Thermal and magnetic quantum discord in Heisenberg models

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Abstract
We investigate how quantum correlations [quantum discord (QD)] of a two-qubit one-dimensional 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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