Evidence of conventional superconductivity in La-Ba-Cu-O compounds

First-principles methods are applied to investigate the electronic structure and electron-phonon coupling in the end-point members (x=0 and x=1) of the new high-transition-temperature ($T_c$) superconductor ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Ba}}_{\mathrm{x}}$ ${\mathrm{CuO}}_4$ and their counterpart cubic perovskites ${\mathrm{La}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Ba}}_{\mathrm{x}}$ ${\mathrm{CuO}}_3$. Alloying with Ba (x=1) induces non-rigid-band changes in the Cu and O density of states, but the Cu charge state changes rather little. The calculated McMillan-Hopfield parameter η coupled with soft phonon modes provides a strong electron-phonon interaction λ∼2.5 that can account for the high $T_c$ of ∼30–40 K in these systems.