NMR study of superconductivity and spin fluctuations in hole-doped   superconductor Ca1-xNaxFe2As2 (Tc =32 K)
  We report both 23Na and 75As NMR studies on hole-doped Ca1-xNaxFe2As2 superconducting single crystals (x\approx 0.67) with Tc =32 K. Singlet superconductivity is suggested by a sharp drop of the Knight shift 75K below Tc. The spin-lattice relaxation rate 1/T1 does not show the Slichter-Hebel coherence peak, which suggests an unconventional pairing. The penetration depth is estimated to be 0.24 {\mu}m at T=2 K. 1/75T1T shows an anisotropic behavior and a prominent low-temperature upturn, which indicates strong low-energy antiferromagnetic spin fluctuations and supports a magnetic origin of superconductivity. 
Long Ma, J. S. Zhang, D. M. Wang, J. B. He, T.-L. Xia, G. F. Chen and   Weiqiang Yu
cond-mat.supr-con