C. Knetter et al., Fractional and integer excitations in quantum antiferromagnetic spin 1/2 ladders - art. no. 167204, PHYS REV L, 8716(16), 2001, pp. 7204
Spectral densities are computed in unprecedented detail for quantum anti fe
rromagnetic spin 1/2 two-leg ladders. These results were obtained due to a
major methodical advance achieved by optimally chosen unitary transformatio
ns. The approach is based on dressed integer excitations. Considerable weig
ht is found at high energies in the two-particle sector. Precursors of frac
tional spinon physics occur supporting the conclusion that there is no nece
ssity to resort to fractional excitations in order to describe features at
higher energies.