  Radioactive $^{129}$Sb, which can be treated as a proton plus semi-magic
$^{128}$Sn core within the particle-core coupling scheme, was studied by
Coulomb excitation. Reduced electric quadrupole transition probabilities,
$B(E2)$, for the $2^+$ $\times$ $\pi g_{{7/2}}$ multiplet members and candidate
$\pi d_{{5/2}}$ state were measured. The results indicate that the total
electric quadrupole strength of $^{129}$Sb is a factor of 1.39(11) larger than
the $^{128}$Sn core, which is in stark contrast to the expectations of the
empirically successful particle-core coupling scheme. Shell-model calculations
performed with two different sets of nucleon-nucleon interactions suggest that
this enhanced collectivity is due to constructive quadrupole coherence in the
wavefunctions stemming from the proton-neutron residual interactions, where
adding one nucleon to a core near a double-shell closure can have a pronounced
effect. The enhanced electric quadrupole strength is an early signal of the
emerging nuclear collectivity that becomes dominant away from the shell
closure.
