Mott, Sir Nevill Francis
(19051996), theoretical physicist
, was born on 30 September 1905 in a nursing home at 20 Clarendon Road, Leeds, the elder child of (Charles) Francis Mott, senior science master at Giggleswick School, and his wife, Lilian Mary, née
Reynolds. A daughter, Joan, was born two years later. Mott's father, who later became director of education for Liverpool, was originally a physicist, and his mother had been the best female mathematician of her year at Cambridge; they had met while working in the Cavendish Laboratory at Cambridge. Mott's early childhood was spent in Giggleswick, and then in Brocton, on the edge of Cannock Chase, being taught at home by his mother until he was ten. He was then sent as a boarder to Baswich House preparatory school where he recalled that the headmaster, G. F. A. Osborne, was an excellent teacher, taking me well on in algebra and Latin before I left, and, as far as I can remember, even introducing me to calculus (Mott, 5). In 1918 Mott won a scholarship to Clifton College, where he spent five unhappy years. Clifton had very good laboratories, but it was a superb mathematics teacher, H. C. Beaven, who made most impression on Mott and turned him towards mathematical physics.
Mott won a major scholarship to St John's College, Cambridge, in December 1923, and began in mathematics in 1924. He compressed the course into two years, and obtained first-class honours with a distinction in 1926. He spent his third year struggling to master the new theories of quantum mechanics, with which no one in Cambridge except Paul Dirac, who was too reticent to help much, was familiar. His first significant paper, using Schrodinger's wave mechanics to prove the scattering law that underpinned Rutherford's deduction of the nuclear atom, was published early in 1928, by which time Mott was officially a research student at the Cavendish, working under R. H. Fowler's supervision. Mott never begrudged the intellectual effort spent on learning German in order to follow quantum mechanics, and he later developed strong views about the desirability of scientists learning foreign languages. He became president of the Modern Languages Association in 19545.
Mott spent the autumn of 1928 at Niels Bohr's Institute in Copenhagen. There, for the first time, he found a lively and stimulating community of theoreticians and realized how [physics] was a social activity and how a teacher should be with students and how beautiful physics could be (Mott, 25), a realization which had a profound effect on the way that he was to run his own laboratories and the collaboration he promoted between experimentalists and theoreticians. While in Copenhagen he continued to investigate the implications of quantum mechanics for Rutherford's scattering law, applying Dirac's new theory of electron spin to the scattering of electrons by nuclei and deducing that their spins would be polarized in the process. Stimulated by J. R. Oppenheimer's experiments on electron scattering, Mott looked, on his return to Cambridge, at the effect of symmetry in the collisions of two identical Fermi-Dirac particles, such as electrons or alpha particles, predicting anomalous scattering at 45° which was later verified by Chadwick.
Chadwick took me along to see Rutherford, who said if you think of anything else like this, come and tell me. This certainly made my day. In fact I think it was on this day that I gained complete confidence in my ability to make a career in theoretical physics. (Mott, 3031)
Mott subsequently spent a term at Göttingen before returning to England to take up a lectureship at Manchester under W. L. Bragg in autumn 1929. His first book, An Outline of Wave Mechanics
(1930), was based on the lecture course he gave there, and his interest in the properties of materials dates from this time, no doubt stimulated by contact with Bragg and his crystallography group. Mott remained in Manchester for only a year, before returning to Cambridge as a fellow of Gonville and Caius College. He had married Ruth Eleanor Horder (19062000), daughter of Gerald Morley Horder, architect's quantity surveyor, on 24 March 1930; she was a classics student at Newnham College to whom he had been engaged for two years, and they bought a house in Sedley Taylor Road. Ruth later taught classics part-time, and gave harpsichord concerts. Through their sixty-six years of marriage, Mott gave Ruth credit for being a marvellous understanding wife who creates the conditions in which I can operate (Independent
, 12 Aug 1996). Mott was ill at ease with Caius, which he thought over-traditional, but found the Cavendish rather more theory-friendly than previously. At Fowler's suggestion, in collaboration with an experimental physicist, Harrie Massey, he wrote The Theory of Atomic Collisions
(1933), the first of thirteen authoritative, co-authored books.
In 1933 Mott succeeded Sir John Lennard-Jones as professor of theoretical physics at Bristol University. He was elected a fellow of the Royal Society in 1936. He went to Bristol determined to build a close collaboration with experimentalists, and found suitable topics in research that Harry Jones, Herbert Skinner, and Clarence Zener were conducting into the structure of metals. Following up Skinner's experiments, they showed in an article published in the Physical Review
of 1934 that the soft X-ray spectra from light metallic elements demonstrate the existence of a sharp Fermi surface (a theoretical concept introduced by Jones and Zener in 1934 to describe the limit to the energy that metallic electrons can have when the metal is in its ground state). Mott recorded: It was a revelation to me that quantum mechanics could penetrate into the business of the metals industry (Mott, 48). He pursued this insight and, with Jones, published The Theory of the Properties of Metals and Alloys
Influenced by the work of R. Pohl on colour centres in crystals, Mott next turned his attention to semiconductors, showing, in an article published in the Proceedings of the Royal Society of London
in 1939, that the process involved in rectifying junctions between semiconductors and metals was one of thermal excitation, rather than tunnelling, as had previously been believed. In another article, written with R. W. Gurney and published in the Proceedings of the Royal Society of London
the previous year, he had elucidated the theory of the photographic latent imagehow light, absorbed all over a grain of silver bromide in a photographic plate, produces a speck of silver somewhere on the surfacewhich remains the foundation of photographic theory today. Together with Gurney he published Electronic Processes in Ionic Crystals
in 1940. Mott and his group opened up the subject of solid-state physics (as it became called) worldwide; the books are notable for their reliance on physical principles and intuitive approximations, and their avoidance of sophisticated mathematics, which horrified some of Mott's continental colleagues; but slowly, slowly … the success of their approach became so evident that no-one questioned seriously their value (R. Smoluchowski, 101).
In Bristol the Motts lived initially in a flat at 4 Caledonia Place, Clifton; they moved in 1939 to 6 Princes Buildings, where they had superb views over the Avon Gorge and the docks. They had two daughters, Elizabeth (Libby) and Alice, born in 1941 and 1943 respectively. Through childhood Libby developed a severe mental handicap and reluctantly they decided in the 1950s that she needed residential care. Alice later married the mathematician Michael Crampin.
During the Second World War, Mott worked first on the propagation of radio waves, and later, from 1943, as superintendent of theory at the armaments research department, where he contributed to the theory of the explosive fragmentation of shell and bomb cases. At the end of the war he was offered professorships at both Oxford and Cambridge. However, he was reluctant to leave Bristol, where he had outstanding support from the university, was assured that he would succeed Arthur Tyndall as head of department, and had just been offered a beautiful Georgian residence, Stuart House, on Royal Fort, right next to the laboratory.
The war stimulated Mott's desire to strengthen physicists' links with industry, and to this end the post-war group he recruited to Bristol had strong interests in the mechanical properties of metals: Charles Frank, Frank Nabarro, J. Mitchell, and Jaques Friedel (who married Ruth Mott's sister, Mary Horder) became world leaders in investigations of dislocations, work-hardening, and fatigue. Mott's own contribution, though impressive, was not as outstanding as his earlier (and later) work on solid-state physics, for he was becoming increasingly involved in university administration. He was also concerned to revive the fortunes of British scientific publishing: [Since the war] English had become the language of science and the American journals … had cashed in on it. I felt that we should have our share (Mott, 76). His increasing involvement with the Philosophical Magazine
dated from this time, as did an interest in science policy. In 1946 he became the first president of the Atomic Scientists' Association, which aimed to investigate proposals for the control of nuclear energy and to put the true facts before the public; he became one of the few British scientists to argue against the UK developing its own atomic bomb, and later, in 1962, he hosted a Pugwash conference on arms control.
In 1954 W. L. Bragg resigned the Cavendish professorship at Cambridge, and Mott was elected to succeed him and moved his family to 31 Sedley Taylor Road, Cambridge. In contrast to Bristol he found that The Cavendish … was a going concern with little opportunity to start new things; on the research side it took me some time to find a role (Mott, 102). Among these going concerns was the nuclear research programme, a relic of Rutherford's tenure at the laboratory; one of Mott's first, difficult, decisions was to cancel ambitious plans for a linear accelerator which, he believed, would still not be adequate to rival the American lead in nuclear physics. He continued Bragg's policy of diversification at the Cavendish, and favoured a very open administration, regularly consulting the heads of research groups. He strongly encouraged the molecular biology group, recently successful in their search for the double-helix structure of DNA, in their search for an identity and site independent of the Cavendish, for he foresaw that they would grow out of all proportion to the rest of the laboratory. Into the space thus created he transferred Bowden's group on surface science from the physical chemistry department, providing a nucleus of solid-state physics in the Cavendish. He was able to promote these moves by his membership of the general board, a committee of twelve that made most of the decisions in Cambridge. Despite this position he constantly felt frustrated by the lack of any central policy-making body for the university.
While still seeking a research role Mott threw himself into education policy, at Cambridge and in schools. He argued strenuously for fundamental changes to the Cambridge natural sciences tripos, and also to the scholarship exam which, he thought, encouraged overspecialization in schools. He became chairman of the education committee of the Institute of Physics, and of the Nuffield advisory committee on physical science which was seeking to modernize science teaching in schools; he sat on the government committee on education between the ages of sixteen and eighteen and the Ministry of Education's standing committee on the supply of teachers; and from 1965 to 1975 he served on the education committee of the Royal Society. In 1962 he received a knighthood.
In 1959 Mott was elected master of his college, Gonville and Caius, on the resignation of James Chadwick. The college at the time was very divided between conservative and progressive parties, and Mott was seen as a unifying candidate. He succeeded in his new role, which he enjoyed for several years. Caius gave him an excellent venue for the meetings and conferences he was so fond of organizing, bringing scientists or policy-makers together to stimulate and progress their views. However, the old divisions surfaced again and Mott in turn resigned in 1966.
The origins of Mott's work on amorphous, or glassy, semiconductors, for which he (together with P. W. Anderson and J. H. Van Vleck) received the Nobel prize in 1977, go back to the 1930s, but Mott took the subject up strongly on resigning from the mastership. In a paper published in the Proceedings of the Physical Society of London
in 1949 he had shown theoretically that certain non-metallic materials would become metals under extreme pressure as the electrons got closer together, and that this Mott transition would be a sharp, rather than gradual one. The required pressure was too high for an experimental test to be practicable, but in 1958 Fritzsche showed experimentally that a comparable effect was seen in the semiconductor germanium as the concentration of impurities (and hence of electrons) increased. In the same year P. W. Anderson suggested that a similar, sharp, metal-insulator transition would occur if solids became disordered (that is, non-crystalline or amorphous), as the randomness localized the electrons, preventing them from moving freely.
These results introduced Mott to the problems of disordered solids, and he introduced new concepts: the mobility edge, which separated the localized and unlocalized states in Anderson's theory, the 8-N rule; and minimum metallic conductivity, to describe their behaviour. He gave a comprehensive account of what was by then a vast literature on this subject in Metal-Insulator Transitions
(1974). In an article published in the Philosophical Magazine
in 1969 he considered electrical conduction on the insulating side of the transition, formulating the Mott T-¼
law, which strikingly displays his intuitive genius and describes the variable range hopping of electrons between localized states over a distance that is temperature dependent. Mott's collaborator, E. A. Davis, recalled:
At the time, few groups showed any interest in them. Although the Xerox Corporation in America achieved great success with their first dry photocopying machine, which used amorphous selenium as the photoreceptor, the physics behind the process was not understood. In the Soviet Union, Kolomiets had been working on the properties of glasses; in Germany, Stuke had a small group; and, in the United Kingdom, Spear was studying transport in thin films of germanium and silicon. What Mott did was to bring together these disparate activities using his by now well-tested methods. Pouncing eagerly on new results, he formulated his ideas and communicated them to interested parties. He organized mini-conferences (in the way he had done at Bristol), visited laboratories for personal discussions, suggested PhD topics, and wrote draft papers for wide circulation and comment. Thereby he rapidly became the father figure of a growing community. (Davis, xxiixxiii)
Mott's understanding that amorphous materials could act as semiconductors ushered in the age of the truly cheap electronic device, for they were much easier to prepare than the crystalline materials used in most previous electronic systems, as they did not need to be anything like as pure. These discoveries quite simply ended the notion of the computer as the preserve of aerospace and defence agencies, big industries and scientific research institutes, and added it to the list of household utensils (The Times
, 12 Aug 1996).
Mott retired from the Cavendish professorship in 1971, but remained active within the department. Although eighty-one when high-temperature superconductors were discovered in 1986, he plunged into a new area of research, co-authoring two books on the subject with A. S. AlexandrovHigh-Temperature Superconductors and other Superfluids
(1994) and Polarons and Bipolarons
(1995)in which they outlined the idea of bipolarons, a virtual particle consisting of two electrons and the associated lattice polarization, as the superconducting mechanism. This remained one of a number of competing theories, unresolved at the time of Mott's death in 1996.
Mott had been brought up as an atheist, but Ruth was an Anglican and Mott was attracted by that tradition as evinced in the beauty of church architecture and of traditional services. However, it was not until the 1950s that he began to think deeply about the relationship between science and Christian faith, impelled largely by Mervyn Stockwood, then vicar of Great St Mary's, the Cambridge University church, who persuaded him to talk on Science and belief in 1957. He began to attend his parish church, occasionally preached, and was eventually baptized and confirmed in the mid-1980s, being admitted to the church on a simple statement of his own rather unorthodox beliefs. Mott rejected a substantial fraction of the creed and refused to believe in an omnipotent God who could perform miracles. He did believe, however, that human consciousness could not be explained or understood through the laws of physics and chemistry, but was a God-given gift that allowed man to understand natural beauty. He outlined his beliefs in 1991 when he edited Can Scientists Believe?
To give meaning to consciousness, a belief in God who is outside us is necessary to me. Without Him life can seem a tale told by an idiot. I believe in God because I wish to do so, to give meaning to human life. (Davis, 323)
In 1980 the Motts left Cambridge to live at Aspley Guise in Bedfordshire, near their daughter Alice and their grandchildren. From there Mott still visited Cambridge for several days a week. Eventually Ruth became too frail to live alone and went to live in a residential home in Cambridge, where Mott would visit her, staying overnight with his sister Joan.
A tall man, Mott was often described as stork-like. He had a wide variety of interests, possessed great charm and friendliness, and loved entertaining. He received many honours, including the Copley, royal, and Hughes medals of the Royal Society, the French Chevalier de l'ordre national du mérite
, and numerous honorary degrees. He was made a Companion of Honour in 1995. He died of heart failure in Milton Keynes General Hospital on 8 August 1996, and was survived by his wife, Ruth, and both his daughters.