= Spin lattice
A <spin lattice> assigns a classical or quantum <spin> degree of freedom to each site of a specified <crystal lattice>. The lattice specifies spatial organization, while the interaction graph specifies which spins couple. Nearest-neighbor models couple adjacent sites, but longer-range interactions are also possible.
For an <Ising model>, each site carries a two-valued spin variable. For a quantum <Heisenberg model>, spin operators have noncommuting components, and a typical interaction is $J\sum_{\langle ij\rangle}\mathbf S_i\cdot\mathbf S_j$. The sign and geometry of the couplings determine whether neighboring preferences can be satisfied together or produce <geometric frustration>.
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