Transport Properties Of Nd_2-xCe_xCuO₄ And Na_xCoO₂ Single Crystals

The high temperature superconductivity (HTSC) is a main topic in the condensed matter physics since it is related to the strongly correlated interaction among electrons. It has been in mystery yet since its discovery in 1986. Recently the study in Na_xCoO₂·yH₂O also becomes a very hot topics in the physics community. The motivation for this effort is two aspects. On the one side, the discovery of superconductivity in this system makes us consider that it may be an alternative example instead of HTSC as a doped Mott insulator becomes superconducting. On the other side, the complex phase diagram in this system may suggest some novel magnetic and electronic phases due to the nature of the strongly correlation and the triangle lattice in this system at a fundamental level.In this dissertation, we have studied the electron-type Nd_2-xCe_xCuO_4-δ and triangle lattice cobaltate Na_xCoO₂ single crystals and the dissertation was arranged as seven part:1. Brief overview of the physical properties of the high-T_c cuprates and triangular cobalt oxidesIn the first chapter, the author reviews the structure and phase diagram of the high temperature superconductors and the new discovered Na_xCoO₂yH₂O superconducting materials. The physics and the mechanism for the transport phenomenon are also introduced. For the cuprates, their anomalous properties are briefly discussed and the recent development in theoretical and experiemental physics about this topic is also mentioned. For the material Na_xCoO₂yH₂O, the recent experiements for the evidence of unconventional superconductivity and the research on its parent compound material Na_xCoO₂ are reviewed.2. Effect of Ce doping and oxygen content on pseudogap and anisotropy in Nd_2-xCe_xCuO_4-δIn this chapter, transport properties of underdoped Nd_2-xCe_xCuO_4-δ single crystals with different oxygen contents have been investigated systematically. In-plane resistivity begins to decrease at a certain temperature T^* (pseudogap opening); at the same temperature the out-of-plane resistivity shows more apparent decrease, in contrast to the hole-doped cuprates. Around the T^*, the curvature changes the sign for both ρ_ab(T) and ρ_c(T). Decrease of oxygen content results in sign change of the curvature from negative to positive at high temperature for the crystals x close to...