André Schella

Controlling strongly correlated dust clusters with lasers

Lasers have been used extensively to manipulate matter in a controlled way ? from single atoms and molecules up to macroscopic materials. They are particularly valuable for the analysis and control of mesoscopic systems such as few-particle clusters. Here we report on recent work on finite size complex (dusty) plasma systems. These are unusual types of clusters with a very strong inter-particle interaction so that, at room temperature, they are practically in their ground state. Lasers are employed as a tool to achieve excited states and phase transitions.The most attractive feature of dusty plasmas is that they allow for a precise diagnostic with single-particle resolution. From such measurements, the structural properties of finite two-dimensional (2D) clusters and three-dimensional (3D) spherical crystals in nearly harmonic traps?so-called Yukawa balls?have been explored in great detail. Their structural features?the shell compositions and the order within the shells?have been investigated and good agreement to theoretical predictions was found. Open questions on the agenda are the excitation behaviour, the structural changes and phase transitions that occur at elevated temperature.Here we report on recent experimental results where laser heating methods were further improved and applied to finite 2D and 3D dust clusters. Comparing to simulations, we demonstrate that laser heating indeed allows to increase the temperature in a controlled manner. For the analysis of thermodynamic properties and phase transitions in these finite systems, we present theoretical and experimental results on the basis of the instantaneous normal modes, pair distribution function and the recently introduced centre-two-particle correlation function.

Crystal and fluid modes in three-dimensional finite dust clouds

The spectral properties of three-dimensional dust clusters confined in gaseous discharges are investigated using both a fluid mode description and the normal mode analysis (NMA). The modes are analysed for crystalline clusters as well as for laser-heated fluid-like clusters. It is shown that even for clusters with low particle numbers and under presence of damping fluid modes can be identified. Laser-heating leads to the excitation of several, mainly transverse, modes. The mode frequencies are found to be nearly independent of the coupling parameter and support the predictions of the underlying theory. The NMA and the fluid mode spectra demonstrate that the wakefield attraction is present for the experimentally observed Yukawa balls at low pressure. Both methods complement each other, since NMA is more suitable for crystalline clusters, whereas the fluid modes allow to explore even fluid-like dust clouds.

Correlation buildup during recrystallization in three-dimensional dusty plasma clusters

The recrystallization process of finite three-dimensional dust clouds after laser heating is studied experimentally. The time-dependent Coulomb coupling parameter is presented, showing that the recrystallization starts with an exponential cooling phase where cooling is slower than damping by the neutral gas friction. At later times, the coupling parameter oscillates into equilibrium. It is found that a large fraction of cluster states after recrystallization experiments is in metastable states. The temporal evolution of the correlation buildup shows that correlation occurs on even slower time scale than cooling.

Introduction to Streaming Complex Plasmas A: Attraction of Like-Charged Particles

Like-charged particles usually interact via a repulsive force. However, in streaming dusty plasmas one can observe that two negatively charged dust particles may attract each other. This is explained by accumulation of positive ions below the dust particles (with respect to the streaming direction). In this chapter, we describe the dependence of this ion focus and the resulting wakes on discharge rf-power, pressure and thermophoretic force, as the three key parameters, that can be varied in dusty plasma experiments. Moreover, we discuss the impact of this attractive force on the collective properties of many dust particles, in particular, on the structure and on the dynamics of spherically confined clusters.

Recrystallization in Finite 3-D Dust Clouds

The recrystallization process of a finite dust cloud is studied experimentally on the kinetic level. The temporal evolution of the coupling parameter allows to reveal that the cluster is molten while laser heated and recrystallizes when heating is switched OFF. The cooling rate can be deduced from the initial stage of recrystallization.

Charging changes contact composition in binary sphere packings

Equal volume mixtures of small and large polytetrafluorethylene spheres are shaken in an atmosphere of controlled humidity which allows one to also control their tribocharging. We find that the contact numbers are charge dependent: As the charge density of the beads increases, the number of same-type contacts decreases and the number of opposite-type contacts increases. This change is not caused by a global segregation of the sample. Hence, tribocharging can be a way to tune the local composition of a granular material.