(1899-1980) American Theoretician, Solid State Physicist
John Housbrook Van Vleck, who developed the first complete quantum mechanical theory of magnetism in matter in the early 1930s, is known as “the father of modern magnetism.” He shared the 1977 Nobel Prize in physics with philip warren anderson and sir nevill francis mott for his fundamental theoretical investigations of the electronic structure of magnetism in the solid state of matter.
He was born on March 13, 1899, in Middle-town, Connecticut, into a family of scientists. As a child, John reacted to having a father who was a professor of mathematics and a grandfather who was a professor of astronomy by rebelling against the academic tradition and vowing not to continue it. When his father accepted a position at the University of Wisconsin, the family moved to Madison, where John attended public schools.
He did his undergraduate work at the University of Wisconsin and his graduate studies at Harvard University, where he took courses with percy williams bridgman and Edwin C. Kemble. He wrote his dissertation, on the binding energy of the helium atom, under Kemble, whom he described as “the one person in America at that time qualified to direct purely theoretical research in quantum atomic physics,” and earned his doctorate in 1922. He had long before outgrown his “childish prejudices,” as he called them, and realized that he was best qualified for the life of a physicist in an academic environment.
For the next year Van Vleck served as an instructor at Harvard; he then took a position as assistant professor at the University of Minnesota, where he had the opportunity, rarely given to a new faculty member, of teaching only graduate courses. In 1927, he was promoted to full professor and married Abigail Pearson; their union would last more than 50 years. The young couple then set out on a bit of academic wandering: in 1928, they moved to Wisconsin, where Van Vleck was a professor of physics at his alma mater and remained until 1934. During this period, Van Vleck was a Guggenheim Foundation Fellow. In 1930, they moved to Harvard, where he did his groundbreaking work on magnetism and published his classic work The Theory of Electric and Magnetic Susceptibilities in 1932.
The focus of his long, illustrious career was magnetism and its relationship to the quantum theory of atomic structure. As quantum wave mechanics was being developed in the early 1930s, Van Vleck set out to understand its implications for magnetism. As a result, he constructed a theory, using erwin schrodinger’s wave mechanics, that offered a precise explanation of the magnetic properties of individual atoms in a series of chemical elements. He went on to propose the notion of temperature-independent susceptibility in paramagnetic materials, that is, materials with small susceptibility to the external magnetic field. This phenomenon, which came to be called Van Vleck paramag-netism, drew attention to the importance of electron correlation (the interaction between the wave mechanical motion of electrons) in the appearance of localized magnetic moments (tiny quantum mechanical magnets in metals). His former student Philip Anderson would further develop these ideas to explain how local magnetic moments can occur in metals such as copper and silver, which in pure form are not magnetic at all.
Van Vleck’s most important work was his formulation of the quantum mechanics of chemical bonding in crystals, used for interpreting the patterns of chemical bonds in complex compounds. This theory was able to explain the magnetic, electrical, and optical properties of many elements and compounds by considering the influences exerted on the electrons in particular atoms by other atoms nearby. It gave the following description of how a perturbing (foreign) ion or atom behaves in a crystal: at first the electrons of a perturbing ion feel the influence of the electric field that is generated by the atomic nuclei and the electrons of the host crystal; then, through the action of its electrons, the perturbing ion enters into chemical bonding with its environment. In this context he also found that a perturbing atom in a crystal could sometimes replace a host atom without essential changes in the surrounding lattice. However, under certain circumstances the electronic structure of the perturbing atom is so incompatible with the symmetry of the environment that it leads to a local distortion of the lattice. This phenomenon, which later became known as the Jahn-Teller effect, played an important role in the development of the physics of the solid state. At age 78, Van de Graaff was awarded the Nobel Prize in physics for this work.
During World War II, he worked on radar, discovering that water molecules in the atmosphere absorb radar waves with a wavelength of about 1.25 cm and that oxygen molecules have a similar effect on 0.5-cm radar waves. This finding played an important role in the design of effective radar systems and later in microwave communication and radio astronomy.
In 1951, Van Vleck became Hollis Professor of Mathematics and Natural Philosophy at Har vard. He also served as Dean of Engineering and Applied Physics from 1951 to 1957. He took sabbaticals from Harvard to serve as Lorentz Professor at Leiden University, Holland, in 1960, and as Eastman Professor at Oxford University, in 1962-1963. In 1969, when he retired, he became professor emeritus. He died in his sleep at his home in Cambridge, Massachusetts, on October 27, 1980, at the age of 81.
Van Vleck’s groundbreaking research on the electronic structure of magnetism in the solid state of matter led to the development of the laser, new industrial uses of glass, and copper spirals used in birth control devices. The quantum chemical methods have now become routine tools, particularly in inorganic chemistry, with important extensions to molecular biology, medicine, and biology. His research into molecular spectra and the theoretical problems associated with their fine structure contributed to advances in radio astronomical investigations of molecular gases in space.
