Alexander Cyril Hewson

Alex Hewson, Emeritus Professor at Imperial College London, died on 19 July 2021 after a disabling three-year illness.

He was born on 21 July 1938 in a small East Yorkshire village where his father was the vicar. He was the eldest of six boys in his father’s second family and joined nine children by his father’s first wife. They lived in a roomy vicarage with a large garden, which was an invaluable source of produce in the needy days during and after the Second World War.

Alex was brought up to milk goats, pluck chickens, collect honey, and pick fruit. His early education was at the village school, where children of all ages were often taught in the same classroom. At 13 he graduated to grammar school in Hull, from where he gained a scholarship to Oxford. He took a first class degree in physics followed by a DPhil with Dirk ter Haar as his adviser. Alex then accepted a lectureship in the mathematics department at Imperial College, joining the research group of Harry Jones.

The theory of magnetism in metals had become an important focus of research at Imperial in the 1960s, with Peter Wohlfarth and me in the mathematics department and with Bryan Coles’s appointment of Seb Doniach and Martin Zuckermann to complement his experimental group in the physics department. Accordingly, Alex switched from his doctoral work on insulators to an investigation of the effect of electron correlation on Philip W. Anderson’s Hartree–Fock solution of the Anderson impurity model. This was the beginning of his life’s work on the physics of magnetic impurities, which soon centered on the Kondo problem. His definitive book on the subject was published in 1993. It covered everything known at the time and pointed to the future.

At this point Alex retired early in order to explore this wide vista, free of departmental duties. He was always a conscientious lecturer but often felt it hampered his creative work. Over the next 25 years his research blossomed and he collaborated with a wide network of students, research associates, visitors, and colleagues abroad. These included J. Bauer, R. Bulla, T. A. Costi, D. J. G. Crow, W. Koller, D. Meyer, Y. Nishikawa, A. Oguri, Y. Ono, G.-M. Zhang, V. Zlatic and, earlier, D. M. Newns and J. W. Rasul.

Many of their projects exploited Kenneth Wilson’s numerical renormalization group (NRG) method, applying it to more general and more realistic Kondo and Anderson models than the original one solved by Wilson. The scope of the impurity model widened enormously with the advent of dynamical mean field theory (DMFT), whereby many-body lattice systems can be mapped onto an impurity model that can be solved by NRG. Alex and his collaborators used this method to study the Hubbard–Holstein model and quantum phase transitions such as charge ordering and antiferromagnetism. They also studied superconductivity in the attractive Hubbard model, embracing the crossover from BCS to Bose–Einstein condensation. Alex introduced an important new renormalized perturbation theory that works directly in terms of fully dressed quasiparticles. This enables many results for the impurity model to be obtained very simply and, in combination with DMFT, throws new light on the spin and charge dynamics of the Hubbard model. This approach has yet to be fully exploited. Another major widening of the scope of the impurity model has come with the current interest in quantum dots and their application in quantum computing. A dot or pair of dots can be modeled by means of the magnetic impurity model in which parameters can be varied using a gate voltage.

Alex was a very active member of the strongly correlated electron systems (SCES) community. He helped Veljko Zlatic organize 12 influential workshops (1999–2017), some supported with NATO grants, on the island of Hvar, Croatia. Alex and I, together with Peter Littlewood and Alexei Tsvelik, organized a six-month program at the Newton Institute in Cambridge (2000). This was a unique opportunity for 71 long-stay participants, with an average stay of six weeks, to meet and work together.

Alex had a deep appreciation of art and music. His wife, Edwina Leapman, is a well-known artist with paintings held in major international collections and museums, and Alex fully supported her in this work. Alex’s musical activity was mainly the Imperial College Choir, in which he exulted in the performance of large-scale choral works with orchestra. Another occupation that gave him great pleasure was tennis; he organized a local group with whom he was playing regularly until his sudden illness. Alex kept all his interests alive during his long years of illness and, a few weeks before his death, was still talking to friends about a possible updated version of his book.

Alex is survived by Edwina, four brothers, and a half-brother.