Quantum correlations versus spin magnitude: Transition to the classical limit

Quantum-classical transitions have long attracted much attention. We study such transitions in quantum spin-($j$,1/2) systems at thermal equilibrium. Unlike previous work [Phys. Rev. A 73, 064302 (2006)], we find that the threshold temperature of quantum entanglement decreases with increasing spin $j$ and completely disappears in the limit $j\ensuremath{\rightarrow}\ensuremath{\infty}$. In the ground state of systems with highly symmetric interactions, the discord-type quantum correlations can exist even for arbitrarily large spin. Such correlations turn out to be unstable and are destroyed by small perturbations that violate the symmetry of the Hamiltonian. The stable quantum correlations gradually degrade as the spin $j$ grows and eventually vanish when the classical limit is reached.