A model for the chain-to-plane charge transfer in YBa2Cu3O6+x
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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 YBa2Cu3O6+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 LT 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 bri...efly discussed.