Theory of Toeplitz Determinants and the Spin Correlations of the Two-Dimensional Ising Model. IV

Abstract

We consider the rectangular Ising model on a half-plane of infinite extent and study some of the consequences connected with the presence of the boundary. Only the spins on the boundary row are allowed to interact with a magnetic field $\mathfrak{H}$. The method of Pfaffians is employed to obtain exact expressions for the partition function. It is found that the free energy is the sum of two terms, one of which is independent of $\mathfrak{H}$ and proportional to the total number of lattice sites, while the other depends on $\mathfrak{H}$ and is proportional to the number of lattice sites on the boundary. This separation makes it possible to define various thermodynamic quantities associated with the boundary. In particular, the boundary magnetization is shown to be discontinuous, in the ferromagnetic case, at zero magnetic field for temperatures below the bulk critical temperature $T_c$. This discontinuity, which is the spontaneous boundary magnetization, goes to zero as ${(1-\frac{T}{T_c})}^{\frac{1}{2}}$ as $T\to T_c-$. For $T=T_c$, the discontinuity is of course absent, and the boundary magnetization behaves as $-\mathfrak{H}\ln \mathfrak{H}$ for small $\mathfrak{H}$. The boundary susceptibility at zero magnetic field in the ferromagnetic case exhibits a logarithmic singularity at $T=T_c$, both above and below transition. An interesting feature is that the ferromagnetic boundary magnetization, although discontinuous for $T<\mathrm{}{T}_c$, may be analytically continued beyond the point $\mathfrak{H}=0\mathrm{}$. We interpret this as a hystersis phenomenon which we study in detail by computing the probability distribution function for the average boundary spin. The correlation function for two spins, both on the boundary row, is also obtained exactly and its asymptotic behavior is given. Finally, we derive an expression for the magnetization in any row and explicitly evaluate it for the second row, i.e., the row next to the boundary.