Spherical-oblate shape coexistence in $^{94}$Zr from a model-independent analysis

TL;DR

This study uses a model-independent approach to analyze low-energy states of $^{94}$Zr, revealing shape coexistence with a spherical ground state and an oblate, deformed excited state, aligning with shell-model predictions.

Contribution

First model-independent determination of shape coexistence and deformation softness in $^{94}$Zr using quadrupole sum rules.

Findings

Ground state is spherical with diffuse shape.

$0_2^+$ state is oblate and more deformed.

Results support shell-model predictions and refine IBM-CM models.

Abstract

Low-lying states of $^{94}$Zr were investigated via low-energy multi-step Coulomb excitation. From the measured $\gamma$-ray yields, 13 reduced transition probabilities between low-spin states were determined, together with the spectroscopic quadrupole moments of the $2_{1,2}^+$ states. Based on this information, for the first time in the Zr isotopic chain, the shapes of the $0_{1,2}^+$ states including their deformation softness were inferred in a model-independent way using the quadrupole sum rules approach. The ground state of $^{94}$Zr possesses a rather diffuse shape associated with a spherical configuration, while the $0_2^+$ state is oblate and more strongly deformed. The observed features of shape coexistence in $^{94}$Zr are in agreement with Monte-Carlo shell-model predictions, and the present results are vital to refine the IBM-CM description of the Zr isotopes around $A\approx 100$ in terms of an intertwined quantum phase transition.