NMR and NQR studies on transition-metal arsenide superconductors LaRu2As2, KCa2Fe4As4F2, and A2Cr3As3

We report 75As-nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on transition-metal arsenides LaRu2As2, 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 ob-served in the normal state. We further find that the anisotropy rate RAF=T1c/T1ab 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.