Experimental evidence for ${}^{56}\mathrm{Ni}$-core breaking from the low-spin structure of the $N=Z$ nucleus ${}_{29}^{58}{\mathrm{Cu}}_{29}$

Low-spin states in the odd-odd $N=Z$ nucleus ${}^{58}\mathrm{Cu}$ were investigated with the ${}^{58}\mathrm{Ni}(p,n\gamma {)}^{58}\mathrm{Cu}$ fusion evaporation reaction at the FN-TANDEM accelerator in Cologne. $\gamma \gamma$-coincidences, $\gamma \gamma$-angular correlations, and signs of $\gamma$-ray polarizations were measured. Seventeen low-spin states below 3.6 MeV and 17 new transitions were observed. Ten multipole mixing ratios and 17 $\gamma$-branching ratios were determined for the first time. New detailed spectroscopic information on the $2_2^{+}$ state, the isobaric analogue state (IAS) of the $2_1^{+},T=1\mathrm{}$ state of ${}^{58}\mathrm{Ni},$ makes ${}^{58}\mathrm{Cu}$ the heaviest odd-odd $N=Z$ nucleus with known ${B(E2;2\mathrm{}}^{+},T=\overrightarrow{1}{0}^{+},T=1)$ value. The $4^{+}$ state at 2.751 MeV, observed here for the first time, is identified as the IAS of the $4_1^{+},T=1\mathrm{}$ state in ${}^{58}\mathrm{Ni}.$ The new data are compared to full $\mathrm{pf}$-shell-model calculations with the GXPF1 residual interaction and to calculations within a ${\mathrm{pf}}_{5/2}$ configurational space with a residual surface $\delta$ interaction. The role of the ${}^{56}\mathrm{Ni}$ core excitations for the low-spin structure in ${}^{58}\mathrm{Cu}$ is discussed.