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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.


Low-temperature band structure of iron chalcogenides in comparison to iron pnictide.Fermi surfaces measured on (a) FeTe0.56Se0.44 (FTS), (b) monolayer FeSe film on SrTiO3 (FS/STO), (c) K0.76Fe1.72Se2 (KFS) and (d) Ba(Fe0.93Co0.07)2As2 (BFCA), shown in BZ notation corresponding to 2-Fe unit cell (For comparison purposes, we use the M point to denote the BZ corner where the electron pockets live for all compounds and LDA, even though for 122 crystal structures, this is the X point.), with schematic outlines shown in cyan (magenta) for hole (electron) Fermi pockets. (e) Spectral image of FTS along the Γ–M high-symmetry direction, taken with 22 eV (26 eV) photons for near the Γ (M) point. Measurements along the same cut for (f) FS/STO, (g) KFS and (h) BFCA, with photon energies of 22, 26 and 47.5 eV, respectively. In-plane polarization was odd with respect to the cut for all measurements, (e–g) has additional out-of-plane polarization. (i–l) Second energy derivatives for the spectral images above. Observable bands are marked with dominant orbital character (red: dxz, green: dyz and blue: dxy).
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f1: Low-temperature band structure of iron chalcogenides in comparison to iron pnictide.Fermi surfaces measured on (a) FeTe0.56Se0.44 (FTS), (b) monolayer FeSe film on SrTiO3 (FS/STO), (c) K0.76Fe1.72Se2 (KFS) and (d) Ba(Fe0.93Co0.07)2As2 (BFCA), shown in BZ notation corresponding to 2-Fe unit cell (For comparison purposes, we use the M point to denote the BZ corner where the electron pockets live for all compounds and LDA, even though for 122 crystal structures, this is the X point.), with schematic outlines shown in cyan (magenta) for hole (electron) Fermi pockets. (e) Spectral image of FTS along the Γ–M high-symmetry direction, taken with 22 eV (26 eV) photons for near the Γ (M) point. Measurements along the same cut for (f) FS/STO, (g) KFS and (h) BFCA, with photon energies of 22, 26 and 47.5 eV, respectively. In-plane polarization was odd with respect to the cut for all measurements, (e–g) has additional out-of-plane polarization. (i–l) Second energy derivatives for the spectral images above. Observable bands are marked with dominant orbital character (red: dxz, green: dyz and blue: dxy).

Mentions: The generic electronic structure of iron-based superconductors (FeSCs) consists of three hole bands at the BZ centre, Γ, and two electron bands at the BZ corner, M. The hole bands are predominantly of dxz, dyz and dxy orbital characters, while the electron bands are dxz and dxy along Γ–M. The relative positions of these bands with respect to each other as well as to the Fermi level (EF) could vary with differences in lattice parameters and doping level. Hence, the FS topology among different FeSCs could be qualitatively different, as shown in Fig. 1a–d, where the Fermi pockets at the BZ centre vary from being hole-like to non-existent to electron like. The measured band structure along the Γ–M high-symmetry direction for the three compounds are shown in Fig. 1e–g, in comparison to that for the optimally Co-doped BaFe2As2 (BFCA) (Fig. 1h), an FePn as a reference. For FTS (Fig. 1i), one of the hole bands crosses EF, and both electron bands cross EF at M, resulting in roughly compensated hole pocket at Γ and electron pockets at M (Fig. 1a), consistent with isovalent substitution for this compound. For both FS/STO (Fig. 1j) and KFS (Fig. 1k) in contrast, only the electron bands cross EF while the hole band tops are well below EF, with an additional small electron pocket at Γ in KFS. Thus, there is heavy electron doping in both compounds as reflected in a FS topology consisting only of electron pockets (Fig. 1b,c). Comparing the band structure of the three FeChs to the FePns, we notice a significant difference near the M point—there is an apparent gap between the bottom of the electron bands and the top of the hole band in all three FeChs, in sharp contrast to BFCA (Fig. 1l), in which the dxz electron band bottom is degenerate with the dyz hole band top.


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)

Low-temperature band structure of iron chalcogenides in comparison to iron pnictide.Fermi surfaces measured on (a) FeTe0.56Se0.44 (FTS), (b) monolayer FeSe film on SrTiO3 (FS/STO), (c) K0.76Fe1.72Se2 (KFS) and (d) Ba(Fe0.93Co0.07)2As2 (BFCA), shown in BZ notation corresponding to 2-Fe unit cell (For comparison purposes, we use the M point to denote the BZ corner where the electron pockets live for all compounds and LDA, even though for 122 crystal structures, this is the X point.), with schematic outlines shown in cyan (magenta) for hole (electron) Fermi pockets. (e) Spectral image of FTS along the Γ–M high-symmetry direction, taken with 22 eV (26 eV) photons for near the Γ (M) point. Measurements along the same cut for (f) FS/STO, (g) KFS and (h) BFCA, with photon energies of 22, 26 and 47.5 eV, respectively. In-plane polarization was odd with respect to the cut for all measurements, (e–g) has additional out-of-plane polarization. (i–l) Second energy derivatives for the spectral images above. Observable bands are marked with dominant orbital character (red: dxz, green: dyz and blue: dxy).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: Low-temperature band structure of iron chalcogenides in comparison to iron pnictide.Fermi surfaces measured on (a) FeTe0.56Se0.44 (FTS), (b) monolayer FeSe film on SrTiO3 (FS/STO), (c) K0.76Fe1.72Se2 (KFS) and (d) Ba(Fe0.93Co0.07)2As2 (BFCA), shown in BZ notation corresponding to 2-Fe unit cell (For comparison purposes, we use the M point to denote the BZ corner where the electron pockets live for all compounds and LDA, even though for 122 crystal structures, this is the X point.), with schematic outlines shown in cyan (magenta) for hole (electron) Fermi pockets. (e) Spectral image of FTS along the Γ–M high-symmetry direction, taken with 22 eV (26 eV) photons for near the Γ (M) point. Measurements along the same cut for (f) FS/STO, (g) KFS and (h) BFCA, with photon energies of 22, 26 and 47.5 eV, respectively. In-plane polarization was odd with respect to the cut for all measurements, (e–g) has additional out-of-plane polarization. (i–l) Second energy derivatives for the spectral images above. Observable bands are marked with dominant orbital character (red: dxz, green: dyz and blue: dxy).
Mentions: The generic electronic structure of iron-based superconductors (FeSCs) consists of three hole bands at the BZ centre, Γ, and two electron bands at the BZ corner, M. The hole bands are predominantly of dxz, dyz and dxy orbital characters, while the electron bands are dxz and dxy along Γ–M. The relative positions of these bands with respect to each other as well as to the Fermi level (EF) could vary with differences in lattice parameters and doping level. Hence, the FS topology among different FeSCs could be qualitatively different, as shown in Fig. 1a–d, where the Fermi pockets at the BZ centre vary from being hole-like to non-existent to electron like. The measured band structure along the Γ–M high-symmetry direction for the three compounds are shown in Fig. 1e–g, in comparison to that for the optimally Co-doped BaFe2As2 (BFCA) (Fig. 1h), an FePn as a reference. For FTS (Fig. 1i), one of the hole bands crosses EF, and both electron bands cross EF at M, resulting in roughly compensated hole pocket at Γ and electron pockets at M (Fig. 1a), consistent with isovalent substitution for this compound. For both FS/STO (Fig. 1j) and KFS (Fig. 1k) in contrast, only the electron bands cross EF while the hole band tops are well below EF, with an additional small electron pocket at Γ in KFS. Thus, there is heavy electron doping in both compounds as reflected in a FS topology consisting only of electron pockets (Fig. 1b,c). Comparing the band structure of the three FeChs to the FePns, we notice a significant difference near the M point—there is an apparent gap between the bottom of the electron bands and the top of the hole band in all three FeChs, in sharp contrast to BFCA (Fig. 1l), in which the dxz electron band bottom is degenerate with the dyz hole band top.

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.