Low-temperature magnetoresistance of layered cobalt oxides NaxCoO2

Abstract: We measured transport properties in layered cobalt oxides , that are parent materials of the superconductor , in order to reveal so-far unsettled origin of these electronic and magnetic features. Various sodium nonstoichiometric samples () have been carefully synthesized to control the carrier density and the electronic state of cobalt ions. We found a large negative magnetoresistance (MR) effect in the vicinity of and 1.0. The and 1.0 samples show negative MR, and , respectively, at 90kOe and 4K. These negative MR suggest the pseudogap formation due to the charge ordering in layers, such as and , all of which exhibit similar behaviors. The suppression of the pseudogap by a magnetic field causes the large negative magnetoresistance. [Copyright &y& Elsevier]