Mott-Kondo insulator behavior in the iron oxychalcogenides

Abstract

We perform a combined experimental-theoretical study of the Fe-oxychalcogenides (FeOCh) series La2O2Fe2OM 2 (M =S, Se), which are among the latest Fe-based materials with the potential to show unconventional high-Tc superconductivity (HTSC). A combination of incoherent Hubbard features in X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS) spectra, as well as resitivity data, reveal that the parent FeOCh 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 FeOCh may constitute a new, ideal testing ground to explore HTSC arising from a strange metal proximate to a novel selective-Mott quantum criticality.

Site-selective electronic structure of pure and doped Ca2O3Fe3S2

“…As a result the orbital-selective Mott gap is smaller due to reduced effective Coulomb repulsion, which is controlled by the intrinsic U/W ratio. Interestingly, (iii) our results at Sites 1 and 2 display a behavior which suggests strong similarities with the Fe-oxychalcogenide systems [22,23]. Namely, all orbitals are partially populated due to U -induced dynamical interorbital entanglement [36].…”

“…We choose values of U = 5.0 eV, J H = 0.7 eV as employed in our earlier works for the Fe-oxychalcogenies [22,23]. Our parameter choice for the on-site Coulomb interaction is consistent with the U value used in Ref.…”

“…2 and 3 confirm that similar to Fe-based superconductors the active electronic states involve Fe 3d carriers. A sizable enhancement of the average LDA bandwidth [23,31], and the significantly larger on-particle bandwidth for Ca 2 O 3 Fe 3 S 2 naturally implies proximity to metallization. Site-and orbital-driven anisotropies in the LDA band structure are also clearly manifested in Fig.…”

“…Here it is shown that this scenario holds true for Ca 2 O 3 Fe 3 S 2 . Additionally, as common to Fe-oxychalcogenides [22,23] and peculiar to the Fe-based superconductors, strong crystal-field splitting effects lift the xz, yz orbitals' degeneracy, leaving an antiferromagnetically ordered state at low temperatures [13] without tetragonal-to-orthorhombic structural phase transition or orbital nematic instabilities. It thus follows that the novelties found in Fe-based superconductors [33], relating to electronic nematic instabilities in the tetragonal phase near the borderline of structural and magnetic transitions, will not play an active role in Ca 2 O 3 Fe 3 S 2 : Only spin nematic phase fluctuations [34] are expected to be seen across the magnetic phase transition of Ca 2 O 3 Fe 3 S 2 at low temperatures.…”

“…This is even more important when the Mott transition is unconventional in nature, as shown here in Ca 2 O 3 Fe 3 S 2 where carrier localization persists on different Fe channels upon doping. Thus, following earlier studies on Fe-oxychalcogenides [22,23], we use the local density approximation plus dynamical mean-field theory (LDA+DMFT) [24] to study the role played by sizable MO electron-electron interactions in the Fe 3d shells of Ca 2 O 3 Fe 3 S 2 . As shown below, all five 3d-bands of two distinct Fe sites (Site 1, Site 2) must be kept in order to satisfactorily resolve the site-selective Mott localization and bad-metallic regimes obtained, respectively, for pure and doped Ca 2 O 3 Fe 3 S 2 .…”