Possible shape coexistence and magnetic dipole transitions in C17 and Ne21

Magnetic dipole (M1) transitions of N=11 nuclei C17 and Ne21 are investigated by using shell model and deformed Skyrme Hartree-Fock + blocked BCS wave functions. Shell model calculations predict well observed energy spectra and magnetic dipole transitions in Ne21, while the results are rather poor to predict these observables in C17. In the deformed HF calculations, the ground states of the two nuclei are shown to have large prolate deformations close to β2=0.4. It is also pointed out that the first Kπ=1/2+ state in Ne21 is prolately deformed, while the first Kπ=1/2+ state in C17 is predicted to have a large oblate deformation close to the ground state in energy, We point out that the experimentally observed large hindrance of the M1 transition between Iπ=1/2+ and 3/2+ in C17 can be attributed to a shape coexistence near the ground state of C17.