Tunneling in double-layered quantum Hall systems.

We study interlayer tunnelling in some double-layered quantum Hall states that contain neutral gapless superfluid mode. A tunnelling current less than a certain critical value will not cause any voltage drop between the two layers. A finite voltage V between the layers will induce an AC current with a frequency ω = eV/h̄. In contrast to tunnelling between superconductors, the critical current here is linear rather than quadratic in the tunnelling amplitude. * Published in Phys. Rev. B47, 2265 (1993)