Identifying valence structure in LiFeAs and NaFeAs with core-level spectroscopy

Kurmaev, E.Z.; McLeod, John A.; Skorikov, N. A.; Finkelstein, L. D.; Moewes, A.; Izyumov, Yu. A.; Clarke, Simon J.

doi:10.1088/0953-8984/21/34/345701

Cited by 21 publications

(38 citation statements)

References 22 publications

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“…These observations lend evidence for a delocalized character of the 3d electrons in these materials [54,57,58]. Likewise, the nonresonant XES spectra are quite similar to those reported in the Fe-pnictide superconductors [22][23][24]27]. By contrast, the spectra for FeO show signs of constant energy loss features and of low-energy shoulders, and the spectral shape depends strongly on the excitation energy.…”

Section: B Spin State and Magnetic Orderingsupporting

confidence: 77%

“…The value of FeSe is closer to FeO than to Fe metal whereas the value for the rest of the series is closer to Fe metal, suggesting again that Fe 1−x Co x Se can be considered at most as a moderate correlated system. This is an important finding if we keep in mind that in most Fe-pnictide superconductors the 3d electrons are mainly itinerant [23,27]. The metallic character of these materials is represented by LiFeAs whose value is quite close to the value of Fe metal.…”

Section: Strength Of Electronic Correlationsmentioning

confidence: 68%

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Electronic structure of Co-substituted FeSe superconductor probed by soft x-ray spectroscopy and density functional theory

et al. 2014

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We study the crystalline and electronic properties of the Fe 1−x Co x Se system (x = 0, 0.25, 0.5, 0.75, and 1.0) using x-ray diffraction, x-ray spectroscopy, and density functional theory. We show that the introduction of Co 3d states in FeSe relaxes the bond strengths and induces a structural transition from tetragonal to hexagonal whose crossover takes place at x ≈ 0.38. This structural transition in turn modifies the magnetic order, which can be related to the spin state. Using resonant inelastic x-ray scattering, we estimate the spin state of the system; FeSe is found to be in a high-spin state (S = 2), but Fe is reduced to a low-spin state upon Co substitution of x 0.25, well below the structural transition. Finally, we show evidence that FeSe is a moderately correlated system but the introduction of Co into the host lattice weakens the correlation strength for x 0.25. These novel findings are important to unravel the mechanisms responsible for the superconducting state in iron-chalcogenide superconductors.

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Section: B Spin State and Magnetic Orderingsupporting

confidence: 77%

Section: Strength Of Electronic Correlationsmentioning

confidence: 68%

Electronic structure of Co-substituted FeSe superconductor probed by soft x-ray spectroscopy and density functional theory

et al. 2014

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“…Clearly, Fe-L edge XAS confirms that in EuFe 2 As 2 Fe has the common 2+ oxidation state. Furthermore, the spectra do not show any multiplet structure common to oxides, confirming the high sample quality and the success of the in situ cleaving [40][41][42]65,66 . The FWHM of this peak is about 3 eV and is unchanged moving from EuFe 2 As 2 to BaFe 2 As 2 , indicating that the electronic structure is similar for the two compounds.…”

Section: Resultsmentioning

confidence: 63%

“…1(b), we plot the Fe-L 2,3 XAS spectrum of EuFe 2 As 2 together with the one of BaFe 2 As 2 . The spectra display two broad peaks at ≈708 and 720 eV typical of Fe pnictides, ascribed to the L 3 and L 2 absorption edges [40][41][42]65,66 . In both compounds, the spectra are rather similar with minor differences in the shoulder at 711 eV.…”

Section: Resultsmentioning

confidence: 99%

Local and collective magnetism ofEuFe2As2

et al. 2017

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We present an experimental study of the local and collective magnetism of EuFe 2 As 2 , that is isostructural with the high temperature superconductor parent compound BaFe 2 As 2 . In contrast to BaFe 2 As 2 , where only Fe spins order, EuFe 2 As 2 has an additional magnetic transition below 20 K due to the ordering of the Eu 2+ spins (J = 7/2, with L = 0 and S = 7/2) in an A-type antiferromagnetic texture (ferromagnetic layers stacked antiferromagnetically). This may potentially affect the FeAs layer and its local and correlated magnetism. Fe-K β x-ray emission experiments on EuFe 2 As 2 single crystals reveal a local magnetic moment of 1.3±0.15 µ B at 15 K that slightly increases to 1.45±0.15 µ B at 300 K. Resonant inelastic x-ray scattering (RIXS) experiments performed on the same crystals show dispersive broad (in energy) magnetic excitations along (0, 0) → (1, 0) and (0, 0) → (1, 1) with a bandwidth on the order of 170-180 meV. These results on local and collective magnetism are in line with other parent compounds of the AFe 2 As 2 series (A = Ba, Ca, and Sr), especially the well characterized BaFe 2 As 2 . Thus, our experiments lead us to the conclusion that the effect of the high magnetic moment of Eu on the magnitude of both Fe local magnetic moment and spin excitations is small and confined to low energy excitations.2

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“…Band-structure calculations [5,6,7] show the Fermi surfaces for 111-system to be comprised of quasitwo-dimensional (2D) hole cylinders centered at the Γ-point and electron ones at the M-point of the first Brillouin zone that can be considered as two effective bands (so called minimal two-band model) [8,9]. The total density of states at the Fermi level is formed mainly by Fe 3d-states [10,11,12,13,14]. As was shown in [15], the superconducting transition temperatures T C for different types of iron-based superconductors correlate with the total density of states at the Fermi level.…”

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confidence: 99%

Investigation of LiFeAs by means of “break-junction” technique

et al. 2012

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In our tunneling investigation using Andreev superconductor -normal metal -superconductor contacts on LiFeAs single crystals we observed two reproducible independent subharmonic gap structures at dynamic conductance characteristics. From these results, we can derive the energy of the large superconducting gap ∆L = (2.5 ÷ 3.4) meV and the small gap ∆S = (0.9 ÷ 1) meV at T = 4.2 K for the T local C ≈ (10.5 ÷ 14) K (the contact area critical temperature which deviation causes the variation of ∆L). The BCS-ratio is found to be 2∆L/kBTC = (4.6 ÷ 5.6), whereas 2∆S/kBTC ≪ 3.52 results from induced superconductivity in the bands with the small gap.The new class of superconducting rare-earth oxypnictides [1] is still not completely understood and therefore requires further investigation. The layered LiFeAs (111-system) [2] is one of the few stoichiometric Febased pnictides which shows neither magnetic nor structural transition but becomes superconducting at 18 K [3,4]. Band-structure calculations [5,6,7] show the Fermi surfaces for 111-system to be comprised of quasitwo-dimensional (2D) hole cylinders centered at the Γ-point and electron ones at the M-point of the first Brillouin zone that can be considered as two effective bands (so called minimal two-band model) [8,9]. The total density of states at the Fermi level is formed mainly by Fe 3d-states [10,11,12,13,14]. As was shown in [15], the superconducting transition temperatures T C for different types of iron-based superconductors correlate with the total density of states at the Fermi level. This fact and the strong Fe isotope effect, which was reported by [16] supports the phonon-mediated coupling importance [17] in these compounds [15,18]. The electron-phonon coupling is enhanced by an extended van Hove singularity [19] which was shown for iron pnictides and, in particular, for LiFeAs [20]. In this work, we present an investigation of the superconducting properties of LiFeAs single crystals by means of Andreev spectroscopy of superconductor -normal metal -superconductor (SNS) contacts, and the corresponding superconducting gaps.The LiFeAs single crystals were obtained by self-flux method. The synthesis and investigation of the composition and properties are detailed in [21]. A mixture of small lumps of the Li metal and powders of Fe and As in a molar ratio of Li : Fe : As = 3 : 2 : 3 was 1) e-mail: kuzmichev@mig.phys.msu.ru placed into an alumina crucible. All work on the reactive mixture preparation was carried out in a dry box under argon atmosphere. The crucible was inserted into a niobium container, which was welded in argon at 1.5 atm. The sealed Nb container was enclosed in a quartz ampoule. The sample was heated up to 1363 K, kept at this temperature and slowly cooled down to 873 K. At this temperature ampoule was extracted from the furnace and cooled in open air. The LiFeAs single crystals in the form of thin plates with lateral dimensions of (12 ± 6) × (12 ± 6) × (0.1 ± 0.05) mm 3 were separated from flux mechanically. According to the XRD, EDX, ICP MS, a...

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Identifying valence structure in LiFeAs and NaFeAs with core-level spectroscopy

Cited by 21 publications

References 22 publications

Electronic structure of Co-substituted FeSe superconductor probed by soft x-ray spectroscopy and density functional theory

Electronic structure of Co-substituted FeSe superconductor probed by soft x-ray spectroscopy and density functional theory

Local and collective magnetism ofEuFe2As2

Investigation of LiFeAs by means of “break-junction” technique

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