摘要
We report both 23Na and 75As nuclear magnetic resonance (NMR ) studies on hole-doped Ca_(1-x)Na_(x)Fe_(2)As_(2) superconducting single crystals (x ≈ 0.67) with the superconducting temperature Tt =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 =2K.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.
基金
Supported by the National Basic Research Program of China under Grant Nos 2010CB923004 and 2011CBA00112
the National Natural Science Foundation of China under Grant Nos 11074304,11104070,11004245 and 10974254
the Program for New Century Excellent Talents in University under Grant No NCET-08-0546
the Fundamental Research Funds for the Central Universities of China.
