Electron Doping The Normal State Of The Cobalt Oxide Superconducting Materials Anomalous Physical Properties And Superconducting Mechanism

In the past three years, the discovery of superconductivity in layered cobalt oxides Na_xCoO₂·yH₂O has attracted much interest from both experimental and theoretical sides in the condensed matter physics society. Two reasons are key: the first one is that electron doped cobaltate may be a new system where superconductivity occurs in the doped Mott insulator like the case of cuprates. It is believed that the investigations in cobaltates are expected to shed light on the mechanism of cuprate superconductors. The second reason is that the triangular lattice based on cobaltates may exhibit some novel electronic and magnetic phase, one may expect the unconventional superconductivity with magnetic frustrations originating from the triangular lattice. In this paper, within the t — J model and charge-spin separation fermion-spin theory, the anomalous charge transport, heat transport, spin dynamics and mechanism of superconductivity of electron doped cobaltates are studied systemically.The paper is organized as follows. In chapter â…¡, we introduce the two-dimensional (2D) t — J model and charge-spin separation fermion-spin theory. It has been argued that the essential low-energy physics of electron doped cobaltats can be described effectively by the 2D t — J model, where the single occupancy local constraint can be treated properly within the charge-spin separation fermion-spin theory. In order to apply the 2D t — J model and charge-spin separation fermion-spin theory in electron doped cobaltate, we have introduced a particle-hole transformation.Within the 2D t — J model and charge-spin separation fermion-spin theory, we study the unusual transport properties including charge transport and heat transport in chapter â…¢. For charge transport, we find that the optical conductivity spectrum shows the low-energy peak and unusual midinfrared band, while resistivity exhibits a crossover from the high-temperature metalliclike behavior to the low-temperature insulatinglike behavior due to the competition between the kinetic energy and magnetic energy, in qualitative agreement with the experimental data. Our result also shows that the optical and transport properties of the elecreon doped cobaltate are...