  Polarized and unpolarized triple-axis spectrometry has been used to study an
underdoped YBa_2Cu3O6.6. Our results indicate that spin excitations in the
superconducting state are essentially the same as those in the fully doped
material except that the unusual 41 meV resonance is observed at 34.8 meV. The
normal state spin excitations are characterized by a weakly energy-dependent
continuum whose temperature dependence shows the clear signature of a
superconducting gap at Tc. The enhancement at the resonance is accompanied by a
suppression of magnetic fluctuations at both higher and lower energies.
