Neutron scattering measurements [#!RenardEPL87!#,#!RenardJAP88!#,#!MaPRB92!#,#!RegnaultPRB94!#,#!MaPRB95!#]

The first report [75] presents two energy gaps at $k=\pi$ ($E_{\rm g}^{xy}\approx 1.1$ meV and $E_{\rm g}^z\approx 2.5$ meV).

The work by Regnault et al. [82] is very detailed; the high energy-resolution measurements resolved the lower energy gap ($E_{\rm g}^{xy}$)to two gaps ($E_{\rm g}^y\approx 1.05$ meV and $E_{\rm g}^x\approx 1.23$ meV); magnetic fields (up to 10 T) shifted the gap energies, as expected for a triplet state; a Lorentzian and/or square-root Lorentzian correlation function was observed with the finite correlation length ($\xi_{xy}/d\approx 8$ and $\xi_z/d\approx 4$). They concluded that the observed gaps were well understood as the Haldane gap of an anisotropic S=1 spin chain.