Normal state ${}^{17}$O NMR studies of Sr${}_2$RuO${}_4$ under uniaxial stress

Abstract

The effects of uniaxial compressive stress on the normal state ${}^{17}$O 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 $\mathbf{a}$-axis strain ${\epsilon }_{aa}\equiv {\epsilon }_v$, that maximizes the superconducting transition temperature, $T_c$. The spin susceptibility weakly increases on lowering the temperature below $T$$\simeq$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 $\gamma$ band, formed by the Ru $d_{xy}$ states hybridized with in-plane O $p_{\pi }$ orbitals, the large Hund's coupling renormalizes the uniform spin susceptibilty, which, in turn, affects the hyperfine fields of all nuclei. We estimate this \textquotedblleft Stoner\textquotedblright\ 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.

15 pages, 9+6 figures, 0+1 table