Primary structural dynamics in graphite

The structural dynamics of graphite and graphene are unique, because of the selective coupling between electron and lattice motions and hence the limit on electric and electro-optic properties. Here, we report on the femtosecond probing of graphite films (1–3 nm) using ultrafast electron crystallography in the transmission mode. Two time scales are observed for the dynamics: a 700 fs initial decrease in diffraction intensity due to lattice phonons in optically dark regions of the Brillouin zone, followed by a 12 ps decrease due to phonon thermalization near the Г and K regions. These results indicate the non-equilibrium distortion of the unit cells at early time and the subsequent role of long-wavelength atomic motions in the thermalization process. Theory and experiment are now in agreement regarding the nature of nuclear motions, but the results suggest that potential change plays a role in the lateral dynamics of the lattice

Structural dynamics of surfaces by ultrafast electron crystallography: Experimental and multiple scattering theory

Recent studies in ultrafast electron crystallography (UEC) using a reflection diffraction geometry have enabled the investigation of a wide range of phenomena on the femtosecond and picosecond time scales. In all these studies, the analysis of the diffraction patterns and their temporal change after excitation was performed within the kinematical scattering theory. In this contribution, we address the question, to what extent dynamical scattering effects have to be included in order to obtain quantitative information about structural dynamics. We discuss different scattering regimes and provide diffraction maps that describe all essential features of scatterings and observables. The effects are quantified by dynamical scattering simulations and examined by direct comparison to the results of ultrafast electron diffraction experiments on an in situ prepared Ni(100) surface, for which structural dynamics can be well described by a two-temperature model. We also report calculations for graphite surfaces. The theoretical framework provided here allows for further UEC studies of surfaces especially at larger penetration depths and for those of heavy-atom materials

Inert gas accumulation in sonoluminescing bubbles

In this paper we elaborate on the idea [Lohse et al., Phys. Rev. Lett. 78, 1359-1362 (1997)] that (single) sonoluminescing air bubbles rectify argon. The reason for the rectification is that nitrogen and oxygen dissociate and their reaction products dissolve in water. We give further experimental and theoretical evidence and extend the theory to other gas mixtures. We show that in the absence of chemical reactions (e.g., for inert gas mixtures) gas accumulation in strongly acoustically driven bubbles can also occur.Comment: J. Chem. Phys., in press (to appear in November 1997), 30 pages, 15 eps-figure

Western Denial and Russian Control: How Russia’s National Security Strategy Threatens a Western-Based Approach to Global Security, the Rule of Law and Globalization.

The Russian National Security Strategy of 2015 aims at achieving autarky from Western influences on global security, the rule of law and global trade. Russia aims at attaining this by applying a holistic mix of military, political and economic means to weaken the West and to strengthen its own role as a global player. The Russian approach builds on a strategy of reflexive control which as such is an old method, but the outcome of the application of this approach results in hybrid warfare which as such is a new emerging concept of warfighting. This short article looks at one particular aspect of this Russian strategy, namely using Hybrid, or non-linear, Warfare against its Western direct neighbours in particular and the West in general. We will discuss the underlying cultural logic in Russia’s actions and will reflect on the impact of Russia’s utilization of the existing cultural asymmetry as a form of warfare in regard to the West. The examples used in this text are taken from the context of the conflicts of Ukraine and Syria, but have to be seen as constituting a part of an on-going global conflict aimed at NATO and the EU. The text builds on years of research within the hybrid threat, warfare respectively, context by both authors

Zur Rolle einer optimierten Verteilung von Sicherheiten im Risikomanagement: Motivation, Modellierung und Implikationen

Die Erfahrungen aus der jüngsten Finanzmarktkrise zeigen, dass der gezielten Abschätzung und Steuerung von Kreditrisiken in Banken eine enorme betriebs- und volkswirtschaftliche Bedeutung zukommt. Sowohl in der internen Kreditrisikomessung als auch bei den zugehörigen gesetzlichen Vorgaben liegt der Schwerpunkt dabei aber insbesondere auf der Prognose zukünftiger Ausfälle von Krediten, wohingegen Sicherheiten nur eine untergeordnete Bedeutung zukommt. Vor diesem Hintergrund analysiert der vorliegende Beitrag die optimierte Verteilung von Sicherheiten zunächst innerhalb bestehender fortgeschrittener Verfahren des Risikomanagements. Darüber hinaus wird auf der Grundlage eines Vergleichs der gängigen Verfahren eine flexible Zuordnungssystematik entwickelt. Diese neue Vorgehensweise über-trifft im Ergebnis die bestehenden Ansätze in ihrem Optimierungsziel, ohne dabei den Grundsatz einer konservativen Abschätzung aufgeben zu müssen

Excitations of Few-Boson Systems in 1-D Harmonic and Double Wells

We examine the lowest excitations of one-dimensional few-boson systems trapped in double wells of variable barrier height. Based on a numerically exact multi-configurational method, we follow the whole pathway from the non-interacting to the fermionization limit. It is shown how, in a purely harmonic trap, the initially equidistant, degenerate levels are split up due to interactions, but merge again for strong enough coupling. In a double well, the low-lying spectrum is largely rearranged in the course of fermionization, exhibiting level adhesion and (anti-)crossings. The evolution of the underlying states is explained in analogy to the ground-state behavior. Our discussion is complemented by illuminating the crossover from a single to a double well.Comment: 11 pages, 10 figure