Jan Korringa

Jan Korringa, one of the most respected condensed matter theorists of the past century, died at the age of one hundred on the ninth of October, 2015, at his home in Laguna Beach, California. He was surrounded by his children and grand-children, who remember him as a wonderful father, a friend, and a compassionate human being whose love of life and physics never ended. He was writing notes to his students in his famous illegible script, correcting their explanations of his scientific discoveries, within weeks of his death.

He was born in Heemstede, the Netherlands, on March 31, 1915. He received his undergraduate degree from the Technische Universiteit Delft and went to the Universiteit Leiden in 1937 to pursue his graduate studies under H. A. Kramers. That university closed in 1940 when the Netherlands was occupied by the Nazis and the faculty went on strike in sympathy with their Jewish colleagues. Korringa returned to Delft to finish his Ph. D., and remained there until the end of the war. In 1946, he was hired as an associate professor at the Universiteit Leiden where he was a colleague and the protégé of Kramers. Kramers had been the first protégé of Niels Bohr, so Korringa’s connection with quantum mechanics started at the source.

He was widely expected to be offered the chair at Leiden upon the retirement of Kramers, but Kramers died young and Korringa was not considered senior enough. For this reason, he was free to accept a full professorship at The Ohio State University in 1953. In addition to his professorship, he was a consultant to the Oak Ridge National Lab and the Chevron Oil Field Research Company for many years. He was awarded a Guggenheim Foundation fellowship in 1962 that he used for a sabbatical at the University of Besançon, Besançon, France.

The discovery by Korringa that has had the most far-reaching influence on the way that condensed matter theory is done today is his use of multiple-scattering equations to calculate the stationary electronic states in ordered and disordered solids. World war II made life difficult for Korringa, but he had to broaden his scientific interests out of necessity. This brought him into contact with the experimental and theoretical work by N. P. Kasterin on the scattering of acoustic waves by an array of spheres. Korringa’s interest had always been in condensed matter physics, so he naturally considered how Kasterin’s multiple-scattering equations could be used in that context. On a train ride from Delft to Heemstede, Korringa had the epiphany that these equations could be applied to electrons scattering from a cluster of atoms. Moreover, when the number of atoms increased without bound, the incoming and outgoing waves could be set equal to zero resulting in a formalism for calculating the stationary states.

Korringa showed how his multiple-scattering theory (MST) could be used to find the energy as a function of wave vector for electrons in a periodic solid in a famous 1947 paper. In 1954, the Nobel laureate Walter Kohn and Norman Rostoker (who went on to have a successful career in nuclear physics) derived the same equations from a different starting point. The Korringa-Kohn-Rostoker (KKR) band theory equations are used all over the world. Two of Korringa’s students, J. S. Faulkner and H. L. Davis, started a program for calculating the electronic states in condensed matter using MST at the Oak Ridge National Laboratory in 1967 that continues to this day.

Korringa realized almost immediately that his equations can be used to calculate the electronic states of non-periodic solids for which Bloch’s theorem does not hold. In 1958 he published an approach that is now called the average t-matrix approximation for the electronic states in random substitutional alloys. This work continued to evolve, and was later connected to the higher-level theory called the coherent potential approximation (CPA). B. L. Gyorffy and G. M. Stocks combined this work with the KKR theory to obtain the KKR-CPA method, the technique that is presently used to do alloy calculations. More recently, Stocks and Y. Wang developed the locally self-consistent multiple-scattering (LSMS) method 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.

It is interesting that Korringa’s work is in the category that 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 of this is the Korringa relation that was published in 1950 and is quoted without attribution in many papers on magnetic resonance and many-body theory. His name has even become an adjective. The nuclear magnetic relaxation of a material can be described as Korringa-like or non-Korringa-like.

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, K, times the temperature Τ. The magnitude of the Korringa constant K and its possible deviation from a constant value is the signature of the effects of strong correlations in the electron gas. These considerations have proved valuable in recent studies of strongly correlated electron materials and high Τ_c superconductors.

Some of Korringa’s studies are best known today as precursors of more complete theories that appeared later. For example, his discussion of the anomalous resistance of noble metals containing paramagnetic ions in 1951 is now part of the literature on the Kondo effect.

Korringa was a quiet man who was not given to self-aggrandizement, but he was highly respected by all who knew him. He was an old-fashioned European intellectual in the sense that he was not only an expert in his field but also was very knowledgeable about art, literature, and classical music. We miss him.