  Double-layer quantum Hall systems with spontaneous broken symmetry can
exhibit a novel manybody quantum Hall effect due to the strong interlayer
coherence. When the layer separation becomes close to the critical value,
quantum fluctuations can destroy the interlayer coherence and the quantum Hall
effect will disappear. We calculate the renormalized isospin stiffness $\rho_s$
due to quantum fluctuations within the Hartree-Fock-RPA formalism. The
activation energy of the topological excitations thus obtained demonstrates a
nice qualitative agreement with recent experiment.
