Solution (energy sketch)

ID: past-exam-of-the-advanced-quantum-physics-course-of-the-university-of-cambridge/2014/paper-1/3/solution-energy-sketch

The square-root branches avoid one another when ; crossings with a parallel-spin branch are allowed because total spin projections differ. At zero field the positive- sketch has one lower singlet and a triplet above it. The source supplies no numerical moment ratio, so the two panels show explicit illustrative signed choices, including the exactly compensated case.
Figure 1.
Two-spin exchange and Zeeman energies depend on the signed moment ratio, with an exact parallel-spin degeneracy for opposite equal moments
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