The width of the Hall plateau and the activation energy were measured in the bilayer quantum Hall state at filling factors ν=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 ν=1 state is found to be stable over all measured range of the density difference. The ν=2/3 state is stable only around the balanced point. The ν=2 state shows a phase transition between these two types of states as the electron density is changed.
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