Berezinskii-Kosterlitz-Thouless transition

The Berezinskii-Kosterlitz-Thouless (BKT) transition is a phase change in certain two-dimensional systems, like thin magnetic or superconducting films. At low temperatures, the system has bound pairs of vortex-like excitations, maintaining order. As temperature rises, these pairs unbind into free vortices, disrupting long-range order without a typical sharp transition. This leads to a sudden change in properties like electrical resistance or magnetic behavior. Unlike traditional phase transitions, the BKT transition involves topological changes—alterations in the overall arrangement rather than local symmetry breaking—making it a unique and important phenomenon in condensed matter physics.