Entanglement and disorder: a mean field approach

The entanglement of symmetric mean-field (MF) ground states is studied for systems of finite size. While a bare MF approximation amounts to considering factorized states, the symmetrization allows one to restore an amount of entanglement, which survives if finite systems are considered. For the isotropic XY model in the presence of a random transverse field, we calculated the one-site entanglement entropy, which is zero both for vanishing and maximum disorder and reaches a maximum for an intermediate value of disorder.