Negative-parity bands in the vicinity of ${}^{156}\mathrm{Gd}$ and ${}^{160}\mathrm{Yb}$ have been suggested as candidates for the rotation of tetrahedral nuclei. We report the observation of the odd and even-spin members of the lowest energy negative-parity bands in ${}^{160}\mathrm{Yb}$ and ${}^{154}\mathrm{Gd}$. The properties of these bands are similar to the proposed tetrahedral band of ${}^{156}\mathrm{Gd}$ and its even-spin partner. Band-mixing calculations are performed and absolute and relative quadrupole moments deduced for ${}^{160}\mathrm{Yb}$ and ${}^{154}\mathrm{Gd}$. The values are inconsistent with zero, as required for tetrahedral shape, and the bands are interpreted as octupole vibrational bands. The failure to observe the in-band $E2$ transitions of the bands at low spins can be understood using the measured $B(E1)$ and $B(E2)$ values.

• Received 2 September 2009

DOI:https://doi.org/10.1103/PhysRevLett.104.022501