Vibron quasi-bound state in the noncentrosymmetry tetragonal heavy fermion compound: CeCuAl3

Strongly correlated electron systems having spin, charge and lattice degrees of freedom exhibit elementary excitations such as single ion crystal field (CF) excitations and dispersive phonons excitations.  Even though both the excitations having very similar energy scale, in general these excitations are considered to be decoupled from each other and the two phenomena, i.e. the CF level schemes and the phonon dispersion relations can be investigated independent of each other.   A signature of a strong coupling between these two type of excitations,  called as CF-phonon (or magnetoelastic) coupling,  is observed only in a handful of compounds, for example in CeAl2, and they exhibit unusual behaviour both in term of allowed number of CEF excitations and their temperature dependence. We have recently discovered another example of strong CF-phonon coupling in the tetragonal heavy fermion antiferromagnetic compound CeCuAl3 (TN=2.5K) through an inelastic neutron scattering study, which reveals three magnetic excitations at 1.4, 9.8 and 20.5 meV at 5 K. Considering Ce3+  4f1, S=1/2 (J=5/2), odd number of 4f-electron  and Kramers’ theorem one can not have three CF excitations in CeCuAl3. We have extended the CF-phonon coupling model (developed for a cubic CeAl2 system) of Thalmeier and Fulde  (1982) to a tetragonal point symmetry that explains very well the positions and intensities of the observed three magnetic excitations in CeCuAl3. Thus observed magnetic excitations in CeCuAl3 can be attributed due to a vibron quasi-bound state formation in this compound. We will also compare the inelastic neutron scattering results of CeCuAl3 with that of other RCuAl3 (R=Pr and Nd) compounds and discus the role of CF-phonon coupling on the pressure induced superconductivity in the noncentrosymmetry tetragonal heavy fermion compounds.