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Condensed matter physics is J. Michael Kosterlitz's primary research area. This field explores the physical properties of matter in its condensed phases, such as solids and liquids, where the constituent particles are closely packed. Kosterlitz's work focuses on phase transitions, particularly in two-dimensional systems, where he discovered the Kosterlitz-Thouless transition, which describes the behavior of matter as it undergoes a phase change from a superfluid to a normal fluid.