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Knapek, Christina (2010): Phase Transitions in Two-Dimensional Complex Plasmas. Dissertation, LMU München: Faculty of Physics
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Abstract

This thesis presents the experimental investigation of the phase state of two-dimensional complex plasmas by means of their dynamical and kinetic properties. The two-dimensional complex plasma consists of negatively charged micron-sized plastic spheres, levitated in the sheath of a radio-frequency noble gas discharge in a single horizontal layer. In two different experiments the thermodynamical state of a crystalline complex plasma (``plasma crystal''), and the process of recrystallization of a molten complex plasma is studied. The experiments were performed on strictly two-dimensional particle systems, and all data analysis builds on the examination of particle coordinates and trajectories. One important aspect of the data analysis is the estimation of uncertainties. A procedure has been developed to obtain reliable estimations of the measurement uncertainties introduced by the recording method and the particle tracking algorithm. The implications of the uncertainties on the scientific interpretation of the experimental results will be considered throughout the thesis. The first experiment aims to estimate the coupling parameter of a two-dimensional, crystalline complex plasma. The coupling parameter of an ensemble of particles is the ratio of their mean potential energy to their mean kinetic energy. It describes the thermodynamical state of the system, and is therefore an important quantity to characterize such a system. To calculate it, not only the particle temperature has to be estimated, but also an expression for the interparticle potential has to be known. For charged particles, this depends on the particle charge, which can often only be obtained with additional experimental effort, and its measurement is usually subject to large uncertainties. A simple, new method to calculate the coupling parameter from solely the spatial particle coordinates will be presented in this thesis, and verified to be consistent with the conventional estimation by charge and temperature measurements. The second experiment involves the creation of a two-dimensional plasma crystal and its shock melting by the application of a short electric pulse. The following phase of rapid recrystallization gives insight into the nature of a non-equilibrium transition of a two-dimensional system of interacting particles from a disordered to an ordered state. The measurements have been performed at a high temporal resolution to ensure the possibility to obtain kinetic energies from particle velocity distributions. The process is investigated thoroughly by means of the time-dependent development of the kinetic particle energy and structural properties of the system, such as translational and orientational long range order, defects fraction and spatial defect arrangements. Finally the connection of structural order parameters to the kinetic energy -- in comparison with conventional models and theories -- gives novel insights into the underlying physical processes determining the two-dimensional phase transition.