Separability and Entanglement of Quantum States Based on Covariance   Matrices
  We investigate the separability of quantum states based on covariance matrices. Separability criteria are presented for multipartite states. The lower bound of concurrence proposed in Phys. Rev. A. 75, 052320 (2007) is improved by optimizing the local orthonormal observables. 
Ming Li, Shao-Ming Fei, Zhi-Xi Wang