Quantum Hall Exciton Condensation at Full Spin Polarization

Using Coulomb drag as a probe, we explore the excitonic phase transition in quantum Hall bilayers at $\nu_T=1$ as a function of Zeeman energy, $E_Z$. The critical layer separation $(d/\ell)_c$ for exciton condensation initially increases rapidly with $E_Z$, but then reaches a maximum and begins a gentle decline. At high $E_Z$, where both the excitonic phase at small $d/\ell$ and the compressible phase at large $d/\ell$ are fully spin polarized, we find that the width of the transition, as a function of $d/\ell$, is much larger than at small $E_Z$ and persists in the limit of zero temperature. We discuss these results in the context of two models in which the system contains a mixture of the two fluids.