Thermoelectric and Thermal Transport in Disordered and Strongly Correlated Electron Systems

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TECHNICAL SUMMARYThis award supports theoretical research of thermoelectric and thermal transport in various systems, for which an essential fraction of the entropy is stored in an ensemble of collective modes. These modes complement quasi-particle excitations and describe different kinds of critical fluctuations. Generally, promising candidate systems for displaying large thermopower are those in which strong fluctuations invalidate the regular Sommerfeld expansion used for finding thermal properties at low temperatures.As a theoretical tool, a newly developed scheme based on quantum kinetic equations will be applied, thereby profiting from the recent experience of the PI in studying the Nernst effect in superconductors. Analysis of the Nernst effect showed that thermal transport can be a more effective tool for studying fluctuation effects than the ordinary electronic transport. The generality of the method makes it possible to account for different kinds of electron-electron interactions mediated by critical fluctuations. The research will enhance the theoretical understanding of thermal transport in systems which exhibit non-Fermi liquid behavior. The issues to be addressed theoretically include thermopower near the metal-insulator transition caused by disorder in combination with electron-electron interactions, and near quantum phase transitions in systems of itinerant electrons. As a result, the research will help to develop a conceptual picture of thermoelectric transport in the vicinity of a quantum critical point or the metal-insulator transition. In particular, it may result in a new simple formula for the thermopower determined by fluctuations that can be used in place of Mott's formula. The fundamental understanding of thermoelectricity in strongly interacting systems acquired in this research will help to make informed choices in the quest for materials with strong thermopower. Such materials can be used for creating new coolers, which are needed for progress in low temperature experimental physics and applied instrumentation.Training junior researchers is of paramount importance in any academic setting. Besides physics of thermal phenomena, the participation in the research will provide training of the young scientists in advanced methods of condensed matter theory and quantum kinetics.NON-TECHNICAL SUMMARYThis award supports theoretical research on the flow of heat and electric charge in certain materials for which contributions from electrons dominate heat flow. The PI will use advanced theoretical techniques to investigate particularly interesting cases, including: (1) materials that are near a transformation from a metal to an insulator due to impurities and imperfections and strong interactions between electrons, and(2) materials that are near a quantum phase transition. Unlike an ordinary phase transition which is driven by thermal fluctuations, for example the transition from ice to water at 32 degrees, a quantum phase transition is driven by the quantum fluctuations of Heisenberg's famous uncertainty principle and can occur at the absolute zero of temperature, a frigid -460 degrees on the familiar Fahrenheit scale. (3) a gas of electrons confined to two dimensions which can be created in artificial semiconductor material structures,(4) spin liquids which are predicted to arise in certain materials where the interactions between electrons cannot all be simultaneously satisfied. Naively, one might think these materials will become magnets, but rather these frustrated interactions lead to a liquid-like state of electrons without magnetic order. The PI will use quantum mechanical equations that can describe the net movement of electrons in states out of equilibrium, such as when there is a difference of temperature across a material. This research will advance our understanding of the transport of heat and charge in materials with strongly interacting electrons which exhibit new states of electronic matter or interesting phenomena. The PI's research may lead to an interesting way to probe the properties of materials and new states of electronic matter. The fundamental understanding of thermoelectricity in strongly interacting systems acquired in this research will help to make informed choices in the quest for materials with strong thermopower. The thermopower is a measure for the temperature difference developing in a sample when a voltage difference is maintained between the contacts. This is why such materials can be used for creating new cooling devices which have a wide range of potential technological applications.Training junior researchers is of paramount importance in any academic setting. Besides physics of thermal phenomena, the participation in the research will provide training of the young scientists in advanced methods of condensed matter theory and quantum kinetics.