Evidence of a first order phase transition between Wigner crystal and   Bubble Phases of 2D electrons in higher Landau levels
  For filling factors $\nu$ in the range between 4.16 and 4.28, we simultaneously detect {\it two} resonances in the real diagonal microwave conductivity of a two--dimensional electron system (2DES) at low temperature $T \approx 35$ mK. We attribute the resonances to Wigner crystal and Bubble phases of the 2DES in higher Landau Levels. For $\nu$ below and above this range, only single resonances are observed. The coexistence of both phases is taken as evidence of a first order phase transition. We estimate the transition point as $\nu=4.22$. 
R. M. Lewis, Yong Chen, L. W. Engel, D. C. Tsui, P. D. Ye, L. N.   Pfeiffer, and K. W. West