水戸 毅

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 Tc = 1.3 K) and Na1−xSn2P2 (Tc = 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 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 (EF) due to the decrease in EF. 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 EF 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.