Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Quantum entanglement at negative temperature   An isolated spin system that is in internal thermodynamic equilibrium and
that has an upper limit to its allowed energy states can possess a negative
temperature. We calculate the thermodynamic characteristics and the concurrence
in this system over the entire range of positive and negative temperatures. Our
calculation was performed for different real structures, which can be used in
experiments. It is found that the temperature dependence of the concurrence is
substantially asymmetrical similarly other thermodynamic characteristics. At a
negative temperature the maximum concurrence and the absolute temperature of
the entanglement appearance are sufficiently larger than those at a positive
temperature. The concurrence can be characterized by two dimensionless
parameters: by the ratio between the Zeeman and dipolar energies and by the
ratio of the thermal and dipolar energies. It was shown that for all considered
structures the dimensionless temperatures of the transition between
entanglement and separability of the first and second spins are independent of
spin structure and number of spins.

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The categories of this paper are: nucl-th
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The categories of this paper are: physics
22
The categories of this paper are: chao-dyn
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The categories of this paper are: q-fin
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The categories of this paper are: cond-mat
0
The categories of this paper are: hep-ph
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The categories of this paper are: ao-sci
5
The categories of this paper are: gr-qc
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The categories of this paper are: math-ph
37
The categories of this paper are: supr-con
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The categories of this paper are: plasm-ph
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The categories of this paper are: chem-ph
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The categories of this paper are: nlin
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The categories of this paper are: dg-ga
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The categories of this paper are: hep-ex
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The categories of this paper are: econ