A model for the chain-to-plane charge transfer in YBa₂Cu₃O_6+x

A model for the chain-to-plane charge transfer is proposed to account for the two plateaus, at 60 K and at 90 K, of the T_c(x) characteristics of the YBa₂Cu₃O_6+x high-T_c superconductor. It is assumed that the number of holes transferred from a CuO chain of length l to two nearby CuO₂ sheets is proportional to l (that is, to the number of oxygen atoms in the chain), if the chain length is greater than, or equal to, a certain critical chain length, l_cr, that is required to trigger the charge transfer process. No holes are assumed to have been transferred from chains of length l<l_cr. The calculated T_c(x) dependence is found to be in excellent agreement with the experimentally reported T_c(x). The critical chain length parameter is estimated to be equal to l_cr=11 (eleven oxygen atoms in a chain), which is a greater value than that obtained in the previously proposed model for the chain-to-plane charge transfer (l_cr=4). The results obtained out of the proposed model are briefly discussed.