Neutron scattering from copper benzoate, $\mathrm{Cu}({\mathrm{C}}_6{\mathrm{D}}_5\mathrm{COO}{)}_2̇3{\mathrm{D}}_2\mathrm{O}$, provides the first direct experimental evidence for field-dependent incommensurate low energy modes in a one-dimensional spin $S=1/2\mathrm{}$ antiferromagnet. Soft modes occur for wave vectors $\tilde{q}=\pi \pm \delta \tilde{q}\left(H\right)$, where $\delta \tilde{q}\left(H\right)\approx 2\pi M\left(H\right)/g{\mu }_B$ as predicted by Bethe ansatz and spinon descriptions of the $S=1/2\mathrm{}$ chain. Unexpected was a field-induced energy gap $\Delta \left(H\right)\propto H^{\alpha }$, where $\alpha =0.65\mathrm{}\left(3\right)$ as determined from specific heat measurements. At $H=7\mathrm{}\mathrm{T}$ ( $g{\mu }_{B}H/J=0.52\mathrm{}$), the magnitude of the gap varies from $0.06J\mathrm{to}0.3J$ depending on the orientation of the applied field.

• Received 31 March 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.1750