NMR Study of Superconductivity and Spin Fluctuations in Hole-Doped Superconductor Ca_1-xNa_xFe₂As₂ (T_c=32 K)

We report both ²³Na and ⁷⁵As nuclear magnetic resonance (NMR) studies on hole?doped Ca_1-xNa_xFe₂As₂ superconducting single crystals (x≈0.67) with the superconducting temperature T_c=32 K. Singlet superconductivity is suggested by a sharp drop of the Knight shift ⁷⁵K below T_c. 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 μm at temperature T=2 K. Here 1/⁷⁵T₁T shows an anisotropic behavior and a prominent low-temperature upturn , with ⁷⁵T₁ denoting the ⁷⁵As spin-lattice relaxation time and T the temperature, which indicates strong low-energy antiferromagnetic spin fluctuations and supports a magnetic origin of superconductivity.