Completeness of boundary conditions for the critical three-state Potts model

We show that the conformally invariant boundary conditions for the three-state Potts model are exhausted by the eight known solutions. Their structure is seen to be similar to the one in a free field theory that leads to the existence of D-branes in string theory. Specifically, the fixed and mixed boundary conditions correspond to Neumann conditions, while the free boundary condition and the new one recently found by Affleck et al. [Boundary critical phenomena in the three-state Potts model, preprint cond-mat/9804117] have a natural interpretation as Dirichlet conditions for a higher-spin current. The latter two conditions are governed by the Lee–Yang fusion rules. These results can be generalized to an infinite series of non-diagonal minimal models, and beyond.