Pairing correlations in N~Z pf-shell nuclei

Abstract

We perform Shell Model Monte Carlo calculations to study pair correlations in the ground states of $N=Z$ nuclei with masses A=48-60. We find that $T=1$, $J^π=0^+$ proton-neutron correlations play an important, and even dominant role, in the ground states of odd-odd $N=Z$ nuclei, in agreement with experiment. By studying pairing in the ground states of $^{52-58}$Fe, we observe that the isovector proton-neutron correlations decrease rapidly with increasing neutron excess. In contrast, both the proton, and trivially the neutron correlations increase as neutrons are added. We also study the thermal properties and the temperature dependence of pair correlations for $^{50}$Mn and $^{52}$Fe as exemplars of odd-odd and even-even $N=Z$ nuclei. While for $^{52}$Fe results are similar to those obtained for other even-even nuclei in this mass range, the properties of $^{50}$Mn at low temperatures are strongly influenced by isovector neutron-proton pairing. In coexistence with these isovector pair correlations, our calculations also indicate an excess of isoscalar proton-neutron pairing over the mean-field values. The isovector neutron-proton correlations rapidly decrease with temperatures and vanish for temperatures above $T=700$ keV, while the isovector correlations among like nucleons persist to higher temperatures. Related to the quenching of the isovector proton-neutron correlations, the average isospin decreases from 1, appropriate for the ground state, to 0 as the temperature increases.