In-plane ordering of oxygen vacancies in a high-Tc cuprate superconductor with compressed Cu-O octahedrons: An automated cluster expansion study

A recently discovered 73-K cuprate superconductor Ba(2)CuO(4-delta )implies that intact CuO2 planes are not absolutely necessary in achieving high-T-c superconductivity. Featuring an exceptional Jahn-Teller distortion, wherein the CuO6 octahedrons are compressed along the c axis, O vacancies in this material prefer to reside in the CuO2 plane, which significantly modifies the two-dimensional (2D) square lattice. By combining first-principles total-energy calculation with the automated structure inversion method, the effective cluster interactions of O vacancies are mapped out. Around delta = 0.8, where the superconductivity was observed experimentally, we predict that the O vacancies form a long-range order, which slice the CuO2 plane into 1D chains and two-leg ladders. The latter was not known to exist in other cuprates. A Monte Carlo simulation is performed based on the effective cluster interaction model, showing that such an ordering pattern is stable up to 900 K. Our results put forth a concrete structural basis to discuss the underlying superconducting mechanism.