Nonhomogeneous charge distribution in layered high-Tcsuperconductors

We consider the problem of the charge distribution of holes among the various ${\mathrm{CuO}}_2$ layers in the unit cell for compounds of the type ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{Ca}}_{\mathit{n}\mathrm{-}1}$ ${\mathrm{Cu}}_{\mathit{n}}$ ${\mathrm{O}}_{2\mathit{n}+4\mathrm{}}$. In analogy with graphite intercalation compounds, the charge distribution is strongly nonhomogeneous (for n≥3) with a depletion in the central layers. This result points to a special importance of the bounding layers and it provides a way to understand the observed maximum ${\mathit{T}}_{\mathit{c}}$ as a function of n.