Berezinskii–Kosterlitz–Thouless transition

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.