Anisotropy of the spin interactions in the spin-Peierls compound CuGeO3: Anew magnetic excitation branch

We present new inelastic neutron scattering data which show that a second m agnetic branch (optic branch) exists in CuGeO3 and gives a non-negligible s pectral weight to the scattering. From the dependencies on the scattering v ector and the magnetic field of the inelastic neutron intensity at scatteri ng vectors (0, Q(k), Q(l) + 1/2), we are led to the conclusion that both th is new branch and the previously reported have anomalous structure factors and polarisations.