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

Abstract

The ${\mathrm{Y}}_{2}$${\mathrm{Ba}}_{4}$${\mathrm{Cu}}_{7}$${\mathrm{O}}_{14+\mathrm{\ensuremath{\delta}}}$ family of superconductors is composed of alternating layers of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathrm{\ensuremath{\delta}}}$ and ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ materials along the c axis. The distribution of the hole-carrier density in the two-dimensional ${\mathrm{CuO}}_{2}$ planes in this compound can be quite complex due to the differing nature of these layers. Results of electronic-structure calculations are presented for three oxygen stoichiometries (\ensuremath{\delta}=0.0, 0.5, 1.0) which shed some light on the distribution of the hole-carrier density in different ${\mathrm{CuO}}_{2}$ planes. We find that the hole density in the ${\mathrm{CuO}}_{2}$ planes associated with the single chains is, generally speaking, lower than for double-chain layers. The difference is, however, relatively small.For the compound ${\mathrm{Y}}_{2}$${\mathrm{Ba}}_{4}$${\mathrm{Cu}}_{7}$${\mathrm{O}}_{14}$, we obtain an average value of \ensuremath{\sim}0.110 hole/${\mathrm{CuO}}_{2}$ for the hole density which is slightly above the threshold value of \ensuremath{\sim}0.06 hole/${\mathrm{CuO}}_{2}$ for the appearance of superconductivity to give it a low value of ${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}15 K, despite the fact that the single chains in this compound have no oxygen atoms. In ${\mathrm{Y}}_{2}$${\mathrm{Ba}}_{4}$${\mathrm{Cu}}_{7}$${\mathrm{O}}_{15}$ and in ${\mathrm{Y}}_{2}$${\mathrm{Ba}}_{4}$${\mathrm{Cu}}_{7}$${\mathrm{O}}_{14.5}$ in the full, empty, full,. . . sequence for single chains, the hole densities are much larger, in agreement with much higher ${\mathit{T}}_{\mathit{c}}$ 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 ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathrm{\ensuremath{\delta}}}$. 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.