Geometry of the faces for separable states arising from generalized Choi   maps
  We exhibit examples of separable states which are on the boundary of the convex cone generated by all separable states but in the interior of the convex cone generated by all PPT states. We also analyze the geometric structures of the smallest face generated by those examples. As a byproduct, we obtain a large class of entangled states with positive partial transposes. 
Kil-Chan Ha, Seung-Hyeok Kye
quant-ph math.OA