CRITICAL ISING LINES OF THE D=2 ASHKIN-TELLER MODEL

The universal critical point ratio Q is exploited to determine positio ns of the critical Ising transition lines on the phase diagram of the Ashkin-Teller model on the square lattice. A leading-order expansion o f the ratio Q in the presence of a nonvanishing thermal field is found from finite-size scaling and the corresponding expression is fitted b y the accurate perturbative transfer-matrix data calculations for the LXL square clusters with L less than or equal to 9.