# Reduction of the $^{17}$O Knight shift in the Superconducting State and the Heat-up Effect by NMR Pulses on Sr$_2$RuO$_4$

## TL;DR

This study investigates the reduction of the $^{17}$O Knight shift in Sr$_2$RuO$_4$'s superconducting state, revealing that RF pulse power influences the observed shift and that previous invariance results were due to RF-induced superconductivity destruction.

## Contribution

The paper demonstrates that RF pulse power affects the $^{17}$O Knight shift measurements, clarifying previous conflicting results and highlighting the heat-up effect during NMR measurements.

## Key findings

- Knight shift decreases in the superconducting state with small RF pulses
- RF pulse power threshold influences spectral peak shifts
- Heat-up effects impact Knight shift measurements

## Abstract

Quite recently, a pronounced drop of $^{17}$O NMR Knight shift in the superconducting (SC) state of an unstrained Sr$_2$RuO$_4$ was reported by Pustogow and Luo ${\it et al.}$ They revealed such behavior from the free-induction decay (FID) signal after a weak RF pulse. We examined this behavior with our single-crystalline Sr$_2$RuO$_4$, and reproduced their result: the peaks of the $^{17}$O-NMR spectra shift in the SC state as long as the RF-pulse power is smaller than a threshold. Furthermore, we measured the temperature variation of the Knight shift by a standard spin-echo method with small-power RF pulses, and found that the spin susceptibility decreases in the SC state. We conclude that our previous results of the invariance of the Knight shift in the SC state were due to instantaneous destruction of superconductivity by the RF pulses. The heat-up effect was characterized by the temperature variation of the Knight shift under various measurement conditions.