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

The relationship between charge-density wave (CDW) orders and superconductivity in arsenide superconductor SrPt2As2 with Tc=5.2K which crystallizes in the CaBe2Ge2-type structure was studied by 75As nuclear magnetic resonance (NMR) measurements up to 520 K, and 75As nuclear quadrupole resonance (NQR) and Pt195-NMR measurements down to 1.5 K. At high temperature, As75-NMR spectrum and nuclear-spin-relaxation rate (1/T1) have revealed two distinct CDW orders, one realized in the As-Pt-As layer below TCDWAs(1)=410K and the other in the Pt-As-Pt layer below TCDWAs(2)=255K. The 1/T1 measured by As75-NQR shows a clear Hebel-Slichter peak just below Tc and decreases exponentially well below Tc. Concomitantly, Pt195 Knight shift decreases below Tc. Our results indicate that superconductivity in SrPt2As2 is in the spin-singlet state with an s-wave gap and is robust under the two distinct CDW orders in different layers.