Properties of the potential energy landscape under shear

In dieser Dissertation wird das Verhalten eines Modell-Glasbildners unter Scherung im Kontext der potentiellen Energielandschaft (PEL) untersucht. Durch die Einführung von minimierten Strukturen (MS) als Erweiterung von inhärenten Strukturen (IS) auf das gescherte System sehen wir die Bedeutung von MS-Übergängen für die Dynamik. Der erste MS-Übergang spielt eine zentrale Rolle beim Spannungsüberschuss, bei der Bestimmung der Fließgrenze und bei zyklischer Scherung. Durch minimierte Strukturen können wir Metabasins (MB) auf konsistente Weise im Übergangsregime zwischen scherungs- und temperaturdominiertem Bereich definieren. Dies führt uns zu einer guten Näherung der Diffusionskonstanten und dem strukturellen Beitrag zur Viskosität im Übergangsregime. Außerdem studieren wir die strukturellen Eigenschaften des gescherten Systems, wie zum Beispiel die Anisotropie und stellen eine Methode vor, wie man die Statistik auf Teilchenebene in Kugelkoordinaten darstellen kann.In this thesis, the behavior of a model glass former under applied shear is explored in the context of the potential energy landscape (PEL). By introducing minimized structures (MS) as an extension of inherent structures (IS) to the sheared system, we see the importance of MS transitions to the dynamics. The first MS transition plays a vital role in the stress overshoot, at yielding and in cyclic shear. Furthermore, the use of minimized structures allows us to define metabasins (MB) in a consistent way for the whole crossover regime, between the shear dominated and the temperature dominated regime. This helps us to give a good approximation of the diffusion constant and the structural contribution to the viscosity in the crossover regime. Last, we explore structural properties of the sheared system like the anisotropy and introduce a method to calculate per-particle statistics in spherical coordinates.<br

Nonlinear response from the perspective of energy landscapes and beyond

The paper discusses the nonlinear response of disordered systems. In particular we show how the nonlinear response can be interpreted in terms of properties of the potential energy landscape. It is shown why the use of relatively small systems is very helpful for this approach. For a standard model system we check which system sizes are particular suited. In case of the driving of a single particle via an external force the concept of an effective temperature helps to scale the force dependence for different temperature on a single master curve. In all cases the mobility increases with increasing external force. These results are compared with a stochastic process described by a 1d Langevin equation where a similar scaling is observed. Furthermore it is shown that for different classes of disordered systems the mobility can also decrease with increasing force. The results can be related to the properties of the chosen potential energy landscape. Finally, results for the crossover from the linear to the nonlinear conductivity of ionic liquids are presented, inspired by recent experimental results in the Roling group. Apart from a standard imidazolium-based ionic liquid we study a system which is characterized by a low conductivity as compared to other ionic liquids and very small nonlinear effects. We show via a real space structural analysis that for this system a particularly strong pair formation is observed and that the strength of the pair formation is insensitive to the application of strong electric fields. Consequences of this observation are discussed