This award supports theoretical and computational research and education on materials that are somewhere between metals and insulators. Material properties are easy to tune in this regime, where several possible ground states compete. Here most physical quantities display unusual behavior, and prove difficult to interpret using conventional ideas and approaches. Recent experiments reveal the significant effects of spatial inhomogeneities. In some cases, intermediate heterogeneous phases may emerge between the metal and the insulator, possibly even in absence of disorder. Such complexity emerges as a new paradigm of the metal-insulator transition region. The PI will carry out a comprehensive research program using advanced computational and theoretical techniques to advance understanding of fundamental physical processes in this regime, including: (1) the interplay of Mott and Anderson routes to localization, and the properties of electronic Griffiths phases preceding the metal-insulator transition; (2) the competition of Ruderman-Kittel-Kasuya-Yosida and Kondo singlets as a mechanism to suppress Fermi liquid coherence; (3) the role of the long-range Coulomb interaction and charge ordering, as a driving force for Wigner-Mott localization in doping-driven metal-insulator transitions. Methods based on dynamical mean-field theory and its extensions will be used to carry out the research.
This project will train graduate students in theoretical and computational condensed matter physics. The PI will continue to promote and enhance scientific and technological understanding and importance of our research to the general public through outreach activities, including: presenting lectures at elementary schools in order to popularize science and technology, and to bring the most recent discoveries within reach of the youngest; acting as a Judge at annual Regional Science Fairs; and participating in the FSU Physics Department and the NHMFL Annual Open House activities. The PI will provide a leading role in developing a major new outreach initiative called the Emergent Labs which aims to take cutting-edge research discoveries in condensed matter science directly into the classrooms of middle and high school teachers.
NONTECHNICAL SUMMARY
This award supports theoretical and computational research and education on materials that are somewhere between metals and insulators. The PI will use advanced computational techniques to study materials which have electrons that interact strongly with each other and contain many imperfections in their structure or chemistry that are reflected in the environment of the electrons. Under certain conditions, increasing the number of defects and imperfections in a conducting material can turn it into an insulator. Recent experiments suggests that in some materials this is not a simple transformation, but rather the route from metal to insulator proceeds through complicated "bad metal" states. The PI will use advanced computational methods to investigate the nature of these states with an aim to understand the role of the strong interactions between electrons and how this transformation differs in bulk materials or in a single layer.
This is fundamental research to understand the nature of metals, insulators, and new states of electronic matter that may exist between them. This research may contribute to the intellectual foundations of future device technologies and to developing new strategies to discover new materials with desired electronic properties.
This project will train graduate students in theoretical and computational condensed matter physics. The PI will continue to promote and enhance scientific and technological understanding and importance of our research to the general public through outreach activities, including: presenting lectures at elementary schools in order to popularize science and technology, and to bring the most recent discoveries within reach of the youngest; acting as a Judge at annual Regional Science Fairs; and participating in the FSU Physics Department and the NHMFL Annual Open House activities. The PI will provide a leading role in developing a major new outreach initiative called the Emergent Labs which aims to take cutting-edge research discoveries in condensed matter science directly into the classrooms of middle and high school teachers.
" (DMR-1005751). In contrast to the weak-coupling approach to the metal-insulator transitions, which had its heyday in mid-1980s, the central theme of our work is the focuson strong-correlation effects associated with Mott localization. This new perspective on the metal-insulator transition is presented in two long review articles, which were published as Chapters 1 and 6 of the multi-author monograph "Conductor Insulator Quantum Phase Transitions", edited by V. Dobrosavljevi?, N. Trivedi, and J. M. Valles Jr. (Oxford University Press, 2012). Our work is based on non-perturbative methods based on recently developed Dynamical Mean-Field Theory (DMFT) methods. This approach relies on assuming that all relevant scattering processes take a relatively local character - spanning only a few unit cells. This key simplification allows a "non-perturbative" solution of the strong correlation problem over a very broad energy range - providing a theoretical description even far outside the Fermi liquid regime. The local ("single-site") DMFT approximation becomes essentially exact above a certain characteristic temperature T*(U), but it can also be systematically improved using the "cluster" methods (e.g. DCA, CDMFT). In strongly correlated systems, this "incoherent" regime is very broad, and often covers much of the experimentally relevant range. In this sense the DMFT based theories for the metal-insulator transition should play the same role as the very useful description the Van der Waals equation provided for liquid-gas critical points. Current and future work of the PI includes not only using the (generalized) DMFT approach, but also employing cluster methods to determine the range of its validity. Real materials always have disorder due to impurities and crystalline defects. Even in absence of disorder, competing interactions can sometimes give rise to "intrinsic" nano-scale phase separation, behavior often observed within the metal-insulator transition region. But what is the impact of such inhomogeneities in presence of strong correlation effects? These questions are rapidly becoming a central issue in material science, not least because of spectacular recent advances in experimental probes that allow direct visualization on the nano-scale. The original development of DMFT methods has focussed on homogeneous systems with strong correlations. In recent years, however, this non-perturbative approach has been further extended to incorporate the interplay between the two fundamental mechanisms for electron localization: the Mott (interaction-driven) and the Anderson (disorder-driven) route to arrest the electronic motion. In addition, the DMFT formulation can be very naturally adapted to also describe strongly inhomogeneous and glassy phases of electrons, and even capture some aspects of the Quantum Griffiths Phase physics found at strong disorder (it is interesting to note that the well-known Parisi-Sompolinsky theory for spin glasse is one of the first examples of the DMFT method). Since the beginning of the DMFT era, the PI has played a leading role in developing theories using these and related ideas, to treat the very difficult problem of strongly correlated electronic systems with disorder. This proposal presents a detailed and carefully prepared program of continued research in this area, which has already produced significant results of direct experimental relevance.
