Direct Observation of Field-Induced Incommensurate Fluctuations in a One-Dimensional S  =  1/2 Antiferromagnet

Neutron scattering from copper benzoate, Cu(C₆D₅COO)₂̇3D₂O, provides the first direct experimental evidence for field-dependent incommensurate low energy modes in a one-dimensional spin S  =  1/2 antiferromagnet. Soft modes occur for wave vectors q̃  =  π±δq̃(H), where δq̃(H)≈2πM(H)/gμ_B as predicted by Bethe ansatz and spinon descriptions of the S  =  1/2 chain. Unexpected was a field-induced energy gap Δ(H)∝H^α, where α  =  0.65(3) as determined from specific heat measurements. At H  =  7 T ( gμ_BH/J  =  0.52), the magnitude of the gap varies from 0.06J to 0.3J depending on the orientation of the applied field.