Evidence of a first order phase transition between Wigner crystal and Bubble Phases of 2D electrons in higher Landau levels

For filling factors $ u$ 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 $ u$ 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 $ u=4.22$.