Triaxiality and Vibrational Modes in Strongly Deformed Nuclei

Abstract

Low-energy vibrational excitations associated with the fluctuation of quadrupole deformed shapes are discussed within the framework of state-of-the-art Configuration Interaction calculations, actually performed via the Quasi-particle Vacua Shell Model version of the Monte Carlo Shell Model. Recently, low-lying $\gamma$ bands in heavy strongly deformed nuclei were shown to be rotational $K^P$ = 2${}^{+}$ excitations of triaxially deformed states (see T. Otsuka {\it et al.}, Eur. Phys. J. A 61, 126 (2025)) rather than vibrational excitations as traditionally interpreted. We present additional illustrations as to how the E2 excitation strength to the gamma band changes as the triaxial deformation parameter $\gamma$ increases, as a systematic indicator for the evolving triaxiality. This is a part of a general formulation of E2 properties of strongly-deformed triaxial states obtained within the quantum many-body scheme. Once the triaxial picture is clarified as a whole, it is important to identify possible low-lying $\gamma$ and other vibrational excitations and to characterize the excitation energy at which they emerge. Focusing on two typical examples, ${}^{166}$Er and ${}^{162}$Dy, vibrational states are indeed identified above the $\gamma$ band using an extended version of the so-called T-plot. Notable agreement with experiment is seen for the $\gamma$-vibrational band in ${}^{166}$Er and for the $\beta$-vibrational band in ${}^{162}$Dy, regarding energy levels and $E2$ transitions. The phenomenon of shape coexistence is also shown to produce low-lying states below such vibrational band heads. These results suggest novel and rich structures in heavy deformed nuclei, opening doors to future dedicated studies.

arXiv comment10 pages, 8 figures