Spin-glass-like behavior of Ge:Mn

We present a detailed study of the magnetic properties of low-temperature-molecular-beam-epitaxy grown Ge:Mn dilute magnetic semiconductor films. We find strong indications for a frozen state of Ge_{1-x}Mn_{x}, with freezing temperatures of T_f=12K and T_f=15K for samples with x=0.04 and x=0.2, respectively, determined from the difference between field-cooled and zero-field-cooled magnetization. For Ge_{0.96}Mn_{0.04}, ac susceptibility measurements show a peak around T_f, with the peak position T'_f shifting as a function of the driving frequency f by Delta T_f' / [T_f' Delta log f] ~ 0.06, whereas for sample Ge_{0.8}Mn_{0.2} a more complicated behavior is observed. Furthermore, both samples exhibit relaxation effects of the magnetization after switching the magnitude of the external magnetic field below T_f which are in qualitative agreement with the field- and zero-field-cooled magnetization measurements. These findings consistently show that Ge:Mn exhibits a frozen magnetic state at low temperatures and that it is not a conventional ferromagnet.Comment: Revised version contains extended interpretation of experimental dat

Computational and mathematical approaches to societal transitions

After an introduction of the theoretical framework and concepts of transition studies, this article gives an overview of how structural change in social systems has been studied from various disciplinary perspectives. This overview first leads to the conclusion that computational and mathematical approaches and their practical form, modeling, up till now, have been almost absent in the research and theorizing of structural change or transitions in social systems. Second, this review of the social science literature suggests numerous theoretical constructs relevant for transition modeling. Relevant concepts include the conceptualization of the micro-to-macro link, the importance of explaining both stability and change, quantitative and qualitative definitions of structural change, the use of dichotomies, synchronic and diachronic reasoning in explaining structural change, definitions of basic patterns of social change, the conceptualization of resistance to change and intentional and normative aspects of social change. This article employs these theoretical concepts to describe and discuss the models presented in this special issue in order to develop an understanding of what exactly entails a computational or mathematical approach to societal transitions

Evolutionary policies for sustainable development: adaptive flexibility and risk minimising

An evolutionary perspective on policies to foster sustainable development is presented. It is argued that policies suggested by the traditional economic theory of environmental policy can stimulate unsustainable socio-economic structures and patterns. In addition, they are unable to remove undesired locked-in systems and technologies. Drawing on evolutionary thinking, characterised by diversity, selection, innovation, path-dependence and bounded rationality, an alternative, partly complementary theory of environmental policy is suggested. Specific attention is given to the role of strategies that are aimed at increasing diversity and adaptive flexibility, and at reducing risk. © 2003 Elsevier B.V. All rights reserved

Emerging migration flows in a changing climate in dryland Africa

Fears of the movement of large numbers of people as a result of changes in the environment were first voiced in the 1980s (ref. 1). Nearly thirty years later the numbers likely to migrate as a result of the impacts of climate change are still, at best, guesswork(2). Owing to the high prevalence of rainfed agriculture, many livelihoods in sub-Saharan African drylands are particularly vulnerable to changes in climate. One commonly adopted response strategy used by populations to deal with the resulting livelihood stress is migration. Here, we use an agent-based model developed around the theory of planned behaviour to explore how climate and demographic change, defined by the ENSEMBLES project(3) and the United Nations Statistics Division of the Department of Economic and Social Affairs(4), combine to influence migration within and from Burkina Faso. The emergent migration patterns modelled support framing the nexus of climate change and migration as a complex adaptive system(5). Using this conceptual framework, we show that the extent of climate-change-related migration is likely to be highly nonlinear and the extent of this nonlinearity is dependent on population growth; therefore supporting migration policy interventions based on both demographic and climate change adaptation

Soziooekonomie unter besonderer Beruecksichtigung des Tourismus in den Grossschutzgebieten Mecklenburg-Vorpommerns und ihren Randbereichen Projektbericht: Thematische Fallstudien

SIGLEAvailable from TIB Hannover: RN 8908(97-028,5,1) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekUmweltbundesamt, Berlin (Germany); Ministerium fuer Landwirtschaft und Naturschutz des Landes Mecklenburg-Vorpommern, Schwerin (Germany); Ministerium fuer Wirtschaft und Angelegenheiten der Europaeischen Union des Landes Mecklenburg-Vorpommern, Schwerin (Germany)DEGerman