Double Gap and Solitonic Excitations in the Spin-Peierls Chain ${\mathrm{CuGeO}}_3$

We have studied magnetic excitations in the spin-Peierls phase of ${\mathrm{CuGeO}}_3$ 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 $\approx 2\mathrm{meV}$, 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- $\frac{1}{2}$ solitons that are separated by a dimerized region of arbitrary length.