DIRECT OBSERVATION OF FIELD-INDUCED INCOMMENSURATE FLUCTUATIONS IN A ONE-DIMENSIONAL S=1 2 ANTIFERROMAGNET/

Neutron scattering from copper benzoate, Cu(C6D5COO)(2) . 3D(2)O, prov ides the first direct experimental evidence for field-dependent incomm ensurate low energy modes in a one-dimensional spin S = 1/2 antiferrom agnet. Soft modes occur for wave vectors (q) over tilde = pi +/- delta (q) over tilde(H), where delta (q) over tilde(H) approximate to 2 pi M(H)/g mu(B) as predicted by Bethe ansatz and spinon descriptions of t he S = 1/2 chain. Unexpected was a field-induced energy gap Delta(H) a lpha H-alpha, where alpha = 0.65(3) as determined from specific heat m easurements. At H = 7 T (g mu(B)H/J = 0.52), the magnitude of the gap varies from 0.06J to 0.3J depending on the orientation of the applied field.