NMR Study of Superconductivity and Spin Fluctuations in Hole-Doped Superconductor Ca1−xNaxFe2As2 (Tc = 32 K)

Abstract

We report both 23Na and 75As nuclear magnetic resonance (NMR) studies on hole-doped Ca1−xNaxFe2As2 superconducting single crystals (x ≈ 0.67) with the superconducting temperature 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 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.