John Hasbrouck Van Vleck

John Hasbrouck Van Vleck was an American physicist whose quantum-mechanical theories of magnetism helped establish modern magnetic spectroscopy. He shared the 1977 Nobel Prize in Physics.

Van Vleck paramagnetism: A form of paramagnetism caused by magnetic-field-induced mixing of a material’s quantum energy states. This magnetic response is named for Van Vleck and follows from his treatment of quantum state mixing.
Quantum mechanics: The physical theory describing matter and energy at atomic and subatomic scales. Van Vleck used its rules to explain magnetic behavior that classical physics could not account for.
Philip W. Anderson: An American condensed-matter physicist whose work on localization and magnetism earned the 1977 Nobel Prize in Physics. Anderson shared the Nobel with Van Vleck for contributions to the electronic structure of magnetic and disordered systems.
Old quantum theory: The collection of pre-1925 models that applied quantization rules to atomic and molecular phenomena. Van Vleck’s early scholarship examined the limits of this framework before modern quantum mechanics replaced it.
Crystal field theory: A model describing how surrounding electric charges split the energy levels of ions in solids. Van Vleck’s analysis of magnetic ions helped establish how their crystal environments shape magnetic properties.
Angular momentum: A conserved physical quantity associated with rotational motion, including intrinsic quantum spin. Atomic angular momentum determines the magnetic moments central to his theories.
Nevill Francis Mott: A British physicist whose work on electronic structure in solids earned the 1977 Nobel Prize in Physics. Mott shared the award that recognized Van Vleck’s contributions to magnetic and disordered materials.
Niels Bohr: A Danish physicist whose atomic model and work on quantum theory shaped twentieth-century physics. Bohr’s correspondence principle provided a bridge between classical mechanics and the quantum theories Van Vleck studied.
Magnetic susceptibility: A measure of how strongly a material becomes magnetized in an applied magnetic field. His theories explained how quantum states determine this central observable of magnetism.
Spin–orbit interaction: The coupling between a particle’s spin and its orbital motion. This interaction helps set the energy levels and magnetic responses of ions in solids.

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