Structural, electrical, magnetic, and optical properties of iron-based ladder compounds BaFe 2 ( S 1 − x Se x ) 3

We performed a comprehensive study on structural, electrical, magnetic, and optical properties for iron-based ladder materials ${\mathrm{BaFe}}_{2}{({\mathrm{S}}_{1\ensuremath{-}x}{\mathrm{Se}}_{x})}_{3}\phantom{\rule{4pt}{0ex}}(0\ensuremath{\le}x\ensuremath{\le}1)$, which shows pressure-induced superconductivity in the vicinity of the Mott transition at $x=0$ and 1. We obtain a complete electronic phase diagram in a temperature-composition plane, which reveals that the magnetic ground state switches from the stripe-type to the block-type phase without any intermediate phase at $x=0.23$ with increasing $x$. This behavior is in sharp contrast to the filling controlled system ${\mathrm{Ba}}_{1\ensuremath{-}x}\phantom{\rule{4pt}{0ex}}{\mathrm{Cs}}_{x}{\mathrm{Fe}}_{2}{\mathrm{Se}}_{3}$, in which a paramagnetic state down to the lowest temperature is realized between two magnetic ordered states. The structural transition, which is considered to be relevant to the orbital order, occurs far above the magnetic transition temperature. The magnetic and structural transition temperatures exhibit a similar composition dependence, indicating a close relationship between magnetic and orbital degrees of freedom. In addition, we found that charge dynamics are considerably influenced not only by the magnetic order but also by the structural change (orbital order) from the detailed measurements of electrical resistivity and optical conductivity spectra. We discuss the magnetism and orbital order by comparing the experimental results with the proposed theory based on the multiorbital Hubbard model. The relationship between the charge dynamics and the magnetic/orbital order is also discussed.