Quantum criticality in AFe₂As₂ (A = K, Rb, Cs) superconductors probed by ⁷⁵As NMR spectroscopy

We present 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 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 for in-plane applied fields 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, magnetically ordered phase.