Observation of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mmultiscripts><mml:mi>Ni</mml:mi><mml:mprescripts/><mml:none/><mml:mn>54</mml:mn></mml:mmultiscripts></mml:math>: Cross-Conjugate Symmetry in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>f</mml:mi><mml:mrow><mml:mn>7</mml:mn><mml:mo>/</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:math>Mirror Energy Differences

Gamma decays from excited states up to ${J}^{\ensuremath{\pi}}={6}^{+}$ in the $N=Z\ensuremath{-}2$ nucleus $^{54}\mathrm{Ni}$ have been identified for the first time. Level energies are compared with those of the isobars $^{54}\mathrm{Co}$ and $^{54}\mathrm{Fe}$ and of the cross-conjugate nuclei of mass $A=42$. The good but puzzling ${f}_{7/2}$ cross-conjugate symmetry in mirror and triplet energy differences is analyzed. Shell model calculations reproduce the new data but the necessary nuclear charge-dependent phenomenology is not fully explained by modern nucleon-nucleon potentials.