A model for the aquasia 60 K plateau in the YBa2Cu3O6+x high-Tc compound

The 60 K plateau of the YBa2Cu3O6+x high-Tc superconducting compound has traditionally been known as a horizontal section in its Tc(x) characteristics within which the critical temperature maintains at a constant level Tc a 60 K. However, recent experiments on a series of high quality single crystals samples of the YBa2Cu3O6+x compound (produced by improved techniques of sample preparation) have shown the 60 K plateau does not manifest itself anymore as a horizontal section of the Tc(x), but emerges as a section over which Tc(x) keeps on continuously rising (the aquasia 60 K plateau). Here we propose a theoretical model that explains how the classical (horizontal) 60 K plateau naturally evolves into the aquasia 60 K plateau, due to the improved orthorhombicity in the high quality single crystal material accompanied by longer CuO chains. The model assumes that short CuO chains do not contribute to the hole transfer from chains to CuO2 layers, but only chains whose length is greater than, or equal to, a certain threshold length lcr. Our calculations point to a conclusion that this minimal number of oxygen atoms in a CuO chain, that is required to trigger the hole transfer, is equal to four. The calculated values for Tc(x) are in a very good correlation with the corresponding experimentally obtained results.