Isospin Properties of Nuclear Pair Correlations From the Level Structure of the Self-Conjugate Nucleus $^{88}$Ru

The low-lying energy spectrum of the extremely neutron-deficient self-conjugate ($N$=Z) nuclide $^{88}_{44}$Ru$_{44}$ has been measured using the combination of the Advanced Gamma Tracking Array (AGATA) spectrometer, the NEDA and Neutron Wall neutron detector arrays, and the DIAMANT charged particle detector array. Excited states in $^{88}$Ru were populated via the $^{54}$Fe($^{36}$Ar,2n$γ$)$^{88}$Ru* fusion-evaporation reaction at the Grand Accélérateur National d’Ions Lourds (GANIL) accelerator complex. The observed $γ$-ray cascade is assigned to $^{88}$Ru using clean prompt $γ$-$γ$-2-neutron coincidences in anticoincidence with the detection of charged particles, confirming and extending the previously assigned sequence of low-lying excited states. It is consistent with a moderately deformed rotating system exhibiting a band crossing at a rotational frequency that is significantly higher than standard theoretical predictions with isovector pairing, as well as observations in neighboring $N>Z$ nuclides. The direct observation of such a “delayed” rotational alignment in a deformed $N$=Z nucleus is in agreement with theoretical predictions related to the presence of strong isoscalar neutron-proton pair correlations.