Defect lines in the Ising model and boundary states on orbifolds

Critical phenomena in the two-dimensional Ising model with a defect line are studied using boundary conformal field theory on the c = 1 orbifold. Novel features of the boundary states arising from the orbifold structure, including continuously varying boundary critical exponents, are elucidated. New features of the Ising defect problem are obtained including a novel universality class of defect lines and the universal boundary to bulk crossover of the spin correlation function.