The results of exact calculations of the critical behaviour of square-lattice Ising models with regularly spaced defects of finite density x=1/nm are reported and shown to satisfy certain scaling relations.
Using the decimation technique,a decorated square Ising lattice with two kinds of interactions K1and K2 was transformed into a regular square Ising lattice with nearest-neighbor(NN),next-nearest-neighbor(NNN),and four-spin interactions.The critical point of the approximate solution in this system was at K′C=0.4406868.The result indicated that the phase transition occurred only when the NN interaction K1 and the NNN interaction K2 satisfied a certain relation.If we only calculated the NN interaction K1,the critical point of the exact solution was at K1C=0.7635.It was also found that if we decorated two spins between the two square lattice points,the answer could be solved exactly on condition that the NN interaction existed on the doubly decorated square Ising lattice.
We address the problem of the definition of the finite-volume correlation length. First, we study the large-N limit of the N-vector model, and we show the existence of several constraints on the definition if regularity of the finite-size scaling functions and correct anomalous behaviour above the upper critical dimension are required. Then, we study in detail a model in which the zero mode is prohibited. Such. a model is a generalization of the fixed-magnetization Ising model which is equivalent to the lattice gas. Also in this case, we find that the finite-volume correlation length must satisfy appropriate constraints in order to obtain regular finite-size scaling functions, and, above the upper critical dimension, an anomalous scaling behaviour. The large-N results are confirmed by a one-loop calculation in the lattice phi (4) theory.
The amplitude, $\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{A}(x,\ensuremath{\tau})$, of the logarithmic specific-heat singularity of a square Ising lattice with bond defects regularly spaced on an $m\ifmmode\times\else\texttimes\fi{}n$ grid (with x=mn1 and $\ensuremath{\tau}=\frac{n}{m}$) is calculated asymptotically with the result $\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{A}(x,\ensuremath{\tau})={A}_{0}\ensuremath{-}{A}_{1}x\mathrm{ln}{x}^{\ensuremath{-}1}\ensuremath{-}|{A}_{2}(\ensuremath{\tau})x\ensuremath{-}{A}_{3}{x}^{2}{\mathrm{ln}}^{2}x+O({x}^{2}\mathrm{ln}x)$, as $x\ensuremath{\rightarrow}0$. The coefficient A1 verifies the scaling prediction A1=C1, where C1 is the amplitude of the (ln∣t∣)2 term in the incremental specific heat due to a single, isolated defect.