Melting Processes and Laser Manipulation of Strongly Coupled Yukawa Systems

The thesis at hand is devoted to the melting processes in spherical dust crystals and mechanisms for the selective heating of the dust particles. The observed dust crystals are composed of micrometer-sized plastic spheres, which become negatively charged inside an rf-plasma, and are therefore subject to strong (screened) Coulomb interactions. In a three-dimensional harmonic confinement potential, so-called Coulomb or Yukawa balls are formed. These ``artificial atoms'' consist of nested spherical shells even at room temperature. In addition to the radial order, the melting process of these three-dimensional shell structures involves the loss of the relative orientation of the shells to each other and the specific arrangement of the dust particles within a shell. For the investigation of the multi-stage melting process of Yukawa balls, classical Monte Carlo simulations are applied. In order to cover a broad temperature range, the so-called parallel tempering method has been implemented which allows exchanges between system replicas at different temperatures. The first topical part of this work is the development of appropriate melting criteria for finite dust clusters that make it possible to resolve different melting processes. For this purpose, the center-two-particle distribution function and the triple-correlation function are introduced. These quantities describe the distributions of particle pairs and triples, respectively, in generalized coordinates. The coordinates are adjusted to the spherical symmetry of the confined Yukawa systems. As a measure of the degree of order in such a distribution, a reduced entropy Sn is introduced, which generalizes the concept of the thermodynamic Boltzmann entropy to pair and three-particle distributions. The application of this quantity allows a systematic analysis of the multi-level structural transitions in the studied dust crystals. In particular the intra-shell order within the spherical shells are proved to be highly stable. The second main topic of this thesis deals with the selective heating of two-dimensional, planar dust crystals by means of moving laser spots. In order to characterize the properties of the considered heating method, Langevin molecular dynamics (LMD) simulations are performed, in which the radiation pressure of the laser is used as an additional, time-dependent force acting on the dust particles. In addition to the laser power, other parameters such as the spot size or the speed at which the spots are moved through the cluster have a significant influence on the heating power. A simple analytical model is presented, which allows for an estimation of the resulting dust temperature in dependence on the parameters of the laser heating. In the final part of the thesis at hand, a realistic experimental setup is considered in which the central part of the dust cluster is selectively heated. The particular temperature profiles obtained in the LMD simulations allow for conclusions about the radial thermal conductivity. The presented quantities are well suited for experimental applications since they are based only on the positions and velocities of the particles which are accessible in experiments. The thermal conductivity does not show any dependency on the dust temperature over wide ranges. At the same time, already a relatively weak perpendicular magnetic field is found to reduce the thermal conductivity.