Impact of chain fragmentation on charge transfer scenario and two-plateaus-like behavior of Tc(x) in Y Ba2Cu3O6+x

A model of charge transfer mechanism from CuO chains to CuO2 planes has been proposed to account for doping of the planes, assuming that only chains containing more than three oxygen atoms can contribute to hole transfer. Only chains with l≥4 are assumed to have transferred a certain fraction, approximately 40%, of the holes created by oxygen added to the chain beyond the first three oxygen atoms. Using the so obtained x dependence of doping, p(x), at constant (room) temperature and utilizing empirical parabolic phase relation Tc(p) (Tc(p)=Tc,max[1−82.6(p(x)−0.16)2]), the Tc versus x dependence is found to have two clearly distinguished plateaus at 60 and 90 K, remarkably fitting to experimental Tc(x). The effect of statistics of CuO chain fragmentation has been included by applying cluster variation method to two dimensional asymmetric next-nearest-neighbor Ising model that is employed to describe oxygen-chain ordering in basal planes. The obtained results indicate that plateaus, coinciding with Tc′(x)=0, emerge either when p′(x)=0 (p(x)≈const), in the region of OII phase formation (the 60 K plateau), or when p=0.16, representing the optimal doping at x≈0.91 in OI phase (the 90 K plateau).