Low-energy spin fluctuations of the heavy-Fermion compound CeNi_2Ge_2: origin of non-Fermi liquid behavior

Neutron scattering shows that non-Fermi-liquid behavior of the heavy-Fermion compound CeNi_2Ge_2 is brought about by development of low-energy spin fluctuations with an energy scale of 0.6 meV. They appear around antiferromagnetic wave vectors $({1/2} {1/2} 0)$ and $(0 0 {3/4})$ at low temperatures, and coexist with high-energy spin fluctuations with an energy scale of 4 meV and a modulation vector $(0.23, 0.23, {1/2})$. The energy dependence of the spin fluctuations is a peculiar character of CeNi_2Ge_2 which differs from typical heavy-Fermion compounds, and suggests importance of low-energy structures of quasiparticle bands.