Isospin nonconserving interaction in the T=1 analogue states of the mass-70 region

Mirror energy differences (MEDs) and triplet energy differences (TEDs) in the T = 1 analogue states are important probes of isospin-symmetry breaking. Inspired by the recent spectroscopic data of Se-66, we investigate these quantities for A = 66-78 nuclei with large-scale shell-model calculations. For the first time, we find clear evidence suggesting that the isospin nonconserving (INC) nuclear force has a significant effect for the upper fp shell region. Detailed analysis shows that, in addition to the INC force, the electromagnetic spin-orbit interaction plays an important role for the large, negative MED in A = 66 and 70 and the multipole Coulomb term contributes to the negative TED in all the T = 1 triplet nuclei. The INC force and its strength needed to reproduce the experimental data are compared with those from the G-matrix calculation using the modern charge-dependent nucleon-nucleon forces.