  Entanglement in quantum XY spin chains of arbitrary length is investigated
via a recently-developed global measure suitable for generic quantum many-body
systems. The entanglement surface is determined over the phase diagram, and
found to exhibit structure richer than expected. Near the critical line, the
entanglement is peaked (albeit analytically), consistent with the notion that
entanglement--the non-factorization of wave functions--reflects quantum
correlations. Singularity does, however, accompany the critical line, as
revealed by the divergence of the field-derivative of the entanglement along
the line. The form of this singularity is dictated by the universality class
controlling the quantum phase transition.
