Bias-voltage-induced phase transition in bilayer quantum Hall ferromagnets

We consider bilayer quantum Hall systems at total filling factor nu=1 in presence of a bias voltage Delta(v) that leads to different filling factors in each layer. We use auxiliary field functional-integral approach to study mean-field solutions and collective excitations around them. We find that at large layer separation, the collective excitations soften at a finite wave vector leading to the collapse of quasiparticle gap. Our calculations predict that as the bias voltage is increased, bilayer systems undergo a phase transition from a compressible state to a nu=1 phase-coherent state with charge imbalance. We present simple analytical expressions for bias-dependent renormalized charge imbalance and pseudospin stiffness that are sensitive to the softening of collective modes.