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Finite Size Effects (finite + size_effects)
Selected AbstractsPolarizability, Susceptibility, and Dielectric Constant of Nanometer-Scale Molecular Films: A Microscopic ViewADVANCED FUNCTIONAL MATERIALS, Issue 13 2010Amir Natan Abstract The size-dependence of the polarizability, susceptibility, and dielectric constant of nanometer-scale molecular layers is explored theoretically. First-principles calculations based on density functional theory are compared to phenomenological modeling based on polarizable dipolar arrays for a model system of organized monolayers composed of oligophenyl chains. Size trends for all three quantities are primarily governed by a competition between out-of-plane polarization enhancement and in-plane polarization suppression. Molecular packing density is the single most important factor controlling this competition and it strongly affects the bulk limit of the dielectric constant as well as the rate at which it is approached. Finally, the polarization does not reach its "bulk" limit, as determined from the Clausius,Mossotti model, but the susceptibility and dielectric constant do converge to the correct bulk limit. However, whereas the Clausius,Mossotti model describes the dielectric constant well at low lateral densities, finite size effects of the monomer units cause it to be increasingly inaccurate at high lateral densities. [source] Corrections for surface X-ray diffraction measurements using the Z -axis geometry: finite size effects in direct and reciprocal spaceJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2000O. Robach X-ray diffraction data have to be corrected by geometrical correction factors prior to any quantitative analysis. Here the case of grazing incidence X-ray diffraction measurements is considered, including the case of high exit angles. First, an approach taking into account the evolution of the diffracting area during an , scan is presented. From the calculation of the effective part of the sample surface that participates in the diffraction phenomena at each step of the scan, a more accurate correction factor than those commonly used is derived and the evolution of the line shape along a zero-width rod is explained. Secondly, the case of finite-width rods, under the point-like sample approximation, is considered: the influence of the partial integration, as a result of the detector in-plane acceptance, of a rod with an anisotropic in-plane shape, is studied and leads to an analytical expression for the corresponding correction factor. Finally, a full numerical simulation is presented, which provides an alternative method for correcting the experimental intensities and shows in which conditions the previous formulae are no longer valid. [source] Self-organization and finite size effects in epitaxial ferromagnetic MnAs films on GaAsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 8 2007L. Däweritz Abstract The self-organized striped structure of coexisting ,-MnAs and ,-MnAs, characteristic for MnAs films grown on GaAs(001) by molecular beam epitaxy under As-rich conditions at low temperature, has been studied in samples prepared under conditions closer to the equilibrium. Whereas the period of the stripe pattern is independent on the preparation procedure, the width of the ferromagnetic ,-MnAs stripes increases. Thus, the aspect ratio p between the stripe width L and the stripe thickness t can be varied over a wide range. The magnetic properties of the finite-size magnetic system are investigated as a function of the ratio p at room temperature. The transition from type-I domains with meander-like contrast in the magnetic force microscopy (MFM) image to type-II domains with a line-shaped contrast due to the division of the stripe into N subdomains across the stripe occurs at p ratios expected from a model based on the shape anisotropy. Besides the well-known N = 3 type-II domains also type-II domains with N = 4, 5 were detected. When the stripe width approaches the period of the self-organized structure, the boundary between two neighboring stripes is imaged as chessboard-like contrast in the MFM pattern. The shape of the magnetic hysteresis loops changes with the p ratio or, in other words, with the predominance of in-plane or out-of plane magnetic moments in the demagnetized state. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Conductance of inhomogeneous systems: Real-time dynamicsANNALEN DER PHYSIK, Issue 9 2010A. Branschädel Numerical time evolution of transport states using time dependent Density Matrix Renormalization Group (td-DMRG) methods has turned out to be a powerful tool to calculate the linear and finite bias conductance of interacting impurity systems coupled to non-interacting one-dimensional leads. Several models, including the Interacting Resonant Level Model (IRLM), the Single Impurity Anderson Model (SIAM), as well as models with different multi site structures, have been subject of investigations in this context. In this work we give an overview of the different numerical approaches that have been successfully applied to the problem and go into considerable detail when we comment on the techniques that have been used to obtain the full I,V-characteristics for the IRLM. Using a model of spinless fermions consisting of an extended interacting nanostructure attached to non-interacting leads, we explain the method we use to obtain the current,voltage characteristics and discuss the finite size effects that have to be taken into account. We report results for the linear and finite bias conductance through a seven site structure with weak and strong nearest-neighbor interactions. Comparison with exact diagonalisation results in the non-interacting limit serve as a verification of the accuracy of our approach. Finally we discuss the possibility of effectively enlarging the finite system by applying damped boundaries and give an estimate of the effective system size and accuracy that can be expected in this case. [source] | ||||||||||