Theory of Multipolar Excitations in CeB6

A theory for the multipolar excitations in the antiferro-quadrupolar (AFQ) phase of CeB 6 is developed. It is based on a pseudo-spin model for the Γ 8 ground state of Ce used previously to explain the B - T phase diagram. An equation of motion method for the multipolar operators is employed and the solutions for the O 2 0 and O x y phases are given. It is shown that the excitation branches are classified as being of mixed quadrupolar-dipolar, quadrupolar-octupolar and octupolar-dipolar character. The former two have strong and the latter only weak dispersion respectively. The transition from the O 2 0 to the O x y phase in the symmetric model is accompanied by the reappearance of a quadrupolar Goldstone mode. In the O x y phase of the asymmetric model some modes are gapped already at zero field and for finite field no Goldstone mode remains. In addition we give a derivation for the dynamical structure function based on the mixing scheme of multipolar excitations and discuss the result for the zero-field case.