Pristine CNO abundances from Magellanic Cloud B stars II. Fast rotators in the LMC cluster NGC 2004

We present spectroscopic abundance analyses of three main-sequence B stars in the young Large Magellanic Cloud cluster NGC 2004. All three targets have projected rotational velocities around 130 km/s. Techniques are presented that allow the derivation of stellar parameters and chemical abundances in spite of these high v sin i values. Together with previous analyses of stars in this cluster, we find no evidence among the main-sequence stars for effects due to rotational mixing up to v sin i around 130 km/s. Unless the equatorial rotational velocities are significantly larger than the v sin i values, this finding is probably in line with theoretical expectations. NGC 2004/B30, a star of uncertain evolutionary status located in the Blue Hertzsprung Gap, clearly shows signs of mixing in its atmosphere. To verify the effects due to rotational mixing will therefore require homogeneous analysis of statistically significant samples of low-metallicity main-sequence B stars over a wide range of rotational velocities.Comment: 12 pages, 5 figures, 2 tables; accepted for publication in ApJ (vol. 633, p. 899

Electron spin relaxation in paramagnetic Ga(Mn)As quantum wells

Electron spin relaxation in paramagnetic Ga(Mn)As quantum wells is studied via the fully microscopic kinetic spin Bloch equation approach where all the scatterings, such as the electron-impurity, electron-phonon, electron-electron Coulomb, electron-hole Coulomb, electron-hole exchange (the Bir-Aronov-Pikus mechanism) and the $s$-$d$ exchange scatterings, are explicitly included. The Elliot-Yafet mechanism is also incorporated. From this approach, we study the spin relaxation in both $n$-type and $p$-type Ga(Mn)As quantum wells. For $n$-type Ga(Mn)As quantum wells where most Mn ions take the interstitial positions, we find that the spin relaxation is always dominated by the DP mechanism in metallic region. Interestingly, the Mn concentration dependence of the spin relaxation time is nonmonotonic and exhibits a peak. This behavior is because that the momentum scattering and the inhomogeneous broadening have different density dependences in the non-degenerate and degenerate regimes. For $p$-type Ga(Mn)As quantum wells, we find that Mn concentration dependence of the spin relaxation time is also nonmonotonic and shows a peak. Differently, this behavior is because that the $s$-$d$ exchange scattering (or the Bir-Aronov-Pikus) mechanism dominates the spin relaxation in the high Mn concentration regime at low (or high) temperature, whereas the DP mechanism determines the spin relaxation in the low Mn concentration regime. The Elliot-Yafet mechanism also contributes the spin relaxation at intermediate temperature. The spin relaxation time due to the DP mechanism increases with Mn concentration due to motional narrowing, whereas those due to the spin-flip mechanisms decrease with Mn concentration, which thus leads to the formation of the peak.... (The remaining is omitted due to the space limit)Comment: 12 pages, 8 figures, Phys. Rev. B 79, 2009, in pres

Scanning Raman spectroscopy of graphene antidot lattices: Evidence for systematic p-type doping

We have investigated antidot lattices, which were prepared on exfoliated graphene single layers via electron-beam lithography and ion etching, by means of scanning Raman spectroscopy. The peak positions, peak widths and intensities of the characteristic phonon modes of the carbon lattice have been studied systematically in a series of samples. In the patterned samples, we found a systematic stiffening of the G band mode, accompanied by a line narrowing, while the 2D mode energies are found to be linearly correlated with the G mode energies. We interpret this as evidence for p-type doping of the nanostructured graphene

Solvable Examples of Drift and Diffusion of Ions in Non-uniform Electric Fields

The drift and diffusion of a cloud of ions in a fluid are distorted by an inhomogeneous electric field. If the electric field carries the center of the distribution in a straight line and the field configuration is suitably symmetric, the distortion can be calculated analytically. We examine the specific examples of fields with cylindrical and spherical symmetry in detail assuming the ion distributions to be of a generally Gaussian form. The effects of differing diffusion coefficients in the transverse and longitudinal directions are included

Mean first passage times for bond formation for a Brownian particle in linear shear flow above a wall

Motivated by cell adhesion in hydrodynamic flow, here we study bond formation between a spherical Brownian particle in linear shear flow carrying receptors for ligands covering the boundary wall. We derive the appropriate Langevin equation which includes multiplicative noise due to position-dependent mobility functions resulting from the Stokes equation. We present a numerical scheme which allows to simulate it with high accuracy for all model parameters, including shear rate and three parameters describing receptor geometry (distance, size and height of the receptor patches). In the case of homogeneous coating, the mean first passage time problem can be solved exactly. In the case of position-resolved receptor-ligand binding, we identify different scaling regimes and discuss their biological relevance.Comment: final version after minor revision

Generic Subsequence Matching Framework: Modularity, Flexibility, Efficiency

Subsequence matching has appeared to be an ideal approach for solving many problems related to the fields of data mining and similarity retrieval. It has been shown that almost any data class (audio, image, biometrics, signals) is or can be represented by some kind of time series or string of symbols, which can be seen as an input for various subsequence matching approaches. The variety of data types, specific tasks and their partial or full solutions is so wide that the choice, implementation and parametrization of a suitable solution for a given task might be complicated and time-consuming; a possibly fruitful combination of fragments from different research areas may not be obvious nor easy to realize. The leading authors of this field also mention the implementation bias that makes difficult a proper comparison of competing approaches. Therefore we present a new generic Subsequence Matching Framework (SMF) that tries to overcome the aforementioned problems by a uniform frame that simplifies and speeds up the design, development and evaluation of subsequence matching related systems. We identify several relatively separate subtasks solved differently over the literature and SMF enables to combine them in straightforward manner achieving new quality and efficiency. This framework can be used in many application domains and its components can be reused effectively. Its strictly modular architecture and openness enables also involvement of efficient solutions from different fields, for instance efficient metric-based indexes. This is an extended version of a paper published on DEXA 2012.Comment: This is an extended version of a paper published on DEXA 201

Defining the Field: Revisiting the ACA 1995 Definition of Communication Studies

This article deals with the problem of defining communication studies in higher education. In 1995, the Association for Communication Administration (ACA) convened a summer conference that produced a two-sentence definition of the field of communication. More than 100 conferees voted their unanimous approval of the definition, which was then disseminated nationally and used by communication scholar/teachers for a multiplicity of purposes. Given the potential utility of that definition and the expansion of communication studies since 1995, the present study surveyed ACA\u27s current members to determine whether they are aware the definition exists, how they have used it, and the extent to which they perceive it as representative of communication studies today. The results of that survey are reported in this article, which begins with a description of why and how this definition was originally developed. In a field as diverse and eclectic as communication, a need exists for some commonality of understanding about what constitutes the discipline\u27s subject matter. Such understanding, in the form of a definition, can serve two functions: it can provide a descriptor of the diversity, breadth, and depth of the field itself; and, it can be used to represent the discipline to an external audience, both inside and outside of academe, many of whom may still hold onto the notion that the field is committed only to the practice and study of speech making

The Expansion in Width for Domain Walls in Nematic Liquid Crystals in External Magnetic Field

The improved expansion in width is applied to curved domain walls in uniaxial nematic liquid crystals in external magnetic field. In the present paper we concentrate on the case of equal elastic constants. We obtain approximate form of the director field up to second order in magnetic coherence length.Comment: 18 pages, Latex 2.09, no figure