Electronic structure and properties of YBa₂Cu₄O₈

The electronic structure and properties of the new high-T_c superconductor, YBa₂Cu₄O₈ (called Y124), have been determined using the local-density full potential linearized augmented plane wave method. Despite the additional layer of Cu(1)-O(1) chains relative to YBa₂Cu₃O₇ (called Y123): (i) the band structure at and near E_F is simplified with the absence of the Cu(1)-O(1) dpπ band which plagued the Y123 results; (ii) the Fermi surface is also simplified, shows greater nestings, but still cannot be represented by a simple nearest neighbor tight-binding Hamiltonian model; (iii) the density of states of E_F is one-half of that in Y123 (in good agreement with experiment). Further, as a result of (i) and (iii) together with the lower Madelung potential at the O(1) sites, which leads to the higher binding energy for O(1) the Cu(1)-O(1) chains are now stable and Y124 should be stable in its oxygen stoichiometry - as its observed.