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 (\(1/T_1\)) have revealed two distinct CDW orders, one realized in the As-Pt-As layer below \(T_{\rm CDW}^{\rm As(1)}\) \(=\) 410 K and the other in the Pt-As-Pt layer below \(T_{\rm CDW}^{\rm As(2)}\) \(=\) 255 K. The \(1/T_1\) 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.