Dissipationless transport in low-density bilayer systems

In a bilayer electronic system the layer index may be viewed as the z compo nent of an isospin-1/2. An XY isospin-ordered ferromagnetic phase was obser ved in quantum Hall systems and is predicted to exist at zero magnetic fiel d at low density. This phase is a superfluid for opposite currents in the t wo layers. At B = 0 the system is gapless but superfluidity is not destroye d by weak disorder. In the quantum Hall case, weak disorder generates a ran dom gauge field which probably does not destroy superfluidity. Experimental signatures include Coulomb drag and collective mode measurements.