Numerical study of the critical behavior of the Ashkin-Teller model at a line defect

We consider the Ashkin-Teller model on the square lattice, which is represented by two Ising models (sigma and tau) having a four-spin coupling of strength, is an element of, between them. We introduce an asymmetric defect line in the system along which the couplings in the sigma Ising model are modified. In the Hamiltonian version of the model we study the scaling behavior of the critical magnetization at the defect, both for sigma and for tau spins, by means of density matrix renormalization. For is an element of > 0 we observe identical scaling for sigma and tau spins, whereas for is an element of < 0 one model becomes locally ordered and the other locally disordered. This is different from the critical behavior of the uncoupled model (is an element of =0) and is in contradiction with the results of recent field-theoretical calculations.