Interlayer tunneling in double-layer quantum Hall pseudoferromagnets

We show that the interlayer tunneling I-V in double-layer quantum Hall stat es displays a rich behavior which depends on the relative magnitude of samp le size, voltage length scale, current screening, disorder, and thermal len gths. For weak tunneling. we predict a negative differential conductance of a power-law shape crossing over to a sharp zero-bias peak. An in-plane mag netic field splits this zero-bias peak, leading instead to a "derivative" f eature at V-B(B-\ \) = 2 pi hvB(\ \)d/e phi (upsilon), which gives a direct measurement of the dispersion of the Goldstone mode corresponding to the s pontaneous symmetry breaking of the double-layer Hall state.