# Increasing stripe-type fluctuations in $A$Fe$_{2}$As$_{2}$ ($A$ = K, Rb, Cs) superconductors probed by $^{75}$As NMR spectroscopy

## TL;DR

This study uses $^{75}$As NMR spectroscopy to show that increasing lattice size in $A$Fe$_{2}$As$_{2}$ superconductors enhances stripe-type spin fluctuations and electronic correlations, approaching a quantum critical point.

## Contribution

It reveals how lattice expansion in $A$Fe$_{2}$As$_{2}$ compounds intensifies spin fluctuations and deviations from Fermi-liquid behavior, indicating proximity to a quantum critical point.

## Key findings

- Enhanced stripe-type spin fluctuations with larger alkali ions.
- Decreased Fermi-liquid behavior evidenced by relaxation rate exponents.
- Approach to a quantum critical point with lattice expansion.

## Abstract

We report $^{75}$As nuclear magnetic resonance measurements on single crystals of RbFe$_{2}$As$_{2}$ and CsFe$_{2}$As$_{2}$. Taking previously reported results for KFe$_{2}$As$_{2}$ into account, we find that the anisotropic electronic correlations evolve towards a magnetic instability in the $A$Fe$_{2}$As$_{2}$ 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 $(1/T_{1})_{H\parallel{ab}}$, 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 $A$Fe$_{2}$As$_{2}$ series tunes the electronic correlations towards a quantum critical point at the transition to a yet unobserved, ordered phase.