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Single-phonon and multi-phonon excitations of the $γ$ vibration in rotating odd-$A$ nuclei

Multi-phonon excitations in atomic nuclei were observed very rarely although collective motions in quantum many-body systems are described as bosonic excitations. In particular, the first two-phonon $γ$ vibrational ($2γ$) excitation in odd-$A$ nuclei was reported in 2006 and only a few have been known. Quite recently, conspicuously enhanced $B(E2)$s feeding $2γ$ states were observed in $^{105}$Nb and conjectured that their parent states are candidates of $3γ$ states. In the present work, the model space is enlarged from the present author's previous calculation for $^{103}$Nb. The purpose is twofold: One is to see how the description of $2γ$ states is improved, and the other is to examine the existence of collective $3γ$ states, and when they exist, study their collectivity through calculating interband $B(E2)$s. The particle-vibration coupling model based on the cranking model and the random-phase approximation is used to calculate the vibrational states in rotating odd-$A$ nuclei. Interband $B(E2)$s are calculated by adopting the method of the generalized intensity relation. The present calculation reproduces the observed spectra of $0γ$ - $2γ$ states well and gives collective $3γ$ states with enhanced $B(E2)$s to $2γ$ states in $^{103}$Nb and $^{105}$Nb. The most collective $3γ$ state with the highest $K$ at zero rotation is thought to be the main component of the observed band.