Nuclear shape transition and oblate-prolate coexistence are investigated in N=Z nuclei of fpg configuration space (2p_3/2, 1f_5/2, 2p_1/2, and 1g_9/2). We perform shell model calculations and constrained Hartree-Fock calculations for N=Z nuclei, 60Zn, 64Ge, and 68Se, employing an effective pairing plus quadrupole residual interaction with monopole interactions. We found shape transition from prolate to oblate deformation and oblate-prolate coexistence. The ground state of 68Se has oblate shape, while the deformation for 60Zn and 64Ge is prolate. It is shown that the isovector monopole matrix element between f5/2 and p1/2 orbitals gives rise to the oblate deformation in 68Se, and four-particle four-hole (4p-4h) excitations are important for the oblate configuration.
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