Ising Model and Mean-Field Theory

12.1 The Ising Model

The Ising model is the simplest model of interacting spins on a lattice. Each site $i$ has a spin variable $\sigma_i \in \{-1, +1\}$.

$\mathcal{H} = -J\sum_{\langle i,j \rangle}\sigma_i\sigma_j - h\sum_i \sigma_i$

Where $J > 0$ is the ferromagnetic coupling, $\langle i,j \rangle$ denotes nearest-neighbor pairs, and $h$ is the external magnetic field.

Partition function (in 1D with periodic boundary conditions, $N$ spins):

$Z = \sum_{\{\sigma\}} \exp\!\left(\beta J \sum_i \sigma_i \sigma_{i+1} + \beta h \sum_i \sigma_i\right)$

This can be evaluated using the transfer matrix method. Define:

$\mathbf{T} = \begin{pmatrix} e^{\beta J + \beta h} & e^{-\beta J} \\ e^{-\beta J} & e^{\beta J - \beta h} \end{pmatrix}$

Then $Z = \text{Tr}(\mathbf{T}^N) = \lambda_+^N + \lambda_-^N$ where $\lambda_\pm$ are the eigenvalues of $\mathbf{T}$.

In the thermodynamic limit ($N \to \infty$), $Z = \lambda_+^N$ where:

$\lambda_+ = e^{\beta J}\cosh(\beta h) + \sqrt{e^{2\beta J}\sinh^2(\beta h) + e^{-2\beta J}}$

Key result: The 1D Ising model has no phase transition at $T > 0$. The magnetization $m = \langle\sigma\rangle \to 0$ as $h \to 0$ for all finite $T$.

12.2 Mean-Field Approximation

The mean-field (Weiss) approximation replaces each neighboring spin by its thermal average:

$\sigma_i\sigma_j \approx \sigma_i\langle\sigma_j\rangle + \langle\sigma_i\rangle\sigma_j - \langle\sigma_i\rangle\langle\sigma_j\rangle$

The effective Hamiltonian becomes:

$\mathcal{H}_{\text{MF} = -\sum_i \left(zJm + h\right)\sigma_i + \frac{1}{2}N zJ m^2}$

Where $z$ is the coordination number and $m = \langle\sigma\rangle$.

Each spin is independent, so:

$m = \tanh\!\left[\beta(zJm + h)\right]$

This is a self-consistency equation for $m$. For $h = 0$:

$m = \tanh(\beta zJm)$

Expanding for small $m$: $m \approx \beta zJ m - \frac{1}{3}(\beta zJ)^3 m^3$. Nonzero $m$ exists when:

$\beta zJ > 1 \implies T_c^{\text{MF} = \frac{zJ}{k_B}}$

12.3 Exact Solution: 2D Ising Model (Onsager, 1944)

Onsager”s exact solution for the square lattice gives:

$T_c = \frac{2J}{k_B \ln(1 + \sqrt{2})} \approx \frac{2.269J}{k_B}$

The spontaneous magnetization below $T_c$:

$m = \left[1 - \sinh^{-4}(2\beta_c J)\right]^{1/8}, \quad T < T_c$

The specific heat diverges logarithmically at $T_c$:

$C \sim -A\ln|T - T_c|$