Kondo effect in underdopedn-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 logT dependence, but saturates toward the lowest temperatures. Notably, the insulating upturn tends to be suppressed by applying magnetic fields. This negative magnetoresistance has a logB dependence, and its anisotropy shows a nonsimple 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.