QUANTUM FERROMAGNETISM AND PHASE-TRANSITIONS IN DOUBLE-LAYER QUANTUM HALL SYSTEMS

Double-layer quantum Hall systems have interesting properties associat ed with interlayer correlations. At nu = 1/m where m is an odd integer they exhibit spontaneous symmetry breaking equivalent to that of spin 1/2 easy-plane ferromagnets, with the layer degree of freedom playing the role of spin. We explore the rich variety of quantum and finite t emperature phase transitions in these systems. In particular, we show that a magnetic field oriented parallel to the layers induces a highly collective commensurate-incommensurate phase transition in the magnet ic order.