Charge transfer mechanism and T-c(x) dependence in Y-0.8(Ca)(0.2)Ba2Cu3O6+x
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We propose a model for the charge transfer mechanism in Y0.8(Ca)0.2Ba2Cu3O6+x to count the hole doping of CuO2 planes and the x dependence of the critical transition temperature Tc. It is assumed that the total number of doped holes in the planes is the sum of the numbers of holes that are introduced through two separate channels: substitution of Y3+ by Ca2+ and from CuO chains that are longer than a minimal (critical) length lmin needed for charge transfer to take place. The Tc(x) dependence is obtained by combining the calculated x dependence of doping, p(x), and the universal Tc versus p relation. Although calculated Tc(x) dependences for lmin = 3 and 4 both correlate remarkably well with the experimental Tc(x), we argue that the value lmin = 3 gives a reasonable overall agreement.