Imaging and Control of Magnetic Domains in a Quasi-One-Dimensional Quantum Antiferromagnet BaCu_{2}Si_{2}O_{7}.

We visualize antiferromagnetic domains in a representative quasi-one-dimensional S=1/2 quantum antiferromagnet, BaCu_{2}Si_{2}O_{7}, using nonreciprocal directional dichroism, which differentiates the optical absorption of a pair of antiferromagnetic domains. Opposite antiferromagnetic domains, each about submillimeter in size, are found to coexist in a single-crystal specimen, and the domain walls run predominantly along the spin chains. We also demonstrate that the domain walls can be moved by an applied electric field through a magnetoelectric coupling and that the direction of the domain walls is maintained during the motion. We explain the domain wall anisotropy by the quasi-one-dimensional nature of the exchange interactions. This Letter will contribute to the understanding of the domain physics of quasi-one-dimensional quantum antiferromagnets.