title: Low-lying quasiparticle states and hidden collective charge instabilities in parent cobaltate superconductors.
abstract: We report a state-of-the-art photoemission (angle-resolved photoemission spectroscopy) study of high quality single crystals of NaxCoO2 the series focusing on the fine details of the low-energy states. The Fermi velocity is found to be small (<0.5 eV A) and only weakly anisotropic over the Fermi surface at all dopings, setting the size of the pair wave function to be on the order of 10-20 nm. In the low-doping regime, the exchange interlayer splitting vanishes and two-dimensional collective instabilities such as 120 degrees -type fluctuations become kinematically allowed. Our results suggest that the unusually small Fermi velocity and the unique symmetry of kinematic instabilities distinguish cobaltates from most other oxide superconductors.
authors.affiliation: Department of Physics, Joseph Henry Laboratories, Princeton University, Princeton, New Jersey 08544, USA.
authors.forename: D
authors.initials: D
authors.lastname: Qian
authors.forename: D
authors.initials: D
authors.lastname: Hsieh
authors.forename: L
authors.initials: L
authors.lastname: Wray
authors.forename: Y-D
authors.initials: YD
authors.lastname: Chuang
authors.forename: A
authors.initials: A
authors.lastname: Fedorov
authors.forename: D
authors.initials: D
authors.lastname: Wu
authors.forename: J L
authors.initials: JL
authors.lastname: Luo
authors.forename: N L
authors.initials: NL
authors.lastname: Wang
authors.forename: L
authors.initials: L
authors.lastname: Viciu
authors.forename: R J
authors.initials: RJ
authors.lastname: Cava
authors.forename: M Z
authors.initials: MZ
authors.lastname: Hasan