Theory of interlayer tunneling in bi-layer quantum Hall ferromagnets
  Spielman et al. have recently observed a large zero-bias peak in the tunnel conductance of a bi-layer system in a quantum Hall ferromagnet state. We argue that disorder-induced topological defects in the pseudospin order parameter limit the peak size and destroy the predicted Josephson effect. We predict that the peak would be split and shifted by an in-plane magnetic field in a way that maps the dispersion relation of the ferromagnet's Goldstone mode. We also predict resonant structures in the DC I-V characteristic under bias by an {\em ac} electric field. 
Ady Stern (Weizmann Institute), S.M. Girvin (Indiana University), A.H.   MacDonald (Indiana University), and Ning Ma (Indian...