1,721,139 research outputs found
Local structural studies of Fe-based superconductors
Get PDFIn the framework of high temperature superconductors, the Fe-based systems are the rst magnetic materials that exhibit a critical temperature above 50 K. These systems are characterized by strong interplay between electronic, spin and lattice degrees of freedom and the macroscopic functional properties arise from the competitions between them. All the theoretically and experimental results point out on the diff iculty to separate the effects of disorder, doping and magnetism on the superconducting properties. The presence of multiple bands crossing the Fermi level makes the electronic structure very sensitive to the structural details, particulary to the changes on the local atomic arrangement in the Fe-Pn/Ch slabs. The aim of this work is to study the local structure and discuss possible implications on the electronic properties in the Fe-based superconductors. This has important implications, both in theoretical and in practical terms: a close relationship between structure and superconductivity, direct or indirect, places constraints on both the theoretical understanding of the pairing interaction and the promise of
superconductors with higher TC values. The proposed sensitivity of electronic structure and/or magnetic interactions to the details of the internal structure of the Fe-As layers is likely relevant to unraveling this puzzle
Photoemission Study of La8?xSrxCu8O20: Impact of the Charge and Spin Density Waves on the Electronic Structure
No full textWe report a photoemission study of La8-xSrxCu8O20 which shows antiferromagnetic (AFM), weakly ferromagnetic (WFM), and paramagnetic (PM) phases. All the samples in the AFM, WFM or PM phases are found to have a sharp Fermi edge with finite density of states at the Fermi level (EF), indicating the metallic nature of the samples at different doping and temperatures studied. In the WFM and AFM phases, the spectral weight near the EF (up to ∼-200 meV) is partially suppressed. In the valence band spectra, the Cu 3d and O 2p derived states around ∼-5 and ∼-2.5 eV show different spectral weights in different magnetic phases. The observed changes in the electronic structure can be due to formation of the charge and/or spin density waves causing the anomalies in the electronic and magnetic transport properties of this system
Study of local disorder in LiMn(Cr,Ni)O2 compounds by extended X-ray absorption fine structure measurements
No full textWe have studied local structure of LiMnO2, LiMn0.65Cr0.35O2 and LiMn0.5Ni0.5O2 compounds by Mn K-edge extended X-ray absorption fine structure measurements. The local structure of LiMnO2 is found to be consistent with Jahn Teller distorted MnO6 octahedra characterized by two different Mn-O bond distances. The Jahn Teller distortions are suppressed in the Cr and Ni substituted compounds, resulting a single Mn-O distance. However, the Cr atoms tend to occupy a site at a longer distance from Mn in the host lattice (Mn-Cr distance is longer than Mn-Mn distance), unlike the Ni atoms which prefer a site closer to the Mn atoms (Mn-Ni distance is shorter than Mn Mn distance). Incidentally, Mn-O and Mn Mn bonds are substantially stiffer in the Cr and Ni substituted compounds. In addition, the static atomic disorder is confined around Cr atoms in the LiMn0.65Cr0.35O2, that is different from the case of LiMn0.5Ni0.5O2 in which larger static disorder appears in the proximity of the Mn atoms. The results suggest that the differences in the local structure of different compounds should be the likely reason for their differing battery characteristics. (C) 2013 Elsevier B.V. All rights reserved
Influence of the extra layer on the transport properties of NdFeAsO1-0.14F0.14 and FeSe0.88 superconductors from magneto dynamic analysis
No full textIn this letter, the electromagnetic response of the NdFeAsO1-0.14F0.14 (T (c) =49 K) superconductor system, characterized by FeAs and NdO alternating layers, has been compared with that of FeSe0.88. We have studied the flux dynamics of these two systems by means of ac multi-harmonic magnetic susceptibility. The analysis shows that although characterized by larger thermal fluctuations due to its higher T (c) , NdFeAsO1-0.14F0.14 exhibits a stronger pinning force relative to FeSe0.88. The further Irreversibility Line (IL) analysis also points out that both superconductors have a 3D flux pinning behavior. We associate the stronger pinning force in the NdFeAsO1-0.14F0.14 structure to the presence of the extra NdO layer. Different pinning contributions can be associated to the structural stress associated to FeAs superconducting layers and/or to the Nd3+ ions magnetic moment (mu similar to 3.6 mu(B)) contribution on the flux cores. We will also show that these pinning are over imposed to a weak collective contribution due to the dopant F atoms
Determination of local structure in FeSe(0.25)Te(0.75) single crystal by polarized EXAFS
No full textTemperature-dependent polarized extended X-ray absorption fine-structure (EXAFS) measurements are made on the superconducting FeSe0.25Te0.75 single crystal. The Fe-Se bond length is found to be significantly shorter than the average crystallographic Fe-Se/Te distance, and almost equal to the one for the system without Te. On the other hand, the Fe-Te bond length is nearly equal to the one known for a binary FeTe system. This suggests that the Se and Te occupy distinct sites in the ternary FeSe0.25Te0.75, indicating the breaking of the average crystal symmetry with locally inhomogeneous atomic distribution
Evidence of local structural inhomogeneity in FeSe1-xTex from extended x-ray absorption fine structure
No full textLocal structure of FeSe1-xTex has been studied by extended x-ray absorption fine-structure (EXAFS) measurements as a function of temperature. Combination of Se and Fe K edge EXAFS has permitted to quantify the local interatomic distances and their mean-square relative displacements. The Fe-Se and Fe-Te bond lengths in the ternary system are found to be very different from the average crystallographic Fe-Se/Te distance, and almost identical to the Fe-Se and Fe-Te distances for the binary FeSe and FeTe systems, indicating distinct site occupation by the Se and Te atoms. The results provide a clear evidence of local inhomogeneities and coexisting electronic components in the FeSe1-xTex, characterized by different local structural configurations, with direct implication on the fundamental electronic structure of these superconductors
Mesoscopic stripes in antiferromagnetic Fe chalcogenide probed by scanning photoelectron spectromicroscopy
No full textWe have performed scanning photoelectron spectromicroscopy measurements in the paramagnetic (PM) and the antiferromagnetic (AFM) phases of Fe1+δTe (δ = 0.09). The spectromicroscopy images reveal stripe modulation of the electronic structure in the AFM monoclinic phase at low temperature, that disappears at high temperature in the PM phase. The stripes, running along the monoclinic crystal axis, are characterized by different density of states around the Γ point and near the Fermi level (EF). Since the near EF electronic states around the Γ point have strong electron–lattice coupling in Fe1+δTe, the observed stripes are likely to be related to the strain modulation introduced by the monoclinic lattice distortion
Local structure response of phase separation and iron-vacancy order inKxFe2−ySe2superconductor
No full textWe have studied the local structure of a KxFe2-ySe2 superconductor across the phase separation and iron-vacancy order-disorder temperatures (respectively at similar to 520 K and similar to 580 K). The combination of Fe K edge and Se K edge extended x-ray absorption fine-structure (EXAFS) measurements reveal the glassy local structure of KxFe2-ySe2, with anomalous behavior of local atomic displacements. We find that the Fe-Se distance remains temperature independent while the Fe-Fe distance suffers a substantial effect of the phase separation. Furthermore, the Fe-Se network shows a reduced disorder in the phase separation regime at lower temperature. The x-ray absorption near-edge structure features follow the local structure anomaly observed by EXAFS and reveal substantially reduced Fe 3d-Se 4p hybridization in the iron-vacancy disordered phase at higher temperature. The results provide further information on the role of nanoscale atomic displacements in the peculiar coexistence of different electronic phases in KxFe2-ySe2 with a filamentary superconducting state embedded in the iron-vacancy ordered magnetic texture
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