Dielectric-branes

We extend the usual world-volume action for a Dp-brane to the case of N coi ncident Dp-branes where the world-volume theory involves a U(N) gauge theor y. The guiding principle in our construction is that the action should be c onsistent with the familiar rules of T-duality. The resulting action involv es a variety of potential terms, i.e. nonderivative interactions, for the n onabelian scalar fields. This action also shows that Dp-branes naturally co uple to RR potentials of all form degrees, including both larger and smalle r than p + 1. We consider the dynamics resulting from this action for Dp-br anes moving in nontrivial background fields, and illustrate how the Dp-bran es are "polarized" by external fields. In a simple example, we show that a system of D0-branes in an external RR four-form field expands into a noncom mutative two-sphere, which is interpreted as the formation of a spherical D 2-D0 bound state.