Electric Monopole Transition from the Superdeformed Band in Ca40

The electric monopole ($E0$) transition strength ${\ensuremath{\rho}}^{2}$ for the transition connecting the third ${0}^{+}$ level, a ``superdeformed'' band head, to the ``spherical'' ${0}^{+}$ ground state in doubly magic $^{40}\mathrm{Ca}$ is determined via ${e}^{+}{e}^{\ensuremath{-}}$ pair-conversion spectroscopy. The measured value ${\ensuremath{\rho}}^{2}(E0;{0}_{3}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})=2.3(5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ is the smallest ${\ensuremath{\rho}}^{2}(E0;{0}^{+}\ensuremath{\rightarrow}{0}^{+})$ found in $A<50$ nuclei. In contrast, the $E0$ transition strength to the ground state observed from the second ${0}^{+}$ state, a band head of ``normal'' deformation, is an order of magnitude larger ${\ensuremath{\rho}}^{2}(E0;{0}_{2}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})=25.9(16)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, which shows significant mixing between these two states. Large-scale shell-model (LSSM) calculations are performed to understand the microscopic structure of the excited states and the configuration mixing between them; experimental ${\ensuremath{\rho}}^{2}$ values in $^{40}\mathrm{Ca}$ and neighboring isotopes are well reproduced by the LSSM calculations. The unusually small ${\ensuremath{\rho}}^{2}(E0;{0}_{3}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})$ value is due to destructive interference in the mixing of shape-coexisting structures, which are based on several different multiparticle-multihole excitations. This observation goes beyond the usual treatment of $E0$ strengths, where two-state shape mixing cannot result in destructive interference.