We study the entanglement features of the ground state of a system composed of spin 1 and 1/2 parts. In the light of the ground state degeneracy, the notion of average entanglement is used to measure the entanglement of the Hilbert subspace. The entanglement properties of both a general superposition as well as the mixture of the degenerate ground states are discussed by means of average entanglement and the negativity respectively.
The spin-1/2 Ising-Heisenberg trimerized chain in a magnetic field is revisited with the aim to explore the quantum entanglement and non-locality within the exactly solved spin system, which exhibits in a low-temperature magnetization curve two intermediate plateaux at zero and one-third of the saturation magnetization. The ground-state phase diagram involves two quantum (antiferromagnetic, ferrimagnetic I) and two classical (ferrimagnetic II, saturated paramagnetic) phases. We have rigorously calculated the concurrence and Bell function in order to quantify the quantum entanglement and non-locality at zero as well as non-zero temperatures. It is demonstrated that the entanglement can be thermally induced also above the classical ground states unlike the quantum non-locality, which means that the thermal entanglement is indispensable for a violation of the locality principle.
The Galois symmetry of exact Bethe Ansatz eigenstates for magnetic pentagonal ring is shown to bear a close analogy to some crystallographic constructions. Automorphisms of number field extensions associated with these eigenstates prove to be related to choices of the Bravais cells in the finite crystal lattice ℤ₂×ℤ₂, responsible for extension of the cyclotomic field by the Bethe parameters.
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