Abstract
Superoxygenated La 2CuO 4+δ, produced at 1800 atm and 550°C, has been investigated by the following means; resistivity, Seebeck coefficient Hall coefficient, magneto-resistance, electron microscopy, AC and DC susceptibility, and specific heat. The results are contrasted with those from “reduced” (stoichiometric) La 2CuO 4. Unlike the latter Mott insulator, the oxygenated material shows mixed valent, delocalized behaviour. That behaviour is rendered more complex because of phase segregation via spinodal decomposition onsetting just below room temperature. The observed microstructure permits detailed interpretation of the resulting modification in properties. The degree to which the observed superconductivity mirrors that of (La/Sr) 2CuO 4 is discussed. Restricting the phase segregation by fast cooling inhibits the superconductive behaviour, by promoting lower effective carrier counts and mobilities and greater magnetic admixture. It is shown that phase segregation is not triggered by or coupled to the point at which long-range 3D magnetic order develops within the insulating component of the two-phase mixture. The super-conductivity is presented in terms of the negative- U charge fluctuation model developed earlier.
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