Spin-polarized Quantum Transport in Mesoscopic Conductors: Computational Concepts and Physical Phenomena

Mesoscopic conductors are electronic systems of sizes in between nano- and micrometers, and often of reduced dimensionality. In the phase-coherent regime at low temperatures, the conductance of these devices is governed by quantum interference effects, such as the Aharonov-Bohm effect and conductance fluctuations as prominent examples. While first measurements of quantum charge transport date back to the 1980s, spin phenomena in mesoscopic transport have moved only recently into the focus of attention, as one branch of the field of spintronics. The interplay between quantum coherence with confinement-, disorder- or interaction-effects gives rise to a variety of unexpected spin phenomena in mesoscopic conductors and allows moreover to control and engineer the spin of the charge carriers: spin interference is often the basis for spin-valves, -filters, -switches or -pumps. Their underlying mechanisms may gain relevance on the way to possible future semiconductor-based spin devices. A quantitative theoretical understanding of spin-dependent mesoscopic transport calls for developing efficient and flexible numerical algorithms, including matrix-reordering techniques within Green function approaches, which we will explain, review and employ.Comment: To appear in the Encyclopedia of Complexity and System Scienc

Robust Helical Edge Transport in Quantum Spin Hall Quantum Wells

We show that burying of the Dirac point in semiconductor-based quantum-spin-Hall systems can generate unexpected robustness of edge states to magnetic fields. A detailed ${\bf k\cdot p}$ band-structure analysis reveals that InAs/GaSb and HgTe/CdTe quantum wells exhibit such buried Dirac points. By simulating transport in a disordered system described within an effective model, we further demonstrate that buried Dirac points yield nearly quantized edge conduction out to large magnetic fields, consistent with recent experiments.Comment: 11 pages, 6 figure

A toolbox for animal call recognition

Monitoring the natural environment is increasingly important as habit degradation and climate change reduce theworld’s biodiversity.We have developed software tools and applications to assist ecologists with the collection and analysis of acoustic data at large spatial and temporal scales.One of our key objectives is automated animal call recognition, and our approach has three novel attributes. First, we work with raw environmental audio, contaminated by noise and artefacts and containing calls that vary greatly in volume depending on the animal’s proximity to the microphone. Second, initial experimentation suggested that no single recognizer could dealwith the enormous variety of calls. Therefore, we developed a toolbox of generic recognizers to extract invariant features for each call type. Third, many species are cryptic and offer little data with which to train a recognizer. Many popular machine learning methods require large volumes of training and validation data and considerable time and expertise to prepare. Consequently we adopt bootstrap techniques that can be initiated with little data and refined subsequently. In this paper, we describe our recognition tools and present results for real ecological problems

Low-Income Demand for Local Telephone Service: Effects of Lifeline and Linkup

This study evaluates the effect of the “Lifeline” and “Linkup” subsidy programs on telephone penetration rates of low-income households. It is the first to estimate low-income telephone demand across demographic groups using location-specific Lifeline and Linkup prices. The demand specifications use a discrete choice model aggregated across demographic groups. GMM estimators correct for the possible endogeneity of subsidized prices. A simulation predicts low-income telephone penetration would be 4.1 percentage points lower without Lifeline and Linkup. Results suggest that Linkup is more cost-effective than Lifeline, and that automatic enrollment in the programs increases penetration.telephone subsidies, low-income telephone usuers

A friendly conquest: German libraries after the fall of the Berlin Wall in 1989

This paper provides an overview of the development of libraries in the geographical area of the former German Democratic Republic (GDR) after it joined the territory of the Federal Republic of Germany (FRG) in 1990. It briefly describes the situation of libraries in the GDR and the major changes that accompanied the unification process. It also touches on a series of three nationwide studies on reading and library-user behavior, and on library legislation and major national-planning initiatives since 1989. For academic libraries, the unification process was mainly favorable, as a structured plan and continuous funding were introduced as part of higher education development. For public libraries, the process was less structured, severely reducing a previously very dense system within a very short time. Recent library statistics indicate, however, that the integration of the two library systems has benefited the remaining libraries and left no clearly visible difference between library systems in the eastern and western parts of Germany.published or submitted for publicatio

An elliptic expansion of the potential field source surface model

Context. The potential field source surface model is frequently used as a basis for further scientific investigations where a comprehensive coronal magnetic field is of importance. Its parameters, especially the position and shape of the source surface, are crucial for the interpretation of the state of the interplanetary medium. Improvements have been suggested that introduce one or more additional free parameters to the model, for example, the current sheet source surface (CSSS) model. Aims. Relaxing the spherical constraint of the source surface and allowing it to be elliptical gives modelers the option of deforming it to more accurately match the physical environment of the specific period or location to be analyzed. Methods. A numerical solver is presented that solves Laplace's equation on a three-dimensional grid using finite differences. The solver is capable of working on structured spherical grids that can be deformed to create elliptical source surfaces. Results. The configurations of the coronal magnetic field are presented using this new solver. Three-dimensional renderings are complemented by Carrington-like synoptic maps of the magnetic configuration at different heights in the solar corona. Differences in the magnetic configuration computed by the spherical and elliptical models are illustrated.Comment: 11 pages, 7 figure

Optimal block-tridiagonalization of matrices for coherent charge transport

Numerical quantum transport calculations are commonly based on a tight-binding formulation. A wide class of quantum transport algorithms requires the tight-binding Hamiltonian to be in the form of a block-tridiagonal matrix. Here, we develop a matrix reordering algorithm based on graph partitioning techniques that yields the optimal block-tridiagonal form for quantum transport. The reordered Hamiltonian can lead to significant performance gains in transport calculations, and allows to apply conventional two-terminal algorithms to arbitrary complex geometries, including multi-terminal structures. The block-tridiagonalization algorithm can thus be the foundation for a generic quantum transport code, applicable to arbitrary tight-binding systems. We demonstrate the power of this approach by applying the block-tridiagonalization algorithm together with the recursive Green's function algorithm to various examples of mesoscopic transport in two-dimensional electron gases in semiconductors and graphene.Comment: 28 pages, 14 figures; submitted to Journal of Computational Physic

Online Product Descriptions – Boost for your Sales?

Product descriptions are a source of information online consumers can use to reduce product uncertainty. Recent research provides evidence that consumers favor using information from other consumers, such as customer reviews, over retailer or manufacturer provided information, such as product descriptions. We complement this research and show that the presence of product descriptions significantly influences products’ sales and that this influence decreases with an increasing number of customer reviews. We furthermore demonstrate that a product description’s information amount positively affects a product\u27s sales. The number of customer reviews available for a product also moderates the effect of the information amount of a product description on sales

Analysis of Interactive Editing Operations for Out-of-Core Point-Cloud Hierarchies

In this paper we compare the time and space complexity of editing operations on two data structures which are suitable for visualizing huge point clouds. The first data structure was introduced by Scheiblauer andWimmer [SW11] and uses only the original points from a source data set for building a level-of-detail hierarchy that can be used for rendering points clouds. The second data structure introduced byWand et al. [WBB+07] requires additional points for the level-of-detail hierarchy and therefore needs more memory when stored on disk. Both data structures are based on an octree hierarchy and allow for deleting and inserting points. Besides analyzing and comparing these two data structures we also introduce an improvement to the points deleting algorithm for the data structure of Wand et al. [WBB+07], which thus allows for a more efficient node loading strategy during rendering