One of the peculiar aspects of iron-based superconductors (IBS) is the presence of several bands crossing the Fermi level, associated with the different Fe 3d orbitals. When the electronic correlations are large, Hund coupling may promote the single band occupancy and decouple the interband charge correlations, yielding to a markedly different behaviour for the electron excitations in the different bands. On the other hand, while in the cuprates it is well established that a charge density wave develops in the underdoped regime, in the IBS the observation of charge orders remains elusive and often the underlying driving mechanism, CDW, orbital or nematic, remains unclear. RbFe2As2 (Tc = 2.7 K) is an IBS close to half-band filling where the significant electronic correlations may lead to an orbital selective behaviour and possibly to a charge order which can be suitably investigated by means of Nuclear Quadrupole Resonance (NQR). We observed that upon cooling RbFe2As2 below T0≃140 K the 75As NQR  spectrum progressively broadens and below 50 K it is characterized by two peaks, with a shape that recalls the one expected for an incommensurate CDW, where the charge order causes a modulation of the electric field gradient (EFG) probed by the nuclei. Even if we cannot exclude that the broadening of the spectrum could results from an orbital or from a nematic order, we can state unambiguosuly that the broadening is associated with a charge order. The change in the local charge distribution at T0was found to affect also the muSR relaxation rate likely due to a change in the mu+implantation site.
The low-energy spin excitations can be conveniently investigated by means of nuclear spin-lattice relaxation rate (1/T1) measurements. While at T ≥ 20K the recovery of 75As nuclear magnetization is described by a single exponential, below T∗≃20 K one observes the appearance of a second component characterized by much longer relaxation times . The appearance of different relaxation rates below T∗is an indication of an orbital selective behaviour which may lead to an electronic phase separation. One observes that while the temperature dependence of 1/T1 for the fast relaxing component is characteristic of a strongly correlated electron system approaching localization, the 1/T1 for the slow relaxing component shows a temperature dependence which is more characteristic of a weakly correlated metal.
 E. Civardi et al., Phys.Rev.Lett. 117, 217001 (2016).