Coexistence of multiple charge-density waves and superconductivity in SrPt2As2 revealed by 75As-NMR/NQR and 195Pt-NMR

The relationship between charge density wave (CDW) orders and superconductivity in arsenide superconductor SrPt${}_2$As${}_2$ with $T_c$ = 5.2 K which crystallizes in the CaBe${}_2$Ge${}_2$-type structure was studied by ${}^{75}$As nuclear magnetic resonance (NMR) measurements up to 520 K, and ${}^{75}$As nuclear quadrupole resonance (NQR) and ${}^{195}$Pt-NMR measurements down to 1.5 K. At high temperature, ${}^{75}$As-NMR spectrum and nuclear spin relaxation rate ($1\mathrm{/}{T}_1$) have revealed two distinct CDW orders, one realized in the As-Pt-As layer below $T_{\mathrm{C}\mathrm{D}\mathrm{W}}^{\mathrm{A}\mathrm{s}(1)}$ $=$ 410 K and the other in the Pt-As-Pt layer below $T_{\mathrm{C}\mathrm{D}\mathrm{W}}^{\mathrm{A}\mathrm{s}(2)}$ $=$ 255 K. The $1\mathrm{/}{T}_1$ measured by ${}^{75}$As-NQR shows a clear Hebel-Slichter peak just below $T_c$ and decreases exponentially well below $T_c$. Concomitantly, ${}^{195}$Pt Knight shift decreases below $T_c$. Our results indicate that superconductivity in SrPt${}_2$As${}_2$ is in the spin-singlet state with an $s$-wave gap and is robust under the two distinct CDW orders in different layers.