Experimental evidence for ⁵⁶Ni-core breaking from the low-spin structure of the N=Z nucleus ₂₉⁵⁸Cu₂₉

Low-spin states in the odd-odd N=Z nucleus ⁵⁸Cu were investigated with the ⁵⁸Ni(p,nγ)⁵⁸Cu fusion evaporation reaction at the FN-TANDEM accelerator in Cologne. γγ-coincidences, γγ-angular correlations, and signs of γ-ray polarizations were measured. Seventeen low-spin states below 3.6 MeV and 17 new transitions were observed. Ten multipole mixing ratios and 17 γ-branching ratios were determined for the first time. New detailed spectroscopic information on the 2₂⁺ state, the isobaric analogue state (IAS) of the 2₁⁺,T=1 state of ⁵⁸Ni, makes ⁵⁸Cu the heaviest odd-odd N=Z nucleus with known B(E2;2⁺,T=1⃗0⁺,T=1) value. The 4⁺ state at 2.751 MeV, observed here for the first time, is identified as the IAS of the 4₁⁺,T=1 state in ⁵⁸Ni. The new data are compared to full pf-shell-model calculations with the GXPF1 residual interaction and to calculations within a pf_5/2 configurational space with a residual surface δ interaction. The role of the ⁵⁶Ni core excitations for the low-spin structure in ⁵⁸Cu is discussed.