The classical XY model (sometimes also called classical rotor (rotator) model or O(2) model) is a lattice model of See more

Given a D-dimensional lattice Λ, per each lattice site j ∈ Λ there is a two-dimensional, unit-length vector sj = (cos θj, sin θj)
The spin configuration, s = (sj)j ∈ Λ is an assignment of the angle −π < θj ≤ π for each j ∈ Λ. … See more

• The existence of the thermodynamic limit for the free energy and spin correlations were proved by Ginibre, extending to this case the Griffiths inequality.
• Using the Griffiths inequality in the formulation of Ginibre, Aiz… See more

As mentioned above in one dimension the XY model does not have a phase transition, while in two dimensions it has the Berezinski-Kosterlitz-Thouless transition between the phases with exponentially and powerlaw d… See more

• Classical Heisenberg model
• Coulomb gas
• Goldstone boson
• Ising model
• Potts model… See more