KTHNY theory
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The KTHNY-theory describes the melting of crystals in two dimensions (2D). The name is derived from the initials of the surnames of John Michael Kosterlitz, David J. Thouless,[1][2] Bertrand Halperin, David R. Nelson,[3][4] and A. Peter Young,[5] who developed the theory in the 1970s. It is, beside the Ising model in 2D and the XY model in 2D,[6][7] one of the few theories, which can be solved analytically and which predicts a phase transition at a temperature .