet trapping and collection and demonstrates nanoparticle synthesis inside these trapped droplets. Since droplet trapping is size-dependent, droplets having various sizes caused by unstable inlet flows can be eliminated. Moreover, the new droplet substitutes the previous one, so we can keep the up-to-date contents of droplets at all times. All trapped droplets can be simply collected at the outlet. We had demonstrated the synthesis of iron oxide nanoparticles inside droplets and collection of them from the device. Due to its simplicity and high efficiency for selective trapping and collection, this method can become one of the key essentials in droplet-based microfluidic assays.
Diffusion-controlled formation mechanism of dual-phase structure during Al induced crystallization of SiGe
Authors: Zhang Tian-Wei , Ma Fei , Zhang Wei-Lin , Ma Da-Yan , Xu Ke-Wei , Chu Paul K.
Published: February 2012

Aluminum induced crystallization of amorphous SiGe at low temperature is studied and a dual-phase stacked structure with different compositions emerges when the annealing temperature is higher than a critical value. This behavior is very sensitive to the oxidization state of the interlayer. A model based on energetics is proposed to elucidate this temperature dependent behavior. Thermodynamically, it can be ascribed to the competition between grain-boundary-mediated and interface-mediated crystallization and kinetically, it stems from the different diffusion rates of Si and Ge. The results are useful to the design and fabrication of high-efficiency solar cells.
ZnO:Al cathode for highly efficient, semitransparent 4% organic solar cells utilizing TiOx and aluminum interlayers
Authors: Bauer Andreas , Wahl Tina , Hanisch Jonas , Ahlswede Erik
Published: February 2012

We report on the fabrication of highly efficient, semitransparent bulk heterojunction solar cells comprising poly[[9-(1-octylnonyl)-9h-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) blended with [6,6]-phenyl-C71-butyric acid methyl (PC71BM) esters as active layer. As semitransparent cathode sputtered aluminum doped ZnO was used in combination with a sputter damage preventing, thin (8 nm) TiOx layer processed from solution and a sputtered aluminum interlayer (ALI). The short circuit current improves for thicker ALIs due to increased reflectance at the cathode leading to average efficiencies of 4.0% for semitransparent solar cells. Comparable results (3.9%) were achieved for devices comprising thick absorbers and thin ALIs.
Electrode band structure effects in thin MgO magnetic tunnel junctions
Authors: Teixeira J. M. , Ventura J. , Fernández-García M. P. , Araujo J. P. , Sousa J. B. , Wisniowski P. , Freitas P. P.
Published: February 2012

In this study, we demonstrate that, beyond the standard magnon excitations, the electronic band structure of the electrodes plays a significant role on the low bias voltage window (0 < |V| < 0.4 V) of the tunnel magnetoresistance (TMR) in thin MgO-CoFeB junctions. The tunneling conductance in the parallel state presents a minimum at about ±0.35 and ±0.3 V for the negative and positive bias, respectively. The presence of this minimum indicates a related decrease in the TMR(V). These observations are explained by the electronic band structures of bcc-Fe and Co.
Contrast Polarity Preservation's Role in Perception: Explained and Unexplained Stimuli
Authors: McCormick Meghan C , Hon Alice J , Huang Abigail E , Altschuler Eric L
Published: February 2012

Roncato and Casco (2003, Perception & Psychophysics65 1252–1272) had shown that in situations where the Gestalt principle of good continuity is put into conflict with preservation of contrast polarity (CP) the perception that preserves CP prevails. Parlangeli and Roncato (2010, Perception39 255–259) have studied this question of preservation of CP more closely and have added an addendum to the rule. They have used stimuli consisting of a checkerboard of perpendicularly arranged rectangular bricks (white, gray, or black) and draughtsmen–white, gray, or black disks placed at the corners of the bricks. This study has caused them to add an addendum to the rule of CP-preserved path-conjunction binding: if there are two contour completions that preserve the CP, the one with the higher contrast will prevail. Parlangeli and Roncato find that, for certain shades of the disks and bricks, the perpendicular lines of the checkerboard appear strikingly to be slanted or undulating. Here we consider all possible arrangements of relative magnitudes of checkerboards consisting of bricks of two different shades and disks of two shades as well, as such arrangements with widely varying differences in the magnitude of brightness. We have found a number of cases where the perception is not explained by the rule and addendum of Roncato and Casco, and Parlangeli and Roncato, and a case where preservation of “distant” as well as local CP plays a role in perception. The previously known cases, and the new exceptional unexplained stimuli we have found, warrant further study.
Protooncogene Ski cooperates with the chromatin-remodeling factor Satb2 in specifying callosal neurons
Authors: Baranek Constanze , Dittrich Manuela , Parthasarathy Srinivas , Bonnon Carine Gaiser , Britanova Olga , Lanshakov Dmitriy , Boukhtouche Fatiha , Sommer Julia E. , Colmenares Clemencia , Tarabykin Victor , Atanasoski Suzana
Published: February 2012

First insights into the molecular programs orchestrating the progression from neural stem cells to cortical projection neurons are emerging. Loss of the transcriptional regulator Ski has been linked to the human 1p36 deletion syndrome, which includes central nervous system defects. Here, we report critical roles for Ski in the maintenance of the neural stem cell pool and the specification of callosal neurons.
Ski
-deficient callosal neurons lose their identity and ectopically express the transcription factor Ctip2. The misspecified callosal neurons largely fail to form the corpus callosum and instead redirect their axons toward subcortical targets. We identify the chromatin-remodeling factor Satb2 as a partner of Ski, and show that both proteins are required for transcriptional repression of
Ctip2
in callosal neurons. We propose a model in which Satb2 recruits Ski to the
Ctip2
locus, and Ski attracts histone deacetylases, thereby enabling the formation of a functional nucleosome remodeling and deacetylase repressor complex. Our findings establish a central role for Ski–Satb2 interactions in regulating transcriptional mechanisms of callosal neuron specification.
Predictive microfluidic control of regulatory ligand trajectories in individual pluripotent cells
Authors: Moledina Faisal , Clarke Geoff , Oskooei Ali , Onishi Kento , Günther Axel , Zandstra Peter W.
Published: February 2012

Local (cell-level) signaling environments, regulated by autocrine and paracrine signaling, and modulated by cell organization, are hypothesized to be fundamental stem cell fate control mechanisms used during development. It has, however, been challenging to demonstrate the impact of cell-level organization on stem cell fate control and to relate stem cell fate outcomes to autocrine and paracrine signaling. We address this fundamental problem using a combined in silico and experimental approach in which we directly manipulate, using laminar fluid flow, the local impact of endogenously secreted gp130-activating ligands and their activation of signal transducer and activator of transcription3 (STAT3) signaling in mouse embryonic stem cells (mESC). Our model analysis predicted that flow-dependent changes in autocrine and paracrine ligand binding would impact heterogeneity in cell- and colony-level STAT3 signaling activation and cause a gradient of cell fate determination along the direction of flow. Interestingly, analysis also predicted that local cell density would be inversely proportional to the degree to which endogenous secretion contributed to cell fate determination. Experimental validation using functional activation of STAT3 by secreted factors under microfluidic perfusion culture demonstrated that STAT3 activation and consequently mESC fate were manipulable by flow rate, position in the flow field, and local cell organization. As a unique demonstration of how quantitative control of autocrine and paracrine signaling can be integrated with spatial organization to elicit higher order cell fate effects, this work provides a general template to investigate organizing principles due to secreted factors.
Heme-copper terminal oxidase using both cytochrome
c
and ubiquinol as electron donors
Authors: Gao Ye , Meyer Björn , Sokolova Lucie , Zwicker Klaus , Karas Michael , Brutschy Bernd , Peng Guohong , Michel Hartmut
Published: February 2012

The cytochrome
c
oxidase Cox2 has been purified from native membranes of the hyperthermophilic eubacterium
Aquifex aeolicus
. It is a cytochrome
ba
3
oxidase belonging to the family B of the heme-copper containing terminal oxidases. It consists of three subunits, subunit I (CoxA2, 63.9 kDa), subunit II (CoxB2, 16.8 kDa), and an additional subunit IIa of 5.2 kDa. Surprisingly it is able to oxidize both reduced cytochrome
c
and ubiquinol in a cyanide sensitive manner. Cox2 is part of a respiratory chain supercomplex. This supercomplex contains the fully assembled cytochrome
bc
1
complex and Cox2. Although direct ubiquinol oxidation by Cox2 conserves less energy than ubiquinol oxidation by the cytochrome
bc
1
complex followed by cytochrome
c
oxidation by a cytochrome
c
oxidase, ubiquinol oxidation by Cox2 is of advantage when all ubiquinone would be completely reduced to ubiquinol, e.g., by the sulfide∶quinone oxidoreductase, because the cytochrome
bc
1
complex requires the presence of ubiquinone to function according to the Q-cycle mechanism. In the case that all ubiquinone has been reduced to ubiquinol its reoxidation by Cox2 will enable the cytochrome
bc
1
complex to resume working.