Necessary and sufficient conditions of freezing phenomena of quantum   discord under phase damping
  We investigate the freezing phenomenon of quantum discord occurring in phase damping noise processes. By relating the expression of the time variation of the discord to the convex function of relative entropy, we obtain the necessary and sufficient conditions of the phenomenon for standard Bell-diagonal states. These conditions are applicable also to the phenomenon occurring in a non-Markovian dephasing process. Moreover, we show that the same condition and phenomenon coincide in a new sort of Bell-diagonal states beyond the standard form. 
Bo You and Li-Xiang Cen
quant-ph