Electronic Structure of FeSe 1– x Te x Studied by X-ray Spectroscopy and Density Functional Theory (original) (raw)
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Journal of Physics: Condensed Matter, 2010
We present first-principles density-functional-theory-based calculations to determine the effects of the strength of on-site electron correlation, magnetic ordering, pressure and Se vacancies on phonon frequencies and electronic structure of FeSe 1−x. The theoretical equilibrium structure (lattice parameters) of FeSe depends sensitively on the value of the Hubbard parameter U of on-site correlation and magnetic ordering. Our results suggest that there is a competition between different antiferromagnetic states due to comparable magnetic exchange couplings between first-and second-neighbor Fe sites. As a result, a short range order of stripe antiferromagnetic type is shown to be relevant to the normal state of FeSe at low temperature. We show that there is a strong spin-phonon coupling in FeSe (comparable to its superconducting transition temperature) as reflected in large changes in the frequencies of certain phonons with different magnetic ordering, which is used to explain the observed hardening of a Raman-active phonon at temperatures (∼100 K) where magnetic ordering sets in. The symmetry of the stripe antiferromagnetic phase permits an induced stress with orthorhombic symmetry, leading to orthorhombic strain as a secondary order parameter at the temperature of magnetic ordering. The presence of Se vacancies in FeSe gives rise to a large peak in the density of states near the Fermi energy, which could enhance the superconducting transition temperature within the BCS-like picture.
Journal of Physics and Chemistry of Solids, 2015
Ab initio X-ray absorption near edge structure (XANES) calculations for FeSe x Te 1 À x , using a structural model that combines FeSe and FeTe phases at the nanoscale, are compared with Fe K-edge XANES measurements in the "pre-edge" region. The important aspects of this model are (i) magnetic order in the FeTe phase; (ii) Se and Te atoms placed randomly in both FeSe and FeTe crystallographic positions and; (iii) the two distinct distances for Fe-Se and Fe-Te of the bulk phases. The calculated spectra reproduce the observed increase of spectral weight in the experiments on FeSe x Te 1 À x with Se concentration. This is consistent with an inhomogeneous local electronic structure of FeSe x Te 1 À x. Additionally, we have calculated projected electronic density of d-states for the Fe atom, revealing increased spectral weight in the "pre-edge" region of the XANES spectra, which correlates with the increase in Se concentration. The decrease of calculated Fe d-density of states for the Fermi level, N(ε F), for high Te content is consistent with the suppression of superconductivity in the title system. & 2014 Elsevier Ltd. All rights reserved. calculated the Fe d-density of states (DOS) in the vicinity of the Fermi energy of this system, using the same computational method. We have performed calculations in real space, where no translational symmetry is invoked. We note that systems with compositional disorder exhibit a broken translational symmetry, Contents lists available at ScienceDirect
Electronic Structure of Superconducting FeSe Studied by High-Resolution Photoemission Spectroscopy
Journal of the Physical …, 2009
We have performed soft x-ray and ultrahigh-resolution laser-excited photoemission measurements on tetragonal FeSe, which was recently identified as a superconductor. Energy dependent study of valence band is compared to band structure calculations and yields a reasonable assignment of partial densities of states. However, the sharp peak near the Fermi level slightly deviates from the calculated energy position, giving rise to the necessity of self-energy correction. We have also performed ultrahigh-resolution laser photoemission experiment on FeSe and observed the suppression of intensity around the Fermi level upon cooling.
Fe-vacancy order and superconductivity in tetragonal -Fe1-xSe
Proceedings of the National Academy of Sciences, 2014
Several superconducting transition temperatures in the range of 30-46 K were reported in the recently discovered intercalated FeSe system (A1-xFe2-ySe2, A = K, Rb, Cs, Tl). Although the superconducting phases were not yet conclusively decided, more than one magnetic phase with particular orders of iron vacancy and/or potassium vacancy were identified, and some were argued to be the parent phase. Here we show the discovery of the presence and ordering of iron vacancy in nonintercalated FeSe (PbO-type tetragonal β-Fe1-xSe). Three types of iron-vacancy order were found through analytical electron microscopy, and one was identified to be nonsuperconducting and magnetic at low temperature. This discovery suggests that the rich-phases found in A1-xFe2-ySe2 are not exclusive in Fe-Se and related superconductors. In addition, the magnetic β-Fe1-xSe phases with particular iron-vacancy orders are more likely to be the parent phase of the FeSe superconducting system instead of the previously assigned β-Fe1+δTe.
Synthesis and characterization of FeSe1−xTex (x=0, 0.5, 1) superconductors
AIP Conference Proceedings, 2015
In this study, FeTe 1-x Se x (x=0,0.5,1) samples were prepared by conventional solid state reaction method and investigated by powder XRD, SEM, Raman and resistivity measurement techniques to reveal the effect of tellurium (Te) substitution in FeSe matrix. Rietveld analysis was performed on room temperature recorded, X-ray diffraction (XRD) patterns of pure FeSe, FeTe and FeSe 0.5 Te 0.5 which shows that all the compounds are crystallized in a tetragonal structure. SEM images show the dense surface morphology. Raman spectra recorded in the range from 100 to 700 cm-1 at ambient temperature has been interpreted by P4/nmm space group of the lattice. The variation in intensity and shift in peak positions of some phonon modes has been discussed on the basis of variation in crystalline field effect by substituting Te in FeSe lattice. The resistivity versus temperature curves reveals that FeSe becomes superconductor at 7 K and FeSe 0.5 Te 0.5 shows superconductivity below14 K while FeTe is non-superconducting compound.
Electron correlation in the FeSe superconductor studied by bulk-sensitive photoemission spectroscopy
Physical Review B, 2010
We have investigated the electronic structures of recently discovered superconductor FeSe by soft-x-ray and hard-x-ray photoemission spectroscopy with high bulk sensitivity. The large Fe 3d spectral weight is located in the vicinity of the Fermi level (EF ), which is demonstrated to be a coherent quasi-particle peak. Compared with the results of the band structure calculation with local-density approximation, Fe 3d band narrowing and the energy shift of the band toward EF are found, suggesting an importance of the electron correlation effect in FeSe. The self energy correction provides the larger mass enhancement value (Z −1 ≃3.6) than in Fe-As superconductors and enables us to separate a incoherent part from the spectrum. These features are quite consistent with the results of recent dynamical mean-field calculations, in which the incoherent part is attributed to the lower Hubbard band.
Physical Review B, 2010
We report density functional studies of the Fe1−xCuxSe alloy done using supercell and coherent potential approximation methods. Magnetic behavior was investigated using the disordered local moment approach. We find that Cu occurs in a nominal d 10 configuration and is highly disruptive to the electronic structure of the Fe sheets. This would be consistent with a metal insulator transition due to Anderson localization. We further find a strong cross over from a weak moment itinerant system to a local moment magnet at x ≈ 0.12. We associate this with the experimentally observed jump near this concentration. Our results are consistent with the characterization of this concentration dependent jump as a transition to a spin-glass.