Domain walls and shape dependence of finite-size correction terms in the two-dimensional Ising-model critical region

Ferdinand and Fisher have analyzed the finite-size behavior of the two-dimensional Ising model with periodic boundary conditions for $T$ near $T_c$. In the thermodynamic limit, the leading correction term to the specific heat has an unusual and unexpected behavior as the shape of the $m\times n$ lattice is changed. For shape parameter $s=\frac{m}{n}=1\mathrm{}$, the peak in the specific-heat correction term occurs at a reduced temperature ${\tau }_{max}=2\mathrm{}{K}_{c}n\frac{(T-T_c)}{T_c}>0\mathrm{}$. With increasing $s$ the peak moves to negative $\tau$ values and then increases to 0. We show that this effect may be understood by keeping only the two largest eigenvalues of the transfer matrix. To this approximation, which is good for $s=1\mathrm{}$ and improves exponentially with increasing $s$, all the shape dependence of the free energy is due to a factor that can be expressed entirely in terms of a domain-wall energy. Furthermore, the functional form of this factor is the same as that of the finite-length correction to the one-dimensional Ising model. Thus the dependence of ${\tau }_{max}$ on $s$ is qualitatively similar to the dependence of $T_{max}^1$ the location of the one-dimensional Ising-model specific-heat peak on $N$, the number of spins. We also argue that the two-dimensional partition function is expressible at least for $s>\mathrm{}1\mathrm{}$ as a factor due to domains, and another due to a one-dimensional Ising array of domain walls. Approximate expressions are obtained for the effective number of spins and the effective coupling of the array. We also point out that there is reason to believe the one-dimensional Ising array of domain walls at fixed $\tau$ connects smoothly with a similar array for $T<\mathrm{}{T}_c$.