*3.3. Shape Coexistence in Z* ≈ *40 Nuclei*

The sudden onset of deformation at *N* = 60 observed in the Zr and Sr isotopic chains has attracted a lot of attention, both from theoretical and experimental points of view. While the energies of the 2<sup>+</sup> <sup>1</sup> states in 90–100Zr were well reproduced by the LSSM calculations reported in [49], the required truncations of the model space made it impossible to account for the enhanced transition probability in 100Zr. Recently, the rapidity of the shape transition in the Zr isotopes has been reproduced, for the first time both in terms of level energies and transition probabilities, using the MCSM [50]. The calculations [50] also predict that 94,96,98,100Zr would present a multitude of low-lying states with various quadrupole shapes. A Coulomb-excitation study of 94Zr aiming to verify this scenario was performed at INFN-LNL [51], and its analysis is in progress. There exists, however, strong experimental evidence for the coexistence of deformed and spherical structures in 96,98Sr, recently reinforced by the results of Coulomb-excitation experiments performed at ISOLDE [2,52]. The rich set of transitional and diagonal *E*2 matrix elements determined in this study provides a consistent picture of a prolate-deformed groundstate band in 98Sr that coexists with an almost spherical structure built on the 0<sup>+</sup> <sup>2</sup> state. Similarity of the *B*(*E*2; 2<sup>+</sup> <sup>2</sup> <sup>→</sup> <sup>0</sup><sup>+</sup> <sup>2</sup> ) = <sup>13</sup>(2) W.u. value in 98Sr with the *<sup>B</sup>*(*E*2; 2<sup>+</sup> <sup>1</sup> <sup>→</sup> <sup>0</sup><sup>+</sup> <sup>1</sup> ) = 17+<sup>4</sup> <sup>−</sup><sup>3</sup> W.u. value in 96Sr, as well as of the quadrupole moments of the 2<sup>+</sup> <sup>2</sup> state in 98Sr and the 2<sup>+</sup> <sup>1</sup> state in 96Sr (both compatible with zero), suggest that the spherical and deformed structures interchange at *N* = 60. Contrary to what is observed in most known cases of shape coexistence, these two structures mix very weakly. This feature is in line with the type-II shell-evolution scenario proposed in [50] that links particular multiparticle– multihole excitations to significant reorganisations of the shell structure, which hinders configuration mixing.

A notable result of [2,52] is the observed reduction of the *Qs*(2<sup>+</sup> <sup>1</sup> ) value in 98Sr with respect to the rotational estimate. This feature may indicate triaxiality of this state, which gives way to a more prolate deformation for higher-spin members of the ground-state band. Detailed Coulomb-excitation studies of 96,98,100Mo [17,53] yielded *Q*2 and *Q*<sup>3</sup> cos <sup>3</sup>*δ* invariants for the ground states and the low-lying 0<sup>+</sup> <sup>2</sup> states, demonstrating their different shapes and confirming that triaxiality is also a key feature of Mo nuclei with *A* ≈ 100. The obtained invariant quantities indicate that, in 96Mo, an almost spherical 0<sup>+</sup> <sup>2</sup> state coexists with a triaxial ground state, while, in 98Mo, both the 0<sup>+</sup> <sup>1</sup> and 0<sup>+</sup> <sup>2</sup> states have approximately the same values of *Q*2. However, the cos <sup>3</sup>*δ* values suggest that the ground state in 98Mo is triaxial and the 0<sup>+</sup> <sup>2</sup> state has a prolate shape. The same pattern of a prolate 0<sup>+</sup> <sup>2</sup> state coexisting with a triaxial ground state appears in 100Mo, but the *Q*2 invariants obtained for both the 0<sup>+</sup> <sup>1</sup> and 0<sup>+</sup> <sup>2</sup> states in this nucleus are significantly greater than those for 98Mo, with that for the 0<sup>+</sup> <sup>2</sup> state being much larger. Given also that the proton vacancies and neutron occupancies for the ground states of 98,100Mo were recently extracted from an extensive series of single-proton and single-neutron transfer reactions [54], these nuclei would represent a stringent test for Shell-Model calculations. Such investigation would also be relevant in the context of neutrinoless double-*β* decay studies, as 100Mo is one of candidate nuclei for this process.
