Entanglement versus mixedness for coupled qubits under a phase damping   channel
  Quantification of entanglement against mixing is given for a system of coupled qubits under a phase damping channel. A family of pure initial joint states is defined, ranging from pure separable states to maximally entangled state. An ordering of entanglement measures is given for well defined initial state amount of entanglement. 
E. S. Cardoso, M. C. de Oliveira, and K. Furuya