Site-Selective NMR=NQR Study on Layered Tin Pnictide Superconductor NaSn₂Pn₂ (Pn = P and As)

We present the results of nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on the layered tin pnictide superconductors NaSn(2)Pn(2) (the superconducting transition temperature T-c = 1.3 K) and Na1-xSn(2)P(2) (T-c= 2.0 K). In NaSn(2)Pn(2), a broad 75As NQR spectrum indicates a large charge distribution at the As site, and a broad 119Sn NMR spectrum indicates a distributed local density of states (DOS) at the Sn site. Almost the same level of microscopic disorder in the electronic state is also found at the Sn site in Na1-xSn2P2 as well. These results clearly indicate the presence of a local atomic disorder in these superconductors. We have also measured the nuclear spin-lattice relaxation rate 1/T1 to gain insight into the partial DOS at each site. Our NMR/NQR results are consistent with previous band calculations, indicating that Na deficiency results in an increase in the DOS at the Fermi energy (E-F) due to the decrease in E-F. The agreement between the experimental and band calculation results makes it more plausible that the increase in the Tc of Na1-xSn2P is due to the increase in the DOS at E-F caused by the Na deficiency. This study thus demonstrates that NMR/NQR measurements can be a very effective tool in revealing the local electronic structure of materials by comparing experimental results with theoretical band calculations.