Orbital order and fluctuations in the two-leg ladder materials BaFe${}_2{X}_3$ ($X$ = S and Se) and CsFe${}_2$Se${}_3$

The electronic structure of BaFe${}_2{X}_3$ ($X$ = S and Se) and CsFe${}_2$Se${}_3$ in which two-leg ladders are formed by the Fe sites are studied by means of x-ray absorption and resonant inelastic x-ray scattering spectroscopy. The x-ray absorption spectra at the Fe L edges for BaFe${}_2{X}_3$ exhibit two components, indicating that itinerant and localized Fe 3$d$ sites coexist. Substantial x-ray linear dichroism (XLD) is observed in polarization dependent spectra, indicating the existence of orbital order or fluctuation in the Fe-ladder even above the N'{e}el temperature $T_N$. Direct exchange interaction along the legs of the Fe-ladder stabilizes the orbital and antiferromagnetic orders in BaFe${}_2$S${}_3$, while the ferromagnetic molecular orbitals are realized between the rungs in CsFe${}_2$Se${}_3$.