Kondo effect in underdoped n-type superconductors
We present high-field magnetotransport properties of high-quality single-crystalline thin films of heavily underdoped nonsuperconducting (La,Ce)2CuO4, (Pr,Ce)2CuO4, and (Nd,Ce)2CuO4. All three materials show identical behavior. They are metallic at high temperatures and show an insulating upturn at low temperatures. The insulating upturn has a log T dependence, but saturates toward the lowest temperatures. Notably, the insulating upturn tends to be suppressed by applying magnetic fields. This negative magnetoresistance has a log B dependence, and its anisotropy shows non simple behavior. We discuss these findings from the viewpoints of Kondo scattering and also two-dimensional weak localization, and demonstrate Kondo scattering as a more plausible explanation. The Kondo scatters are identified as Cu2+ spins in the CuO2 planes.