The influence of dipole-dipole interaction on entanglement in   nondegenerate two-photon Tavis-Cummings model with atomic coherence
  The entanglement of two dipole-coupled atoms with nondegenerate two-photon transitions interacting with two-mode field in lossless cavity has been investigated. It shows that the entanglement is dependent on the initial atomic states. The possibility of considerable growth of atomic entanglement due atomic coherence and dipole-dipole interaction is shown in the case of great mean values of thermal photons. 
E.K. Bashkirov, M.S. Mastuygin
quant-ph