First Evidence of Axial Shape Asymmetry and Configuration Coexistence in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>Zn</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>74</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:math>: Suggestion for a Northern Extension of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>N</mml:mi><mml:mo>=</mml:mo><mml:mn>40</mml:mn></mml:math> Island of Inversion

The excited states of $N=44$ $^{74}\mathrm{Zn}$ were investigated via $\ensuremath{\gamma}$-ray spectroscopy following $^{74}\mathrm{Cu}$ $\ensuremath{\beta}$ decay. By exploiting $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ angular correlation analysis, the ${2}_{2}^{+}$, ${3}_{1}^{+}$, ${0}_{2}^{+}$, and ${2}_{3}^{+}$ states in $^{74}\mathrm{Zn}$ were firmly established. The $\ensuremath{\gamma}$-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 ${2}_{3}^{+}\ensuremath{\rightarrow}{0}_{2}^{+}$ and ${2}_{3}^{+}\ensuremath{\rightarrow}{4}_{1}^{+}$ 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.