Synopsis: Pairing in nuclei

Paired states between neutrons and protons were thought to only occur in nuclei beyond the proton dripline, but calculations suggest they may be seen in stable nuclei, too.
Synopsis figure
Credit: A. Gezerlis et al., Phys. Rev. Lett. (2011)

It is well known that nucleons can form paired states, analogous to the way electrons pair in superconducting metals. Typically, this pairing occurs between identical nucleons (proton-proton or neutron-neutron) and forms a spin-singlet state. However, when the nucleon number is large and there are an equal number of neutrons (N) and protons (Z), spin-triplet or neutron-proton pairing is favored. This pairing, which is similar to that in the deuteron, is projected to only occur beyond the proton dripline—the line of nuclear stability that determines the maximum number of protons that can be in a nucleus for a given number of neutrons. As a result, researchers have assumed that spin-triplet pairing would be unobservable in stable nuclei.

In a paper in Physical Review Letters, Alexandros Gezerlis and colleagues at the University of Washington, Seattle, show this assumption may not be fully correct. They investigated the stability and symmetry of pairing in nuclei where N is not equal to Z. Employing the Bogoliubov-de Gennes equations for a many-body nuclear model, Gezerlis et al. find that the domain where spin-triplet pairing dominates actually extends well off the N=Z line. The condensate changes smoothly from a pure spin-triplet on the N=Z line to pure spin-singlet at large neutron excess. Further, mixed-spin pairing condensates (spin-triplet and spin-singlet) are found to coexist below the proton dripline. In principle, low-energy excitations characteristic of these mixtures should be experimentally accessible. – Sarma Kancharla


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