Electronic mechanism of critical temperature variation in RBa_2Cu_3O_(7− δ)

We have performed systematic studies of the trend of the critical temperature T_c due to both Madelung site potential difference between in-plane oxygen and copper sites ΔV_M and interlayer effect in the optimally doped 123 superconductors RBa_2Cu_3O_(7−δ). ΔV_M is found to decrease with the increase of the trivalent rare-earth ionic radius r_(R^(3^+)). This change enhances the next-nearest-neighbor hopping integral t′, which results in the experimentally observed increase of T_c with r_(R^(3^+)). The coherent interlayer single-particle hopping t_⊥ has a more profound effect than t′ on the nearly linear trend of T_c as a function of r_(R^(3^+)). These results reveal the importance of the electronic origin of the rare-earth ionic size effect on T_c in this family