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.[ref]
Quantum Mechanics: The Key to Understanding Magnetism by John Hasbrouck Van Vleck. Nobel lecture, atomic susceptibility and crystal-field magnetic response
A physicist.[ref]
Professional profile of John Hasbrouck Van Vleck. Independent professional identity
Portrait of John Hasbrouck Van Vleck
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John Hasbrouck Van Vleck (1899–1980) was an influential American theoretical physicist known for his contributions to quantum mechanics and solid-state physics. He is particularly recognized for his work on the behavior of electrons in solids and the development of theories related to magnetism and superconductivity.