GIANT QUADRUPOLAR INTERACTION IN THE HEAVY-FERMION COMPOUND YBAS

Inelastic neutron scattering has been employed to study the phonon and crystal-field excitation spectra in a single crystal of the heavy-fer mion compound YbAs. Below a critical temperature of about 80 K we obse rve an unusual splitting of the ground-state crystal-field transition GAMMA6 --> GAMMA8 into two lines. Although the optical-phonon branches turn out to be accidentally degenerate with the crystal-field transit ion, there is no evidence for a coupling of the two elementary excitat ions. Instead we derive a large quadrupolar interaction to be origin o f the observed splitting of the crystal-field transition. Our model ex cellently describes similar features observed in the other ytterbium m onopnictides YbX (X = N, P, Sb).