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 NaSn₂As₂ (the superconducting transition temperature T_c = 1.3 K) and Na_1−xSn₂P₂ (T_c = 2.0 K). In NaSn₂As₂, 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 Na_1−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 T_c of Na_1−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.