Non-interacting electrons and the metal-insulator transition in 2D with correlated impurities

While standard scaling arguments show that a system of non-interacting electrons in two dimensions and in the presence of uncorrelated disorder is insulating, in this work we discuss the case where inter-impurity correlations are included. We find that for point-like impurities and an infinite inter-impurity correlation length a mobility edge exists in 2D even if the individual impurity potentials are random. In the uncorrelated system we recover the scaling results, while in the intermediate regime for length scales comparable to the correlation length, the system behaves like a metal but with increasing fluctuations, before strong localization eventually takes over for length scales much larger than the correlation length. In the intermediate regime, the relevant length scale is not given by the elastic scattering length but by the inter-impurity correlation length, with important consequences for high mobility systems.Comment: 4 page

Populationsgenetische Differenzierung beim Eichenwickler (Tortrix viridana L.) und seiner Wirtspflanze (Quercus robur L.) anhand nukleärer Genmarker

In Western Europe pedunculate oak (Quercus robur L.) is the forest tree with the highest number of phytophagous insect species (Yela & Lawton 1997). One of these, the green oak leaf roller Tortrix viridana L. is an oligophagous herbivorous moth with a host range limited to the genus Quercus (Hunter 1990, Du Merle 1999). During outbreaks, T. viridana often leads to defoliation of oaks in spring. The abundance of T. viridana is subject to the population size fluctuations typical for herbivorous insects, where periods of small population sizes (latent periods) alternate with periods of high population sizes (outbreak) (e.g. Schütte 1957, Horstmann 1984). Apart from many experimental studies on population dynamics of the moth (e.g. Hunter 1990, Du Merle 1999, Ivashov & al. 2002) so far little attention has been paid to the genetic variation within the species as an important aspect of the genetics of this host-parasite interaction. Simchuk & al. (1999) found changes in the heterozygosity level of different isozyme loci during outbreaks in T. viridana and molecular markers for T. viridana have been developed for analyses of genetic variation within and among populations (Schroeder & Scholz 2005). But, investigations of genetic variation within and among populations of forest pest species are important to predict future pest outbreaks. So far the processes outbreaks based on are not entirely clarified, however it is known that migration plays a major role. Using molecular markers investigations of the genetic variation are possible and offer the opportunity to analyse distribution events. In this paper first results are presented concerning the genetic variation of the green oak leaf roller at three geographic scales: (1) among trees within a population, (2) among populations at a small spatial scale of about 150 km and (3) among populations at a broader geographic scale up to 3000 km. Furthermore results of the genetic variation of oaks at the small spatial scale are represented.Populationen des grünen Eichenwicklers (Tortrix viridana L., Lepidoptera: Tortricidae) und der Stieleiche (Quercus robur L., Fagaceae) wurden auf genetische Differenzierungen mit Hilfe von Kern-DNA-Markern untersucht. Insgesamt wurden 14 Eichenbestände in Nordrhein-Westfalen (NRW) und 7 weitere in verschiedenen Regionen Europas in die Untersuchung einbezogen. Weder für die Eiche noch für den Eichenwickler sind genetische Differenzierungen der Populationen auf einer kleinen geographischen Skala (etwa 150 km) nachweisbar, jedoch zeigen die Eichenpopulationen eine zunehmende genetische Distanz mit zunehmender geographischer Distanz. Für den Eichenwickler konnten innerhalb von Populationen in einem Radius von etwa 40 m Familienstrukturen ermittelt werden und auf einer kontinentalen Skala lassen sich die Ergebnisse für den Eichenwickler mit einer „isolation by distance“ erklären

Seeing Anderson Localization

Anderson localization was discovered 50 years ago to describe the propagation of electrons in the presence of disorder. The main prediction back then, was the existence of disorder induced localized states, which do not conduct electricity. Many years later it turns out, that the concept of Anderson localization is much more general and applies to almost any type of propagation in time or space, when more than one parameter is relevant (like phase and amplitude). Here we propose a new optical scheme to literally see Anderson localization by varying the optical wavelength or angle of incidence to tune between localized and delocalized states. The occurrence of Anderson localization in the propagation of light, in particular, has become the focus of tremendous interest due to the emergence of new optical technologies and media, such as low dimensional and disordered optical lattices. While several experiments have reported the measurement of Anderson localization of light, many of the observations remain controversial because the effects of absorption and localization have a similar signature, i.e., exponential decrease of the transmission with the system size. In this work, we discuss a system, where we can clearly differentiate between absorption and localization effects because this system is equivalent to a perfect filter, only in the absence of any absorption. Indeed, only one wavelength is perfectly transmitted and all others are fully localized. These results were obtained by developing a new theoretical framework for the average optical transmission through disordered media.Comment: 5 pages, 4 figure

Why Are Middle-Aged People so Depressed?: Evidence from West Germany

Does happiness vary with age? The evidence is inconclusive. Some studies show happiness to increase with age (Diener et al. 1999; Argyle 2001). Others hold that the association is U-shaped with either highest depression rates (Mroczek and Christian, 1998; Blanchflower and Oswald, 2008) or highest happiness levels occurring during middle age (Easterlin, 2006). Current studies suffer from two shortcomings. Firstly, they do not control for three confounding time variables: age, period and cohorteffects. Secondly, all empirical research lacks a theoretical explanation as to why age affects happiness. The purpose of our analysis is to contribute to closing both of these research gaps. A social investment model frames the dynamics of happiness across the life-span. The empirical test draws on West German panel data that followed individuals from 1984 to 2005. Descriptive analysis shows a cubic age function with the lowest level at middle age. However, hierarchical three-level variance component models (Rabe-Hesketh and Skrondal, 2005), find significant differences across pre-war and post-war cohorts, baby boomers and offspring of the baby bust as well as deviations during reunification. Yet, cohort and period effects account for less than 10% of the variance. (Un)happiness in midlife is more strongly determined by gender-specific occasional influences and individual characteristics. Both define objective and subjective returns of professional and personal life investments. Thesesocial investment decisions date back to early adulthood and bear a high risk of failure during midlife. Unforeseen consequences and long-term private and professional commitments make it costly to adjust, but at the same time new investments may pay off in a pro-longed future. This dilemma turns many middle-aged people into "frustrated achievers".Happiness, subjective well-being, gendered life-course, inequality, APC effects (Age-Period-Cohort effects), multi-level analysis, Germany

Why is health treatment for the elderly less expensive than for the rest of the population? Health care rationing in Germany

The consequences of population ageing for the health care system and health care costs may be less severe than is commonly assumed. Hospital discharge data from Germany’s largest health insurer (AOK) show that the care of patients during their last year of life is less costly if they die at an advanced age. As a multivariate analysis reveals, oldest old patients receive less costly treatment for the same illness than younger patients. Health care is informally rationed according to the age of the patient. The data also indicate that age-related rationing may be more pronounced in Germany than in the United States. (AUTHOR)