STOCKHOLM, Sweden, Oct. 7 — Two American citizens and a Russian won the 2003 Nobel Prize in physics Tuesday for their work in how matter can behave at extremely low temperatures. The Royal Swedish Academy of Sciences cited Alexei A. Abrikosov, Anthony J. Leggett and Vitaly L. Ginzburg for their contributions concerning two phenomena in quantum physics: superconductivity and superfluidity. Abrikosov is a Russian and American citizen based at the Argonne National Laboratory in Illinois; Ginzburg is a Russian based at the P.N. Lebedev Physical Institute in Moscow; and Leggett is a British and American citizen based at the University of Illinois. Superconductivity is the ability of some materials to conduct electricity without resistance when they are chilled to extremely low temperatures. Superconducting magnets are used to produce powerful magnetic fields for magnetic resonance imaging. Other discoveries concerning MRI were honored Monday with the Nobel medicine prize. Abrikosov and Ginzburg were honored for theories about superconductivity that they started developing in the 1950s. Leggett applied ideas about superconductivity to explain how atoms behave in one kind of "superfluid" in the 1970s. His theory has proven useful for other fields of physics, like the study of particles and of the universe, the Swedish academy said. For more information, visit: www.nobel.se