  The long-standing problem of dramatic structure change near N=40 in Ge
isotopes is investigated by means of large-scale shell model calculation. The
analysis of simulated calculations suggests a possible understanding of the
problem in terms of rapid increase in the $g_{9/2}$ proton and neutron
occupation. The observed variation in excitation of the second $0^+$ state in
$^{70,72,74}$Ge appears to correlate closely with the $g_{9/2}$ occupations
induced by strong proton-neutron interactions. The enhancement of the $g_{9/2}$
occupancies is probably due to correlations in the $1g2d3s$ shell.
