Neutron scattering from copper benzoate, Cu(C 6 D 5 COO) 2 ·3D 2 O, 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 wavevectorsq = π ± δq(H) where δq(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 ∆(H) ∝ H α , where α = 0.65(3) as determined from specific heat measurements. At H = 7 T (gµ B H/J = 0.52), the magnitude of the gap varies from 0.06−0.3J depending on the orientation of the applied field. To search for incommensurate spin correlations, we used inelastic neutron scattering to probe the wavevector-dependent spin susceptibility of the model one-dimensional S = 1/2 antiferromagnet (AFM) copper benzoate, Cu(C 6 D 5 COO) 2 · 3D 2 O. In contrast to other systems that have been studied in this context [7,8], in copper benzoate the exchange constant J is small enough to permit the large values of the reduced field, h = gµ B H/J ≈ 1 needed to observe this effect, while still being large enough to allow the interesting low energy part of the spectrum to be explored with a cold neutron triple-axis spectrometer [9]. Our most important result is that a magnetic field induces new low-energy modes in the excitation spectrum of copper benzoate at incommensurate wavevectorsq = π ± δq, where δq(H) ≈ 2πM(H)/gµ B , with M(H) being the magnetization per spin, as predicted by theory [1][2][3]. The modes are not completely soft, however, because the field also induces a gap in the excitation spectrum both at the incommensurate wave vector and atq = π. A, c = 6.30Å, and β = 89.5• [11]. Copper ions within a spin chain are separated by c/2 and coordinated by edge sharing, tetragonally distorted oxygen octahedra. The near-neighbor intrachain exchange interaction is J = 1.57 meV [9]. For this work, deuterated single crystals were prepared as described previously [9]. Specific heat measurements in magnetic fields up to H = 8.8 T were made on individual single crystals of typical mass 0.01 g using the relaxation method. For the neutron scattering measurements, approximately 500 crystals with total mass 3.82 g were mutually aligned to within 5• in the horizontal (h0l) scattering plane. Neutron scattering measurements in fields up to H = 7 T b were performed on the SPINS cold neutron triple-axis spectrometer at NIST. FWHM beam divergences were′ , and the fixed final energy was 2.5 meV, yielding energy and wavevector resolution ∆E = 69 µeV and ∆q/π = 0.02, respectively. We refer to wave vector transfer along the chain asq = Q · c/2 = lπ. The normalized magnetic scattering intensitỹ I(q, ω) [9], was derived from the detector count rate by subtracting T = 25 K data as a background, dividing by the squared magnetic form factor for copper, and normalizing to
