In-plane ordering of O vacancies in a high-<i>T_c</i> cuprate superconductor with compressed Cu-O octahedrons: an automated cluster expansion study

A recently discovered 73K cuprate superconductor Ba₂CuO_4-δ implies that intact CuO₂ planes are not absolutely necessary in achieving high-Tc 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 modify the 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 δ=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.