Increasing stripe-type fluctuations in $A$Fe${}_2$As${}_2$ ($A$ = K, Rb, Cs) superconductors probed by ${}^{75}$As NMR spectroscopy

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.

5 pages, 4 figures