Tunneling gap collapse andv=2quantum Hall state in a bilayer electron system

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We have investigated the evolution of quantum Hall states in a GaAs-${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ bilayer electron system by low-temperature magnetoresistivity measurements as the system was driven from a balanced to an off-balanced configuration. At low magnetic fields, odd integer filling factor quantum Hall states were observed on balance owing to the symmetric-antisymmetric tunneling gap. However, at high magnetic fields, in the regime of tunneling gap collapse, we observed anomalous quantum Hall states at $v=2$ off balance and $v=3$ on balance. At $v=2,$ an energy gap was present all the way from the balanced configuration to far off balance, when only one quantum well was occupied. This is attributed to a transition from a spin-polarized state on balance to a spin-singlet state off balance, either by an abrupt exchange-driven phase transition or a continuous phase transition via a series of interlayer phase coherent states.