Empirical Identification of the Superconducting Carriers in YBa_2Cu_3O_<7-δ> : Electrical Properties of Condensed Matter

概要

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From consideration of the empirical δ-dependence of the electrical conductivity of both the normal and superconducting phases of YBa_2Cu_3O_<7-δ> -as well as from the T-dependence of the thermoelectric power-it is argued that the superconducting carriers are holes in a fairly broad oxygen-based band, formed by charge transfer to the Cu(I) ions to render them divalent-like those of the Mott-insulating Cu(II) subsystem. Simple valence/stoichiometry considerations indicate that for δ≠0, the number of such holes can be maintained at its maximum (δ=0) value by the introduction of Y-vacancies or divalent Y-substitutes; to increase further the carrier concentration and hence-according to a recent empirical study of T_c(δ)-also T_c requires, however, accessing the Cu(II) subsystem, which is apparently not involved in the 90 K superconductivity of YBa_2Cu_3O_<7-δ>; ways of doing this using fluorine or nitrogen are suggested. Finally, attention is drawn to an interesting coincidence between the T_c's of La_>1.85>Ba_<0.15>CuO_4 and YBa_2Cu_3O_<6.5+ε> for ε=0.075.
社団法人応用物理学会の論文
1988-01-20