No description available.

The objective of this study was to analyze patterns, dynamics and processes of land-use/cover changes in the transboundary Mara River Basin in East Africa. We specifically focused on deforestation and expansion of agriculture in the watershed. The intensity analysis approach was used to analyze data from satellite imagery-derived land-use/cover maps. Results indicate that swap change accounted for more than 50% of the overall change, which shows a very dynamic landscape transformation. Transition from closed forest to open forest was found to be a dominant landscape change, as opposed to a random change. Similarly, transition from open forest to small-scale agriculture was also found to be a dominant transition. This suggests a trend (pathway) of deforestation from closed forest to small-scale agriculture, with open forest as a transitional land cover. The observed deforestation may be attributed to continuous encroachment and a series of excisions of the forest reserve. Transition from rangeland to mechanized agriculture was found to be a dominant land-use change, which was attributed to change in land tenure. These findings are crucial for designing strategies and integrated watershed management policies to arrest further deforestation in the forest reserves as well as to sustainably control expansion of agriculture.

No description available.

The objective of this study was to analyze patterns, dynamics and processes of land-use/cover changes in the transboundary Mara River Basin in East Africa. We specifically focused on deforestation and expansion of agriculture in the watershed. The intensity analysis approach was used to analyze data from satellite imagery-derived land-use/cover maps. Results indicate that swap change accounted for more than 50% of the overall change, which shows a very dynamic landscape transformation. Transition from closed forest to open forest was found to be a dominant landscape change, as opposed to a random change. Similarly, transition from open forest to small-scale agriculture was also found to be a dominant transition. This suggests a trend (pathway) of deforestation from closed forest to small-scale agriculture, with open forest as a transitional land cover. The observed deforestation may be attributed to continuous encroachment and a series of excisions of the forest reserve. Transition from rangeland to mechanized agriculture was found to be a dominant land-use change, which was attributed to change in land tenure. These findings are crucial for designing strategies and integrated watershed management policies to arrest further deforestation in the forest reserves as well as to sustainably control expansion of agriculture.

The influence of disorder is an important area in materials research. A supersaturated solid solution is in a metastable state, in which the host lattice is forced to solve guest atoms over the thermodynamical stability limit. The guest atoms are point defects, which amount can be varied by changing the concentration. The increasing amount of point defects makes the lattice unstable against schare, which may ends up in an order-disorder transition. In this work especially the lattice dynamics was studied at the atomic level by means of inelastic neutron scattering. In case of both studied systems (ZrAl, CuFe) a softening of especially the tranverse phonons could be shown, which supports the existing theory. / Der Einfluß von Unordnung ist ein wichtiges Forschungsgebiet der Materialwissenschaft. Bei übersättigten festen Lösungen handelt es sich um einen metastabilen Zustand, bei dem das Wirtsgitter gezwungen ist, mehr Fremdatome aufzunehmen, als thermodynamisch stabil ist. Die Fremdatome im Wirtsgitter sind Punktdefekte, deren Menge mit der Konzentration variiert werden kann. Die Zunahme substitutionaler Defekte bewirkt eine Gitterinstabilität gegenüber Scherkräften und anschliessend möglicherweise einen Ordnungs - Unordnungs - Übergang. In dieser Arbeit wurden vor allem dynamische Aspekte betrachtet: die Gitterdynamik wurde auf atomarer Ebene mittels inelastischer Neutronenstreuung untersucht. Bei beiden untersuchten Systemen (ZrAl, CuFe) konnte ein Weichwerden (softening) von Phononen beobachtet werden, insbesondere von transversalen Moden, die einen direkten Nachweis der oben erwähnten Gitterinstabilität ergeben.

The orientation changes of perpendicular cylindrical microdomains in polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) thin films upon annealing in different solvent vapors were investigated by in situ grazing incidence small-angle X-ray scattering (GISAXS) and ex situ scanning force microscopy (SFM). The swelling of P4VP perpendicular cylinders (C⊥) in chloroform, a non-selective solvent vapor, leads to the reorientation to in-plane cylinders through a disordered state in a particular kinetic pathway in the phase diagram upon drying. On the other hand, the swelling of the P4VP perpendicular cylinders in a selective solvent vapor (i.e., 1,4-dioxane) induces a morphological transition from cylindrical to ellipsoidal as a transient structure to spherical microdomains; subsequent solvent evaporation resulted in shrinkage of the matrix in the vertical direction, merging the ellipsoidal domains into the perpendicularly aligned cylinders. In this paper, we have discussed the mechanism based on the selectivity of the solvent to the constituting blocks that is mainly responsible for the orientation changes.

Background: Early substance use (SU) in adolescence is known to be associated with an elevated risk of developing substance use disorders (SUD); it remains unclear though whether early SU is associated with more rapid transitions to SUD. Objective: To examine the risk and speed of transition from first SU (alcohol, nicotine, cannabis) to SUD as a function of age of first use. Methods: N = 3021 community subjects aged 14–24 years at baseline were followed-up prospectively over 10-years. SU and SUD were assessed using the DSM-IV/M-CIDI. Results: (1) The conditional probability of substance-specific SU-SUD transition was the greatest for nicotine (36.0%) and the least for cannabis (18.3% for abuse, 6.2% for dependence) with alcohol in between (25.3% for abuse; 11.2% for dependence). (2) In addition to confirming early SU as a risk factor for SUD we find: (3) higher age of onset of any SU to be associated with faster transitions to SUD, except for cannabis dependence. (4) Transitions from first cannabis use (CU) to cannabis use disorders (CUD) occurred faster than for alcohol and nicotine. (5) Use of other substances co-occurred with risk and speed of transitions to specific SUDs. Conclusion: Type of substance and concurrent use of other drugs are of importance for the association between age of first use and the speed of transitions to substance use disorders. Given that further research will identify moderators and mediators affecting these differential associations, these findings may have important implications for designing early and targeted interventions to prevent disorder progression.

This contribution discusses scaling aspects of individually gated nanowire Schottky junctions which are essential parts of reconfigurable field effect transistors (RFETs). The applicability of the screening (or natural) length theory in relation to the carrier transport is discussed first. Various geometrical parameters of the device were investigated to find the optimal structure in terms of performance. For this purpose, electrostatic properties and the dynamic behavior of the RFET were studied. Finally the increase in performance due to an additional substitution of the silicon by germanium is analyzed.

This article presents a learning portfolio written during the Basic Teaching Qualification in Higher Education at the Centre for Teaching and Learning in Higher Education Saxony (HDS). This document provides insights into my personal learning process and contains reflections on my teaching practice and how it was influenced by attending various didactics-related courses of the HDS. Hence, it is a compilation of material and information from these courses as well as of results of exercises. My individual comments and insights show how I adapted, inter-connected, and applied the contents and experiences of these courses to my teaching. I also discuss challenges in applying some guidelines and ideas for future application of didactic methods and theories.

The vicinity of continuous quantum phase transitions displays unique properties such as scaling behavior and incoherent excitation spectra which are not found in any stable phase of matter. This fascinating quantum critical regime is crucial for progress on key problems of modern condensed matter physics. The three research projects of this thesis challenge and refine our understanding of quantum criticality in different ways. Part I concerns unexpected quantum critical behavior near the Mott transition. The bandwidth-controlled Mott transition in the half-filled one-band Hubbard model is one of the most paradigmatic phenomena of strongly correlated physics. Within the approximation of dynamical mean-field theory (DMFT) this metal-insulator transition is of first order at low temperatures, with the transition line ending at a critical temperature. Surprisingly, numerical calculations with DMFT and experiments in organic salts consistently found quantum critical scaling of the resistivity above the critical temperature. The aim of this project is to explain this unexpected scaling in the absence of a quantum critical point in the phase diagram. To this end, we perform extensive DMFT simulations with the numerical renormalization group as a state-of-the-art impurity solver. We find that the quantum critical scaling can be traced back to the metastable insulator at the boundary of the coexistence region at T = 0 which exhibits previously unknown scale-invariance on the frequency axis. In Part II we study how magnetic quantum criticality is affected by the coupling to additional non-critical degrees of freedom. Considering typical electronic energy scales the study of quantum critical phenomena in magnets requires very low temperatures in the sub-100mK range. In this regime additional effects which are typically neglected in the theoretical modeling may become important. Here we focus on one particular example, which is the hyperfine coupling to nuclear spins. We investigate the fate of the quantum critical behavior at lowest temperatures and determine crossover scales below which a purely electronic description is no longer sufficient. Explicit calculations for paradigmatic models on the level of mean-field theory plus Gaussian fluctuations reveal that the quantum phase transition can be shifted or smeared in the presence of nuclear spins. More exotic effects of nuclear spins, e.g. in spin liquids, are discussed on a qualitative level. Part III is devoted to the discussion of domain-driven phase transitions in easy-axis ferromagnets.This work is motivated by an experimental study of LiHoF4, a dipolar easy-axis ferromagnet that displays a well-studied quantum phase transition from a ferromagnetic to a paramagnetic phase as function of a transverse field. Measurements of the ac susceptibility found a well-defined phase transition even in tilted fields where the Ising symmetry is explicitly broken and Landau theory of the microscopic order parameter predicts a crossover. We are able to explain and model the transition in tilted fields by the inclusion of domain effects, i.e., by taking into account the spontaneous breaking of translational symmetry by mesoscale pattern formation in the ferromagnetic phase. The modeling of stray-field energies as effective antiferromagnetic couplings between magnetization components in different domains is in excellent quantitative agreement with the experimental results.:1 Phases and their transitions . . . . . . . . . . . . . . . . . . . . 4 1.1 Thermal and quantum phase transitions . . . . . . . . . . . . . . . . . . . . 4 1.2 Theoretical description of phase transitions . . . . . . . . . . . . . . . . . . 8 1.3 Project overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 I Mott quantum criticality in the one-band Hubbard model . . . . . . . . . . .15 2 Introduction to the Mott transition . . . . . . . . . . . . . . . . . . . . 16 2.1 Metal-insulator transitions and the Hubbard model . . . . . . . . . . . . . . 16 2.2 A local perspective: the idea of dynamical mean-field theory . . . . . . . . . 19 2.3 Quantum critical scaling near the Mott transition . . . . . . . . . . . . . . . 21 3 Dynamical mean-field theory (DMFT) . . . . . . . . . . . . . . . . . . . . 25 3.1 Single-impurity Anderson model . . . . . . . . . . . . . . . . . . . . . . . . 25 3.2 Theoretical foundations of DMFT . . . . . . . . . . . . . . . . . . . . . . . 28 3.3 Wilson's numerical renormalization group . . . . . . . . . . . . . . . . . . . 32 3.4 Implementation and choice of parameters . . . . . . . . . . . . . . . . . . . 36 4 Power-law spectra and quantum critical scaling . . . . . . . . . . . . . . . . . . 38 4.1 Scale-invariant solutions of DMFT . . . . . . . . . . . . . . . . . . . . . . . 38 4.2 Spectral power laws at T=0 in the metastable insulator . . . . . . . . . . . 40 4.3 Finite-temperature crossovers in the spectral function . . . . . . . . . . . . 47 4.4 Resistivity scaling driven by spectral power laws . . . . . . . . . . . . . . . 50 4.5 Scaling analysis of the dynamic susceptibility . . . . . . . . . . . . . . . . . 58 4.6 Ideas and obstacles towards an analytical understanding . . . . . . . . . . . 62 4.7 Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 II Limits on magnetic quantum criticality from nuclear spins . . . . . . . . . . . . .65 5 Stability of magnetic transitions to hyperfine coupling . . . . . . . . . . . . . . . .66 5.1 Nuclear spins near quantum criticality . . . . . . . . . . . . . . . . . . . . . 66 5.2 Introduction to nuclear spins and hyperfine coupling . . . . . . . . . . . . . 67 5.3 Magnetic phases in the presence of nuclear spins . . . . . . . . . . . . . . . 69 5.4 Two scenarios for magnetic quantum criticality plus nuclear spins . . . . . . 70 6 Paradigmatic models for magnetic quantum phase transitions . . . . . . . . . 73 6.1 Transverse-field Ising model . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 6.2 Coupled-dimer model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 6.3 Frustrated spin models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 7 Crossover scales introduced by nuclear spins . . . . . . . . . . . . . . . . . . .83 7.1 Shifted transitions: transverse-field Ising magnets . . . . . . . . . . . . . . . 83 7.2 Smeared transitions: coupled-dimer magnets . . . . . . . . . . . . . . . . . . 90 7.3 Additional transitions due to nuclear spins . . . . . . . . . . . . . . . . . . . 98 7.4 Exotic magnetic quantum phase transitions plus nuclear spins . . . . . . . . 101 7.5 Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 III Domain-driven phase transitions in easy-axis ferromagnets . . . . . . . . 105 8 Easy-axis ferromagnet LiHoF4 . . . . . . . . . . . . . . . . . . . . 106 8.1 Easy-axis ferromagnets in tilted fields . . . . . . . . . . . . . . . . . . . . . 106 8.2 LiHoF4 and its phase transitions . . . . . . . . . . . . . . . . . . . . . . . . 109 9 Modeling of microscopic degrees of freedom in LiHoF4 . . . . . . . . . . . . 112 9.1 Landau theory in tilted fields . . . . . . . . . . . . . . . . . . . . . . . . . . 112 9.2 Crystal field effects and microscopic Hamiltonian . . . . . . . . . . . . . . . 113 9.3 Crossovers in the microscopic model . . . . . . . . . . . . . . . . . . . . . . 118 10 Modeling of mesoscopic degrees of freedom in LiHoF4 . . . . . . . . . . . . . . .123 10.1 Domains in ferromagnets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 10.2 Modeling of domain effects as effective interactions . . . . . . . . . . . . . . 127 10.3 Combined mean-field Hamiltonian and domain optimization . . . . . . . . . 130 10.4 Nature of the phase transition in tilted fields . . . . . . . . . . . . . . . . . 132 10.5 Domain-driven phase transition at T = 0 . . . . . . . . . . . . . . . . . . . . 135 10.6 Domain-driven phase transition at finite temperatures . . . . . . . . . . . . 141 10.7 Comparison with experimental results . . . . . . . . . . . . . . . . . . . . . 146 10.8 Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 IV Summary & Outlook . . . . . . . . . . . . . . . . . . . . 151 V Appendices . . . . . . . . . . . . . . . . . . . . 155 A Part I: NRG level spectra . . . . . . . . . . . . . . . . . . . . 156 B Part I: Analytical properties of scale-invariant DMFT solutions . . . . . . . . . . .159 B.1 Kondo perturbation theory as an impurity solver . . . . . . . . . . . . . . . 159 B.2 Analytical properties of a power-law self-energy . . . . . . . . . . . . . . . . 166 C Part I: Scaling analysis of the resistivity . . . . . . . . . . . . . . . . . . 168 D Part II: Solution of the transverse-field Ising model with nuclear spins . . . . . . 172 D.1 Holstein-Primakoff representation of the electronic and nuclear spins . . . . 172 D.2 Determination of the classical reference state . . . . . . . . . . . . . . . . . 174 D.3 Excitation spectrum of the coupled nuclear-electronic model . . . . . . . . . 175 D.4 Magnetization, susceptibility, and heat capacity . . . . . . . . . . . . . . . . 177 E Part II: Solution of the coupled-dimer model with nuclear spins . . . . . . . . . . . 181 E.1 Bond-operator description of the electronic spins . . . . . . . . . . . . . . . 181 E.2 Determination to the electronic ground state . . . . . . . . . . . . . . . . . 185 E.3 Holstein-Primakoff representation of the nuclear spins . . . . . . . . . . . . 188 E.4 Excitation spectrum of the coupled nuclear-electronic model . . . . . . . . . 189 E.5 Staggered magnetization and susceptibility . . . . . . . . . . . . . . . . . . 192 F Part III: Calculation of domain-induced effective interactions . . . . . . . . . . . . . 198 Bibliography . . . . . . . . . . . . . . . . . . . . 203