ULTRA-HEAVY QUASI-PARTICLES IN METALLIC ND2-YCEYCUO4

The specific heat C(p) and the magnetic susceptibility x of compounds Nd2-yCeyCuO4, 0 less-than-or-equal-to y less-than-or-equal-to 0.2, were measured in the temperature range from 70 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 gamma approximately 5 J/K2 per mole Nd. A constant value of chi(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.