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Origin of Low-Lying Enhanced E1 Strength in Rare-Earth Nuclei

The experimental $E1$ strength distribution below 4 MeV in rare-earth nuclei suggests a local breaking of isospin symmetry. In addition to the octupole states, additional $1^-$ states with enhanced E1 strength have been observed in rare-earth nuclei by means of ($γ,γ'$) experiments. By reproducing the experimental results, the spdf interacting boson model calculations provide further evidence for the formation of an $α$ cluster in medium-mass nuclei and might provide a new understanding of the origin of low-lying E1 strength.