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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-fermion compound YbAs. Below a critical temperature of about 80 K we observe an unusual splitting of the ground-state crystal-field transition Γ6 → Γ8 into two lines. Although the optical-phonon branches turn out to be accidentally degenerate with the crystal-field transition, there is no evidence for a coupling of the two elementary excitations. Instead we derive a large quadrupolar interaction to be origin of the observed splitting of the crystal-field transition. Our model excellently describes similar features observed in the other ytterbium monopnictides YbX (X = N, P, Sb).