Suppression of ferromagnetic spin fluctuations in the filled skutterudite superconductor SrOs4As12 revealed by 75As NMR-NQR measurements

TL;DR

This study uses $^{75}$As NMR and NQR measurements to show that the superconductor SrOs$_4$As$_{12}$ exhibits suppressed ferromagnetic spin fluctuations compared to its iron-based counterpart, indicating an $s$-wave pairing mechanism.

Contribution

It provides the first NMR/NQR evidence of suppressed ferromagnetic fluctuations in SrOs$_4$As$_{12}$, highlighting differences from SrFe$_4$As$_{12}$ and clarifying its superconducting nature.

Findings

No significant ferromagnetic spin correlations in SrOs$_4$As$_{12}$

Temperature-independent Knight shift consistent with Pauli paramagnetism

Evidence of $s$-wave superconductivity from relaxation rate data

Abstract

Motivated by the recent observation of ferromagnetic spin correlations in the filled skutterudite SrFe$_4$As$_{12}$ [Ding et al., Phys. Rev. B 98, 155149 (2018)], we have carried out $^{75}$As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements to investigate the role of magnetic fluctuations in a newly discovered isostructural superconductor SrOs$_4$As$_{12}$ with a superconducting transition temperature of $T_{\rm c}$ $\sim$ 4.8 K. Knight shift $K$ determined by the NQR spectrum under a small magnetic field ($\le$ 0.5 T) is nearly independent of temperature, consistent with the temperature dependence of the magnetic susceptibility. The nuclear spin-lattice relaxation rate divided by temperature, 1/$T_1T$, is nearly independent of temperature above $\sim$ 50 K and increases slightly with decreasing temperature below the temperature. The temperature dependence is reasonably explained by a simple model where a flat band structure with a small ledge near the Fermi energy is assumed. By comparing the present NMR data with those in SrFe$_4$As$_{12}$, we found that the values of $|K|$ and $1/T_1T$ in SrOs$_4$As$_{12}$ are smaller than those in SrFe$_4$As$_{12}$, indicating no obvious ferromagnetic spin correlations in SrOs$_4$As$_{12}$. From the temperature dependence of 1/$T_1$ in the superconducting state, an $s$-wave superconductivity is realized.