Experimental evidence for 56 Ni -core breaking from the low-spin structure of the N = Z nucleus 29 58 Cu 29

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