DOUBLE GAP IN THE SPIN-PEIERLS COMPOUND CUGEO3

We report on inelastic high resolution neutron scattering measurements in the spin-Peierls compound CuGeO3. We observed the sharp dispersive mode which is gapped throughout the whole Brillouin zone. We also obs erved that this mode is separated by an unexpected second gap of appro ximate to 2 meV from a continuum of magnetic excitations that extends to higher energy. The first gap (or ''triplet gap'') and its associate d dispersive mode are due to the breaking of a singlet dimer into a de localized triplet as illustrated by polarized neutron under magnetic f ield. We propose that the second gap (or ''solitonic gap'') and the co ntinuum correspond to dissociation of the triplet into two unbound spi n-1/2 solitons that are separated by a dimerized region of arbitrary l ength. (C) 1997 American Institute of Physics.