The Existence of the 60 K Plateau in the YBa2Cu3O6+y Phase Diagram: the Role of Oxygen Ordering and Charge Imbalance

We propose a semi-microscopic model of YBa2Cu3O6+y to investigate the origin of the 60 K plateau in its phase diagram. Our model is a “phase only ” approach to the high-temperature superconducting system in terms of collective variables. It is able to capture characteristic energy scales present in YBa2Cu3O6+y by using adjustable parameters representing phase stiff-nesses and allows for strong anisotropy within basal planes to simulate oxy-gen ordering. We solve the model calculated Tc for chosen system parameters investigating the influence of oxygen ordering and doping imbalance on the shape of YBa2Cu3O6+y phase diagram. Our results suggest that the oxy-gen ordering alone does not seem to be responsible for the existence of the 60 K plateau. However, relying on experimental data unveiling that oxygen doping of YBa2Cu3O6+y may introduce a significant charge imbalance be-tween CuO2 planes and other sites, we show that simultaneously the former are underdoped, while the latter — strongly overdoped almost in the whole region of oxygen doping in which YBa2Cu3O6+y is superconducting. This provides two natural counter acting factors, which possibly lead to rise the 60 K plateau with increasing oxygen doping. PACS numbers: 74.20.–z, 74.72.Bk, 74.62.–c 1