CHAMPAIGN, Illinois — Theoretical physicist Sir Anthony James Leggett, widely recognized as a world leader in condensed matter physics and for his pioneering work on superfluidity and the quantum mechanics of macroscopic systems, died on March 8. The University of Illinois at Urbana-Champaign professor was 87 years old.
Leggett had been the John D. and Catherine T. MacArthur Professor of Physics at the University of Illinois Urbana-Champaign since 1983. His research in theoretical condensed matter physics and the foundations of quantum mechanics won the 2003 prize. Nobel Prize in Physics for his contributions to the theory of superconductors and superfluids.
“Professor Leggett was a brilliant mind, always working on innovative ideas, and had a humble, down-to-earth personality,” said Rashid Bashir, dean of the Grainger College of Engineering and professor of bioengineering at the U. of I. “The world has lost a legend and a wonderful person. He will be greatly missed.”
Leggett was a master at explaining superfluidity, the property of some fluids to flow freely without viscosity. His research expanded the legacy of the U. of I.’s contribution to the theory of superconductivity (that some materials lose all electrical resistance when cooled to temperatures near absolute zero), developed by researchers John Bardeen, Leon Cooper and J. Robert Schrieffer, for which they received the Nobel Prize in Physics in 1972.
His research was broad and included considerable theoretical work on applications of quantum mechanics to collective variables and on ways to incorporate quantum dissipation, tunneling, and coherence into the behavior of macroscopic quantum systems. Leggett’s theories led directly to experiments on coherence and macroscopic quantum tunneling by John Clarke, John Martinis, and Michel Devoret, who shared the 2025 Nobel Prize in Physics for this work.
Since the early 1960s, Leggett was interested in the problem of superfluid liquid helium-3, a rare isotope of helium that liquefies just above absolute zero. It could not be explained by the then existing theory, which explained the superfluidity of helium-4. Leggett’s theory, developed between 1972 and 1975, provided the explanation of the phenomenon in terms of a new physical mechanism, a new form of symmetry breaking that he himself introduced. This was the basis for understanding the superfluid phases of helium-3 as anisotropic pairing of helium-3 atoms.
In 1986, when high-temperature superconductors were discovered, Leggett proposed a symmetry test of the new superconductors, which led to an experiment performed at the U. of I. by Dale Van Harlingen, Donald Ginsberg, and David Wollman. The 1993 experiment established that the new superconductors had “D-wave” symmetry, unlike conventional low-temperature superconductors, which are isotropic.
Leggett was born in Camberwell, south London, in 1938. His mother and father were the first in their families to attend university, where they met and became engaged while studying at the University of London.
He began his university education majoring in classics and graduated from the University of Oxford with a bachelor’s degree in 1959. After completing his first bachelor’s degree, he began a second bachelor’s degree at Oxford, this time in physics. In 1964, after completing his PhD at Oxford, he joined the U. of I. physics department as a postdoctoral researcher with David Pines and John Bardeen.
“Having Tony as a warm and generous colleague over the years made it truly worth being in Urbana,” said Illinois physics professor Gordon Baym, who worked closely with Leggett during his time as a postdoctoral researcher. “He was always miles ahead of the rest of us, not only by being smarter, but also by his boundless energy and harder work. Tony’s enthusiasm for physics and people continued until the end.”
From 1967 to 1983, Leggett was a professor at the University of Sussex, England. During that period, he visited Japanese universities on several occasions, as well as the University of Ghana, and then returned to the U. of I. to join the physics faculty until his retirement in 2018.
While at the U. of I., he also held a position at the Institute for Quantum Computing in Waterloo, Canada. In 2013, he became founding director of the Shanghai Center for Complex Physics and in 2023, he became the chief scientist of the Institute for the Theory of Condensed Matter, a research institute at the U. of I., which in 2023 was renamed the Anthony J. Leggett Institute for the Theory of Condensed Matter.
Leggett is survived by his wife, Haruko Kinase-Leggett, married in 1973; his daughter, Elizabeth Asako Kinase-Leggett; and his sisters, Judith Leggett and Claire Prangley.
Among his many awards are the Wolf Prize in Physics (2022-23), the Eugene Feenberg Memorial Medal (1999), the Paul Dirac Medal and Prize (1991), the Simon Memorial Prize (1981), the Fritz London Memorial Prize (1981), and the James Clerk Maxwell Medal and Prize (1975).
Leggett was a member of the National Academy of Sciences, the American Philosophical Society and the American Academy of Arts and Sciences, the American Philosophical Society, and the Literary and Historical Society of Ireland. He was a member of the Royal Society, the American Physical Society and the American Institute of Physics. He was an honorary member of the Institute of Physics and a foreign member of the Russian Academy of Sciences. He was knighted by Queen Elizabeth II in 2004 “for services to physics.”
“Tony was a much-loved colleague. He was so soft-spoken that it was easy to overlook the remarkable scope of his accomplishments,” said Vidya Madhavan, chair of Illinois’ physics department. “He felt a deep loyalty to both the department and the university. No matter how busy he was, he always made time for his colleagues and students, and despite his extensive travels, he maintained a constant presence in the physics department. Together, he and Haruko opened their home to generations of colleagues, students, and postdocs, for which we are all very grateful.”