Comparison of Ab initio Low-Energy Models for LaFePO, LaFeAsO, BaFe₂As₂, LiFeAs, FeSe, and FeTe: Electron Correlation and Covalency

Abstract

Effective low-energy Hamiltonians for several different families of iron-based superconductors are compared after deriving them from the downfolding scheme based on first-principles calculations. Systematic dependences of the derived model parameters on the families are elucidated, many of which are understood from the systematic variation of the covalency between Fe-3d and pnictogen-/chalcogenp orbitals. First, LaFePO, LaFeAsO (1111), BaFe 2 As 2 (122), LiFeAs (111), FeSe, and FeTe (11) have overall similar band structures near the Fermi level, where the total widths of 10-fold Fe-3d bands are mostly around 4.5 eV. However, the derived effective models of the 10-fold Fe-3d bands (d model) for FeSe and FeTe have substantially larger effective onsite Coulomb interactions U $ 4:2 and 3.4 eV, respectively, after the screening by electrons on other bands and after averaging over orbitals, as compared to $2:5 eV for LaFeAsO. The difference is similar in the effective models containing p orbitals of As, Se or Te (dp or dpp model), where U ranges from $4 eV for the 1111 family to $7 eV for the 11 family. The exchange interaction J has a similar tendency. The family dependence of models indicates a wide variation ranging from weak correlation regime (LaFePO) to substantially strong correlation regime (FeSe). The origin of the larger effective interaction in the 11 family is ascribed to smaller spread of the Wannier orbitals generating larger bare interaction, and to fewer screening channels by the other bands. This variation is primarily derived from the distance h between the pnictogen/chalcogen position and the Fe layer: The longer h for the 11 family generates more ionic character of the bonding between iron and anion atoms, while the shorter h for the 1111 family leads to more covalent-bonding character, the larger spread of the Wannier orbitals, and more efficient screening by the anion p orbitals. The screened interaction of the d model is strongly orbital dependent, which is also understood from the Wannier spread. The dp and dpp models show much weaker orbital dependence. The larger h also explains why the 10-fold 3d bands for the 11 family are more entangled with the smearing of the ''pseudogap'' structure above the Fermi level seen in the 1111 family. While the family-dependent semimetallic splitting of the bands primarily consists of d yz =d zx and d x 2 Ày 2 orbitals, the size of the pseudogap structure is controlled by the hybridization between these orbitals and d xy =d 3z 2 Àr 2 : A large hybridization in the 1111 family generates a large ''band-insulating''-like pseudogap (hybridization gap), whereas a large h in the 11 family weakens them, resulting in a ''half-filled'' like bands of orbitals. This may enhance strong correlation effects in analogy with Mott physics and causes the orbital selective crossover in the three orbitals. On the other hand, the geometrical frustration t 0 =t, inferred from the ratio of the next-nearest transfer t 0 to the nearest one t of the d model is relatively larger for the...

Photoemission studies of the near EFspectral weight shifts in FeSe1−xTexsuperconductor

“…It should be noted that, depletion of these states from the near E F clearly indicates an opening up of a pseudogap as the temperature is lowered from 300 K to 77 K. Figure 2(a) shows the calculated total density of states (DOS) over the valence band region. The calculated DOS matches with the earlier theoretical study [21,22] and also with the observed experimental data. Fig 2) corresponds to the experimentally obtained features at 0.1 eV and 0.5 eV, respectively.…”

“…Peak C at 4 eV is due to the hybridized Fe 3d -Se 4p/Te 5p states while D at 6 eV corresponds to Se 4p/Te 5p states. Our calculations based on the TBLMTO-ASA (fig 2(a)) and also other's calculations [21,22] conform to these assignments. The sharp feature A seen in the case of FeSe transforms into a broadened one for FeTe.…”

Fermi Surface, Pressure-Induced Antiferromagnetic Order, and Superconductivity in FeSe

“…From the first-principles downfolding scheme given by the cRPA method, Ref. 46 revealed that U ll , U ll ′ , J ll ′ , J ll ′ in H dd are orbital-dependent and that the average of U ll isŪ d = 7.2 eV for FeSe.…”

“…46. We use the exact diagonalization method for a finite-size cluster as an impurity solver to obtain the local quantities such as the self-energy, which is calculated at T = 0 as the explicit T -dependence is expected to be small in the intermediate correlation regime with Z > ∼ 0.5.…”

“…Furthermore, the average value of the intersite Coulomb integrals is large for FeSe relative to those for the 1111 and the systems. 46) Thus, by considering suitable intersite d-p Coulomb integrals, FeSe is expected to have larger orbital-polarization interaction than the 1111 and 122 systems.…”