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 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 R_AF = T₁^c/T₁^ab 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, we calculate the electric field gradient by first-principle method and assign the ⁷⁵As-NQR peaks with two crystallographically different As sites, paving the way for further NMR investigation.