Intertwined quantum phase transitions in the even-even $^{90-100}$Sr isotopes

The even-even $^{90-100}$Sr isotopes are identified as a region of intertwined quantum phase transitions (IQPTs). In this scenario, a quantum phase transition involving the crossing of normal and intruder configurations is accompanied by a shape evolution within the intruder configuration. Using the interacting boson model with configuration mixing (IBM-CM), we show that the strontium chain exhibits coexisting Type I and Type II QPTs, where the intruder configuration evolves from a near-spherical structure in $^{90-96}$Sr to a deformed one in $^{98,100}$Sr, while the normal and intruder configurations cross between $^{96}$Sr and $^{98}$Sr. As a result, the ground state changes abruptly from a weakly collective normal configuration to a deformed intruder configuration. Evidence for this scenario is provided by a detailed comparison with experimental excitation energies, spectroscopic quadrupole moments, isotope shifts, and monopole $E0$ transition strengths, together with the configuration and $n_d$ decompositions of the calculated wave functions. The results place the strontium isotopes alongside the neighboring zirconium chain as another realization of IQPTs in the intricate $A\approx100$ region.