Normal State O 17 NMR Studies of Sr 2 RuO 4 under Uniaxial Stress

摘要

The effects of uniaxial compressive stress on the normal state O 17 nuclear-magnetic-resonance properties of the unconventional superconductor Sr 2 RuO 4 are reported. The paramagnetic shifts of both planar and apical oxygen sites show pronounced anomalies near the nominal a -axis strain ? a a ≡ ? v that maximizes the superconducting transition temperature T c . The spin susceptibility weakly increases on lowering the temperature below T ? 10 K , consistent with an enhanced density of states associated with passing the Fermi energy through a van Hove singularity. Although such a Lifshitz transition occurs in the γ band formed by the Ru d x y states hybridized with in-plane O p π orbitals, the large Hund’s coupling renormalizes the uniform spin susceptibility, which, in turn, affects the hyperfine fields of all nuclei. We estimate this “Stoner” renormalization S by combining the data with first-principles calculations and conclude that this is an important part of the strain effect, with implications for superconductivity.