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

Yi M, Liu ZK, Zhang Y, Yu R, Zhu JX, Lee JJ, Moore RG, Schmitt FT, Li W, Riggs SC, Chu JH, Lv B, Hu J, Hashimoto M, Mo SK, Hussain Z, Mao ZQ, Chu CW, Fisher IR, Si Q, Shen ZX, Lu DH - Nat Commun (2015)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: 1] Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory and Stanford University, Menlo Park, California 94025, USA [2] Departments of Physics and Applied Physics, and Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California 94305, USA.

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.

No MeSH data available.


Schematics of the effect of orbital-dependent band renormalizations.(a) LDA calculations for KFS41. (b) Schematic based on a with dxy orbital strongly renormalized. (c) Schematic based on b by introducing hybridization between dxy band and dxz electron band.
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f2: Schematics of the effect of orbital-dependent band renormalizations.(a) LDA calculations for KFS41. (b) Schematic based on a with dxy orbital strongly renormalized. (c) Schematic based on b by introducing hybridization between dxy band and dxz electron band.

Mentions: Generally in FeSC, this degeneracy between the dxz electron band bottom and dyz hole band top at the zone corner is protected by the C4 rotational symmetry, as seen in BFCA (Fig. 1l) and corresponding LDA calculations (Fig. 2a). This degeneracy is only lifted with the breaking of C4 symmetry, as in the orthorhombic phase of underdoped BFCA23, NaFeAs2425, bulk FeSe26272829 and multilayer FeSe film3031, where a splitting between the corresponding dxz bands and dyz bands occurs, in addition to a doubling of the bands from twinning effects due to the orthorhombic distortion. However, no static C4 symmetry breaking has been reported for any of the FeChs studied here, nor are twinning effects observed here that is expected from a broken symmetry due to orthorhombic distortion. Rather, this apparent gap can be explained by a strong orbital-dependent band renormalization and band hybridization. As the schematic shown in Fig. 2, the LDA-calculated dxy electron band bottom is deeper than that of the dxz band. If the dxy orbital is strongly renormalized compared with the other orbitals, the dxy electron band bottom, that is, the corresponding dxy hole band top, would rise above that of the dxz electron band (Fig. 2b). The heavily renormalized dxy hole band then crosses the dxz electron band and the two bands hybridize such that a gap appears at the M point without lifting the dxz/dyz degeneracy protected by C4 symmetry (Fig. 2c). Evidence for two nearly degenerate electron bands can be seen in the high-resolution spectra acquired on FS/STO (Fig. 3a). As an aside, we note that in the unrenormalized case (Fig. 2a), a hybridization gap between the dxy electron band and dyz hole band is not observed. This is because when considering the hopping via the chalcogen atoms along this high-symmetry direction (x), both dxy and dxz are odd while dyz is even. Hence dxy does not mix with dyz to produce a hybridization gap32.


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

Yi M, Liu ZK, Zhang Y, Yu R, Zhu JX, Lee JJ, Moore RG, Schmitt FT, Li W, Riggs SC, Chu JH, Lv B, Hu J, Hashimoto M, Mo SK, Hussain Z, Mao ZQ, Chu CW, Fisher IR, Si Q, Shen ZX, Lu DH - Nat Commun (2015)

Schematics of the effect of orbital-dependent band renormalizations.(a) LDA calculations for KFS41. (b) Schematic based on a with dxy orbital strongly renormalized. (c) Schematic based on b by introducing hybridization between dxy band and dxz electron band.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4525196&req=5

f2: Schematics of the effect of orbital-dependent band renormalizations.(a) LDA calculations for KFS41. (b) Schematic based on a with dxy orbital strongly renormalized. (c) Schematic based on b by introducing hybridization between dxy band and dxz electron band.
Mentions: Generally in FeSC, this degeneracy between the dxz electron band bottom and dyz hole band top at the zone corner is protected by the C4 rotational symmetry, as seen in BFCA (Fig. 1l) and corresponding LDA calculations (Fig. 2a). This degeneracy is only lifted with the breaking of C4 symmetry, as in the orthorhombic phase of underdoped BFCA23, NaFeAs2425, bulk FeSe26272829 and multilayer FeSe film3031, where a splitting between the corresponding dxz bands and dyz bands occurs, in addition to a doubling of the bands from twinning effects due to the orthorhombic distortion. However, no static C4 symmetry breaking has been reported for any of the FeChs studied here, nor are twinning effects observed here that is expected from a broken symmetry due to orthorhombic distortion. Rather, this apparent gap can be explained by a strong orbital-dependent band renormalization and band hybridization. As the schematic shown in Fig. 2, the LDA-calculated dxy electron band bottom is deeper than that of the dxz band. If the dxy orbital is strongly renormalized compared with the other orbitals, the dxy electron band bottom, that is, the corresponding dxy hole band top, would rise above that of the dxz electron band (Fig. 2b). The heavily renormalized dxy hole band then crosses the dxz electron band and the two bands hybridize such that a gap appears at the M point without lifting the dxz/dyz degeneracy protected by C4 symmetry (Fig. 2c). Evidence for two nearly degenerate electron bands can be seen in the high-resolution spectra acquired on FS/STO (Fig. 3a). As an aside, we note that in the unrenormalized case (Fig. 2a), a hybridization gap between the dxy electron band and dyz hole band is not observed. This is because when considering the hopping via the chalcogen atoms along this high-symmetry direction (x), both dxy and dxz are odd while dyz is even. Hence dxy does not mix with dyz to produce a hybridization gap32.

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: 1] Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory and Stanford University, Menlo Park, California 94025, USA [2] Departments of Physics and Applied Physics, and Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California 94305, USA.

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.

No MeSH data available.