Thermal Entanglement of Anisotropic XY Chains in a Transverse Field

By using the concept of concurrence, we numerically investigate the
thermal entanglement between any two nearest-neighbour spins in uniform
and periodic anisotropic XY chains in a transverse field at finite
temperature T. It is found that the entanglement has more than one
critical temperatures on some parameter regions for uniform and periodic
chains. We also discuss the behaviour of the thermal entanglement at the
vicinity of quantum phase transition of periodic anisotropic XY chains
and find that all the derivatives ∂_λ have similar behaviour as that of the uniform chain.