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

TL;DR

This study combines RIXS experiments and theoretical calculations to identify an orbital-selective Mott phase in BaFe$_2$Se$_3$, revealing complex orbital and magnetic behaviors in this ladder compound.

Contribution

The paper demonstrates that the orbital-selective Mott phase explains the RIXS data and magnetic order in BaFe$_2$Se$_3$, providing new insights into its electronic structure.

Findings

RIXS spectra show a two-peak structure linked to orbital excitations.

Theoretical models reproduce experimental data only in the OSMP regime.

Predictions for momentum-resolved charge and orbital responses are provided.

Abstract

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$_3$.