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The classical XY model (sometimes also called classical rotor (rotator) model or O(2) model) is a lattice model of statistical mechanics. It is the special case of the n-vector model for . ==Definition== Given a -dimensional lattice , per each lattice site there is a two-dimensional, unit-length vector The spin configuration, is an assignment of the angle for each . Given a ''translation-invariant'' interaction and a point dependent external field , the configuration energy is : The case in which except for nearest neighbor is called ''nearest neighbor'' case. The configuration probability is given by the Boltzmann distribution with inverse temperature : : where is the normalization, or partition function. The notation indicates the expectation of the random variable in the infinite volume limit, after ''periodic boundary conditions'' have been imposed. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Classical XY model」の詳細全文を読む スポンサード リンク
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