Dr. Akihiro Tanaka (National Institute for Materials Science, Computational Material Science Center)
"Geometric phases and the magnetization process in quantum antiferromagnets"
We revisit the physics underlying the magnetization process of quantum antiferromagnets from the viewpoint of geometric phases. First we present a continuum variant of the Lieb-Schultz-Mattis-type approach to the problem, where the commensurability condition of Oshikawa {\it et al} derives from a Berry connection formulation for the system's crystal momentum, similar to that developed by Haldane for ferromagnets. Building on the physical picture which arises, we go on to formulate an effective field theory which can deal with higher dimensional cases. We find that a topological term associated with vortex-like excitations ultimately controlls the magnetic behavior of the system.