Observation of universal strong orbital-dependent correlation effects in iron chalcogenides

Abstract

Establishing the appropriate theoretical framework for unconventional superconductivity in the iron-based materials requires correct understanding of both the electron correlation strength and the role of Fermi surfaces. This fundamental issue becomes especially relevant with the discovery of the iron chalcogenide superconductors. Here, we use angle-resolved photoemission spectroscopy to measure three representative iron chalcogenides, FeTe0.56Se0.44, monolayer FeSe grown on SrTiO3 and K0.76Fe1.72Se2. We show that these superconductors are all strongly correlated, with an orbital-selective strong renormalization in the dxy bands despite having drastically different Fermi surface topologies. Furthermore, raising temperature brings all three compounds from a metallic state to a phase where the dxy orbital loses all spectral weight while other orbitals remain itinerant. These observations establish that iron chalcogenides display universal orbital-selective strong correlations that are insensitive to the Fermi surface topology, and are close to an orbital-selective Mott phase, hence placing strong constraints for theoretical understanding of iron-based superconductors.

Orbital-selective pairing and superconductivity in iron selenides

“…These materials include the alkaline iron selenides, the Te-doped "11" iron selenides FeSe, and the monolayer FeSe on the SrTiO 3 substrate. [22][23][24][25][26] The effective quasiparticle mass normalized by its non-interacting counterpart, m * /m band is on the order of 3-4 for the 3d xz,yz orbitals, but is as large as 20 for the 3d xy orbital. 22,23,28 Such orbital selectivity has also been the subject of extensive recent theoretical studies.…”

“…[22][23][24][25][26] The effective quasiparticle mass normalized by its non-interacting counterpart, m * /m band is on the order of 3-4 for the 3d xz,yz orbitals, but is as large as 20 for the 3d xy orbital. 22,23,28 Such orbital selectivity has also been the subject of extensive recent theoretical studies. [29][30][31] All these aspects make it natural to study orbital-dependent [32][33][34] and -related 35 superconducting pairing.…”

“…We do so in terms of a strong-coupling approach to superconductivity, in light of the strong correlation effects 9, 10, 31, 40-48 that are especially clear-cut for the iron selenides. 22,23,28 This approach is described in Supplementary Information, with superconductivity driven by short-range interactions. The latter include the antiferromagnetic interactions between the nearest-neighbor (J α 1 ) and next-nearestneighbor (J α 2 ) Fe sites on their square lattice, for the three most relevant orbitals, α = 3d xz , 3d yz , and 3d xy .…”

“…Thus, there is reason to expect that correlation effects will be different for different orbitals. In fact, there is evidence for orbitally selective Mott behavior in the iron selenides [22][23][24][25][26] and, thus, orbital selectivity is to be expected for pairing as well.…”

Heavy electron doping induced antiferromagnetic phase as the parent for the iron oxypnictide superconductorLaFeAsO1−xHx

“…So far, the crossover phenomenon from a metal into a bad metal or insulator on heating has been observed in several compounds. Indeed, recently, Yi et al directly observed a loss of the quasiparticle spectral weight of the 3dxy orbital above the specific temperature, TOSMP, in FeTe0.56Se0.44, K0.76Fe1.72Se2, and mono-layer FeSe by using ARPES [37]. A common feature of these compounds is that the less-occupied Fe 3dxy orbital is responsible for the electronic correlation.…”