Spontaneous interlayer exciton coherence in quantum Hall bilayers at   nu=1 and nu=2: a tutorial
  This tutorial paper reviews some of the physics of quantum Hall bilayers with a focus on the case where there is low or zero tunnelling between the two layers. We describe the interlayer coherent states at filling factors nu=1 and nu=2 as exciton condensates and discuss some of the theory associated with these states. 
Steven H. Simon