High-energy spin excitations inYBa2Cu3O6.5

Inelastic neutron scattering has been used to obtain a comprehensive description of the absolute dynamical spin susceptibility ${\chi }^{″}(\mathbf{q},\omega )$ of the underdoped superconducting cuprate ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.5}$ ${(T}_c=52\mathrm{}$ K) over a wide range of energies and temperatures (2 meV $<~ħ\omega <~$ 120 meV and 5 K$<~T<~$200 K). Spin excitations of two distinct symmetries (even and odd under the exchange of two adjacent ${\mathrm{CuO}}_2$ layers) are observed which exhibit two different gaplike features (rather than a single “spin pseudogap”). The excitations show dispersive behavior at high energies.