Short-time critical dynamics of the two-dimensional random-bond Ising model.

Ferromagnetic systems with quenched randomness have been studied intensively in recent years. For such systems, a main subject is whether the quenched randomness changes the universal class of the phase transition. In 1974 Harris had proposed a criterion @1#: if the critical exponent a is positive for the pure system, the quenched randomness changes the critical exponents, but if a is negative, the universal class of the disordered system remains the same. This criterion works well for most systems. However, for the two-dimensional ~2D! Ising model where a50, one cannot draw a definite conclusion. Theoretical analysis predicted that for the 2D Ising model the randomness could only induce a logarithmic correction to the critical behavior, and all the critical exponents are not changed @2–5#. For example, in the critical region the following behavior has been proposed for the correlation length: