Spin gap and antiferromagnetic correlations in the kondo insulator CeNiSn

Neutron scattering measurements show that the crossover (at T≈≤10 K) from metallic heavy-fermion to semiconducting behavior coincides with the formation of a gap in the magnetic excitation spectrum of CeNiSn. In contrast to the simple band picture of an insulator, the gap is well defined only at particular values of the momentum transfer Q. While substantial anti ferromagnetic correlations in the a-c plane characterize the low-T state, the corresponding zero-frequency response function is Q independent