The development of deformation and shape coexistence in the vicinity of doubly-magic 48Ca, related to the weakening of the N=28 shell closure, was addressed in a low-energy Coulomb excitation experiment using a radioactive 44Ar beam from the SPIRAL facility at GANIL. The 2+1 and 22+ states in 44Ar were excited on 208Pb and 109Ag targets at two different beam energies. B(E2) values between all observed states and the spectroscopic quadrupole moment of the 2+1 state were extracted from the differential Coulomb excitation cross sections, indicating prolate shape of the 44Ar nucleus, and giving evidence for an onset of deformation already two protons and two neutrons away from doubly-magic 48Ca. New Hartree-Fock-Bogoliubov based configuration mixing calculations have been performed with the Gogny D1S interaction for 44Ar and neighboring nuclei using two different approaches: the angular-momentum projected generator coordinate method considering axial quadrupole deformations and a five-dimensional approach including the triaxial degree of freedom. The experimental values and new calculations are furthermore compared to the shell model and to relativistic mean field calculations. The new results give insight into the weakening of the N=28 shell closure and the development of deformation in this neutron-rich region of the nuclear chart.
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