Vibron quasibound state in the noncentrosymmetric tetragonal heavy-fermion compound CeCuAl 3

We have investigated the noncentrosymmetric tetragonal heavy-fermion antiferromagnetic compound CeCuAl 3 (T N=2.5K) using inelastic neutron scattering (INS). Our INS results unequivocally reveal the presence of three magnetic excitations centered at 1.3, 9.8, and 20.5 meV. These spectral features cannot be explained within the framework of crystal-electric-field models and recourse to Kramers' theorem for a 4f1 Ce3 + ion. To overcome these interpretational difficulties, we have generalized the vibron model of Thalmeier and Fulde for cubic CeAl 2 to tetragonal point-group symmetry with the theoretically calculated vibron form-factor. This extension provides a satisfactory explanation for the position and intensity of the three observed magnetic excitations in CeCuAl 3, as well as their dependence on momentum transfer and temperature. On the basis of our analysis, we attribute the observed series of magnetic excitations to the existence of a vibron quasibound state. © 2012 American Physical Society.