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At least 199 records · Page 11Linked to original sources
arXiv2008

Quantitative imaging of colloidal flows

We present recent advances in the instrumentation and analysis methods for quantitative imaging of concentrated colloidal suspensions under flow. After a brief review of colloidal imaging, we describe various flow geometries for two and and three-dimensional (3D) imaging, including a `confocal rheoscope'. This latter combination of a confocal microscope and a rheometer permits simultaneous characterization of rheological response and 3D microstructural imaging. The main part of the paper discusses in detail how to identify and track particles from confocal images taken during flow. After analyzing the performance of the most commonly used colloid tracking algorithm by Crocker and Grier extended to flowing systems, we propose two new algorithms for reliable particle tracking in non-uniform flows to the level of accuracy already available for quiescent systems. We illustrate the methods by applying it to data collected from colloidal flows in three different geometries (channel flow, parallel plate shear and cone-plate rheometry).

cond-mat.soft↗
arXiv2008

Competing jump cycles for vacancy diffusion in binary alloys

The mean-first-passage-times (MFPTs) for a vacancy that diffuses (via one- and six-jump cycles) in a two dimensional ordered binary alloy are evaluated using the properties of random walks on networks. We investigate the effect of temperature and relative barrier height on the ratio between the MFPTs of the two cycles. We find that the six-jump cycle takes shorter time than the one-jump cycle for the range of parameters considered.

cond-mat.mtrl-sci↗
arXiv2008

On the Depletion Effect in Colloids: Correlated Brownian Motions

Our object is to formulate and analyze a physically plausible and mathematically sound model to better understand the phenomenon of clumping in colloid dispersions. Our model is stochastic but rigorously derived from a deterministic setup in a Newtonian setting. A rigorous transition from deterministic mean-field dynamics of several large particles and infinitely many small particles to the stochastic motion of the large particles is invoked. Then the stochastic motion of the large particles is described by a system of correlated Brownian motions. The scaling in the transition preserves a characteristic correlation length. From the limiting stochastic equations we compute the probability fluxes for the separation between two large particles. We show that, for short times, two particles sufficiently close together tend to be attracted to each other. This agrees with the observed depletion phenomena in colloids. To do this computation, we extend the notion of Van Kampen's one-dimensional flux rate in an appropriate way to account for higher dimensional effect.

cond-mat.mtrl-sci↗
arXiv2008

The Laser Ablation Production of Platinum Nitride and its Possible Structure

The synthesis of platinum nitride by the laser ablation method is reported. The spectroscopic results show that nitrogen is in interstitial sites of platinum as N-units, contradicting the accepted configuration for PtN2 where it is as N2-units. To elucidate this point we did density functional calculations to correlate composition with nitrogen sites. For dilute nitrogen concentrations, x < 0.2, nitrogen would be in six-fold coordinated sites (octahedral interstices) as N-units. For 0.2 < x < 1.5, nitrogen would be in tetrahedral interstices as N-units. Only for x > 1.5 the N2-configuration in octahedral sites is attained.

cond-mat.mtrl-sci↗
arXiv2008

Non-linear graphene optics for terahertz applications

The linear electrodynamic properties of graphene -- the frequency-dependent conductivity, the transmission spectra and collective excitations -- are briefly outlined. The non-linear frequency multiplication effects in graphene are studied, taking into account the influence of the self-consistent-field effects and of the magnetic field. The predicted phenomena can be used for creation of new devices for microwave and terahertz optics and electronics.

cond-mat.mes-hall↗
arXiv2009

Graphite intercalation compound KC$_8$ revisited: a key to graphene

Electrons in isolated graphene layers are a two-dimensional gas of massless Dirac Fermions. In realistic devices, however, the electronic properties are modified by elastic deformations, interlayer coupling and substrate interaction. Here we unravel the electronic structure of doped graphene, revisiting the stage one graphite intercalation compound KC$_8$ using angle--resolved photoemission spectroscopy and ab--initio calculations. The full experimental dispersion is in excellent agreement to calculations of doped graphene once electron correlations are included on the $GW$ level. This highlights that KC$_8$ has negligible interlayer coupling. Therefore Dirac Fermion behaviour is preserved and we directly determine the full experimental Dirac cone of doped graphene. In addition we prove that superconductivity in KC$_8$ is mediated by electron--phonon coupling to an iTO phonon, yielding a strong kink in the quasiparticle dispersion at 166 meV. These results are key for understanding, both, the unique electronic properties of graphene and superconductivity in KC$_8$.

cond-mat.mtrl-sci↗
arXiv2009

Effect of self-affine fractal characteristics of surfaces on wetting

The relation between the contact angle of a liquid drop and the morphological parameters of self-affine solid surfaces have been investigated. We show experimentally that the wetting property of a solid surface crucially depends on the surface morphological parameters such as: (1) root mean square (rms) roughness $σ$, (2) in-plane roughness correlation length $ξ$ and (3) roughness exponent $α$ of the self-affine surface. We have shown that the contact angle monotonically decreases with the increase in the rms local surface slope $ρ$ ($\propto σ/ξ^α$) for the cases where the liquid wets the crevices of the surface upon contact. We have shown that the same solid surface can be made hydrophobic or hydrophilic by merely tuning these self-affine surface morphological parameters.

cond-mat.mtrl-sci↗
arXiv2008

Half-metallic ferromagnetism in binary compounds of alkali metals with nitrogen: Ab initio calculations

The first-principles full-potential linearized augmented plane-wave method based on density functional theory is used to investigate electronic structure and magnetic properties of hypothetical binary compounds of I$^{A}$ subgroup elements with nitrogen (LiN, NaN, KN and RbN) in assumed three types of cristalline structure (rock salt, wurtzite and zinc-blende). We find that, due to the spin polarized \textit{p} orbitas of N, all four compounds are half-metallic ferromagnets with wide energy bandgaps (up to 2.0 eV). The calculated total magnetic moment in all investigated compounds for all three types of crystal structure is exactly 2.00 $μ_{\text{B}}$ per formula unit. The predicted half-metallicity is robust with respect to lattice-constant contraction. In all the cases ferromagnetic phase is energetically favored with respect to the paramagnetic one. The mechanism leading to half-metallic ferromagnetism and synthesis possibilities are discussed.

cond-mat.mtrl-sci↗
arXiv2008

High-output CPP-GMR sensor with synthetic-ferrimagnet free layer and enhanced spin-torque critical currents

It is shown that the maximum stable output of a CPP-GMR sensor is increased significantly by using a synthetic ferrimagnet free layer, provided the electron current flows from free layer to reference layer. This free layer allows a larger magnetoresistance ratio for a given free layer magnetic moment, and in addition results in a greater than three-fold increase in the critical current above which spin-torque instability of the free layer occurs. In read heads with net free layer moments equivalent to only 4.5nm of Ni80Fe20, this effect is shown to result in sustainable sense current densities above 2e8 A/cm2.

cond-mat.mtrl-sci↗
arXiv2009

Correlation Energy and the Spin Susceptibility of the Two-Valley Two-dimensional Electron Gas

We find that the spin susceptibility of a two-dimensional electron system with valley degeneracy does not grow critically at low densities, at variance with experimental results [A. Shashkin et al., Phys. Rev. Lett. 96, 036403 (2006)]. We ascribe this apparent discrepancy to the weak disorder present in experimental samples. Our prediction is obtained from accurate correlation energies computed with state of-the-art diffusion Monte Carlo simulations and fitted with an analytical expression which also provides a local spin density functional for the system under investigation.

cond-mat.str-el↗
arXiv2008

Graphene - A rising star in view of scientometrics

We have carried out a scientometric analysis of the literature dealing with graphene, a material which has been identified as a new carbon allotrope. The investigation is based on the CAplus database of Chemical Abstracts Service, the INSPEC database of the Institute of Electrical and Electronics Engineers and the Web of Science (WoS) of Thomson Reuters. The time evolution of the publications shows a dramatic increase since 2004 when graphene has been isolated for the first time. The graphene literature has been analyzed with respect to the most productive authors, research organizations, countries of authors, and the leading journals. Furthermore, the time evolution of the major graphene based research topics and the top 10 most productive research organizations with respect to their specific topics have been analyzed and are shown as heat maps. In addition, the graphene research landscape has been established using the analysis tool STN AnaVist. Finally, a brief citation analysis has been performed. The time evolution of the impact reveals that, rather similar to the output evolution, the impact increase of the graphene papers within the starting period is possibly going to outrun the impact increase of the fullerenes and nanotubes papers.

physics.soc-ph↗
arXiv2008

Universality in the Photophysics of pi-Conjugated Polymers and Single-Walled Carbon Nanotubes

We describe both pi-conjugated polymers and semiconducting single-walled carbon nanotubes because of the remarkable similarities in their behavior under photoexcitation. The common themes between these two seemingly different classes of materials are pi-conjugation, quasi one-dimensionality, and strong Coulomb interactions. Theoretical understanding of both systems requires going beyond traditional band theory. We present our current understanding of the electronic structures and excited state absorptions in both systems within a common theoretical model. Our aim is to give physical interpretations of experiments that have been performed and to give guidance to future experimental work.

cond-mat.mtrl-sci↗
arXiv2009

A lattice approach to model flow in cracked concrete

This paper presents a lattice approach to model the influence of cracking on inviscid flow in concrete. A mechanical lattice model based on a damage-plasticity constitutive model was combined with a new dual lattice of conduit elements for flow analysis. The diffusivity of the conduit elements depends on the crack-opening obtained from the mechanical lattice. The coupled lattice model was applied to several benchmark tests for aligned and random lattices. The results for mechanical loading and flow analysis obtained with the new approach were shown to be independent of the size of lattice elements used.

cond-mat.mtrl-sci↗
arXiv2008

Manganese-diffusion-induced n-doping in semiconductor structures containing Ga(Mn)As layers

Semiconductor structures using ferromagnetic semiconductors as spin injectors are promising systems for future spintronic devices. Here, we present combined photoluminescence (PL) and time-resolved magneto-optical experiments of a nominally nonmagnetic quantum well(QW) separated by a thin barrier from a ferromagnetic Ga(Mn)As layer. Due to the partial quenching of the PL, we conclude that there is a significant Mn backdiffusion into the QW. Moreover, from the time-resolved measurements, we infer that the Mn leads to n-type doping within the QW, and, in addition, strongly increases the electron spin dephasing time.

cond-mat.other↗
arXiv2009

Analytic solution for electrons and holes in graphene under electromagnetic radiation: gap appearance and non-linear current effects

We find the exact solution of graphene s carriers under electromagnetic radiation. To obtain such solution, we combine Floquet theory with a trial solution. Then the energy spectrum is obtained without using any approximation. Using such results, we prove that the energy spectrum presents a gap opening which depends on de radiation frequency and electric wave intensity, whereas the current shows a strongly non-linear behaviour.

cond-mat.mtrl-sci↗
arXiv2009

Nonlinear bending of molecular films by polarized light

A theory of photoinduced directed bending of non-crystalline molecular films is presented. Our approach is based on elastic deformation of the film due to interaction between molecules ordered through polarized light irradiation. The shape of illuminated film is obtained in the frame of the nonlinear elasticity theory. It is shown that the shape and the curvature of the film depend on the polarization and intensity of the light. The curvature of an irradiated film is a non-monotonic function of the extinction coefficient.

cond-mat.soft↗
arXiv2013

The modified Becke-Johnson potential analyzed

Recently in the Wien2k code, the modified Becke-Johnson potential (mBJLDA) was implemented. As the authors [{\em Phys.Rev.Lett.} 102, 226401 (2009)] point, this potential reproduces the band gap of semiconductors with improved accuracy. In this paper we present our analysis of this potential in two directions. First, we checked whether this potential reproduces the band structure for metals, an analysis that lacked in the literature. We calculated the band gap of a group of semiconductors. We observed that the Linear Density Approximation (LDA) give rise to a shorter lattice constant as compared to experiment. The Generalized Gradient Approximation behaves oppositely. Using the average, $a_{Avg}$, in the mBJLDA potential, we obtained a closer to experiment value for the gap. We conclude that the new mBJLDA potential represent an important improvement as compared to the results from the previous version of the Wien2k code. Also the mBJLDA potential can be a very useful tool for the theoretical study of complex systems containing semiconductor compounds such as surfaces, superlattices and interfaces.

cond-mat.mtrl-sci↗