Revealed by Neutron Spectroscopy

We report a high-resolution neutron spectroscopy study of the archetypal heavy fermion material CeIn₃ that exhibits an antiferromagnetic ground state below T_N = 10.1 K with a commensurate propagation vector k = (½, ½, ½). Previous neutron results with moderate energy resolution reported a substantial spin gap of 1.3 meV, which suggest large magnetic anisotropy even though this is not supported by the observed bulk properties. Our results unambiguously demonstrate that CeIn₃ does not exhibit a spin gap. Instead, we find that the spin waves disperse quasi-vertically up to almost 1 meV. When such as steep dispersion is probed with not sufficiently high resolution, this will result in an artifact in the neutron spectrum that looks like a spin gap, and thus our findings are in agreement with previous reports. More interestingly, the steep spin wave dispersion of CeIn₃ reveals truly long-range magnetic exchange interactions and suggests Fermi surface nesting.