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.

中文翻译：

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层状锡磷族元素超导体 NaSn2Pn2 (Pn = P 和 As) 的位点选择性 NMR/NQR 研究

我们展示了层状锡磷族元素超导体 NaSn ₂ As ₂（超导转变温度T _c = 1.3 K）和 Na _{1− x} Sn ₂ P ₂ ( Tc ₌ 2.0 K）。在NaSn ₂ As ₂中，宽的⁷⁵ As NQR谱表明As位点处的大电荷分布，并且宽的¹¹⁹ Sn NMR谱表明Sn位点处的分布局部态密度(DOS)。_{在Na 1− x} Sn ₂ P ₂的Sn位点处也发现了几乎相同程度的电子态微观无序。这些结果清楚地表明这些超导体中存在局部原子无序。我们还测量了核自旋-晶格弛豫率 1/ T ₁以深入了解每个位点的部分 DOS。我们的 NMR/NQR 结果与之前的能带计算结果一致，表明 Na 缺乏会导致费米能 ( E _F ) 的 DOS 由于E _F的降低而增加。实验结果和能带计算结果之间的一致性使得Na _{1− x} Sn _{2 P的}T _c增加是由于Na缺乏导致E _F处的DOS增加而更加合理。因此，这项研究表明，通过将实验结果与理论能带计算进行比较，NMR/NQR 测量可以成为揭示材料局部电子结构的非常有效的工具。