This award supports theoretical work on the physics of strongly correlated electronic systems. It is known that many important phenomena in materials science originate from the strong repulsion between electrons in solids. These include all of magnetism, metal-insulator transitions and, more recently, superconductivity. A particularly exciting recent development is the discovery of materials which are candidates for the long sought quantum spin liquid. In these systems the electrons are localized by correlation but their spins do not form an ordered state down to the lowest temperature due to quantum fluctuations. Past theoretical work has predicted that many novel phenomena may emerge at low temperatures, such as specific heat and thermal conductivity which behave like metals in these materials which are charge insulators. These predictions have recently been observed experimentally. The plan is to build on the past success to achieve a deeper understanding of this novel phenomenon. Theory will be developed which aims to explain in detail the experimental observations and to predict new ones. It is possible that an understanding of the quantum spin liquid will pave the way towards an understanding of the high temperature superconductors, especially in the under doped region, where the formation of quantum spin liquids may be the driving force behind the many anomalous properties observed there. Theoretical work on the high temperature superconductors will be pursued armed with new insights gained from the studies of quantum spin liquids. Progress on these long standing problems represent a new paradigm and will have strong impact on condensed matter physics and materials science.
NONTECHNICAL SUMMARY This award supports theoretical research and education in condensed matter physics. The theoretical work takes it inspiration from experimental discovery in new materials and aims to explain and predict novel phenomena. Such novel phenomena often arise in materials where electrons are strongly interacting with each other, and this work will focus on this rich arena. Past examples include Nobel winning discoveries such as the fractional quantum Hall effect and high temperature superconductivity. This research area is particularly well suited for the training of graduate students and postdoctoral fellows because both mathematical sophistication and an understanding of real materials are required to make progress.
This project deals with materials with exotic properties, due to quantum mechanics which underlies the description of the world around us. One example is the appearance of superconductivity in certain oxides at high temperatures. Since its discovery 27 years ago, there has been intense and long standing efforts to understand this phenomenon. It has been recognize that the high temperature superconductivity should be considered in the context of "emergence", ie the appearance of novel particles at low temperature scale from a relatively simple system. This line of thinking has led to a new class of magnets that have been proposed theoretically and discovered in the laboratory; these are called quantum spin liquids. This project makes detailed theoretical studies of these magnets and proposes new experiments to further probe the exotic properties which emerge. We are now at at stage where we can apply what we learn from the spin liquid study to the high temperature superconductor. New experiements have been proposed to test the new theoretical proposal. The project also supports the training of graduate students and postdoctoral researchers. Several of the postdocs have moved on to faculty positions in Universities and will contribute to research and to the education of the next genereration of scientists. To broaden the graduate student training beyond my own students, I have also given lectures to graduate students at a summer school held at Princeton University.
