Interlayer Coherence in the $\nu=1$ and $\nu=2$ Bilayer Quantum Hall   States
  We have measured the Hall-plateau width and the activation energy of the bilayer quantum Hall (BLQH) states at the Landau-level filling factor $\nu=1$ and 2 by tilting the sample and simultaneously changing the electron density in each quantum well. The phase transition between the commensurate and incommensurate states are confirmed at $\nu =1$ and discovered at $\nu =2$. In particular, three different $\nu =2$ BLQH states are identified; the compound state, the coherent commensurate state, and the coherent incommensurate state. 
A. Sawada, Z.F. Ezawa, H. Ohno, Y. Horikoshi, A. Urayama, Y. Ohno, S.   Kishimoto, F. Matsukura, N. Kumada