  A model for charge transfer mechanism in YBa2Cu3O6+x high-Tc cuprate based on
critical chain length concept is proposed to account for 60K and 90K plateaus
in Tc(x) dependence. It has been shown, when the statistics of CuO chain
formation was described in terms of two dimensional asymmetric next-to-nearest
neighbor Ising (ASYNNNI) model, that at any constant temperature below the top
of OII phase there exists a uniquely defined value of critical chain length
lcr(T) that yields a constant doping p(x)=const over the regime of OII phase
(related to 60K plateau of Tc(x)), while 90K plateau coincides with the
monotonously increasing p(x) over optimal doping level p=0.16 in the regime of
OI phase. Short length chains (l<lcr(T)), together with the first lcr(T)-2
holes in longer chains (l>lcr(T)), are taken as not capable of attracting
electrons from CuO2 planes.. It is shown that only a part (41%) of the
remaining l-lcr(T)+1 holes in the long chains can capture electrons. The
results obtained indicate that the ASYNNNI model and two-plateaus-like behavior
of Tc(x) in YBa2Cu3O6+x are closely connected.
