Session L8: J. H. Van Vleck: Quantum Theory and Magnetism

Van Vleck from Spectroscopy to Susceptibilities: Kuhn Losses Regained

As a young assistant professor in Minneapolis, John H. Van Vleck spent much of his time between 1923 and 1926 writing a book-length Bulletin for the National Research Council. As its title, Quantum Principles and Line Spectra, suggests, the book focuses almost exclusively on spectroscopy, the core pursuit of the old quantum theory. By the time it finally appeared in 1926, the old quantum theory had given way to the new quantum mechanics. Van Vleck soon realized that matrix mechanics reinstated some well-confirmed results of the classical theory of susceptibilities that had been lost in the old quantum theory. In the history and philosophy of science literature, such losses are called 'Kuhn losses'. Using mathematical techniques similar to those presented in his NRC Bulletin, Van Vleck started to work on the theory of susceptibilities. In 1929, now a full professor in Madison, he began writing another book, which appeared in 1932 and has become a classic: The Theory of Electric and Magnetic Susceptibilities. In this talk I follow Van Vleck's trajectory from spectroscopy to susceptibilities and examine how his two books reflect and helped shape research traditions.The talk is based on joint work with Charles Midwinter.   

J. H. Van Vleck and Magnetism at the University of Wisconsin: 1928 -1934

In 1928, John Van Vleck returned to his alma mater to take a position in the Physics Department. Six years later he left to join the faculty of Harvard University. While Van Vleck was at Wisconsin, he began a series of theoretical studies that helped lay the foundation for the modern theory of magnetism in solids. In 1932 Van Vleck published his celebrated monograph, \textit{The Theory of Electric and Magnetic Susceptibilities, }in which he made use of the new theory to explain the results of experimental studies in a variety of magnetic materials. In my talk, I will review the accomplishments of Van Vleck and his students during this period and also comment briefly on his notes for a second edition of the book.   

My interactions with J.H. Van Vleck as a student and colleague at Harvard

In the summer of 1947 I participated as a graduate student in discussions with professors J.H. Van Vleck, C.J. Gorter and E.M. Purcell on exchange narrowing of magnetic resonance lines. Subsequent work on exchange broadening and narrowing in nuclear spin systems will be reviewed and interspersed with personal reminiscences.   

Remembering Van: Three Madison families, and other tales

The talk will present some history, in part personal, in part scientific, of Van's influence on other scientists and on magnetic resonance.   

Van Vleck and the magnetic susceptibilities of gaseous molecules

In his 1927 Physical Review article and in his 1932 book, ~\textit{The Theory of Electric and Magnetic Susceptibilities,~}Van Vleck used the new quantum theory to derive the magnetic susceptibilities of O$_{2}$ and NO in their gaseous form ~and compared them with experiments. ~He was therefore very interested in low temperature susceptibility experiments on O$_{2}$ at Oxford University in 1954 where individual O$_{2}$ molecules were trapped in small, almost spherical cages in organic clathrates. Correspondence between him and this speaker, then at Oxford, led to further measurements of O$_{2}$ and also of NO in such clathrates, to theory and to subsequent publications and correspondence. Later communication with Van Vleck on the magnetism in rare earth iron garnets, a subject of long-term interest to him, will be described ~in connection with experiments carried out at Duke University. Some fond personal ~recollections of this speaker of his interaction with Van Vleck - both while at Harvard, during visits and through correspondence which extended into the seventies - will be presented.