CAREER: Quantum Phase Transitions in Electronic Systems

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This CAREER award supports theoretical and computational research in condensed matter physics and developing computational physics courses that use computation to advance understanding of physics.Research will be conducted along three separate but related lines with a common objective to investigate electronic systems near quantum phase transitions. In the first line, magnetic and superconducting transitions of clean and weakly disordered itinerant electrons will be studied using field-theoretic methods. Emphasis will be on the coupling of critical and noncritical soft modes which can dramatically change critical behavior. The PI aims to develop a combined fermionic and bosonic renormalization group for these transitions that includes all soft modes on equal footing. This theory will also permit a quantum mechanical description of transport close to the transition. This work will not only be of fundamental interest for the quantum phase transition community, it also has direct experimental relevance for heavy fermion materials, weak ferromagnets, and other exotic superconductors. The second line of research is devoted to non-perturbative effects of quenched disorder. The PI recently found that some quantum phase transitions with Ising symmetry (including the itinerant quantum antiferromagnetic one) are smeared by disorder. It is proposed to investigate the properties and consequences of this exciting effect by extremal statistics, real-space renormalization and percolation ideas as well as Monte-Carlo simulations. Special sampling techniques will be developed to investigate broad distributions associated with strongly disordered systems. Separate projects will be devoted to transitions with continuous symmetry where no smearing is expected but rather unconventional strong-disorder fixed points. Beyond its direct relevance for some heavy fermion systems, where indications of a rounded transition have been observed, this work will be of interest for the statistical physics of disordered systems and for other areas dealing with rare events and large fluctuation.The third research line, involving computer simulations of microscopic models of disordered electrons, complements the work on non-perturbative disorder effects. A first application will be to investigate the properties of a single rare region of an itinerant magnetic quantum phase transition and its relation to a correlation-induced localized magnetic moment in a dirty metallic system.The PI intends to develop a sequence of computational physics courses aimed at upper level undergraduate and beginning graduate students. In these courses, the students will learn how to use computation and simulation to advance the understanding of physics. This involves skills that are typically not taught in introductory physics courses where students often lack the background in programming and that are typically not taught in computer science and mathematics courses which emphasize technical aspects of computation. These physics-centered courses are aimed at filling this gap. Their role for teaching computational physics is similar to that of traditional laboratory courses for experiment, giving the students hands-on experience in all aspects of computational work. Students who successfully finish the courses should acquire highly marketable interdisciplinary skills in today's job market and should be well prepared to participate in the PI's research as well as other research projects in the physics department. This will give incoming graduate students a head start and help to provide research opportunities for undergraduate students at the forefront of science.In addition, it is also hoped that these course will help attract new students to the physics program.There are broader impacts in the area of educating undergraduates and in attracting new talent to the field of physics. Advances in understanding strongly correlated electron systems and quantum critical phenomena contribute concepts that may lay the foundation for future technologies and may impact other fields, particularly other areas of physics.%%%This CAREER award supports theoretical and computational research in condensed matter physics and developing computational physics courses that use computation to advance understanding of physics.The PI will investigate perfect and disordered electronic systems near quantum phase transitions. Quantum phase transitions occur at zero temperature. Ordinary phase transitions are driven by thermal fluctuations. By contrast, quantum phase transitions are driven by quantum mechanical fluctuations. They can give rise to unusual properties at finite temperature and may provide insight into exotic properties of strongly correlated electron materials that have so far resisted explanation. The PI plans to study ferromagnetic and superconducting quantum phase transitions and to investigate the effect of disorder.The PI intends to develop a sequence of computational physics courses aimed at upper level undergraduate and beginning graduate students. In these courses, the students will learn how to use computation and simulation to advance the understanding of physics. This involves skills that are typically not taught in introductory physics courses where students often lack the background in programming and that are typically not taught in computer science and mathematics courses which emphasize technical aspects of computation. These physics-centered courses are aimed at filling this gap. Their role for teaching computational physics is similar to that of traditional laboratory courses for experiment, giving the students hands-on experience in all aspects of computational work. Students who successfully finish the courses should acquire highly marketable interdisciplinary skills in today's job market and should be well prepared to participate in the PI's research as well as other research projects in the physics department. This will give incoming graduate students a head start and help to provide research opportunities for undergraduate students at the forefront of science.In addition, it is also hoped that these course will help attract new students to the physics program.There are broader impacts in the area of educating undergraduates and in attracting new talent to the field of physics. Advances in understanding strongly correlated electron systems and quantum critical phenomena contribute concepts that may lay the foundation for future technologies and may impact other fields, particularly other areas of physics.***