Increasing stripe-type fluctuations in AFe2As2 (A = K, Rb, Cs) superconductors probed by 75As NMR spectroscopy

We report As-75 nuclear magnetic resonance measurements on single crystals of RbFe2As2 and CsFe2As2. Taking previously reported results for KFe2As2 into account, we find that the anisotropic electronic correlations evolve towards a magnetic instability in the AFe(2)As(2) series (with A = K, Rb, Cs). Upon isovalent substitution with larger alkali-metal ions, a drastic enhancement of the anisotropic nuclear spin-lattice relaxation rate and decreasing Knight shift reveal the formation of pronounced spin fluctuations with stripe-type modulation. Furthermore, a decreasing power-law exponent of the nuclear spin-lattice relaxation rate (1/T-1) (H) (parallel to) (ab), probing the in-plane spin fluctuations, evidences an emergent deviation from Fermi-liquid behavior. All these findings clearly indicate that the expansion of the lattice in the AFe(2)As(2) series tunes the electronic correlations towards a quantum critical point at the transition to a yet unobserved ordered phase.