Anharmonic double-$γ$ vibrations in nuclei and their description in the interacting boson model

Double-$\gamma$ vibrations in deformed nuclei are studied in the context of the interacting boson model with special reference to their anharmonic character. It is shown that large anharmonicities can be obtained with interactions that are (at least) of three-body nature between the bosons. As an example the $\gamma$ vibrations of the nucleus $^{166}_{\phantom{0}68}$Er$^{\phantom{00}}_{98}$ are studied in detail.