Correlations of the Nd Magnetic Moments and Their Influence on the Specific Heat inNd2−xCexCuO4

We study the effect of Ce substitution on the magnetic excitations of Nd in ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ by inelastic neutron scattering. A strong softening of the spin waves upon doping is observed, due to a decrease of the Cu-Nd exchange field at the Nd site. Consequently, a high spin-wave density of states is created at low energies, which leads to giant $\gamma =C/T$ values at low temperatures. This demonstrates that the magnetism of the Nd ions is the only source of the large $\gamma$ values found in specific-heat experiments, i.e., ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ has to be excluded as a prototype of a novel heavy-fermion system.