The origin of the 60 K plateau in YBa2Cu3O6+x

A model for the charge transfer mechanism in YBa2Cu3O6+x high-Tc cuprate based on the critical chain length concept is proposed to account for the 60 and 90 K plateaus in the Tc(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 lcr(T) that yields a constant doping p(x)const over the regime of ortho-II phase (related to the 60 K plateau of Tc(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=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-plateau-like behavior of Tc(x) in YBa2Cu3O6+x are closely connected.