The excited states of $N=44$ ${}^{74}\mathrm{Zn}$ were investigated via $\gamma$-ray spectroscopy following ${}^{74}\mathrm{Cu}$ $\beta$ decay. By exploiting $\gamma \text{−}\gamma$ angular correlation analysis, the $2_2^{+}$, $3_1^{+}$, $0_2^{+}$, and $2_3^{+}$ states in ${}^{74}\mathrm{Zn}$ were firmly established. The $\gamma$-ray branching and $E2/M1$ mixing ratios for transitions deexciting the $2_2^{+}$, $3_1^{+}$, and $2_3^{+}$ states were measured, allowing for the extraction of relative $B(E2)$ values. In particular, the $2_3^{+}\to 0_2^{+}$ and $2_3^{+}\to 4_1^{+}$ transitions were observed for the first time. The results show excellent agreement with new microscopic large-scale shell-model calculations, and are discussed in terms of underlying shapes, as well as the role of neutron excitations across the $N=40$ gap. Enhanced axial shape asymmetry (triaxiality) is suggested to characterize ${}^{74}\mathrm{Zn}$ in its ground state. Furthermore, an excited $K=0$ band with a significantly larger softness in its shape is identified. A shore of the $N=40$ “island of inversion” appears to manifest above $Z=26$, previously thought as its northern limit in the chart of the nuclides.

• Received 2 August 2022
• Revised 28 November 2022
• Accepted 9 February 2023

DOI:https://doi.org/10.1103/PhysRevLett.130.122502