Impact of chain fragmentation on charge transfer scenario and two-plateaus-like behavior of [formula omitted] in Y Ba 2Cu 3O 6+ x

Abstract

A model of charge transfer mechanism from CuO chains to CuO 2 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 T c ( p ) ( T c ( p ) = T c , max [ 1 − 82.6 ( p ( x ) − 0.16 ) 2 ] ), the T c versus x dependence is found to have two clearly distinguished plateaus at 60 and 90 K, remarkably fitting to experimental T c ( 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 T c ′ ( 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).