Electronic structure of Co-substituted FeSe superconductor probed by soft x-ray spectroscopy and density functional theory (original) (raw)
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Electronic Structure of FeSe 1– x Te x Studied by X-ray Spectroscopy and Density Functional Theory
The Journal of Physical Chemistry C, 2014
We study the electronic properties of the FeSe 1−x Te x system (x = 0, 0.25, 0.5, 0.75, and 1) from the perspective of X-ray spectroscopy and density functional theory (DFT). The analysis performed on the density of states reveals marked differences in the distribution of the 5p states of Te for x > 0. We think that this finding can be associated with the fact that superconductivity is suppressed in FeTe. Moreover, using resonant inelastic X-ray scattering, we estimate the spin state of our system which can be correlated to the magnetic order. We find that the spin state of the FeSe 1−x Te x system fluctuates, as a function of x, between S = 0 and S = 2 with Fe in FeSe in the highest spin state. Finally, our DFT calculations nicely reproduce the X-ray emission spectra performed at the Fe Ledge (which probe the occupied states) and suggest that the FeSe 1−x Te x system can be considered at most as a moderately correlated system.
Journal of the Korean Physical Society, 2011
This paper reports the synthesis and superconducting behaviors of the tetragonal iron-chalcogenide superconductor FeSe. The electrical resistivity and magnetic moment measurements confirmed its superconductivity with a T zero c and T mag c at 9.4 K under ambient pressure. EPMA indicated the sample to have a stoichiometric Fe:Se ratio of 1:1 (±0.02). The Seebeck coefficient which was 12.3 µV/K at room temperature, changed to a negative value near 200 K, indicating it to be a two carriers material. Above T c , the ρ(T) curve revealed an 'S' shape. Hence dρ(T)/dT , and d 2 ρ(T)/dT 2 showed pseudogap-like behavior at T * =110 K according to the resistivity curvature mapping (RCM) method for high T c cuprates. Moreover, the magnetoresistance ρ H (T)/ρ H=0 under a magnetic field and the Seebeck coefficient S(T) revealed revealed pseudogap-like behavior near T *. Interestingly, at the same temperature, 30 K, the sign of S(T) and all signs of d 2 ρ(T)/dT 2 changed from negative to positive above T c .
Electronic structure of Fe-based superconductors
Pramana, 2015
Fe-based superconductors have drawn much attention during the last decade due to the presence of superconductivity in materials containing the magnetic element, Fe, and the coexistence of superconductivity and magnetism. Extensive study of the electronic structure of these systems suggested the dominant role of d states in their electronic properties, which is significantly different from the cuprate superconductors. In this article, some of our studies of the electronic structure of these fascinating systems employing high-resolution photoemission spectroscopy is reviewed. The combined effect of electron correlation and covalency reveals an interesting scenario in their electronic structure. The contribution of ligand p states at the Fermi level is found to be much more significant than indicated in earlier studies. Temperature evolution of the energy bands reveals the signature of transition akin to Lifshitz transition in these systems.
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.
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.
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.
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.