XXX Heisenberg spin chain: Bethe-ansatz diagonalisation

Heisenberg 1928 (Z. Physik 49:619) introduced the isotropic XXX Hamiltonian H = -J·Σ S_i·S_{i+1} as a model of ferromagnetism / antiferromagnetism (sign of J) on a 1D chain. Bethe 1931 (Z. Physik 71:205) diagonalised it via the *nested…