Experimental observation of torsions in a single-layer complex plasma crystal

Kurzfassung

A complex plasma is a weakly ionized gas which includes small particles of solid matter. Particles charge up (usually negatively) by collecting electrons and ions from the plasma and interact with each other through the screened Coulomb potential. They can be suspended in the plasma regions where gravity is compensated by other Forces (e.g., electrostatic, thermophoretic). Due to the high particle charge, their suspensions are often strongly coupled and can be in a liquid or even crystalline state. The latter is called a plasma crystal. In experiments with monodisperse particles, single-layer (2D) plasma crystals can be produced. They are popular model systems where various phenomena can be studied in real time at the Level of individual particles. In this presentation, we report on a new type of collective excitation in 2D plasma crystals, a spinning pair of particles, which we call a "torsion". A torsion is a pair of identical particles oriented along the flow of ions (orthogonal to the plane of plasma crystal), which is spinning around its axis. It is a dissipative structure where energy input is provided by the flow of ions and energy dissipation is through the neutral gas friction on particles. Non-reciprocal attraction between the particles in a torsion is due to the plasma wake effect.