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

We have studied magnetic excitations in the dimerized spin-Peierls phase of CuGeO3, by high resolution inelastic neutron scattering. We measured the well-defined spin triplet dispersive mode which is gapped throughout the whole Brillouin zone. We also observed that this mode is separated by an unexpected second gap of order 2 meV from a continuum of magnetic excitations extending to higher energy. The first gap (or 'triplet gap') and its associated dispersive mode is 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.