We perform a combined experimental-theoretical study of the Fe-oxychalcogenides $\left(\mathrm{FeO}Ch\right)$ 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 $\mathrm{FeO}Ch$ 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 $\mathrm{FeO}Ch$ may constitute an ideal testing ground to explore HTSC arising from a strange metal proximate to a novel selective-Mott quantum criticality.

• Received 27 November 2014
• Revised 9 September 2015

DOI:https://doi.org/10.1103/PhysRevB.92.155139