# Eight-vertex model

In statistical mechanics, the eight-vertex model is a generalization of the ice-type (six-vertex) models. It was discussed by Sutherland and Fan & Wu, and solved by Rodney Baxter in the zero-field case.

## Description

As with the ice-type models, the eight-vertex model is a square lattice model, where each state is a configuration of arrows at a vertex. The allowed vertices have an even number of arrows pointing towards the vertex; these include the six inherited from the ice-type model (1-6), sinks (7), and sources (8).
We consider a $N\times N$ lattice, with $N^{2}$ vertices and $2N^{2}$ edges. Imposing periodic boundary conditions requires that the states 7 and 8 occur equally often, as do states 5 and 6, and thus can be taken to have the same energy. For the zero-field case the same is true for the two other pairs of states. Each vertex $j$ has an associated energy $\epsilon _{j}$ and Boltzmann weight
    $w_{j}=\exp \left(-{\frac {\epsilon _{j}}{k_{\mathrm {B} }T}}\right)$
giving the partition function over the lattice as
    $Z=\sum \exp \left(-{\frac {\sum _{j}n_{j}\epsilon _{j}}{k_{\mathrm {B} }T}}\right)$
where the outer summation is over all allowed configurations of vertices in the lattice. In this general form the partition function remains unsolved.

## Solution in the zero-field case

The zero-field case of the model corresponds physically to the absence of external electric fields. Hence, the model remains unchanged under the reversal of all arrows. The states 1 and 2, and 3 and 4, consequently must occur as pairs. The vertices may be assigned arbitrary weights
    ${\begin{aligned}w_{1}=w_{2}&=a\\w_{3}=w_{4}&=b\\w_{5}=w_{6}&=c\\w_{7}=w_{8}&=d.\end{aligned}}$
The solution is based on the observation that rows in transfer matrices commute, for a certain parametrization of these four Boltzmann weights. This came about as a modification of an alternate solution for the six-vertex model which makes use of elliptic theta functions.

### Commuting transfer matrices

The proof relies on the fact that when $\Delta '=\Delta$ and $\Gamma '=\Gamma$, for quantities
    ${\begin{aligned}\Delta &={\frac {a^{2}+b^{2}-c^{2}-d^{2}}{2(ab+cd)}}\\\Gamma &={\frac {ab-cd}{ab+cd}}\end{aligned}}$
the transfer matrices $T$ and $T'$ (associated with the weights $a$, $b$, $c$, $d$ and $a'$, $b'$, $c'$, $d'$) commute. Using the star-triangle relation, Baxter reformulated this condition as equivalent to a parametrization of the weights given as
    $a:b:c:d=\operatorname {snh} (\eta -u):\operatorname {snh} (\eta +u):\operatorname {snh} (2\eta ):k\operatorname {snh} (2\eta )\operatorname {snh} (\eta -u)\operatorname {snh} (\eta +u)$
for fixed modulus $k$ and $\eta$ and variable $u$. Here snh is the hyperbolic analogue of sn, given by
    ${\begin{aligned}\operatorname {snh} (u)&=-i\operatorname {sn} (iu)=i\operatorname {sn} (-iu)\\{\text{where }}\operatorname {sn} (u)&={\frac {H(u)}{{\sqrt {k}}\Theta (u)}}\end{aligned}}$
and $H(u)$ and $\Theta (u)$ are theta functions of modulus $k$. The associated transfer matrix $T$ thus is a function of $u$ alone; for all $u$, $v$
    $T(u)T(v)=T(v)T(u).$

### The matrix function $Q(u)$

The other crucial part of the solution is the existence of a nonsingular matrix-valued function $Q$, such that for all complex $u$ the matrices $Q(u),Q(u')$ commute with each other and the transfer matrices, and satisfy
| $\zeta (u)T(u)Q(u)=\phi (u-\eta )Q(u+2\eta )+\phi (u+\eta )Q(u-2\eta )$ | (1) |
where
    ${\begin{aligned}\zeta (u)&=[c^{-1}H(2\eta )\Theta (u-\eta )\Theta (u+\eta )]^{N}\\\phi (u)&=[\Theta (0)H(u)\Theta (u)]^{N}.\end{aligned}}$
The existence and commutation relations of such a function are demonstrated by considering pair propagations through a vertex, and periodicity relations of the theta functions, in a similar way to the six-vertex model.

### Explicit solution

The commutation of matrices in (1) allow them to be diagonalised, and thus eigenvalues can be found. The partition function is calculated from the maximal eigenvalue, resulting in a free energy per site of
    ${\begin{aligned}f=\epsilon _{5}-2kT\sum _{n=1}^{\infty }{\frac {\sinh ^{2}((\tau -\lambda )n)(\cosh(n\lambda )-\cosh(n\alpha ))}{n\sinh(2n\tau )\cosh(n\lambda )}}\end{aligned}}$
for
    ${\begin{aligned}\tau &={\frac {\pi K'}{2K}}\\\lambda &={\frac {\pi \eta }{iK}}\\\alpha &={\frac {\pi u}{iK}}\end{aligned}}$
where $K$ and $K'$ are the complete elliptic integrals of moduli $k$ and $k'$. The eight vertex model was also solved in quasicrystals.

## Equivalence with an Ising model

There is a natural correspondence between the eight-vertex model, and the Ising model with 2-spin and 4-spin nearest neighbor interactions. The states of this model are spins $\sigma =\pm 1$ on faces of a square lattice. The analogue of 'edges' in the eight-vertex model are products of spins on adjacent faces:
    ${\begin{aligned}\alpha _{ij}&=\sigma _{ij}\sigma _{i,j+1}\\\mu _{ij}&=\sigma _{ij}\sigma _{i+1,j}.\end{aligned}}$
The most general form of the energy for this model is
    ${\begin{aligned}\epsilon &=-\sum _{ij}(J_{h}\mu _{ij}+J_{v}\alpha _{ij}+J\alpha _{ij}\mu _{ij}+J'\alpha _{i+1,j}\mu _{ij}+J''\alpha _{ij}\alpha _{i+1,j})\end{aligned}}$
where $J_{h}$, $J_{v}$, $J$, $J'$ describe the horizontal, vertical and two diagonal 2-spin interactions, and $J''$ describes the 4-spin interaction between four faces at a vertex; the sum is over the whole lattice.
We denote horizontal and vertical spins (arrows on edges) in the eight-vertex model $\mu$, $\alpha$ respectively, and define up and right as positive directions. The restriction on vertex states is that the product of four edges at a vertex is 1; this automatically holds for Ising "edges." Each $\sigma$ configuration then corresponds to a unique $\mu$, $\alpha$ configuration, whereas each $\mu$, $\alpha$ configuration gives two choices of $\sigma$ configurations.
Equating general forms of Boltzmann weights for each vertex $j$, the following relations between the $\epsilon _{j}$ and $J_{h}$, $J_{v}$, $J$, $J'$, $J''$ define the correspondence between the lattice models:
    ${\begin{aligned}\epsilon _{1}&=-J_{h}-J_{v}-J-J'-J'',\quad \epsilon _{2}=J_{h}+J_{v}-J-J'-J''\\\epsilon _{3}&=-J_{h}+J_{v}+J+J'-J'',\quad \epsilon _{4}=J_{h}-J_{v}+J+J'-J''\\\epsilon _{5}&=\epsilon _{6}=J-J'+J''\\\epsilon _{7}&=\epsilon _{8}=-J+J'+J''.\end{aligned}}$
It follows that in the zero-field case of the eight-vertex model, the horizontal and vertical interactions in the corresponding Ising model vanish.
These relations gives the equivalence $Z_{I}=2Z_{8V}$ between the partition functions of the eight-vertex model, and the (2,4)-spin Ising model. Consequently a solution in either model would lead immediately to a solution in the other.