The origin of the 60 K plateau in YBa(2)Cu(3)O(6+x)

A model for the charge transfer mechanism in YBa(2)Cu(3)O(6+x) high-T(c) cuprate based on the critical chain length concept is proposed to account for the 60 and 90 K plateaus in the T(c)(x) dependence. It has been shown that, when the statistics of CuO chain formation was described in terms of the two-dimensional asymmetric next-to-nearest neighbor Ising (ASYNNNI) model, at any constant temperature below the top of ortho-II phase there exists a uniquely defined value of critical chain length l(cr)(T) that yields a constant doping p(x) approximate to const over the regime of ortho-II phase (related to the 60 K plateau of T(c)(x)), while the 90 K plateau coincides with the monotonically increasing p(x) over the optimal doping level p = 0.16 in the regime of the ortho-I phase. Short length chains (l LT l(cr)(T)), together with the first l(cr)(T)-2 holes in longer chains (l GT = l(cr)(T)), are taken as not capable of attracting electrons from CuO(2) planes. It is shown that only a part (approximate to 41%) of the remaining l - l(cr)(T)+1 holes in the long chains can capture electrons. The results obtained indicate that the ASYNNNI model and two-plateau-like behavior of T(c)(x) in YBa(2)Cu(3)O(6+x) are closely connected