Island of Rare Earth Nuclei with Tetrahedral and Octahedral Symmetries: Possible Experimental Evidence

Calculations using realistic mean-field methods suggest the existence of nuclear shapes with tetrahedral T_d and/or octahedral O_h symmetries sometimes at only a few hundreds of keV above the ground states in some rare earth nuclei around ¹⁵⁶Gd and ¹⁶⁰Yb. The underlying single-particle spectra manifest exotic fourfold rather than Kramers’s twofold degeneracies. The associated shell gaps are very strong, leading to a new form of shape coexistence in many rare earth nuclei. We present possible experimental evidence of the new symmetries based on the published experimental results—although an unambiguous confirmation will require dedicated experiments.