Superconductivity emerging from an electronic phase separation in the charge ordered phase of RbFe$_2$As$_2$

TL;DR

This study uses NQR and NMR techniques to reveal charge order and electronic phase separation in RbFe$_2$As$_2$, linking these phenomena to the emergence of superconductivity and complex local electronic dynamics.

Contribution

It provides the first detailed NQR and NMR analysis of charge order and electronic phase separation in RbFe$_2$As$_2$, highlighting their role in superconductivity.

Findings

Charge order onset at ~140 K causes NQR spectrum broadening.

Below 20 K, different local electronic dynamics are observed.

Electronic correlations exhibit orbital selective behavior.

Abstract

$^{75}$As, $^{87}$Rb and $^{85}$Rb nuclear quadrupole resonance (NQR) and $^{87}$Rb nuclear magnetic resonance (NMR) measurements in RbFe$_2$As$_2$ iron-based superconductor are presented. We observe a marked broadening of $^{75}$As NQR spectrum below $T_0\simeq 140$ K which is associated with the onset of a charge order in the FeAs planes. Below $T_0$ we observe a power-law decrease in $^{75}$As nuclear spin-lattice relaxation rate down to $T^*\simeq 20$ K. Below that temperature the nuclei start to probe different dynamics owing to the different local electronic configurations induced by the charge order. A fraction of the nuclei probes spin dynamics associated with electrons approaching a localization while another fraction probes activated dynamics possibly associated with a pseudogap. These different trends are discussed in the light of an orbital selective behaviour expected for the electronic correlations.