The Berezinskii–Kosterlitz–Thouless (BKT) transition is a phenomenon in statistical physics and condensed matter physics that describes a type of phase transition that occurs in two-dimensional systems with a continuous symmetry, such as the XY model. It was first proposed by Vladimir Berezinskii, J. Michael Kosterlitz, and David Thouless in the 1970s.
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The Berezinskii–Kosterlitz–Thouless transition in the two-dimensional XY model separates a low-temperature phase with algebraically decaying correlations from a high-temperature phase with unbound vortices and exponentially decaying correlations. It does not produce conventional long-range order.