Defect dynamics and plastic deformations in complex plasmas

Abstract

Summary form only given. Complex or dusty plasmas are ordinary ion-electron plasmas with added microparticles. The microparticles are charged by collisions with the electrons and ions, predominantly negatively due to higher mobility of the electrons. The grains interact with each other electrostatically via a Yukawa potential and often form ordered structures. Similar to colloids, complex plasmas can exist in solid, liquid or gaseous states and exhibit phase transitions. As the grains are weakly damped by gas friction and traceable individually, dynamic phenomena can be observed in complex plasmas at the kinetic level in real time. The molecular dynamics simulation code1 that we have developed solves the three-dimensional equations of motion for each particle moving in a global confinement potential and interacting with every other particle via a Yukawa potential. The grain motions are damped by neutral gas drag. After seeding the grains randomly the code is run until the equilibrium is reached and a monolayer crystal lattice is formed. Then different excitation forces are applied on the lattices. The experiments are performed in a capacitively coupled radio-frequency discharge. Monodisperse plastic micro spheres are levitated in the sheath above the lower electrode. They are confined radially in a bowl shaped potential and form a monolayer hexagonal lattice. A horizontal thin sheet of laser light illuminates the particles, which are imaged by a digital video camera. These grains are excited by voltage pulses applied to wires stretched above the lower electrode. Here we study the dynamics of dislocations, which are pairs of 5and 7-fold defects. We focus on the influence of the excitation force on the dynamics of single dislocations and on the plastic deformations of complex plasma lattices. We show that the motion of dislocations is made of either smooth displacements in the direction parallel to the excitation force or jumps from a pair of particles to another. Under compression the lattice releases this applied strain by defect motions and shear slips, which correspond to the movements of neighbouring lines of particles in opposite directions.