SPring-8, the large synchrotron radiation facility

Date & Time: 14:00-15:00, March 31, 2006

Speaker: Prof. Christoph Janowitz
Affiliation: The Institute for Solid State Physics (ISSP) of the University of Tokyo / Humboldt University, Berlin

Abstract
　　The experimental dispersion of the first removal state in the sparsely studied insulating regime of Bi₂Sr₂Ca_1-xY_xCu₂O_8+δ for Yttrium concentrations of 0.92≥x≥0.55, i.e. between the antiferromagnetic and the superconducting phase, is found to differ significantly from that of protytypical oxychlorides, generally assumed to represent the parent compound of the cuprates. While the widely applied and commonly accepted t-t'-t"-J - or three-band Hubbard models are found unable to account for this, a theoretical treatment of the many body effects with the generalized tight binding (GTB) method and a five band Hubbard model, i.e. including non planar orbitals, show increased two hole spin triplet contributions and are in excellent agreement to our data. These findings deliver new insight on the issue of the appropriate description of HTc- parent compounds.
　　In the second part of the talk the modulation of the charge density in Pb-Bi2201 at optimum Pb doping will be discussed. Upon optimum doping with Pb it is well known that the (1x5)  superstructure of Bi cuprate superconductors is suppressed. Nevertheless, a Fermi surface map of Bi_2-yPb_ySr_2-xLa_xCuO_6+δ with x=0.4 and y=0.4 determined by high-resolution photoemission reveals, regardless the optimum Pb content, besides the main Fermi surface centered at X(Y) (π,π) additional Fermi surface (FS) features. These are obviously due to modulations of the electron density beyond the detections limits discussed before.
　　Low energy electron diffraction of the samples taken at different electron energies showed no sign of any superstructure. Scanning tunneling microscopy with high depth resolution, on the other hand, revealed directly a long-range modulation of (1x17) periodicity. In addition, a second weaker modulation of the electron density with (7x7) periodicity has been derived from the autocorrelation of a large topological area of 50 nm x 50 nm. The corresponding k-vectors of both modulations have been extracted and used to simulate the corresponding Fermi surface which is compared with the experimental one from photoemission. The correspondence is strikingly good and opens the pathway to a reexamination and completion of results obtained on other Bi derived cuprates.