NMR and NQR studies on transition-metal arsenide superconductors LaRu2As2, KCa2Fe4As4F2, and A2Cr3As3**Project supported by the National Natural Science Foundation of China (Grant Nos. 11674377, 11634015, and 11974405), the National Key R&D Program of China (Grant Nos. 2017YFA0302904 and 2016YFA0300502), and J. Y. also acknowledges support by the Youth Innovation Promotion Association of Chinese Academy of Sciences.

Abstract

We report 75As-nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on transition-metal arsenides LaRu2As 2, KCa2Fe4As4F2, and A2Cr3As3. In the superconducting state of LaRu2As2, a Hebel–Slichter coherence peak is found in the temperature dependence of the spin-lattice relaxation rate 1/T1 just below Tc, which indicates that LaRu2As2 is a full-gap superperconducor. For KCa2Fe4As4F2, antiferromagnetic spin fluctuations are observed in the normal state. We further find that the anisotropy rate is small and temperature independent, implying that the low energy spin fluctuations are isotropic in spin space. Our results indicate that KCa2Fe4As4F2 is a moderately overdoped iron-arsenide high-temperature superconductor with a stoichiometric composition. For A2Cr3As3 (A = Na, K, Rb, Cs), we calculate the electric field gradient by first-principle method and assign the 75As-NQR peaks to two crystallographically different As sites, paving the way for further NMR investigation.