Obituary: Sir Nevill Mott

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Nevill Mott was one of the great European scientists of this - or any - century. A theoretical physicist, in 1977 he was a joint winner of the Nobel Prize for his work on the electronic properties of disordered materials.

As Cavendish Professor of Physics at Cambridge from 1954 to 1971, he followed in the imperial line of Clark Maxwell, Lord Rayleigh, J.J. Thompson, Lord Rutherford and Sir Lawrence Bragg. His immediate successor, Sir Brian Pippard said yesterday:

In his younger days, Mott had an extraordinary range of interests and could keep many issues in his mind at the same time. As he aged, he tended to concentrate on single issues and to such effect that he was able to go on making important advances. By this economy, he continued right to the end to derive enormous enjoyment from his work; always tackling problems that he and others thought too hard and inspiring his younger colleagues to new efforts.

Nevill Mott's father, C.F. Mott, was a formidable director of education for the City of Liverpool, and his mother, Lilian Reynolds, was one of the earliest lady mathematicians. They sent him to Clifton College, where his interest in physics and mathematics was awakened. In his eighties he would reflect that individual teachers, like individual colleagues, could make all the difference to a scholar's progress and this was part of the reason he felt a driving compulsion to do something about science education.

Going up to St John's College, Cambridge, where he read Mathematics, he had the good fortune to come under the influence of Sir Lawrence Bragg, later Master of John's and a joint Nobel prize winner with his father. His first job was at Manchester University in 1929-30, which brought him into close contact with, and the lifelong friendship of, another formida- ble Nobel prizewinner, Patrick Blackett, later to be Rector of Imperial College, London and President of the Royal Society.

On the instigation of Sir James Chadwick, Mott became a Fellow and Lecturer at Gonville and Caius College, Cambridge (where he worked under Rutherford) between 1930 and 1933, when he was plucked by Bristol, at the age of 28, to be the Melville Wills Professor of Theoretical Physics and to work with the controversial, Communist C.F. Powell.

At Bristol he continued work begun in Cambridge with H.S.W. Massey on the theory of atomic collisions and he then worked on the theory of the properties of metals and alloys. At the beginning of the Second World War, having published Electronic Processes in Ionic Crystals with R.W. Gurney in 1940, he was directed to the most sensitive war work including research on radar and wave mechanics.

His first post-war publication was Wave Mechanics and Its Applications with I.N. Snedden (1948). This was followed in 1952 by his important work on Elements of Wave Mechanics.

One of the remarkable aspects of Mott's life was the way he could run a department, take part in public life and still relentlessly pursue his own research. I asked him once how he managed to combine such a range of work. He said: "There is one factor above all others. I have a marvellous understanding wife who creates the conditions in which I can operate." Ruth Horder, whom he married at the age of 25 in 1930, was a very remarkable partner.

At Cambridge, when he was given the Cavendish chair in 1954, Mott continued to work on the electronic processes in non-crystalline materials (publishing a book on them with E.A. Davis in 1971). In the last two years of his tenure, when most professors would be "demob happy", at least as far as publications were concerned, Mott wrote his Elementary Quantum Mechanics (1972) as a help to students and, in the last year of his professorship, Metal Insulator Transitions, which his astonished contemporaries recognised as serious science by a man at pensionable age.

But Mott was much more than a famous scientist. In the spring of 1965, when still Cavendish Professor, he invited a group of new Labour MPs, Dr Jeremy Bray, Edmund Dell, Shirley Williams, and myself to a weekend at the Master's lodge at Caius, to discuss the science and education policy of the incoming Labour government. Mott outlined his views on the mechanism of shrinkage by which university departments which had been important in previous decades were less so than the emerging multidisciplinary sciences and new topics such as molecular biology. Recognising the pain this would inflict on blameless researchers, he had proposals as to what could be done. He was a man with solutions.

What was clear to us was that he had insatiable curiosity about public policy and demonic energy to improve education in the country. Not only was he President of the Physical Society (1956-58) and a member of the governing board of the National Institution for Research in Nuclear Science (1957-60), but also President of the Modern Languages Association (1955), because he believed in the European dimension of education and he had found the time to be an active chairman of the Ministry of Education's Standing Committee on the sup-ply of schoolteachers (1959-62).

Until his ninth decade Mott gave the impression to his friends that while he always had time for them he never wasted a minute in the pursuit of anything that was less than worthwhile and serious. The last time I saw him he was still exuding energy and discussing the reaction to his latest interest, a synthesis of the philosophy of science and religion, explained in his book Can Scientists Believe? (published in 1991).

Autobiographies by scientists are rare and even rarer is an autobiography which can be read by lay people with great interest. Mott's A Life In Science, published in 1986 when he was 81 years of age, is remarkably interesting. It should be read by anyone who is interested in British physics and European science in this century.

Tam Dalyell

Almost as remarkable as the range of Nevill Mott's researches was his style of working, writes Volker Heine. Right until the end, one might find him in a laboratory, learning from a young experimentalist the latest data and developing a theoretical understanding of them. For this he was always loved and respected.

Solids, metals, alloys, insulators, semi-conductors, are very complex in the phenomena they show and the different processes going on simultaneously. There are electrons bonding the atoms together and carrying electrical current (or not, in insulators) and perhaps magnetism, atomic vibrations activated by the temperature, structural defects such as dislocations, and always impurities or additives which may be beneficial as in alloys or transistors but which more often confuse what one is studying. All these interact with one another in many ways, and Mott had an almost unique gift for going into such a complex situation, finding intuitively the dominant features and stitching them together into a coherent theory.

In this he really opened up the subject of solid state physics world- wide from about 1932, though in most countries and most universities, it remained deeply unfashionable until after the Second World War and the invention of the transistor, or much much later in some cases. He would approach a new puzzling phenomenon by holding up to it all the pieces of physics he had ever thought deeply about, noting similarities and differences, and finally concluding: well, the explanation must be such and such. Theoreticians with a more formal mathematical approach could find discussion with Mott very frustrating!

His style was already evident in his book with H. Jones in 1935, The Theory of Properties of the Metals and Alloys. More than any other work world-wide, it applied the newly developed quantum theory to the complex phenomena in solids. The crystal structures of the elements, soft X-ray emission, alloy phase diagrams, electrical and thermal conductivity, optical properties, thermoelectric power, molten metals and other topics were included. These were dealt with in a mixture of of rigorous theory supplemented by intuitive leaps where there were still huge gaps in the infant theory of solids.

Traditionally theoretical physics in Britain had developed as an offshoot of mathematics, so that it was a most unusual appointment when in 1933 he went to a chair of Theoretical Physics in Bristol in a department that was wholly or predominantly devoted to experimental research (which he headed after 1948). But he was that sort of theoretician. He gathered round him in Bristol a galaxy of collaborators whom he inspired and with whom he interacted closely, including R.W. Gurney, C. Frank, J. Mitchell, J. Friedel, N. Cabrera, and F. Nabarro.

When in 1954 he moved to Cambridge as Cavendish Professor, the leading position in Physics in Britain, as well as building up the whole department he started a group on the theoretical physics of solids (or condensed matter as it is now called). This has poured forth a stream of distinguished scientists and continues his tradition of close involvement with experiment and interpretation of the observed phenomena.

Although he enjoyed talking with young physicists, he had very few graduate students of his own, preferring to work with experimentalists and mroe senior colleagues. Before the advent of computing allowed a much more detailed connection between theory and experiment, theoretical physics involved having the right ideas and there was not much for a graduate student to assist with. Thus when I arrived from New Zealand in 1954, it took some persuasion for him to take me on, and the research project he suggested to me was characteristic: "Why don't you go over to the Low Temperature group and see if you can make yourself useful."

Two pieces of work showing his most profound insights are the variable range hopping, and the metal/insulator transition now called the Mott transition. Neither is easy to explain, even to a graduate class. In semiconductors, impurities at a very low concentration bind one electron (or hole) to themselves, all with slightly different energies in a partially compensated sample due to the randomness. Vibrations of the parent semiconductor are needed to help an electron hop from one side to another over a variable range of distances to conduct electricity. In spite of the two types of randomness, Mott was able to derive a most unusual law for the conductivity.

The Mott transition may be described as follows. When a series of centres such as donor or acceptor impurities in semiconductors are far apart and hold one electron or hole each, they form an insulator at zero Kelvin temperature because it requires a finite energy to ionise one such centre and add the electron to another centre which is already occupied. But when the centres overlap with one by more than a critical amount, they become a metal as for example in sodium metal where each atom also has one electron. The reason lies in the mobility of the electrons, which enables them to screen one another's charges and thus reduce a finite energy jump into an infinitesimal one.

Nevill Francis Mott, physicist: born Liverpool 30 September 1905; Lecturer, Manchester University 1929-30; Fellow and Lecturer, Gonville and Caius College, Cambridge 1930-33; Melville Wills Professor Theoretical Physics, Bristol University 1933-48, Henry Overton Wills Professor and Director, Henry Herbert Wills Physical Laboratories 1948-54; FRS 1936; Cavendish Professor of Physics, Cambridge University 1954-71, Master, Gonville and Caius College 1959-66; Kt 1962; Nobel Prize for Physics 1977; CH 1995; married 1930 Ruth Horder (two daughters); died Milton Keynes 8 August 1996.