Influence of oxygen ordering and charge imbalance on the existence of the 60-K plateau in the YBa2Cu3O6+y phase diagram

We study a 'phase only' model of YBa2Cu3O6+y, which captures characteristic energy scales present in YBCO and allows for strong anisotropy within basal planes to investigate the influence of oxygen ordering and charge imbalance on the critical temperature Tc(y). We describe the high-temperature superconducting system in terms of collective variables utilizing a three-dimensional semi-microscopic XY model with two-component vectors that involve phase variables and adjustable parameters representing microscopic phase stiffnesses. With the help of the spherical closure relation and the transfer matrix method we solve the XY model and determine Tc for chosen system parameters and discuss a possible origin of characteristic feature of YBCO phase diagram: the 60-K plateau. We find it unlikely that the plateau is a result of oxygen ordering within Cu-O chains, however can be caused by a charge redistribution among CuO2 planes and apical sites