Double gap and solitonic excitations in the spin-Peierls chain CuGeO3

We have studied magnetic excitations in the spin-Peierls phase of CuGeO3 by inelastic neutron scattering. There is a dispersive mode which is gapped throughout the Brillouin zone. This mode is separated by an unexpected second gap of approximate to 2 meV, from a continuum of magnetic excitations. The first gap (or ''triplet gap'') and its associated dispersive mode are due to the breaking of a singlet dimer into a delocalized triplet. We propose that the second gap (or ''solitonic gap'') and the continuum correspond to dissociation of that triplet into two unbound spin-1/2 solitons that are separated by a dimerized region of arbitrary length.