Jan Korringa
Jan Korringa (31 March 1915 – 9 October 2015) was a Dutch-American physicist, specializing in theoretical condensed matter physics. He was writing notes to his students in his famous illegible script, correcting their explanations of his scientific discoveries, within weeks of his death. Education and career. Korringa received his bachelor's degree from Delft University of Technology and then his PhD there in 1942 under Bram van Heel with thesis "Onderzoekingen op het gebied algebraïsche optiek" (Essays in the area of algebraic optics). He became in 1946 an associate professor at the University of Leiden. He was a  protégé of Hendrik Kramers, who had been the first protégé of Niels Bohr; so Korringa’s connection with quantum mechanics started at the source. Korringa came to the United States in 1952 and accepted a full professorship at the Ohio State University. He was a consultant to the Oak Ridge National Laboratory for many years. During the summers, he collaborated with a group at the Chevron Oil Field Research Co that developed an important method for oil exploration known as nuclear magnetic resonance logging. In 1962 he was awarded a Guggenheim Foundation fellowship that he used for a sabbatical at  the University of Besançon in France. Korringa’s discovery having the most far-reaching influence on condensed-matter theory is his use of multiple-scattering equations to calculate the stationary electronic states in ordered and disordered solids. He was aware of the work of Nikolai Kasterin on the scattering of acoustic waves by an array of spheres. It was natural for Korringa to consider how Kasterin’s multiple-scattering equations could be used in the context of condensed-matter physics. On a train ride from Delft to Heemstede, Korringa had the epiphany that the equations could be applied to electrons  scattering from a cluster of atoms. Moreover, when the number of atoms increases without bound, the incoming and outgoing waves could be set equal to zero, which yielded a formalism for calculating the stationary states. In a famous 1947 paper, Korringa showed how his multiple scattering theory (MST) could be used to find the energy as a function of wavevector for electrons in a periodic solid. In 1954, Nobel laureate Walter Kohn and Norman Rostoker, who went on to have a successful career in nuclear physics, derived the same equations using the Kohn variational method. Two of Korringa's students , Sam Faulkner. and Harold Davis, started a program at the Oak Ridge National Laboratory using the Korringa-Kohn-Rostoker (KKR) band-theory equations to calculate the properties of solids. The KKR equations are now used around the world and are the subject of several books. Korringa realized that his equations could be used to calculate the electronic states of nonperiodic solids for which Bloch’s theorem does not hold. In 1958 he published an approach, now called the average t-matrix approximation, for calculating the electronic states in random substitutional alloys. That work continued to evolve, and was later connected  to the higher-level theory called the coherent potential approximation (CPA). Balázs Győrffy and Malcolm Stocks. combined it with the KKR theory to obtain the KKR–CPA method, which is presently used for alloy calculations. Korringa’s MST is the basis for numerous theoretical developments, including the locally self-consistent multiple scattering theory developed by Malcolm Stocks and Yang Wang that can be used to obtain the electronic and magnetic states of any ordered or disordered solid. State of the art computer codes, developed by a community of scholars from the USA, Germany, Japan and the UK, that encapsulate the  equations of KKR and KKR-CPA are now available to the materials community. They include relativistic extensions to the solution of the Dirac equation, are all-electron, and exploit the powers of massively parallel state of the art supercomputers. Korringa’s work is outside the usual measures for the evaluation of scientific success in that his theories are referred to much more often than they are referenced. For example, the acronyms KKR and MST are used all the time without referencing the original 1947 paper. Another example is the Korringa relation, quoted without attribution in many papers on nuclear magnetic  resonance and many-body theory. In 1950, Korringa showed that the spin relaxation rate divided by the square of the magnetic resonance field shift (the Knight shift) obtained from an NMR experiment is equal to a constant, κ, times the temperature T. The magnitude of the Korringa constant κ and its possible deviation from a constant value is the signature of the effects of strong correlations in the electron gas. Those considerations have proved valuable in recent studies of strongly correlated electron materials and high-temperature superconductors. His name has even become an adjective: The nuclear magnetic relaxation of a  material can be described as Korringa-like or non-Korringa-like.