Rare earth ion effects on the pseudo-gap in electron-doped superconductors and possible nodeless d-wave gap

We report angle resolved photoemission (ARPES) studies on electron-doped cuprate superconductor Sm2-xCexCuO4Sm2-xCexCuO4 (x=0.14x=0.14 and 0.18). A wide energy range scan shows clear “waterfall” effect at an energy scale close to 500 meV which is consistent with the value found in Nd2-xCexCuO4Nd2-xCexCuO4 (NCCO) but larger than that from hole-doped superconductors. High resolution results from both dopings show pseudo-gap effects that were observed in NCCO. However, the effects are found to be stronger than that observed in optimally doped NCCO. The overall electronic structure is well understood within a simple model in which a 2×2 static order is assumed. Both ARPES and optical measurements give the coupling strengths to the QQ=(π/2,π/2π/2,π/2) (due to the 2×2 order) to be about 0.1 eV, compatible with each other. The effect is strong enough to push the band near the nodal region below the Fermi energy, resulting in possible nodeless d-wave superconductivity where zero energy quasi-particle excitation is inhibited.