Driving XXZ spin chains: Magnetic-field and boundary effects

Using the time-evolving block decimation, we study the spin transport through spin-1/2 and spin-1 XXZ chains subjected to an external magnetic field and contacted to noninteracting fermionic leads. For generic system-lead couplings, the spin conductance exhibits several resonances as a function of the magnetic-field strength. In the spin-1/2 but not the spin-1 case, the coupling to the leads can be fine-tuned to reach a conducting fixed point, where the peak structure is washed out and the spin conductance is large throughout the gapless Luttinger-liquid phase. For the Haldane phase of the spin-1 chain, we analyse how the spin transport is affected by spin-1/2 edge states, and argue that two-impurity Kondo physics is realised.Comment: 7 pages, 8 figures, revised versio

Nonlinear dynamics of river runoff elucidated by horizontal visibility graphs

Horizontal Visibility Graphs (HVGs) are a recently developed method to construct networks from time series. The values of the time series are considered as the nodes of the network and are linked to each other if there is no larger value between them, such as they can “see” each other. The network properties reflect the nonlinear dynamics of the time series. For some classes of stochastic processes and for periodic time series, analytical results can be obtained for network-derived quantities such as the degree distribution, the local clustering coefficient distribution, the mean path length, and others. HVGs have the potential to discern between deterministic-chaotic and correlated-stochastic time series. Here, we investigate the sensitivity of the HVG methodology to properties and pre-processing of real-world data, i.e., time series length, the presence of ties, and deseasonalization, using a set of around 150 runoff time series from managed rivers at daily resolution from Brazil with an average length of 65 years. We show that an application of HVGs on real-world time series requires a careful consideration of data pre-processing steps and analysis methodology before robust results and interpretations can be obtained. For example, one recent analysis of the degree distribution of runoff records reported pronounced sub-exponential “long-tailed” behavior of North American rivers, whereas another study of South American rivers showed hyper-exponential “short-tailed” behavior resembling correlated noise. We demonstrate, using the dataset of Brazilian rivers, that these apparently contradictory results can be reconciled by minor differences in data-preprocessing (here: small differences in subtracting the seasonal cycle). Hence, data-preprocessing that is conventional in hydrology (“deseasonalization”) changes long-term correlations and the overall runoff dynamics substantially, and we present empirical consequences and extensive simulations to investigate these issues from a HVG methodological perspective. After carefully accounting for these methodological aspects, the HVG analysis reveals that the river runoff dataset shows indeed complex behavior that appears to stem from a superposition of short-term correlated noise and “long-tailed behaviour,” i.e., highly connected nodes. Moreover, the construction of a dam along a river tends to increase short-term correlations in runoff series. In summary, the present study illustrates the (often substantial) effects of methodological and data-preprocessing choices for the interpretation of river runoff dynamics in the HVG framework and its general applicability for real-world time series.Fil: Lange, Holger. Norwegian Institute of Bioeconomy Research; NoruegaFil: Sippel, Sebastian. Norwegian Institute of Bioeconomy Research; NoruegaFil: Rosso, Osvaldo Aníbal. Instituto Universidad Escuela de Medicina del Hospital Italiano; Argentina. Universidad de Los Andes; Chile. Universidade Federal de Alagoas; Brasil. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Social background, civic education and political participation of young people - The german case

Due to social and political change the process of young citizens' political socialization was put on a new basis in West European democracies over the last decades. In this article we discuss some aspects of this development and show their consequences. We analyse empirical findings from Germany, focussing on the relevant social factors which influence the individual propensity to participate in politics. The impact of the financial and economic crisis in Europe on political attitudes will also be considered, taking in account sociological aspects. Based on the empirical findings we discuss implications for civic education. In contrast to many discussions in literature about this issue, in which the focus is on the need to put the various influences of political socialization into a broader context, we argue that the parental social background is the crucial upstream factor, prior to e.g. civic education. The conclusion indicates that a group-specific educational approach, taking into account the social background, is the most promising one for reaching the normative goal of civic education: Politically self-determined citizens

Growth of surface undulations at the Rosensweig instability

We investigate the growth of a pattern of liquid crests emerging in a layer of magnetic liquid when subjected to a magnetic field oriented normally to the fluid surface. After a steplike increase of the magnetic field, the temporal evolution of the pattern amplitude is measured by means of a Hall-sensor array. The extracted growth rate is compared with predictions from linear stability analysis by taking into account the proper nonlinear magnetization curve M(H). The remaining discrepancy can be resolved by numerical calculations via the finite-element method. By starting with a finite surface perturbation, it can reproduce the temporal evolution of the pattern amplitude and the growth rate. The investigations are performed for two magnetic liquids, one with low and one with high viscosity.Comment: 12 pages, 12 figure