Low-energy excitations and Fermi surface topology of parent cobaltate superconductor

The essential framework for cuprate superconductivity is that of a spin-1/2 electron system in the vicinity of a half filled (Mott limit) lattice. Of all oxide superconductors, this framework is most closely matched in the sodium doped cobalt oxides except that it is realized on a triangular lattice. We employ angle-resolved photoemission spectroscopy to study the quasiparticle dynamics of the parent cobaltate superconductor. Results reveal a single hole-like Fermi surface generated by the crossing of heavy (similar to 15 m(e) similar to 3m(LDA)) quasiparticles with a negative effective hopping (t(eff) < 0). The observed ground state as given by the topology of the Fermi surface is found be very close to a collective charge instability with root 3 x root 3 symmetry. The measured electron dynamic parameters reveal the unusual character of the parent cobaltate class likely due to small and almost isotropic Fermi velocity (v(F)((k) over bar) similar to v(F) similar to 0.4 +/- 0.1 eV angstrom) observed. ARPES data is consistent with bulk thermodynamic specific heat and quantum oscillation measurements. (c) 2007 Published by Elsevier B.V.