First identification of large electric monopole strength in well-deformed rare earth nuclei

Excited states in the well-deformed rare earth isotopes $^{154}$Sm and $^{166}$Er were populated via ``safe Coulomb excitation at the Munich MLL Tandem accelerator. Conversion electrons were registered in a cooled Si(Li) detector in conjunction with a magnetic transport and filter system, the Mini-Orange spectrometer. For the first excited $0^+$ state in $^{154}$Sm at 1099 keV a large value of the monopole strength for the transition to the ground state of $rho^2(text{E0}; 0^+_2 to 0^+_text{g}) = 96(42)cdot 10^{-3}$ could be extracted. This confirms the interpretation of the lowest excited $0^+$ state in $^{154}$Sm as the collective $beta$-vibrational excitation of the ground state. In $^{166}$Er the measured large electric monopole strength of $rho^2(text{E0}; 0^+_4 to 0^+_1) = 127(60)cdot 10^{-3}$ clearly identifies the $0_4^+$ state at 1934 keV to be the $beta$-vibrational excitation of the ground state.