PHASE-TRANSITION IN THE NU=2 BILAYER QUANTUM HALL STATE

The width of the Hall plateau and the activation energy were measured in the bilayer quantum Hall state at filling factors nu = 2, 1, and 2/ 3 by changing the total electron density and the density ratio in the two quantum wells. Their behaviors are remarkably different. The nu = 1 state is found to be stable over all measured range of the density d ifference. The nu = 2/3 state is stable only around the balanced point . The nu = 2 state shows a phase transition between these two types of states as the electron density is changed.