Electronic structure of FeS

Abstract

Here we report the electronic structure of FeS, a recently identified iron-based superconductor. Our high-resolution angle-resolved photoemission spectroscopy studies show two hole-like (α and β) and two electron-like (η and δ) Fermi pockets around the Brillouin zone center and corner, respectively, all of which exhibit moderate dispersion along kz. However, a third hole-like band (γ) is not observed, which is expected around the zone center from band calculations and is common in iron-based superconductors. Since this band has the highest renormalization factor and is known to be the most vulnerable to defects, its absence in our data is likely due to defect scattering -and yet superconductivity can exist without coherent quasiparticles in the γ band. This may help resolve the current controversy on the superconducting gap structure of FeS. Moreover, by comparing the β bandwidths of various iron chalcogenides, including FeS, FeSe1−xSx, FeSe, and FeSe1−xTex, we find that the β bandwidth of FeS is the broadest. However, the band renormalization factor of FeS is still quite large, when compared with the band calculations, which indicates sizable electron correlations. This explains why the unconventional superconductivity can persist over such a broad range of isovalent substitution in FeSe1−xTex and FeSe1−xSx.

Abrupt change of the superconducting gap structure at the nematic critical point in FeSe 1−x S x

“…Isoelectronic sulfur substitution in FeSe provides the most suitable route to study this issue. In the series of FeSe1−x Sx , the density of states at the Fermi level (or bandwidth) can be tuned significantly through chemical pressure, leading to a change of the electron correlation effect (34)(35)(36)(37)(38).…”

Electronic Nematic States Tuned by Isoelectronic Substitution in Bulk FeSe1−xSx

“…There are two predicted cylindrical electron bands which hardly change in shape across this series, similar to the isoelectronic series BaFe 2 (As 1−x P x ) 2 [56]. The positive chemical pressure in FeSe 1−x S x results in a lattice contraction and the reduction of the c axis [41] and it would bring the Fe(Se/S) layers closer together, increasing the bandwidth and potentially leading to the suppression of the electronic correlations [29,57]. As discussed below, DFT calculations provide essential guide to understand the origin of the observed Fermi surfaces of FeSe 1−x S x , but the size are smaller, the number of hole bands is reduced compared with calculations and the k z dependence is changed.…”

“…The observed energy dispersions of FeSe 1−x S x are all renormalized and shifted, as compared to the DFT dispersions, leading to much smaller hole and electron pockets, as compared with calculations [62]. The renormalization values corresponding to the two main hole dispersions (with d xz /d xz orbital character) are around 3-4 and hardly change for any compositions inside the nematic phase toward x ∼ 0.180.18, but they are reduced to a factor of ∼1-2.3 for FeS, suggesting that the suppression of electronic correlations occurs from FeSe toward FeS [29,57]. Additionally, the highly renormalized d xy band, found at ∼50 meV below the Fermi level, remains relatively unaffected across the nematic phase transition to x ∼ 0.18 and it cannot be resolved for FeS due to the disorder effects [29].…”

“…In the end compound FeS, quantum oscillations have revealed very light effective masses ranging from 0.6-2.1 m e [67,95]. The overall trends shows that cyclotron masses are larger inside the nematic phase of FeSe 1−x S x but they are getting lighter with the increasing bandwidth [57]. The reduction of the electronic correlations toward FeS is supported by the enhanced velocities from ARPES, shown in Figure 4E [29].…”

“…With sulfur substitution, the inner hole band is shifted gradually up and it crosses the Fermi level only at the Z point from x ∼ 0.11 [40] and grows in size at x ∼ 0.18. The inner hole band does not cross the Fermi level at the Γ point for any compositions up to x ∼ 0.18; however, it has been suggested that this pocket could grow in size and become a two-dimensional cylinder for FeS ( Figures 1D,E) [29,57,67]. Interestingly, the Fermi surface of the tetragonal phase for x ∼ 0.18 is very similar to the Fermi surface of FeSe at high temperature.…”