Ultra-heavy quasiparticles in metallic Nd_2-yCe_yCuO₄

The specific heat C_p and the magnetic susceptibility χ of compounds Nd_2-yCe_yCuO₄, 0 ≤ y ≤ 0.2, were measured in the temperature range from 50 mK to 10 K. In the undoped compound, a Schottky anomaly in C_p results from the magnetic splitting of the Nd - 4f ground state Kramers doublet due to exchange interactions with the ordered Cu moments. Upon Ce doping, the anomaly is broadened, and the maximum is shifted to lower temperatures. The anomaly persists in the metallic and even in the superconducting state (y = 0.15). In overdoped metallic compounds (y = 0.2), the temperature dependence of C_p is linear below 300 mK with a large value of the coefficient γ ∼ 5 J/K² per mole Nd. A constant value of χ(T) in the same temperature range and the changes of C_p induced by an applied magnetic field corroborate the interpretation in terms of a Heavy-Fermion system due to an effective coupling between the Nd moments and the conduction electrons at the Cu sites.