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

We report ⁷⁵As 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 AFe₂As₂ 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/T₁)_HIIab, 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 AFe₂As₂ series tunes the electronic correlations towards a quantum critical point at the transition to a yet unobserved ordered phase.