O NMR Shifts*

The unconventional superconductor and Fermi liquid Sr₂RuO₄ exhibits clear effects of electronic correlations associated with Hund’s Rule coupling. In the superconducting (SC) state, several experiments produced evidence for time reversal symmetry breaking, and nuclear magnetic resonance (NMR) Knight shift measurements were consistent with an equal-spin pairing triplet state. To that end, Sr₂RuO₄ is widely considered as a potential candidate for a chiral p-wave pairing with an order parameter of p_x ± ip_y symmetry - a solid state analog to ³He-A.
Motivated by the recent observation of a strong peak in T_c across a strain-induced Lifshitz transition associated with a van Hove singularity, we studied the ¹⁷O NMR shifts in the SC state of Sr₂RuO₄. Our low-temperature experiments at high strain reveal a drop of spin susceptibility below T_c evidenced by unambiguous reduction of Knight shifts for the in-plane O sites. Most pronounced at the Lifshitz point, the loss of spin susceptibility on entering the SC state is reduced on lowering the strain, while no evidence for a phase transition between distinct SC states is observed.