# Charge and nematic orders in AFe$_2$As$_2$ (A= Rb,Cs) superconductors

## TL;DR

This study uses NQR and muon spin relaxation to explore charge and nematic orders in Rb and Cs iron-based superconductors, revealing a charge disproportionation crossover above nematic transition temperatures and nematic fluctuations near the transition.

## Contribution

It provides new insights into charge and nematic order interplay in AFe$_2$As$_2$ superconductors through detailed spectroscopic measurements.

## Key findings

- Charge disproportionation occurs above nematic transition temperatures.
- A crossover in spin-lattice relaxation rate indicates charge localization-delocalization.
- NQR $1/T_1$ peaks suggest critical nematic fluctuations near the transition.

## Abstract

We discuss the results of $^{75}$As Nuclear Quadrupole Resonance (NQR) and muon spin relaxation measurements in AFe$_2$As$_2$ (A= Cs, Rb) iron-based superconductors. We demonstrate that the crossover detected in the nuclear spin-lattice relaxation rate $1/T_1$ (around 150 K in RbFe$_2$As$_2$ and around 75 K in CsFe$_2$As$_2$), from a high temperature nearly localized to a low temperature delocalized behaviour, is associated with the onset of an inhomogeneous local charge distribution causing the broadening or even the splitting of the NQR spectra as well as an increase in the muon spin relaxation rate. We argue that this crossover, occurring at temperatures well above the phase transition to the nematic long-range order, is associated with a charge disproportionation at the Fe sites induced by competing Hund's and Coulomb couplings. In RbFe$_2$As$_2$ around 35 K, far below that crossover temperature, we observe a peak in the NQR $1/T_1$ which is possibly associated with the critical slowing down of electronic nematic fluctuations on approaching the transition to the nematic long-range order.