Magnetoelastic hybrid excitations in CeAuAl3

A cornerstone of condensed-matter physics is the Born-Oppenheimer approximation, which assumes that the motion of the atomic nuclei and electrons in solids may be treated separately. We report the observation of two distinct magneto-elastic hybrid excitations of the phonons and crystal electric fields (CEFs) in the paramagnetic state of the Kondo lattice compound C_{e A u A l 3}: (i) a vibronic bound state and (ii) a pronounced anticrossing. The formation of both excitations due to acoustic phonons in the presence of small coupling constants, as well as considerable damping of the CEF excitations by the conduction electrons, suggests that similar hybrid excitations must generically exist in a wide range of materials. This identifies C_{e A u A l 3} as a showcase for the development of a predictive understanding of magneto-elastic instabilities.