Fingerprints of an Exotic Orbital-Selective Mott Phase in the Block Magnetic State of BaFe$_2$Se$_3$ Ladders.

Resonant Inelastic X-Ray Scattering (RIXS) experiments on the iron-based ladder BaFe$_2$Se$_3$ unveiled an unexpected two-peak structure associated with local orbital ($dd$) excitations in a block-type antiferromagnetic phase. A mixed character between correlated band-like and localized excitations was also reported. Here, we use the density matrix renormalization group method to calculate the momentum-resolved charge- and orbital-dynamical response functions of a multi-orbital Hubbard chain. Remarkably, our results qualitatively resemble the BaFe$_2$Se$_3$ RIXS data, while also capturing the presence of long-range magnetic order as found in neutron scattering, $only$ when the model is in an exotic orbital-selective Mott phase (OSMP). In the calculations, the experimentally observed zero-momentum transfer RIXS peaks correspond to excitations between itinerant and Mott insulating orbitals. We provide experimentally testable predictions for the momentum-resolved charge and orbital dynamical structures, which can provide further insight into the OSMP regime of BaFe$_2$Se$_3$.