Mott-Kondo insulator behavior in the iron oxychalcogenides

We perform a combined experimental-theoretical study of the Fe-oxychalcogenides $(\mathrm{FeO}\phantom{\rule{0.16em}{0ex}}Ch)$ series ${\mathrm{La}}_{2}{\mathrm{O}}_{2}{\mathrm{Fe}}_{2}\mathrm{O}{M}_{2}$ ($M=\mathrm{S}$, Se), which are among the latest Fe-based materials with the potential to show unconventional high-${T}_{c}$ superconductivity (HTSC). A combination of incoherent Hubbard features in x-ray absorption and resonant inelastic x-ray scattering spectra, as well as resistivity data, reveal that the parent $\mathrm{FeO}\phantom{\rule{0.16em}{0ex}}Ch$ are correlation-driven insulators. To uncover microscopics underlying these findings, we perform local density approximation-plus-dynamical mean field theory (LDA+DMFT) calculations that reveal a novel Mott-Kondo insulating state. Based upon good agreement between theory and a range of data, we propose that $\mathrm{FeO}\phantom{\rule{0.16em}{0ex}}Ch$ may constitute an ideal testing ground to explore HTSC arising from a strange metal proximate to a novel selective-Mott quantum criticality.