BOUNDARY CONFORMAL FIELD-THEORY APPROACH TO THE CRITICAL 2-DIMENSIONAL ISING-MODEL WITH A DEFECT LINE

We study the critical two-dimensional Ising model with a defect line ( altered bond strength along a line) in the continuum limit. By folding the system at the defect line, the problem is mapped to a special cas e of the critical Ashkin-Teller model, the continuum limit of which is the Z(2) orbifold of the free boson, with a boundary. Possible bounda ry states on the Z(2) orbifold theory are explored, and a special case is applied to the Ising defect problem, We find the complete spectrum of boundary operators, exact two-point correlation functions and the universal term in the free energy of the defect line for arbitrary str ength of the defect, We also find a new universality class of defect l ines, It is conjectured that we have found all the possible universali ty classes of defect lines in the Ising model. Relative stabilities am ong the defect universality classes are discussed. (C) 1997 Elsevier S cience B.V.