Ca₂O₃Fe_2.6S₂: an antiferromagnetic Mott insulator at proximity to bad metal

Abstract

We report here the first layered iron oxychalcogenide Ca2O3Fe2.6S2 that contains both planar [Ca2FeO2](2+) and [Fe2OS2](2-) layers with the shortest Fe-Fe bond length. This compound is a narrow band gap (~0.073 eV) Mott insulator. The observed antiferromagnetic (AFM) transition at 77 K is due to the ordered Fe vacancies, which can be suppressed by partial substitution of Se for S. We show that the vacancy-free phase Ca2O3Fe3S2 may become a metal with moderate electron correlation comparable to the parent compound LaOFeAs of corresponding superconductors. Our results imply that iron oxychalcogenide can be converted from an AFM Mott insulator into a bad metal like iron pnictides through Fe-Fe bond length shrinking.

Site-selective electronic structure of pure and doped Ca2O3Fe3S2

“…They can induce a pseudogap regime, where the chemical potential lies in an energy region of vanishing density of states (DOS) near the Fermi energy (E F ) [25], or orbital-selective incoherent states, naturally yielding coexistent insulating and bad-metallic states as in cuprates [26] and Fe-pnictide superconductors [27]. In this work, we also use our results to discuss the influence of Mottness in lightly [13], as shown in Fig. 1.…”

“…Given telling similarities, along with notable differences, between the ironand cuprate-based superconductors [8], an especially important issue has been to confirm whether the parent compounds are itinerant metals or if they lie in close proximity to a Mott metal-to-insulator transition [7,9]. Particularly interesting for the latter view are Fe-oxychalcogenides, which have been synthesized with various compositions [2,[10][11][12][13][14], showing different degrees of electron localization.…”

“…The Fe-oxychalcogenides [2] thus help to sharpen the fundamental debate [21] on the degree of electronic correlations in Fe-based superconductors in general. However, finding Mott electrons with different degrees of activation energies [11][12][13][14]18] suggests strong multiorbital (MO) electronic localization in this material class. In contrast to Fe-based superconductors, the consequences of Mottness (or the proximity to a insulator-metal transition) upon external perturbations in the Fe-oxychalcogenides has not received the theoretical attention it deserves.…”

“…in order to minimize their overlap. The LDA calculation we performed using the cell parameter values reported elsewhere for the Ca 2 O 3 Fe 3 S 2 parent compound [13].…”

“…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.…”