Magnetic Field Effects on Copper Benzoate:A Heisenberg Antiferromagnetic Chain with a Dzyaloshiskii-Moriya Interaction Xiaoqun Wang Institute of Theoretical Physics Chinese Academy of Sciences We have investigated the physical effects of the Dzyaloshinskii-Moriya (DM) interaction in copper benzoate using the density matrix renormalization group. In the low field limit, the spin excitation gap is found to vary as $H^{2/3}\ln ^{1/6}(J/\mu_BH_s)$ (here $H$ is the external field and $H_s$ the staggered magnetic field induced by $H$), in agreement with the prediction of conformal field theory. However, the low field staggered magnetization varies as $H^{1/3}$, and the $\ln^{1/3}(J/\mu_BH_s)$ correction predicted by conformal field theory is not confirmed within the same range of magnetic field where the $\ln^{1/6}(J/\mu_BH_s)$ correction to the gap is observed. The linear scaling behavior between the momentum shift of the incommensurate peak $\delta q$ and the spin magnetization $M$, i.e. $\delta q = 2\pi M$, is broken in copper benzoate. We have determined the coupling constant of the DM interaction in copper benzoate and have given a complete quantitative account for the field dependence of the spin gaps along all three principal axes, without resorting to additional interactions like interchain coupling. A crossover to strong applied field behavior due to the competition of the staggered and uniform fields is predicted for further experimental verification.