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 c0ex0ex=0ex0ex1 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.