Configuration crossing, shape evolution, and odd-proton polarization in yttrium isotopes
The odd-mass $^\text{91-101}$Y isotopes provide a testing ground for how an unpaired proton modifies an abrupt collective structural evolution. A configuration-mixing Bose-Fermi description shows that the lowest negative- and positive-parity states undergo a crossing of normal and intruder configurations near neutron number $N = 60$, intertwined with an evolution from weak coupling of a quasiparticle to a near-spherical core toward strong coupling to a deformed core. To isolate the role of the odd proton, a differential charge-radius polarization observable is introduced as the difference between the isotope shifts of the odd-mass chain with those of the corresponding even-even cores. Its pronounced peak at $N = 58$ and sign reversal at $N = 60$ reveal a localized modification of the core evolution in the critical region. Energy levels, wave-function content, electromagnetic moments, two-neutron separation energies, and coherent-state energy surfaces support this interpretation.