Effects of chemical pressure on the Fermi surface and band dispersion of the electron-doped high-T<SUB>c</SUB> superconductors

We have performed angle-resolved photoemission spectroscopy measurements and first-principles electronic-structure calculations on the electron-doped high-Tc superconductors (HTSCs) Ln1.85Ce0.15CuO4 (Ln=Nd, Sm, and Eu). The observed Fermi surface and band dispersion show such changes that with decreasing ionic size of Ln3+ (increasing chemical pressure), the curvature of the Fermi surface or -t'/t decreases, where t and t' are transfer integrals between the nearest-neighbor and next-nearest-neighbor Cu sites, respectively, explaining the apparently inconsistent behavior seen in the hole-doped HTSC La2-xSrxCuO4 under epitaxial strain. Around the node, the antiferromagnetic gap is opened with increasing chemical pressure. We propose that the nodal gap opening is possibly due to the decrease in -t'/t through the improved nesting, leading to the decrease in Tc