Rotating plasma crystals and their transitions to dynamic non-crystalline states*

Complex systems often experience oscillations between quiescent and highly dynamic states. This phenomenon can also be found in dusty plasmas or complex plasmas, which are composed of weakly ionized gas and micron-size charged dust particles. When levitated in a plasma sheath, dust particles can form crystalline structures (plasma crystals). Here we investigate rotating plasma crystals with 5-20 particles by applying a non-uniform magnetic field perpendicular to the sheath above a driven electrode in an argon rf plasma. By tuning the pressure and rf power, the system can undergo transitions between chaotic gas-like states and ordered rotating crystal states. Interestingly, we observe reentrant behavior: as the neutral gas pressure increases from below 1 Pa to above 4 Pa, the plasma crystals undergoes a transition from a rotating crystal state to a dynamic gas-like state, and then back to a rotating crystal state again. This behavior correlates with regimes of small and large damping. The two rotating crystal states exhibit significant differences in terms of crystal structure, angular velocity, and the frequency of small oscillations, all likely due to variations in confinement potential, Debye length, and particle charge. Lastly, we show how this transition can occur spontaneously at constant pressure and power, and depends sensitively on the heterogeneity of particle sizes.