Anomalous momentum dependence of the quasiparticle scattering rate in overdoped Bi2Sr2CaCu2O8

The question of the anisotropy of the electron scattering in high temperature superconductors is investigated using high resolution angle-resolved photoemission data from Pb-doped Bi2Sr2CaCu2O8 (Bi2212) with suppressed superstructure. The scattering rate of low energy electrons along two bilayer-split pieces of the Fermi surface is measured (via the quasiparticle peak width), and no increase of scattering towards the antinode (pi,0) region is observed, contradicting the expectation from Q=(pi,pi) scattering. The results put a limit on the effects of Q=(pi,pi) scattering on the electronic structure of this overdoped superconductor with still very high T(c).     

Asymmetric local displacements in the Bi2Sr2CaCu2O8+β superconductor

Instantaneous in-plane Cu-O bond distribution in the Bi₂Sr₂CaCu₂O_8+β (Bi2212) superconductor has been investigated by high k-resolution Cu K-edge extended X-ray absorption fine structure (EXAFS) measured with polarized vector parallel to the two orthogonal Cu-O-Cu bonds of the CuO₂ square plane. The results show an anisotropic Cu-O distribution in the two directions and provide further information on the local atomic displacements in the lattice-charge stripes. Received 5 June 2000     

From manganites to cuprates: a comparative study of the local lattice instability

Recent studies have shown that structural instabilities play a vital role in segregation of charge carriers in the perovskites. We have used extended x-ray absorption fine structure (EXAFS) to investigate the instability of the CuO₆ octahedra of the La_1:85Sr_0:15CuO₄ (LSCO) system showing high Tc superconductivity and the MnO₆ octhahedra of the La_0:75Ca_0:25MnO₃ (LCMO) system showing GMR (giant-magneto resistance) properties. Temperature dependent Cu K-edge EXAFS spectra on the LSCO and Mn K-edge on the LCMO systems are reported. The results show a similarity in the lattice instability of the two systems providing evidence for a similar phase segregation of localized and itinerant charge carriers in the two families of perovskite systems.     

Evidence for a striped phase of high T c superconductor at the Fermi surface

A complete Fermi surface in the normal state of superconducting Bi₂Sr₂CaCu₂O_8+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:0:23Å^?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 ～ 14Å large stripes.     

Elastic scattering susceptibility of the high temperature superconductor Bi2Sr2CaCu2O(8+delta): a comparison between real and momentum space photoemission spectroscopies

The joint density of states of Bi2Sr2CaCu2O(8+delta) is calculated by evaluating the autocorrelation of the single particle spectral function A(k, omega) measured from angle resolved photoemission spectroscopy (ARPES). These results are compared with Fourier transformed (FT) conductance modulations measured by scanning tunneling microscopy (STM). Good agreement between the two experimental probes is found for two different doping values examined. In addition, by comparing the FT-STM results to the autocorrelated ARPES spectra with different photon polarization, new insight on the form of the STM matrix elements is obtained. This shines new light on unsolved mysteries in the tunneling data.     

Approximate approximations from scattered data

The aim of this paper is to extend the approximate quasi-interpolation on a uniform grid by dilated shifts of a smooth and rapidly decaying function to scattered data quasi-interpolation. It is shown that high order approximation of smooth functions up to some prescribed accuracy is possible, if the basis functions, which are centered at the scattered nodes, are multiplied by suitable polynomials such that their sum is an approximate partition of unity. For Gaussian functions we propose a method to construct the approximate partition of unity and describe an application of the new quasi-interpolation approach to the cubature of multi-dimensional integral operators.     

Metallic behavior of lightly doped La2-xSrxCuO4 with a Fermi surface forming an arc

Lightly doped La2-xSrxCuO4 in the so-called "insulating" spin-glass phase has been studied by angle-resolved photoemission spectroscopy. We have observed that a "quasiparticle" (QP) peak crosses the Fermi level in the node direction of the d-wave superconducting gap, forming an "arc" of Fermi surface, which explains the metallic behavior at high temperatures of the lightly doped materials. The QP spectral weight of the arc smoothly increases with hole doping, which we attribute to the n approximately x behavior of the carrier number in the underdoped and lightly doped regions.     

ALS User Meeting Highlights Accomplishments and Challenges

The 2015 User Meeting brought together 405 ALS users from around the world, many of whom shared insights and sparked discussion with presentations of their ALS research highlights. UEC Chair Chris Cappa launched the meeting with a welcome, followed by another from Berkeley Lab Director Paul Alivisatos. Speaking of the vitality and community that's key to the ALS, Alivisatos also touched on how the ALS could be utilized and optimized with a high-coherence upgrade. ALS Director Roger Falcone outlined short-term and long-term planning for the ALS, with ongoing efforts to develop new beamlines and capabilities within the next three years and beyond.