The essential framework for cuprate superconductivity is that of a spin-1/2 electron system in the vicinity of a half filled (Mott limit) correlated 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. Besides superconductivity, the system exhibits spin-thermopower, metal-insulator transition, spin-density-wave, charge-inhomogeneity/order and A-type antiferromagnetism. Since its discovery in 2003, we have employed state-of-the-art angle-resolved photoemission spectroscopy to study the topology of the Fermi surface, quasiparticle renormalization and its temperature evolution in most of these novel phases of cobaltates [1-4]. We have reported the Fermi surface with its hexagonal topology, determined the sign of single-particle hopping to be negative ruling out a few instabilities in the system, and found an unusual temperature dependence of the quasiparticle weight providing the basic foundation for the low-energy electronic structure of cobaltates [1]. These results significantly narrow down the choices for the superconducting order parameters. In this talk, I plan to focus on some intriguing behavior obtained by studying recent high quality samples of the parent superconductor [3] and the unusual insulator in a nearby doping [4].

References:

[1] M. Z. Hasan et. al., Phys. Rev. Lett. 92, 246402 (2004).
[2] A. Kuprin et. al.,  J. Phys. Chem. Sol. 67, 235 (2006).
[3] D. Qian et. al., Phys. Rev. Lett. 96, 216405 (2006).
[4] D. Qian et. al., Phys. Rev. Lett. 96, 046407 (2006).