Vibron Quasibound State in the Noncentrosymmetric Tetragonal Heavy-Fermion Compound CeCuAl3

We have investigated the noncentrosymmetric tetragonal heavy-fermion antiferromagnetic compound CeCuAl3 (T-N = 2.5 K) 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 4f(1) Ce3+ ion. To overcome these interpretational difficulties, we have generalized the vibron model of Thalmeier and Fulde for cubic CeAl2 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 CeCuAl3, 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.