Tunneling in double-layered quantum Hall systems

We study interlayer tunneling in some double-layered quantum Hall states that contain a neutral gapless superfluid mode. A tunneling 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/{}. In contrast to tunneling between superconductors, the critical current here is linear rather than quadratic in the tunneling amplitude.