Diagrammatic Expansion for the Ising Model with Arbitrary Spin and Range of Interaction

A new and simple diagrammatic expansion is developed for the free energy of the Ising model with arbitrary spin and range of interaction. The contribution of each diagram is a product of (1) the reciprocal of the order of the symmetry group of the diagram, (2) a product of semi-invariants, with a factor $M_n$ for each spin in the diagram $n$ being the number of bonds joined to that spin) and (3) a sum of products $\Sigma {i,j,k,\cdots }^{}\left(2\mathrm{}\beta J_{\mathrm{ij}}\right)\left(2\mathrm{}\beta J_{\mathrm{kl}}\right)\cdots$, where $2\beta J_{\mathrm{ij}}$ corresponds to a bond between spins $i$ and $j$, and where the summation is carried out with no restrictions on the summation indices (i.e., no "excluded volume" corrections). The expansion variables are the spin deviation operators ${\sigma }_i\equiv \overline{S}-S_{\mathrm{iz}}$. The quantity $\overline{S}$ is chosen to eliminate a large class of diagrams; this choice also minimizes the corresponding free-energy contribution and implies $\overline{S}=〈S_{\mathrm{iz}}〉$. By further renormalization of the vertices all reducible (i.e., simply articulated) diagrams are eliminated. To zero order the molecular field solution is obtained. The next approximation consists of the summation of renormalized simple loop diagrams. The justification of this choice of diagrams rests, at low temperature, on the decrease of the value of the higher order semi-invariants, on the relationship of these loop diagrams to the random phase approximation, and on the agreement with the rigorous low-temperature series result. At the Curie temperature the same choice of diagrams is justified by a modification of Brout's $\frac{1}{z}$ criterion, so that the expansion can be viewed as an expansion in $\frac{1}{z}$, where $z$ is the effective number of spins interacting with a given spin. Finally the choice of loop diagrams is justified at high temperature by exact agreement to second order in $\frac{T_c}{T}$ (and by a very close agreement to fourth order) with the rigorous high-temperature series expansion. Thus the theory agrees with rigorous results in the low-temperature and paramagnetic regions and has a well-defined criterion of validity in the intermediate temperature region. For nearest-neighbor interactions and spin ½ the specific heat is continuous through the Curie temperature, with an infinite slope on the low-temperature side of the transition.