Thermal and magnetic quantum discord in Heisenberg models

We investigate how quantum correlations [quantum discord (QD)] of a two-qubit one-dimensional $\mathit{XYZ}$ Heisenberg chain in thermal equilibrium depend on the temperature $T$ of the bath and also on an external magnetic field $B$. We show that the behavior of thermal QD differs in many unexpected ways from thermal entanglement. For example, we show situations where QD increases with $T$ when entanglement decreases, cases where QD increases with $T$ even in regions with zero entanglement, and that QD signals a quantum phase transition even at finite $T$. We also show that by properly tuning $B$ or the interaction between the qubits we get nonzero QD for any $T$ and we present an effect not seen for entanglement, the ``regrowth'' of thermal QD.

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