Impact of chain fragmentation on charge transfer scenario and two-plateaus-like behavior of Tc(x) in YBa2Cu3O6+x

TL;DR

This paper models the charge transfer from CuO chains to CuO2 planes in YBa2Cu3O6+x, explaining the two-plateau behavior of Tc(x) through chain fragmentation and oxygen ordering effects.

Contribution

It introduces a chain fragmentation model combined with oxygen ordering to explain the Tc(x) plateaus in YBa2Cu3O6+x, linking microscopic chain behavior to macroscopic superconducting properties.

Findings

Two distinct Tc plateaus at 60K and 90K match experimental data.

Plateaus correspond to regions of constant doping p(x) or optimal doping.

Chain fragmentation and oxygen ordering are key to understanding Tc(x) behavior.

Abstract

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 length l greater than 4 are assumed to have transferred a certain fraction, approximately 40%, of the holes created by oxygen atoms 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 obtained to have two clearly distinguished plateaus at 60K and 90K, 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 dTc(x)/dx=0, emerge either when dp(x)/dx=0 (p(x)=const), in the region of OII phase formation (the 60K plateau), or when p=0.16, representing the optimal doping at x=0.91 in OI phase (the 90K plateau).