Distribution of the hole-carrier density in Y2Ba4Cu7O14+δ (δ=0.0, 0.5, 1.0) superconductors

Abstract

The Y2Ba4Cu7O14+δ family of superconductors is composed of alternating layers of YBa2Cu3O6+δ and YBa2Cu4O8 materials along the c axis. The distribution of the hole-carrier density in the two-dimensional CuO2 planes in this compound can be quite complex due to the differing nature of these layers. Results of electronic-structure calculations are presented for three oxygen stoichiometries (δ=0.0, 0.5, 1.0) which shed some light on the distribution of the hole-carrier density in different CuO2 planes. We find that the hole density in the CuO2 planes associated with the single chains is, generally speaking, lower than for double-chain layers. The difference is, however, relatively small. For the compound Y2Ba4Cu7O14, we obtain an average value of ∼0.110 hole/CuO2 for the hole density which is slightly above the threshold value of ∼0.06 hole/CuO2 for the appearance of superconductivity to give it a low value of Tc∼15 K, despite the fact that the single chains in this compound have no oxygen atoms. In Y2Ba4Cu7O15 and in Y2Ba4Cu7O14.5 in the full, empty, full,. . . sequence for single chains, the hole densities are much larger, in agreement with much higher Tc values observed in these compounds. The effect of oxygen disorder in the single chains on the hole densities has also been investigated for the latter two compounds, and is found to be much less drastic than in YBa2Cu3O6+δ. This is due to the presence of double chains which act as a cushion. The effective valence of Cu at different sites, which is an alternative way of presenting the hole densities, is also calculated and is shown to depend intimately not only on the oxygen stoichiometry but also on the degree of oxygen ordering in the single chains.

Source-reported materials — not catalogue approval

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