The possibility of two-exciton pairing in singlet and in triplet states
  We propose a two-dimensional exciton model showing the possibility of two-exciton pairing in cuprates, both in singlet and in triplet states. The former is assumed to be responsible for the superconductivity and the latter for the unusual normal state properties. The ground state is found to be a projection of the Bose condensation of N/2 mobile bosons with a \eta projection operator which is a generalization of the Gutzwiller projection operator. The phase diagram of the model shows insulator-superconductivity and superconductivity-metal transitions with doping. 
Fajian Shi