Quadruple perovskites L′L″Ba₂Cu₂Ti₂O₁₁ as good candidates for high-temperature superconductors: Role of oxygen defects

The electronic structure and properties of LaYBa₂Cu₂Ti₂O₁₁, as a typical member of the L′L″Ba₂Cu₂Ti₂O₁₁ family, were determined by means of the full-potential linear muffin-tin method. From the results obtained it appears that L′L″Ba₂Cu₂Ti₂O₁₁ may be a good candidate for high-T_c superconductivity since its band structure at the Fermi energy is dominated by a quasi-two-dimensional Cu-O dpσ band with a prominent van Hove singularity. We suggest that hole doping of about 0.6 holes per unit cell (or 0.3 holes per Cu-O layer) through mono- and/or divalent metal substitution for lanthanides may lead to an optimum T_c in these materials. The role of different structural imperfections on the T_c that apparently could be created in the process of chemical doping - oxygen impurities between the Cu-O layers and oxygen vacancies in the Ti-O plane - was also determined. We found that these likely structural imperfections are unfavorable for the creation of the superconducting state in these materials.