The measurement of the Cu-O distances by a local and fast probe, polarized Cu K-edge extended x-ray absorption fine structure (EXAFS) in La 1.85 Sr 0.15 CuO 4 crystal shows two different conformations of the CuO 6 octahedra below 100 K assigned to two types of stripes with different lattice. This experiment supports a model of "two components" spatially separated in a superlattice of quantum stripes for the anomalous properties of cuprate superconductors. [S0031-9007(96)00119-6] PACS numbers: 74.72. Dn, 61.10.Ht, 78.70.Dm Experimental methods probing the local structure have shown that the structure of the metallic CuO 2 plane in high T c cuprate superconductors is not homogeneous at a mesoscopic scale length [1][2][3][4]. It has been proposed that an anharmonic 1D modulation of the CuO 2 plane is a key feature for the mechanism of high T c superconductivity [5]. A superstructure of the type q pb ‫ء‬ 1 ͑1͞n͒c ‫ء‬ , in the orthorhombic notation, seems to be a common feature of the superconducting cuprates close to the optimum doping. It has been observed in Bi 2 Sr 2 CaCu 2 O 81d (Bi2212) [6,7] and in Bi 2 Sr 2 Cu 2 O 61d (Bi2201) [8] with p ϳ 0.21 and n 2 considering doubling of the c axis; in La 2 CuO 4.1 (LCO) with p 0.22 and n 3 [9] and with p 0.2 and n 3 [10]; in La 22x Sr x Cu 2 O 4 (LSCO) for x ϳ 0.075 with p ϳ 0.16 and n ϳ 2.5 [11] and a similar superstructure for x 0.1, 0.15 but much weaker in intensity for the overdoped sample, i.e., x 0.2 [12]; in Tl 2 Ba 2 CaCu 2 O 8 (Tl2212) with p ϳ 0.2 [13] and a similar one in Tl 2 Ba 2 Ca 2 Cu 3 O 10 (Tl2223) [14]. This superstructure is difficult to identify in some of the compounds (for example, in the case of LSCO it could be identified only after about 9 yr of the discovery of high T c superconductivity in this material), and it is more clear at temperatures lower than 100-200 K (e.g., in Tl2212, Tl2223, LCO, LSCO). On the other hand, the superstructure is stable even at high temperatures in Bi2212. The c-axis modulation, different from sample to sample, is due to ordering of dopants in the rock-salt block layers as it is clear in the isostructural compounds, e.g., La 2 NiO 41d . The long wavelength incommensurate modulation of the CuO 2 plane along the 45 ± direction from the Cu-Cu direction, involving ϳ10 Cu sites, appears to be a common feature of cuprate superconductors at optimum doping.A "two-component" model has been proposed [5] where at optimum doping (0.2 hole per Cu sites) a first component with hole density d i ϳ 1 1 0.16 coexists with a second component of impurity states, with hole density d ᐉ ϳ 0.04, spatially separated in two different types of stripes forming a superlattice of quantum wires. A com-mensurate superstructure with lower period (4 Cu sites) was predicted [5] where all doped holes form a single electronic component, a pinned Wigner polaronic charge density wave (CDW), that will suppress superconductivity, and it has been observed at the 1͞8 critical doping and in the nickelates [15].In the case of Bi2212 we have shown [5] that th...

The experimental determination of the quantum critical point (QCP) that triggers the self-organization of charged striped domains in cuprate perovskites is reported. The phase diagram of doped cuprate superconductors is determined by a first variable, the hole doping δ, and a second variable, the micro-strain ε of the Cu-O bond length, obtained from the Cu K-edge extended x-ray absorption fine structure. For a fixed optimum doping, δ c = 0.16, we show the presence of the QCP for the onset of local lattice distortions and stripe formation at the critical micro-strain ε c. The critical temperature T c (ε, δ) reaches its maximum at the quantum critical point (ε c , δ c) for the formation of bubbles of superconducting stripes. The critical charge, orbital and spin fluctuations near this strain QCP provide the interaction for the pairing.

We have studied the variation of superconducting critical temperature Tc as a function of charge density and lattice parameters in Mg1-xAlxB2 superconducting samples. The AB2 heterostucture of metallic boron layers (intercalated by A = magnesium, aluminum layers, playing the role of spacers) is made by direct chemical reaction. The spacing between boron layers and their charge density are controlled by chemical substitution of Mg by Al atoms. We show that high Tc superconductivity is realized by tuning the chemical potential at a `shape resonance' according with the patent for `high-temperature superconductors made by metal heterostructures at the atomic limit'. The energy width of the superconducting shape resonance is found to be about 400 meV.

We report experimental evidence fur the phase diagram of doped cuprate superconductors as a function of the micro-strain epsilon of the planar Cu-O bund length, measured by Cu K-edge EXAFS, and hole doping delta. The local lattice distortions are measured by EXAFS and the charge ordering is measured by synchrotron radiation diffuse X-ray diffraction. This phase diagram shows a QCP at P(epsilon (c)delta (c)) where for epsilon > epsilon (c) charge-orbital-spin stripes and free carriers co-exist. The superconducting phase occurs in the region of critical fluctuations around this QCP. Thr function T(c) (epsilon, delta) of two variables shows its maximum at the strain QCP. The critical fluctuations near tills strain QCP give the self-organization of a metallic superlattice of quantum wires "superstripes" that favors the amplification of the critical temperature

Rare earth L(3)-edge x-ray absorption near-edge structure (XANES) spectroscopy has been used to study REOFeAs (RE = La, Pr, Nd, Sm) oxypnictides. The Nd L(3) XANES due to the [Formula: see text] transition shows a substantial change in both white line (WL) spectral weight and the higher energy multiple scattering resonances with the partial substitution of O by F. A systematic change in the XANES features is seen due to varying lattice parameters with ionic radius of the rare earth. On the other hand, we hardly see any change across the structural phase transition. The results provide timely information on the local atomic correlations showing the importance of the local structural chemistry of the REO spacer layer and interlayer coupling in the competing superconductivity and itinerant striped magnetic phase of the oxypnictides.

We have investigated the electronic structure of BiS2-based CeO0.5F0.5BiS2 superconductor using polarization-dependent angle-resolved photoemission spectroscopy (ARPES), and succeeded in elucidating the orbital characters on the Fermi surfaces. In the rectangular Fermi pockets around X point, the straight portion parallel to the ky direction is dominated by Bi 6px character. The orbital polarization indicates the underlying quasi-one-dimensional electronic structure of the BiS2 system. Moreover, distortions on tetragonally aligned Bi could give rise to the band Jahn-Teller effect.

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Charge ordering, dielectric permittivity and local structure of La 5/3 Sr 1/3 NiO 4 system have been explored X-ray charge scattering, complex dielectric impedance spectroscopy, and extended Xray absorption fine structure (EXAFS) measurements, made on the same single crystal sample.The local structure measured by the temperature dependent polarized Ni K-edge EXAFS shows significant distortions in the NiO 2 planes. These local distortions could be reasonable cause of high dielectric permittivity of the title system (ε≈100 at 5K) with the charge ordering in this system being a ferroelectric-like second order transition.