NMR and NQR studies on transition-metal arsenide superconductors LaRu₂As₂, KCa₂Fe₄As₄F₂, and A ₂Cr₃As₃

We report 75As-nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on transition-metal arsenides LaRu2As 2, KCa2Fe4As4F2, and A 2Cr3As3. In the superconducting state of LaRu2As2, a Hebel-Slichter coherence peak is found in the temperature dependence of the spin-lattice relaxation rate 1/T 1 just below T c, 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 higherature superconductor with a stoichiometric composition. For A 2Cr3As3 (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.