Stick-Slip Dynamics of a Stressed Ion Crystal

We study the control of the rotation of a laser-cooled ion crystal in a Penning trap by a rotating electric field perturbation. We show that application of a small torque produces sudden angular jumps or ``slips'' of the crystal orientation spaced by intervals when the crystal is phase locked or ``stuck'' relative to the rotating perturbation. The distribution of angular slips is described by a power law, where the power-law exponent depends on the applied torque. We believe this system is driven by a constant force and small perturbations or thermal effects trigger the slips.