Professor Kenneth Stevens, a distinguished theoretical physicist who spent most of his career at the University of Nottingham, died on 16 July 2010 at the age of 87. He began his research in magnetism at Oxford with a PhD thesis under the supervision of Professor M H L Pryce and his subsequent postdoctoral position was in Pryce’s Theoretical Physics Group. The thesis explained, in a mathematical tour de force, how exchange interactions cause ‘motional’ narrowing of magnetic resonance lines. His landmark paper dealing with a quantum mechanical formulation of the magnetism of rare earth ions remains a heavily cited article nearly sixty years after its publication. The operators which he introduced to understand the properties of these materials still bear his name, the so-called ‘Stevens Operators’. His work was highly influential in helping experimentalists to understand paramagnetic resonance, and his joint papers with his Oxford collaborators, Sir Roger Elliott and the late Professor Brebis Bleaney, on this topic are still regularly cited by researchers. He was co-author with Professor Sir Nevill Mott (Nobel Laureate in Physics, 1977) of a paper on the band structure of transition metals and, later in his career, made important contributions to the problem of intermediate valence and to understanding the speed at which a quantum particle tunnels through a potential barrier.
Ken Stevens played a leading administrative role at the University of Nottingham, serving as Dean of Science (1965-68) and Head of the Department of Physics (1975-78). Shortly after coming to Nottingham he helped to design and commission a new building which has housed the Department of Physics since 1964. Many of his PhD students and postdoctoral assistants have gained senior academic appointments in universities in the UK and abroad.
Kenneth was educated at Magdalen College School (1934-40) and at Jesus and Merton Colleges, Oxford, with an interruption for war service at the Admiralty on the development of radar (1942-45). From 1949 to 1953, he held research fellowships at Oxford and Harvard. He was appointed to a readership at Nottingham in 1953 and promoted to a Professor of Theoretical Physics in 1958. In recognition of his research on the theory of magnetism, he was awarded the Maxwell Medal and Prize of the Institute of Physics in 1968. He was a member of the Commission on Magnetism of the International Union of Physics and Applied Physics from 1984 to 1987. Throughout the 1970s and 80s, he made regular visits to IBM’s research laboratory in up-state New York. He retired in 1987, when he became Emeritus Professor at Nottingham. Ken excelled at tennis and was a keen hill walker and musician. He is survived by his wife Audrey and their son, Richard, and daughter, Judith.
Obituary written with input from Professor Sir Roger Elliott, Emeritus Professor of Theoretical Physics, University of Oxford, who was a colleague of Professor Stevens in the 1950s.