Dissipationless Transport in Low-Density Bilayer Systems

In a bilayer electronic system the layer index may be viewed as the $z$ component of an isospin- $\frac{1}{2}$. An $\mathrm{XY}$ isospin-ordered ferromagnetic phase was observed in quantum Hall systems and is predicted to exist at zero magnetic field at low density. This phase is a superfluid for opposite currents in the two layers. At $B=0\mathrm{}$ the system is gapless but superfluidity is not destroyed by weak disorder. In the quantum Hall case, weak disorder generates a random gauge field which probably does not destroy superfluidity. Experimental signatures include Coulomb drag and collective mode measurements.