The spectrum of the continuous isotropic quantum Heisenberg chain: quantum solitons as magnon bound states

The continuous Heisenberg chain is quantised semiclassically. The spectrum consists of unit spin bosons (magnons) and integer spin bosons (solitons) with dispersion laws E=JS(ka) 2 and E n =16JS 3 sin 2 (ka/4S)/n (n=1...) respectively. It is proposed that the n=1 soliton is the magnon and that the spectrum is exact, including all quantum corrections. For S= 1 / 2 the spectrum agrees with results for the discrete chain and it is inferred that the soliton is a multi-magnon bound state.