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...

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We present high resolution angle resolved photoemission data of the bilayer superconductor Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi2212) showing a clear doubling of the near E(F) bands. This splitting approaches zero along the (0,0)-->(pi,pi) nodal line and is not observed in single layer Bi(2)Sr(2)CuO(6+delta) (Bi2201), indicating that the splitting is due to the long sought after bilayer splitting effect. The splitting has a magnitude of approximately 75 meV near the middle of the zone, extrapolating to about 110 meV near the (pi,0) point. The existence of these two bands also helps to clear up the recent controversy concerning the topology of the Fermi surface.

A new low photon energy regime of angle resolved photoemission spectroscopy is accessed with lasers and used to study the superconductor Bi 2 Sr 2 CaCu 2 O 8+δ . The low energy increases bulk sensitivity, reduces background, and improves resolution. With this we observe spectral peaks which are sharp on the scale of their binding energy -the clearest evidence yet for quasiparticles in the normal state. Crucial aspects of the data such as the dispersion, superconducting gaps, and the bosonic coupling kink and associated weight transfer are robust to a possible breakdown of the sudden approximation. 74.72.Hs, 74.00.00, 74.25.Jb, 73.90.+f High T c superconductivity has been at the forefront of solid state physics research since its discovery in 19861 . Tunneling spectroscopy 2 3 and angle resolved photoemission spectroscopy (ARPES) 4 5 have been among the key techniques for studying the electronic structure of the cuprates in the quest to understand the many-body interactions responsible for high T c superconductivity. Unfortunately, both of these techniques are surface sensitive, making unclear their detailed applicability to bulk physics such as superconductivity. Here we introduce laser-based ARPES for studies of superconductors, which is expected to have significantly greater bulk sensitivity and which also offers superior energy and momentum resolution.The bulk-sensitivity of ARPES is limited by the electron mean free path in the solid, which depends on the electron kinetic energy in a roughly universal way 6 . Typically, there is a broad minimum of the mean free path in the 20-50 eV kinetic energy range with a sharp increase at lower energy and a slower increase at high energy, which is thought to hold true for Bi2212 7 . With the current interest in using ARPES to study bulk-physics such as superconductivity, the surface sensitivity has become a real hindrance. In order to improve the bulk-sensitivity of ARPES, one may go to very high photon energy such as a few thousand eV, though photoelectron cross sections decrease, and it becomes prohibitively difficult to obtain high energy and momentum resolution. Moving to low energy is thus a more attractive rout to increase bulk sensitivity, though there are some limitations to the extent of k-space that can be accessed.A critical question for ARPES, especially at low photon energy, is whether the sudden approximation, in which one assumes that the electron leaves the sample prior to relaxation of the created photo-hole, is valid 7 8 9 . If this is so, then the photoelectron spectrum should be directly proportional to the spectral function A(k,ω) which is in principle calculable using many-body techniques. This is consistent with a recent theoretical calculation which argues that the energy at which the adiabatic-sudden transition occurs should be a function of the type of excitation -in particular the transition is expected to occur at lower energies for more localized excitations 9 .We have built a high resolution ARPES system centered around a Scienta 10 SES...