Origin of the heavy fermion behavior in Ca2-xSrxRuO4: Roles of Coulomb interaction and the rotation of RuO6 octahedra

We study the electronic states for Ca2-xSrxRuO4 in 0.5 <= x <= 2 within the Gutzwiller approximation (GA) on the basis of the three-orbital Hubbard model for the Ru t(2g) orbitals. The main effects of the Ca substitution are taken into account as the changes of the dp hybridizations between the Ru 4d and O 2p orbitals. Using the numerical minimization of the energy obtained in the GA, we obtain the renormalization factor (RF) of the kinetic energy and total RF, which estimates the inverse of the mass enhancement, for three cases with the effective models of x = 2 and 0.5 and a special model. We find that the inverse of the total RF becomes the largest for the case of x = 0.5, and that the van Hove singularity, which is located on (below) the Fermi level for the special model (the effective model of x = 0.5), plays a secondary role in enhancing the effective mass. Our calculation suggests that the heavy fermion behavior around x = 0.5 comes from the cooperative effects between moderately strong Coulomb interaction compared to the total bandwidth and the modification of the electronic structures due to the rotation of RuO6 octahedra (i.e., the variation of the dp pi hybridizations and the downward shift for the d(xy) orbital). We propose that moderately strong electron correlation and the orbital-dependent modifications of the electronic structures due to the lattice distortions play important roles in the electronic states for Ca2-xSrxRuO4.