= John Hasbrouck Van Vleck
{c}
{tag=Physicist}
{wiki}
= John H. Van Vleck
{c}
{synonym}
= J. H. Van Vleck
{c}
{synonym}
<John Hasbrouck Van Vleck> developed the quantum theory of magnetic response in atoms and solids. <magnetic susceptibility> depends on the spectrum and magnetic matrix elements of the system, not only on a classical picture of permanent dipoles. An applied field can admix excited states into the ground state and thereby change its <energy> and <magnetic moment>. His treatment connected atomic quantum mechanics, <crystal> fields and measured magnetism, explaining effects that a purely classical thermal average misses.<Quantum Mechanics: The Key to Understanding Magnetism by John Hasbrouck Van Vleck>[Nobel lecture, atomic susceptibility and crystal-field magnetic response]{ref}
A <physicist>.<Professional profile of John Hasbrouck Van Vleck>[Independent professional identity]{ref}
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{description=Photograph by E.B. Boatner, courtesy AIP Emilio Segrè Visual Archives, Bainbridge Collection; American Institute of Physics (AIP); existing Commons crop by Artem.G. https://commons.wikimedia.org/wiki/File:John_H._Van_Vleck.jpg#Licensing[American Institute of Physics attribution-only permission], via Wikimedia Commons.}
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