Ab Initio Emergence and Collapse of Nuclear Collectivity near N = Z = 40
We present an \textit{ab initio} description of the emergence and collapse of enhanced quadrupole collectivity in nuclei near $N=Z=40$ using multi-shell valence-space Hamiltonians derived from chiral two- and three-nucleon forces with the in-medium similarity renormalization group. Projected generator-coordinate calculations with consistently evolved $E2$ operators capture the strong collectivity in $^{80}$Zr and its decrease toward $^{86}$Mo and $^{88}$Ru without empirical effective charges. While the $N,Z=40$ effective single-particle energy gaps remain open, the $1g_{9/2}$--$2d_{5/2}$ spacing is smallest near $^{76}$Sr and $^{80}$Zr, with its variation governed mainly by the proton-neutron monopole contribution. The associated occupancies are consistent with quadrupole correlations involving pseudo-SU(3) $pf$ holes and quasi-SU(3) particles. Excluding the occupation of the proton and neutron $2d_{5/2}$ from the variational space strongly suppresses the deformation and $B(E2;2_1^+\rightarrow0_1^+)$ in $^{80}$Zr. These results mark a steady step toward \textit{ab initio} computations of heavy open-shell nuclei.