John Michael Kosterlitz is a Scottish born British-American physicist. He was awarded the 2016 Nobel Prize in physics along with David Thouless and Duncan Haldane for work on condensed matter physics.
To describe phases and phase transitions Michael Kosterlitz used the concepts of topology, a branch of mathematics. For example, in the early 1970s he and David Thouless described phase transitions in thin layers at low temperatures. Physicists thought that two-dimensional materials would not have phase transitions, since any order that would arise would be wiped out by random thermal fluctuations. Phenomena like superfluidity and superconductivity could not happen without phase transitions. Kosterlitz and Thouless found a topological phase transition in which pairs of vortices form at cold temperatures and then disperse as the temperature increases. This change is known as the Kosterlitz-Thouless (KT) transition and appears in many other areas of physics.