Transition temperature and oxygen isotope effect of Nd doped cuprates: a three square well model

Abstract

Using a three square well model, we investigate the possibility that in low energy plasmons there could be an additional boson responsible for the attractive force binding the Cooper pairs that lead to superconductivity in layered electron-doped cuprates. The three square well model is, in principle, characterized by three coupling strengths, the electron-phonon (λ ph ), the electron-plasmon (λ pl ) and Coulomb screening parameter (µ * ), that estimate the transition temperature (T c ) and oxygen isotope effect coefficient (α). Starting with the three square well model within the framework of the Eliashberg theory, the energy gap kernels allow us to visualize the relative interplay of the Coulomb, electron-phonon and electron-plasmon interactions and we correlate the T c with these three coupling strengths. For a set of parameters (λ ph ≈ 1.0, λ pl ≈ 0.7 and µ * ≈ 0.18), a T c of 28 K is estimated for optimally doped Nd 1.85 Ce 0.15 CuO 4 . The present approach also explains the reported sizeable oxygen isotope effect in electron-doped cuprates. We suggest that the extended attractive force in the proposed three square well scheme consistently explains the superconductivity in electron-doped cuprates.