Critical dynamics in thin films

Critical dynamics in film geometry is analyzed within the field-theoretical approach. In particular we consider the case of purely relaxational dynamics (Model A) and Dirichlet boundary conditions, corresponding to the so-called ordinary surface universality class on both confining boundaries. The general scaling properties for the linear response and correlation functions and for dynamic Casimir forces are discussed. Within the Gaussian approximation we determine the analytic expressions for the associated universal scaling functions and study quantitatively in detail their qualitative features as well as their various limiting behaviors close to the bulk critical point. In addition we consider the effects of time-dependent fields on the fluctuation-induced dynamic Casimir force and determine analytically the corresponding universal scaling functions and their asymptotic behaviors for two specific instances of instantaneous perturbations. The universal aspects of nonlinear relaxation from an initially ordered state are also discussed emphasizing the different crossovers that occur during this evolution. The model considered is relevant to the critical dynamics of actual uniaxial ferromagnetic films with symmetry-preserving conditions at the confining surfaces and for Monte Carlo simulations of spin system with Glauber dynamics and free boundary conditions.Comment: 64 pages, 21 figure

Untersuchung der Rotation des NH4-Ions im Ammoniumchlorid mittels hochauflösender Neutronenspektroskopie

Mit Hilfe inkohärenter quasielastischer Neutronenstreuung wurde am Ordnungs-Unordnungs-Phasenübergang des Ammoniumchlorid das dynamische Verhalten der NH$_{4}$-Tetraeder in der Nähe derÜUbergangstemperatur T$_{c} \approx$ -30,5$^\circ$C untersucht. Die Ergebnisse sind inter-pretierbar mit einern Modell, bei dem die Tetraeder bevorzugt Drehsprünge um die 3- bzw. 4-zähligen Symmetrieachsen des Systems ausführen, wobei im Falle der 4-zähligen Sprünge eine starke Diskontinuität der Sprungwahrscheinlichkeit bei T$_{c}$ auftritt. In Zusammenhang mit dieser Messung wurde mit Hilfe des Bragg-Reflexes, dessen Intensität von der Ordnung der NH$_{4}$-Ionen abhängt, der Ordnungsparameter bestimmt

Nonlocal anomalous Hall effect in ternary alloys based on noble metals

We present a theoretical study of the nonlocal anomalous Hall effect induced by heavy-metal impurities in dilute magnetic alloys based on noble metals. The results of our first-principles calculations are shown in comparison to those obtained within a model consideration via Matthiessen’s rule. Based on the transport properties of the constituent binary alloys, we reveal optimal host-impurity combinations to enhance the phenomenon. In particular, this allows us to explain experimental findings showing a strong effect in Cu-based alloys but a vanishing effect in the case of the Au host