We measured transport properties in layered cobalt oxides Nax CoO2, that are parent materials of the superconductor Nax CoO2 · y H2 O, in order to reveal so-far unsettled origin of these electronic and magnetic features. Various sodium nonstoichiometric samples Nax CoO2 ( x = 0.47, 0.62, 1.0) 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 x ∼ 0.5 and 1.0. The x = 0.47 and 1.0 samples show negative MR, - 45 % and - 10 %, respectively, at 90 kOe and 4 K. These negative MR suggest the pseudogap formation due to the charge ordering in CoO2 layers, such as Ca3 Co4 O9 and Bi2 - x Pbx Sr2 Co2 Oy, all of which exhibit similar behaviors. The suppression of the pseudogap by a magnetic field causes the large negative magnetoresistance.
ASJC Scopus subject areas