We have compared the stability of the ν = 2, 1 and 2/3 quantum Hall states in a bilayer two-dimensional electron system by changing the total density and the density ratio of electrons in the two quantum wells. The following experimental results have been obtained: The ν = 1 state is stable over a wide range of the density difference. It is least stable when the electron density is balanced. The state collapses when the total density is more than a critical value. The density ratio has a small influence on the stability of ν = 1. These features are not shared by the other quantum Hall states. We argue that they give an indication of the interlayer quantum coherence.
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