1977 Nobel Prize in Physics The Royal Swedish Academy of Sciences has awarded the 1977 prize equally to Dr. Philip W. Anderson, Bell Telephone Laboratories, Professor Sir Nevill F. Mott, A. R. Mackintosh, Cambridge University and Professor John H. Van Vleck, Harvard University for “their fundamental theoretical contributions concerning the electronic structure of magnetic Copenhagen and disordered systems”. Although the 1977 prizewinners states in magnetic and disordered damental significance for a number of have worked in a wide variety of areas systems. It is characteristic of them technological applications. For exam­ within solid state theory, a common all that they work closely with their ple, Van Vleck’s ideas have played element in their research has been experimental colleagues, helping to an important part in the development their concern with the electron-elec­ interpret their results and proposing of the laser, while the work of Mott tron and electron-lattice interactions new lines of research. The advances and Anderson has made a vital contri­ in solids, and the influence of these which they have made in our under­ bution to the increasing technical ex­ interactions on localized electron standing of solids have been of fun­ ploitation of amorphous materials. John H. Van Vleck Van Vleck was born in 1899 and were later developed by his former received his Ph.D from Harvard Uni­ student, P.W. Anderson, and influen­ versity in 1922. He worked at the ced Mott’s theory of metal-insulator University of Minnesota from 1923-28 transitions. Van Vleck also developed and at the University of Wisconsin much of the microscopic theory of from 1928-34. He then became Profes­ antiferromagnetism. sor of Physics at Harvard, where he Perhaps his greatest impact on the has remained ever since. He retired theory of magnetism has been through in 1969. his work on the crystalline electric Van Vleck is generally regarded as field which, together with the ex­ the founder of the modern theory of change interaction, determines the magnetism, and has been responsible great majority of the magnetic pro­ for many advances in the subject. His perties of solids. Van Vleck introdu­ book Electric and Magnetic Suscepti­ ced the concept into magnetism and bilities from 1932 is a classic work has shown how it is responsible for which is still extensively used and has a variety of static and dynamic pheno­ stimulated an enormous amount of mena. For example, he demonstrated research. In it he presented the quan­ how the crystal field can quench the tum theory of the magnetism of atoms orbital angular momentum and, toge­ and ions, of which he was the prin­ ther with the spin-orbit coupling, cipal architect, and which is used determine the magnetic properties of med, many of his most fundamental essentially unchanged today. He intro­ transition and rare earth ions in solids. ideas were proposed decades ago. duced a new type of temperature- He furthermore drew attention to the However, the full significance of many independent paramagnetism, now unusual effect which can occur when of these ideas has only been appre­ called “Van Vleck paramagnetism”, the ground-state is a singlet, and ciated during the flourishing period of which allowed him to explain the ma­ therefore non-magnetic. He elucidated experimental studies on magnetic gnetic properties of, for example, eu­ the role of the crystal field in giving materials in the last decade. In particu­ ropium and samarium, which had pre­ rise to magnetic anisotropy and to lar, the technique of inelastic neutron viously defied analysis. the Jahn-Teller distortion of the lattice scattering has allowed the investiga­ Van Vleck carried out pioneering about a localized magnetic moment. tion of many of the phenomena work on the influence of conduction He constructed the first successful inherent in his work. The intensive electrons on the magnetism of metals. theory of the interaction between examination in recent years of the For example, he made important con­ local moments and the lattice, through properties of, for example, singlet tributions to the theory of the indirect the phonon modulation of the crystal ground-state systems, rare earth exchange interaction, mediated by the field. This work is of fundamental metals and Jahn-Teller systems has conduction electrons, which is basic importance for the interpretation of shown the crucial effect of the crystal for the understanding of rare earth paramagnetic resonance and relax­ field on these materials. In such metals, and considered the effect of ation. studies, as in so many in the field of electron correlation in producing Although Van Vleck remained active magnetism, the interpretation of the localized magnetic moments. The until the 1960’s when much of his results is based upon extensions of physical ideas which he propounded work on rare earth metals was perfor­ Van Vleck’s original efforts. 5 Nevill F. Mott Mott was born in 1905. He was transform it into accessible form and educated at the University of Cam­ build a host of new ideas upon it. An bridge following which he became a excellent example is disordered ma­ university lecturer, first at Manchester terials where, in the 1960’s, he took and then at Cambridge. In 1933 he Anderson’s very important, but at the began his long and fruitful association time little appreciated paper on ran­ with the University of Bristol, where dom lattices from 1958 (discussed he remained as Professor of Physics further below) and used it as a buil­ until appointed Cavendish Professor ding block for an essentially new way at Cambrige in 1954. He retired in of thinking about a whole class of 1971. solids. His book with E.A. Davis In his long and productive career, Electronic Processes in Non-Crystal- Mott has had a crucial influence on line Materials from 1971 illustrates his the development of solid state phy­ qualities of physical intuition and clear sics. His most important achievements exposition. lie perhaps in his theories of the phy­ Perhaps the most remarkable of his sical and mechanical properties of achievements was the proposal, first metals and alloys, the metal-insulator made in 1949 and elaborated in 1956 transition and, most recently, electro­ and 1961, of the existence of the has been strikingly manifested by nic phenomena in disordered mate­ metal-insulator transition which now quite recent work on the properties of rials. He has contributed decisively to normally goes under the name “Mott electron-hole gases and liquids in in­ the basic understanding of such im­ transition’’. Although it had already tensely illuminated germanium. The portant technical matters as fatigue, been suggested before the war that whole field is summarized in his re­ oxidation, photography and solid state the insulating properties of NiO must cent book Metal-Insulator Transitions devices. be due to correlation, and the locali­ of 1974. It has always been characteristic of zed nature of the 4f-electrons in the It is perhaps worth emphazing the Mott’s mode of working that he dis­ rare earth metals was implicitly reco­ novelty of Mott’s ideas on this sub­ cusses his ideas constantly with gnized, Mott was the first to place the ject. Although they are now almost experimentalists, and provides a fund subject on a sound footing, making universally accepted, they were ini­ of suggestions for rewarding new predictions which have since been tially received with considerable scep­ investigations. His way of thinking has shown experimentally to be generally ticism, because they implied that the especially in later years, been essen­ correct, and stimulating a large band-theory of solids, which had such tially physical rather than mathema­ amount of fruitful research over the impressive successes to its credit, is tical and this has allowed him to com­ past two decades. The basic applica­ not applicable in some perfect solids municate with experimentalists on bility of his ideas to ordered systems under ordinary conditions. In this their own terms. He has furthermore has been perhaps most clearly de­ sense, Mott must indeed be credited been able to take the sometimes monstrated by experiments on vana­ with causing a profound and far-rea­ abstruse work of solid state theorists dium oxides, while their relevance for ching change in the way in which we and, with his exceptional insight, the general understanding of solids think about solids. Philip W. Anderson Anderson was born in 1923. He Bell Telephone Laboratories the same effect of his penetrating and original received his Ph. D. from Harvard Uni­ year. He has remained there ever ideas. versity in 1949 and began working at since and now occupies the position As has been mentioned above, of Assistant Director of Physical Re­ Anderson’s paper “Absence of Diffu­ search. He was part-time visiting pro­ sion in Certain Random Lattices” pu­ fessor at the University of Cambridge blished in 1958 provided the basis from 1967-1975, since when he has upon which most of the modern theo­ held a similar position at Princeton ry of electronic states in disordered University. systems rests. In it he demonstrated Anderson is an extremely versatile that randomness in the potential can physicist who has made distinguished itself be sufficient to cause localiza­ contributions to a large range of topics; tion of the electron states, and hence for example, superexchange inter­ the absence of conductivity. The quan­ actions, impure superconductors, the titative calculation of this effect is properties of superfluid 3He, disorde­ very difficult, and Anderson’s original red materials and localized magnetic estimates have been improved, but the moments. His impact on solid state basic idea is accepted as correct and physics has been very great, both of fundamental importance.