Spin gap in optimally-doped YBCO

Spin gap in optimally-doped YBCO was investigated by inelastic neutron scattering. We found that q-integrated spectral weight in the vicinity of the spin gap has a shape of a step function at 10 K with the sharp resolution-limited onset at 33 meV, i.e. there is no observable spectral weight at 31 meV but full spectral weight at 35 meV. The spin gap does not soften or 'fill in' as the temperature is increased to T-c. Above T-c we find no evidence for q-dependent magnetic signal at 35 meV. (c) 2005 Elsevier Ltd. All rights reserved.