We show that two interacting physical systems may admit entangled pure or nonseparable mixed states evolving in time as if the mutual interaction Hamiltonian were absent. In this paper we define these interaction-free evolving (IFE) states and characterize their existence for a generic binary system described by a time-independent Hamiltonian. A comparison between IFE subspace and the decoherence-free subspace is reported. The set of all pure IFE states is explicitly constructed for a nonhomogeneous spin-star-system model
We report a set of experimental data on third-harmonic generation at microwave frequencies in YBa&Cu307 single crystals near T, at zero applied field. A theory is also developed which fully accounts for the experimental data. The theory is based on the two-fluid model and the hypothesis that the microwave magnetic field modulates the partial concentrations of both normal and superconducting electrons.
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