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Spin-triplet pairing in large nuclei

The nuclear pairing condensate is expected to change character from spin-singlet to spin-triplet when the nucleus is very large and the neutron and proton numbers $Z,N$ are equal. We investigate the transition between these two phases within the framework of the Hartree-Fock-Bogoliubov equations, using a zero-range interaction to generate the pairing. We confirm that extremely large nucleus would indeed favor triplet pairing condensates, with the Hamiltonian parameters taken from known systematics. The favored phase is found to depend on the specific orbitals at the Fermi energy. The smallest nuclei with a well-developed spin-triplet condensate are in the mass region A ~ 130-140.