Band-Selective Modification of the Magnetic Fluctuations in Sr2RuO4: Study of Substitution Effects

We report a study of magnetic, thermal, and transport properties of La(3+) substituted Sr2RuO4, performed in order to investigate the effects of additional electron doping in this correlated metal. A gradual enhancement of the electronic part of specific heat and a more drastic increase of the static magnetic susceptibility were observed in Sr(2-y)La(y)RuO(4) with increasing y. Furthermore, the quasi-two-dimensional Fermi-liquid behavior seen in pure Sr2RuO4 breaks down near the critical concentration y ~ 0.20. Combined with a realistic tight-binding model with rigid-band shift of Fermi level, the enhancement of the density of states can be ascribed to the elevation of the Fermi energy toward a van Hove singularity of the thermodynamically dominant \gamma Fermi-surface sheet. On approaching the van Hove singularity, the effective nesting-vector of the \gamma band shrinks and further enhances the susceptibility near the wave vector q ~ 0. We attribute the non-Fermi-liquid behavior to two-dimensional ferromagnetic fluctuations with short range correlations at the van Hove singularity. The observed behavior is in sharp contrast to that of Ti(4+) substitution in Sr2RuO4 which enhances antiferromagnetic fluctuations and subsequently induces incommensurate magnetic ordering associated with the nesting between the other Fermi-surface sheets (\alpha and \beta). We thus establish that substitution of appropriate chemical dopants can band-selectively modify the spin-fluctuation spectrum in the spin-triplet superconductor Sr2RuO4.