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 (∼15 m_e ∼ 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 sqrt(3) × sqrt(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 (over(k, →)) ∼ v_F ∼ 0.4 ± 0.1 eV Å) observed. ARPES data is consistent with bulk thermodynamic specific heat and quantum oscillation measurements.