686 research outputs found
Detection of the flux dynamical regimes in Bi4O4S3 by multiharmonic AC susceptibility
No full textThe temperature dependent first and third harmonics of the AC magnetic susceptibility χ1,3(T) have been measured on Bi4O4S3. The χ1(T) magnetic response is due to the intergranular contribution with a transition temperature Tc ≈ 4.4 K. The dependencies of the χ3(T) on the AC field amplitude and frequency reveal a critical state with the presence of the flux creep and the flux flow regimes. In fact, the field dependencies of the effective activation energy and of the frequency dependent critical current density have been found to be described with power laws characteristic of the collective creep regime involving large flux bundles
A study of the electronic structure of FeSe1-xTex chalcogenides by Fe and Se K-edge x-ray absorption near edge structure measurements
No full textFe K-edge and Se K-edge x-ray absorption near edge structure (XANES) measurements are used to study the FeSe1-xTex electronic structure of chalcogenides. An intense Fe K-edge pre-edge peak due to Fe 1s -> 3d (and admixed Se/Te p states) is observed, showing substantial change with Te substitution and x-ray polarization. The main white line peak in the Se K-edge XANES due to Se 1s -> 4p transition appears similar to the one expected for Se2- systems and changes with Te substitution. Polarization dependence reveals that unoccupied Se orbitals near the Fermi level have predominant p(x,y) character. The results provide key information on the hybridization of Fe 3d and chalcogen p states in the Fe-based chalcogenide superconductors
Evidence of local structural inhomogeneity in FeSe1-xTex from extended x-ray absorption fine structure
No full textLocal structure of FeSe1-x Tex has been studied by extended x-ray absorption fine-structure (EXAFS) measurements as a function of temperature. Combination of Se and FeK 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-x Tex, characterized by different local structural configurations, with direct implication on the fundamental electronic structure of these superconductors. © 2010 The American Physical Society
X-ray absorption and photoemission spectroscopy of electronic phase separation inKxFe2−ySe2
No full textThe electronic structure of phase separated KxFe2-y Se-2 has been studied by means of x-ray absorption spectroscopy (XAS) and photoemission spectroscopy. Both Fe L-2,L-3-edge XAS and Fe 2p photoemission spectra show a signature of the metallic phase that is embedded in the insulating texture. Valence-band spectra, measured with different excitation energies, reveal a finite density of states at the Fermi level driven by the Fe 3d orbitals. A combined analysis of the Se L-2,L-3-edge and the Se K-edge XAS provides further information on a significant density of states of Se 4p character near the Fermi level, consistent with a large hybridization of these states with the Fe 3d orbitals. The K L-2,L-3-edge XAS spectrum is also presented to show element specific partial density of states. The results are discussed in connection with the electronic phase separation in K-x Fe2-y Se-2 driven by the Fe 3d states
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
Large local disorder in superconducting K0.8Fe1.6Se2 studied by extended x-ray absorption fine structure
No full textWe have measured the local structure of superconducting K 0.8Fe 1.6Se 2 chalcogenide (T c=31.8K) by temperature dependent polarized extended x-ray absorption fine structure (EXAFS) at the Fe and SeK-edges. We find that the system is characterized by a large local disorder. The FeSe and FeFe distances are found to be shorter than the distances measured by diffraction, while the corresponding mean square relative displacements reveal large Fe-site disorder and relatively large c-axis disorder. The local force constant for the FeSe bondlength (k∼5.8eV Å 2) is similar to the one found in the binary FeSe superconductor, however, the FeFe bondlength appears to be flexible (k∼2.1eV Å 2) in comparison to the binary FeSe (k∼3.5eV Å 2), an indication of partly relaxed FeFe networks in K 0.8Fe 1.6Se 2. The results suggest a glassy nature for the title system, with the superconductivity being similar to that in the granular materials. © 2012 IOP Publishing Ltd
Probing unconventional pairing in LaO0.5F0.5BiS2 layered superconductor by point contact spectroscopy
No full textWe performed point contact spectroscopy experiments on the layered superconductor LaO0.5F0.5BiS2 by pushing mechanically etched gold tip on the surface of polycristalline samples. We measured low temperature differential conductance curves for several contacts obtained by tuning the tip pressure/position. Conductance features appeared with different shape, amplitude and energy. Non conventional d -wave symmetry of the superconducting order parameter is argued. A comprehensive scenario explaining all the conductance features is discussed, considering the possible formation of intergrain Josephson junction in series with the point contact due to the polycrystalline nature of the sample. Superconducting energy gap is estimated in the range 4.2 ÷ 4.8 meV, showing a BCS-like temperature behavior and signature of strong coupling with a BCS ratio 9.7<2Δ/(KBTC)<11.1
Electronic properties of FeSe1-xTex probed by x-ray emission and absorption spectroscopy
No full textThe electronic structure of FeSe1-xTex chalcogenide superconductors has been studied by x-ray emission (XES) and x-ray absorption (XAS) as a function of Te substitution. The Fe K beta XES spectra reveal a relatively low spin state for Fe in FeSe1-xTex superconductors, persisting in the whole range of Te substitution. The Fe K-edge high-resolution XAS shows systematic spectral changes due to the evolving hybridization between the Fe 3d (4p) and chalcogen p (d) orbitals. The resonant inelastic x-ray scattering (RIXS) spectra hardly show any feature except the one due to charge transfer from occupied to unoccupied bands, that changes substantially from FeSe to FeTe. The results provide important information on the electronic states and their evolution in the FeSe1-xTex chalcogenides
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
Large local disorder in superconducting K0.8Fe1.6Se2studied by extended x-ray absorption fine structure
No full textWe have measured the local structure of superconducting K0.8Fe1.6Se2 chalcogenide (Tc = 31.8 K) by temperature dependent polarized extended x-ray absorption fine structure (EXAFS) at the Fe and Se K-edges. We find that the system is characterized by a large local disorder. The Fe–Se and Fe–Fe distances are found to be shorter than the distances measured by diffraction, while the corresponding mean square relative displacements reveal large Fe-site disorder and relatively large c-axis disorder. The local force constant for the Fe–Se bondlength (k ∼ 5.8 eV A ̊ −2) is similar to the one found in the binary FeSe superconductor, however, the Fe–Fe bondlength appears to be flexible (k ∼ 2.1 eV A ̊ −2) in comparison to the binary FeSe (k ∼ 3.5 eV A ̊ −2), an indication of partly relaxed Fe–Fe networks in K0.8Fe1.6Se2. The results suggest a glassy nature for the title system, with the superconductivity being similar to that in the granular materials
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