Phase Transition in \nu=2 Bilayer Quantum Hall State
  The Hall-plateau width and the activation energy were measured in the bilayer quantum Hall state at filling factor \nu=2, 1 and 2/3, by changing the total electron density and the density ratio in the two quantum wells. Their behavior are remarkably different from one to another. The \nu=1 state is found stable over all measured range of the density difference, while the \nu=2/3$ state is stable only around the balanced point. The \nu=2 state, on the other hand, shows a phase transition between these two types of the states as the electron density is changed. 
A. Sawada, Z.F. Ezawa, H. Ohno, Y. Horikoshi, Y. Ohno, S. Kishimoto,   F. Matsukura, M. Yasumoto, A. Urayama