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We report the experimental results of temperature dependent polarized As K-edge extended x-ray absorption fine structure (EXAFS) of LaFe1-xCoxAsO (x=0.0 and 0.11) single-crystals. By aligning the Fe-As bond direction in the direction of the x-ray beam polarization we have been able to identify an anomaly in the Fe-As bond correlations at the tetragonal to orthorhombic transition at 150K, while previous investigations with standard unpolarized EXAFS of undoped LaFeAsO powder samples were not able to detect any such anomaly. Using our approach we have been able to identify in the superconducting doped sample, LaFe0.89Co0.11AsO, a broad anomaly around 60 K. The low temperature anomaly has good correlations with the temperature dependence of several properties like resistivity, magnetic susceptibility, linear thermal expansion, etc indicating the emergence of the dynamical oscillations of the Fe - As pairs, 7 pages, 3 figures

Ab initio Fe K-edge X-ray absorption near edge structure (XANES) calculations on FeTe1−x Se x , based on its crystallographic structure, are compared with experimental data. The calculated XANES spectra are inconsistent with the observed increase of spectral weight in the preedge region of the experimental spectra on the ternary system in which Te is substituted by Se. However, the weighted average of the XANES of FeTe and FeSe binaries reproduce well the observed trend in the experimental spectra of the ternary systems. This suggests that the FeTe1−x Se x is characterized by an inhomogeneous local electronic structure, resulting from a random-alloy like local atomic structure. The calculated unoccupied Fe d-density of states at the Fermi level increases monotonically with Te content unlike the observed change in T c with Te content. T c increases with increasing Te content up to x=0.5; however, the superconductivity is suppressed in the FeTe due to magnetic order, which is not considered in these calculations.

We have studied the electronic structure of FeSe1−xTex and Ir1−xPtxTe2 using photoemission spectroscopy. For FeSe1−xTex, angle-resolved photoemission results indicate that the Fe 3d yz/zx orbital degeneracy at Γ point and orbitally induced Peierls effect in the tetragonal lattice play important roles for the superconductivity. It is suggested that the Jahn-Teller instability of the yz/zx states couples with local lattice distortion derived from the Te substitution for Se and provides an inhomogeneous electronic state. Photoemission results of IrTe2 with triangular lattice are also consistent with the orbitally induced Peierls scenario. The Pt substitution for Ir suppresses the static band Jahn–Teller effect and induces an inhomogeneous electronic state in which orbital (or bond or nematic) fluctuations may help superconductivity through the Peierls effect.

In 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 (μ∼3.6 μB) 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.

We report both measurements and local density approximation calculations of the Fe K-edge X-ray absorption near edge (XANES) spectra of LnOFeAs high Tc superconductors. Calculations based on the local structure centered at the Fe site obtained by crystallographic investigations reproduce observed changes in the spectra under switching of the crystal structure from the tetragonal to the orthorhombic striped phase. Although the local structure changes are very small, their effect is revealed by clear signatures at the absorption edge, indicating the variations of the Fe local unoccupied electronic states. Calculated Fe density of states that do not include spin ordering in the orthorhombic phase do not show a significant modification, implying that the striped spin ordering in the orthorhombic phase is necessary to describe its electronic properties.

International audience; High resolution synchrotron X-ray diffraction is used to study the dynamics of oxygen ordering in a La2CuO4.08 single crystal as a function of the temperature and of the photon flux. We have observed the occurrence of a phase separation between disordered and 3D long-range ordered domains of interstitial oxygen atoms (the latter being commensurate with the lattice). We are able to study and control the phase separation by irradiating the sample while varying the temperature in a range between 350 K and 250 K. These results shed new light on the oxygen ordering in cuprates; in addition, we show that in such systems it is possible to manipulate and to control structural inhomogeneities at the atomic scale using X-ray diffraction.

Local lattice anomalies in optimally doped T-(La,Sr)2CuO4 single crystal like thin films (T c = 43.4 K) grown by molecular-beam epitaxy have been studied by the in-plane polarized Cu K-edge extended X-ray absorption fine structure (EXAFS). The results indicate temperature-dependent local atomic displacements which are anomalous at the T c and below a higher temperature T s as demonstrated by a change in the mean square relative displacement of the Cu–O bond $$\sigma _{{\rm Cu}-{\rm O}}^2$$ , i.e., a sharp drop at the T c and a gradual deviation from a noncorrelated Debye-like behavior below T s where the spatial inhomogeneity appears. We find that the magnitude of the Cu–O displacement changes at the T c, $$\Delta \sigma _{{\rm Cu}-{\rm O}}^2$$ is enhanced by compressive strain while the tendency of charge segregation is suppressed. The results suggest that the uniaxial pressure effects stabilize the system by decreasing the onset temperature and magnitude of spatial heterogeneity.

The interstitial oxygen ordering in a La2CuO4+ y (y = 0.08) single crystal with a T c = 38 K has been investigated. We have studied the growth dynamics of the three dimensional (3D) ordered phase, called Q2, at different temperatures under X-ray illumination. The critical temperature T 0∼334 K of the order-disorder transition for the Q2 phase has been determined. We have found that the illumination of the sample with a continuous polarized synchrotron radiation X-ray flux stimulates the oxygen ordering kinetics. The photoinduced ordering process shows a threshold characteristic of cooperative phenomena and its kinetics shows a temperature dependence that is controlled by the activation energy E A = 100 meV.

We have studied doping evolution of the temperature dependent local Cu-O displacements in the La2-x Sr x CuO4 superconductor by polarized Cu K-edge extended X-ray absorption fine structure (EXAFS) measurements. While temperature dependent Debye-Waller factor of the Cu-O bonds, measuring the local Cu-O displacements, shows an anomalous increase at low temperature for the underdoped single crystals, we do not find such a dependence for the case of the overdoped system. The results, which are discussed in the light of recent angle resolved photoemission measurements, provide an evidence for some important correlation between the doping dependent electron-lattice interaction, the charge inhomogeneities and the local Cu-O displacements in the copper oxide superconductors.

A complete Fermi surface in the normal state of superconducting Bi2Sr2CaCu2O8+δ at the optimum doping (T c = 91K) has been measured by angle scanning photoemission spectroscopy using synchrotron radiation. The Fermi surface reveals broken segments and hot spots of high photointensity along lines of k y = n:0:23A−1 in the (π; π) direction. The pseudogap observed at (π,0) in underdoped materials is one of this missing segments. The results indicate the presence of characteristic subbands of a superlattice of quantum stripes. This is formed by an ordered array of ∼ 14A large stripes.

The temperature dependence of local Cu site conformations in single-domain crystals of Bi2Sr2CaCu2O8+y (Bi-2212) and La1.85Sr0.15CuO4 (La-214) has been determined by EXAFS (Extended X-ray Absorption Fine Structure), a fast (10−15 s) and local probe. Two sets of Cu site conformations characterized by the Cu−O(apical) bond distances and tilting of copper square plane have been found at T