  Recent STM measurements have revealed the existence of periodic charge
modulations at the surface of certain cuprate superconductors. Here we show
that the observed patterns are compatible with the formation of a
three-dimensional crystal of doped holes, with space correlations extending
between different Cu-O layers. This puts severe constraints on the dynamical
stability of the crystallised hole structure, resulting in a close relationship
between the periodicity of the electronic modulation and the interlayer
distance.
