Intrinsic phase separation in superconducting K0.8Fe1.6Se2 (Tc= 31.8 K) single crystals

Temperature dependent single-crystal x-ray diffraction (XRD) in transmission mode probing the bulk of the newly discovered K0.8Fe1.6Se2 superconductor (Tc = 31.8 K) using synchrotron radiation is reported. A clear evidence of intrinsic phase separation at 520 K between two competing phases, (i) a first majority magnetic phase with a ThCr2Si2-type tetragonal lattice modulated by the iron vacancy ordering and (ii) a minority non-magnetic phase having an in-plane compressed lattice volume and a weak superstructure, is reported. The XRD peaks due to the Fe vacancy ordering in the majority phase disappear by increasing the temperature at 580 K, well above phase separation temperature confirming the order-disorder phase transition. The intrinsic phase separation at 520K between a competing first magnetic phase and a second non-magnetic phase in the normal phase both having lattice superstructures (that imply different Fermi surface topology reconstruction and charge density) is assigned to a lattice-electronic instability of the K0.8Fe1.6Se2 system typical of a system tuned at a Lifshitz critical point of an electronic topological transition that gives a multi-gaps superconductor tuned a shape resonance.Comment: 10 pages, 4 figure

Fermi surface features and stripes in Bi2Sr2CaCu2O8+d superconductor

We have studied momentum distribution of spectral weight around the Fermi level of Bi2Sr2CaCu2O8+delta (Bi2212) superconductor by angle resolved photoemission to understand implication of the lattice and spin fluctuations on the excitation spectrum of cuprate superconductors. The photointensity reveals shadow bands at (0.5 pi,0.5 pi) and equivalent locations, related to the spin density wave (SDW) and suppression of spectral weight around (pi,0) and equivalent locations, related to an anharmonic and incommensurate charge density wave (ICDW) in the diagonal direction. The work further uncovers the fact that the high T-c appears in a complex phase of coexisting lattice and spin fluctuations

Nanoscale structure and atomic disorder in the iron-based chalcogenides

The multiband iron-based superconductors have layered structure with a phase diagram characterized by a complex interplay of charge, spin and lattice excitations, with nanoscale atomic structure playing a key role in their fundamental electronic properties. In this paper, we briefly review nanoscale structure and atomic disorder in iron-based chalcogenide superconductors. We focus on the Fe(Se,S)(1-x)Te-x (11-type) and K0.8Fe1.6Se2 (122-type) systems, discussing their local structure obtained by extended x-ray absorption fine structure. Local structure studies on the Fe(Se,S)(1-x)Te-x system reveal clear nanoscale phase separation characterized by coexisting components of different atomic configurations, similar to the case of random alloys. In fact, the Fe-Se/S and Fe-Te distances in the ternary Fe(Se,S)(1-x)Te-x are found to be closer to the respective distances in the binary FeSe/FeS and FeTe systems, showing significant divergence of the local structure from the average one. The observed features are characteristic of ternary random alloys, indicating breaking of the local symmetry in these materials. On the other hand, K0.8Fe1.6Se2 is known for phase separation in an iron-vacancy ordered phase and an in-plane compressed lattice phase. The local structure of these 122-type chalcogenides shows that this system is characterized by a large local disorder. Indeed, the experiments suggest a nanoscale glassy phase in K0.8Fe1.6Se2, with the superconductivity being similar to the granular materials. While the 11-type structure has no spacer layer, the 122-type structure contains intercalated atoms unlike the 1111-type REFeAsO (RE = rare earth) oxypnictides, having well-defined REO spacer layers. It is clear that the interlayer atomic correlations in these iron-based superconducting structures play an important role in structural stability as well as superconductivity and magnetism

Different temperature dependent local Cu-O displacements in the underdoped and overdoped regimes of cuprate superconductors

Cu K-edge extended X-ray absorption fine-structure (EXAFS) measurements have been used to study the temperature-dependent local structure of the underdoped and overdoped superconducting La2 − xSrxCuO4 system. The Debye-Waller factor of the in-plane Cu-O bonds has been used as an order parameter to measure the local displacements, revealing an anomalous behavior for the underdoped case while the overdoped system shows a negligible change as a function of temperature. The results provide an evidence for a close relationship between local Cu-O displacements, electron-lattice interaction and charge inhomogeneity in the copper oxide superconductors

Nanoscale Lattice Fluctuations in Cuprates and Manganites

{Local lattice displacements giving a nanoscale inhomogeneous pattern due to mesoscopic phase separation in the correlated transition metal perovskites play a distinct role for high T c superconductivity (HTcS) in the copper oxides and for colossal magneto resistance (CMR) in manganese oxides. Experimentally the local structure physics of these correlated oxides owes a lot to the extended X-ray absorption fine structure (EXAFS) using polarized synchrotron radiation sources, a local and fast experimental probe that has revealed several key structural aspects. Here we have briefly reviewed some of our studies on the local displacements in the electronically active Cu-O lattice in the copper oxide superconductors and the Mn-O lattice of the manganese oxide CMR and charge ordered systems, exploiting the EXAFS technique. In the copper oxides the determination of polaronic distortions by polarized Cu-K-edge EXAFS has revealed superlattice of distorted stripes intercalated with undistorted stripes with the chemical potential tuned near a shape resonance for the interband scattering in a system with two electronic components that can amplify the critical temperature. On the other hand, the Mn K-edge EXAFS has been applied to underline the importance of the local Mn-O displacements to characterize various phases in the perovskite manganese oxides.

Local Structure and Stripes in the Cuprate Superconductors

We have performed several experiments to explore the role of local lattice in the cuprate superconductors. The Cu K-edge X-ray absorption fine structure (EXAFS) results provide a direct evidence for the Jahn-Teller polaronic distortions of the CuO2 lattice in these materials. The Jahn-Teller polarons in the cuprates are found to be associated with Q(2) mode which is degenerate with the Q(3) Jahn-Teller mode. It is discussed that the Jahn-Teller polarons get ordered in the stripes. The temperature dependent Cu K-edge X-ray absorption near edge structure (XANES) measurements reveal a particular change in the local lattice at the charge stripe ordering temperature in different superconducting cuprate systems. This response of the local lattice to the stripe charge ordering has been used to study oxygen isotope effect (O-16-->O-18) in the La1.94Sr0.06CuO4 superconducting system. The results show that the isotope substitution introduces a large increase to the onset temperature of the charge-stripe ordering by similar to 60K, providing a compelling evidence for the vital role of the electron-lattice interactions to be an important ingredient for the charge-stripe ordering in the high T-c cuprate superconductors

On the local lattice displacements in the correlated transition metal oxides

IN POLARONS IN BULK MATERIALS AND SYSTEMS WITH REDUCED DIMENSIONALITY EDITED BY G. IADONISI, J. RANNINGER AND G. DE FILIPPIS (2005

Superstripes by anomalous X-ray diffraction and angle resolved photoemission in Bi2212

We show an evidence for the superconducting stripes (superstripes) in the Bi2212 system by joint x-ray diffraction and angle resolved photoemission. The kink observed at k(y)0.4 pi in the energy distribution curves is shown to be related to a modulation of the Cu displacement out of the oxygen plane with a wavevector Q similar to (0.4 pi ,0.4 pi) that modulates the next-nearest neighbor hopping integral t'. The resulting Fermi surface reveals broken segments around the M points due to the modulation of the t, associated with modulation of the electron-lattice coupling lambda(epsilon) that depends on the micro strain epsilon of the CuO(2) plane. The present findings further enlightens the fact that the micro-strain, controlling the electron-lattice coupling lambda(epsilon) is a critical parameter for the superstripes