Quantum phase transitions in an effective Hamiltonian: fast and slow   systems
  An effective Hamiltonian describing interaction between generic "fast" and a "slow" systems is obtained in the strong interaction limit. The result is applied for studying the effect of quantum phase transition as a bifurcation of the ground state of the "slow subsystem" in the thermodynamic limit. Examples as atom-field and atom-atom interactions are analyzed in detail. 
Isabel Sainz, Andrei B. Klimov and Luis Roa