fcc-bcc phase transition in plasma crystals using time-resolved measurements

Three-dimensional plasma crystals are often described as Yukawa systems for which a phase transition between the crystal structures fcc and bcc has been predicted. However, experimental investigations of this transition are missing. We use a fast scanning video camera to record the crystallization process of 70 000 microparticles and investigate the existence of the fcc-bcc phase transition at neutral gas pressures of 30, 40, and 50 Pa. To analyze the crystal, robust phase diagrams with the help of a machine learning algorithm are calculated. This work shows that the phase transition can be investigated experimentally and makes a comparison with numerical results of Yukawa systems. The phase transition is analyzed in dependence on the screening parameter and structural order. We suggest that the transition is an effect of gravitational compression of the plasma crystal. Experimental investigations of the fcc-bcc phase transition will provide an opportunity to estimate the coupling strength $\mathrm{\Gamma }$ by comparison with numerical results of Yukawa systems.

Figure 1

Sketch of the experimental setup and in particular the camera system used for data acquisition.

Figure 2

Time resolved structural analysis of the plasma crystal at 30 Pa (a), 40 Pa (b), and 50 Pa (c). The percentages show the amount of particles found to be bcc-like (blue diamond), fcc-like (red triangle), or hcp-like (green circle) relative to the maximum amount of particles in the respective scan.

Figure 3

Phase diagrams for 30 Pa (a), 40 Pa (b), and 50 Pa (c) depending on structural order ($S$) and screening parameter ($\kappa$). Every pixel represents a probability density vector for bcc-like (blue), fcc-like (red), or hcp-like (green). The bold tick numbers of $\kappa$ mark the fcc-bcc phase transition.

Figure 4

Phase diagram in the $\mathrm{\Gamma }\text{−}\kappa$ plane. The solid-liquid line is approximated by an analytical expression according to [23] and the connected fcc-bcc transition data is from Hamaguchi et al. [13]. The measured phase transitions are represented by red symbols for different pressures and the blue crosses show the projection on the linearly interpolated fcc-bcc line.