Ultra-heavy quasiparticles in metallic Nd_2-yCe_yCuO₄

The specific heat C_p(T) and the magnetic susceptibility χ(T) of compounds Nd_2-yCe_yCuO₄, 0 ≤ y ≤ 0.2, were measured in the temperature range from 50 mK to 10 K. A Schottky anomaly in C_p(T) for Nd₂CuO₄, resulting from the magnetic splitting of the Nd-4f ground state Kramers doublet due to exchange interactions with the ordered Cu moments, is broadened upon Ce doping, and the maximum is shifted to lower temperatures. In overdoped metallic compounds (y = 0.2), the temperature dependence of C_p(T) is linear below 300 mK with a large value of the coefficient γ ∼ 4 J/K² per mole Nd. A constant value of γ(T) in the same temperature range and the changes of C_p(T) induced by an applied magnetic field corroborate the interpretation in terms of a heavy-fermion system due to the coupling between Nd moments and conduction electrons at the Cu sites.