Superconductivity in lanthanum cuprates: A layered-electron-gas model.

Abstract

The nature of the superconducting state of lanthanum cuprate superconductor is discussed using strong-coupling theory, when electron-electron, electron-phonon, and electron-plasmon mechanisms are simultaneously present within a layered-electron-gas model. Treating the system as a two-component plasma, the effect of two-dimensional (2D) acoustic phonons as well as plasmons has been investigated. The approach is developed for one conducting copper oxide layer, which is an isolated free-electron layer and is well separated from insulating layers in a unit cell. A pair potential is constructed and the model parameters deduced are used to evaluate the transition temperature (${\mathit{T}}_{\mathit{c}}$) as a function of Ba and Sr doping. The approach is further applied to estimate the oxygen isotope coefficient \ensuremath{\alpha} and the energy gap parameter \ensuremath{\beta}. From these results it is argued that both 2D acoustic phonons and plasmons generated along the ${\mathrm{CuO}}_{2}$ layer play a significant role in copper oxide superconductors. In particular, 2D acoustic plasmons strongly influence ${\mathit{T}}_{\mathit{c}}$ and there is 60% enhancement over the phonon contribution to ${\mathit{T}}_{\mathit{c}}$. The computed values of ${\mathit{T}}_{\mathit{c}}$, \ensuremath{\alpha}, and \ensuremath{\beta} are consistent with the reported experimental data.