This work is a generalization of the Hohenberg-Kohn-Sham theory of the inhomogeneous electron gas, with emphasis on spin effects. An argument based on quantum electrodynamics is used to express the ground-state energy of a system of interacting electrons as a functional of the current density. Expressions are derived for coefficients appearing in an expansion of the correlation functional in terms of the linear-response functions of the homogeneous system, for a gas of almost constant four-current density. The current density contains a spin-dependent term which leads, in the nonrelativistic limit, to a local potential which is also spin dependent.This potential is applied to the problems of spin splitting of energy bands in ferromagnets and spin-density-wave antiferromagnets.The relations between the present approach, that of Slater, and the collective electron theory of ferromagnetism of Stoner are described.

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Based on the form invariance of the structures given by Khinchin's axiomatic foundations of information theory and the pseudoadditivity of the Tsallis entropy indexed by q, the concept of conditional entropy is generalized to the case of nonadditive (nonextensive) composite systems. The proposed nonadditive conditional entropy is classically nonnegative but can be negative in the quantum context, indicating its utility for characterizing quantum entanglement. A criterion deduced from it for separability of density matrices for validity of local realism is examined in detail by employing a bipartite spin-1/2 system. It is found that the strongest criterion is obtained in the limit q going to infinity.Comment: 12 pages, 1 figur

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Uncertainty relations capture the essence of the inevitable randomness associated with the outcomes of two incompatible quantum measurements. Recently, Berta et al. [Nature Phys. 6, 659 (2010)] have shown that the lower bound on the uncertainties of the measurement outcomes depends on the correlations between the observed system and an observer who possesses a quantum memory. If the system is maximally entangled with its memory, the outcomes of two incompatible measurements made on the system can be predicted precisely. Here, we obtain an uncertainty relation that tightens the lower bound of Berta et al. by incorporating an additional term that depends on the quantum discord and the classical correlations of the joint state of the observed system and the quantum memory. We discuss several examples of states for which our lower bound is tighter than the bound of Berta et al. On the application side, we discuss the relevance of our inequality for the security of quantum key distribution and show that it can be used to provide bounds on the distillable common randomness and the entanglement of formation of bipartite quantum states.