Normal state $^{17}$O NMR studies of Sr$_{2}$RuO$_{4}$ under uniaxial stress

TL;DR

This study investigates how uniaxial stress affects the normal state $^{17}$O NMR properties of Sr$_{2}$RuO$_{4}$, revealing anomalies near optimal strain and linking them to changes in electronic structure and spin susceptibility.

Contribution

It provides new insights into the strain-induced electronic and magnetic changes in Sr$_{2}$RuO$_{4}$, highlighting the role of a Lifshitz transition and Hund's coupling in its superconducting properties.

Findings

Paramagnetic shifts show anomalies near optimal strain.

Spin susceptibility increases below 10 K, indicating a van Hove singularity.

Strain effects are significantly influenced by Stoner renormalization.

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 $\varepsilon_{aa}\equiv\varepsilon_v$, 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.