Thermal entanglement in three-qubit Heisenberg models

We study pairwise thermal entanglement in three-qubit Heisenberg models and obtain analytic expressions for the concurrence. We find that thermal entanglement is absent from both the antiferromagnetc XXZ model, and the ferromagnetic XXZ model with anisotropy parameter Delta greater than or equal to 1. Conditions for the existence of thermal entanglement are discussed in detail, as is the role of degeneracy and the effects of magnetic fields on thermal entanglement and the quantum phase transition. Specifically, we find that the uniform magnetic field can induce entanglement in the antiferromagnetic XXX model, but cannot induce entanglement in the ferromagnetic XXX model.