Entanglement spectrum and number fluctuations in the spin-partitioned   BCS ground state
  We study entanglement between the spin components of the Bardeen-Cooper-Schrieffer (BCS) ground state by calculating the full entanglement spectrum and the corresponding von Neumann entanglement entropy. The entanglement spectrum is effectively modeled by a generalized Gibbs ensemble (GGE) of non-interacting electrons, which may be approximated by a canonical ensemble at the BCS critical temperature. We further demonstrate that the entanglement entropy is jointly proportional to the pairing energy and to the number of electrons about the Fermi surface (an area law). Furthermore, the entanglement entropy is also proportional to the number fluctuations of either spin component in the BCS state. 
Xavier M. Puspus, Kristian Hauser Villegas, Francis N. C. Paraan
quant-ph cond-mat.stat-mech cond-mat.str-el