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 L8L9Ba2Cu2Ti2O11 family, were determined by means of the full-potential linear muffin-tin method. From the results obtained it appears that L8L9Ba2Cu2Ti2O11 may be a good candidate for high-Tc superconductivity since its band structure at the Fermi energy is dominated by a quasi-two-dimensional Cu-O dps 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 monoand/or divalent metal substitution for lanthanides may lead to an optimum Tc in these materials. The role of different structural imperfections on the Tc 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.