Anomalous magnetic fluctuations in superconducting Sr2RuO4 revealed by Ru101 nuclear spin-spin relaxation

Abstract

We carried out $^{101}\mathrm{Ru}$ nuclear quadrupole resonance (NQR) measurement on superconducting (SC) ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ under zero magnetic field $(H=0)$ and found that the nuclear spin-spin relaxation rate $1/{T}_{2}$ is enhanced in the SC state. The $1/{T}_{2}$ measurement in the SC state under $H=0$ is effective for detecting slow magnetic fluctuations parallel to the quantized axis of the nuclear spin. Our results indicate that low-energy magnetic fluctuations perpendicular to the ${\mathrm{RuO}}_{2}$ plane emerge when the superconductivity sets in, which is consistent with the previous $^{17}\mathrm{O}\text{-NQR}$ result that the nuclear spin-lattice relaxation rate $1/{T}_{1}$ of the in-plane O site exhibits anomalous behavior in the SC state. The enhancement of the magnetic fluctuations in the SC state is unusual and suggests that the fluctuations are related to the unconventional SC pairing. We suggest that this phenomenon is a consequence of the spin degrees of freedom of the spin-triplet pairing.