Rapid heating of a strongly coupled plasma near the solid-liquid phase transition

Between 10⁴ and 10^{6 9}B^e+ ions were trapped in a Penning trap and laser cooled to ∼1 mK, where they formed a crystalline plasma. We measured the ion temperature as a function of time after turning off the laser cooling and observed a rapid temperature increase as the plasma underwent the solid-liquid phase transition at T ≃10 mK (Γ ∼ 170). We present evidence that this rapid heating is due to a sudden release of energy from weakly cooled degrees of freedom involving the cyclotron motion of trapped impurity ions. This equilibration of cyclotron motion with motion parallel to the magnetic field is more than 10 orders of magnitude faster than that predicted by currently available theory, which is valid only in the absence of correlations (Γ ≪1).