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, $\epsilon$, 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 density matrix renormalization. For $\epsilon>0$ we observe identical scaling for $\sigma$ and $\tau$ spins, whereas for $\epsilon<0$ one model becomes locally ordered and the other locally disordered. This is different of the critical behavior of the uncoupled model ($\epsilon=0$) and is in contradiction with the results of recent field-theoretical calculations.Comment: 6 pages, 4 figure