This CAREER award supports theoretical and computational research and education, aimed at advancing the understanding of emergent quantum phenomena in materials with strongly interacting electrons. Strongly correlated metals behave differently from ordinary metals such as aluminum or copper because the electrically charged mobile particles of which they consist are unlike ordinary electrons. When an electric current or thermal heat flow travels through an ordinary metal, to a first approximation the electrons can be treated as moving independently and interacting only very weakly with each other. However, when a current travels through a strongly correlated metal, the electrons lose their individuality and form collective excitations. Accordingly, researchers call such strongly correlated metals "strange metals." A classical analogy of a collective behavior is the complex pattern of a flying flock of birds, seemingly behaving as one unit despite comprising thousands of individual birds.
A fascinating class of strange metals is harbored by the so-called "heavy fermion" materials, in which the interactions between the constituent particles are so strong that they acquire a very heavy mass, sometimes several hundred times greater than that of a bare electron. As a result of this heaviness, many material properties, including electrical and thermal conductivity, are profoundly affected. Unfortunately, the very feature that makes these materials so interesting - strong interactions between electrons - also makes them very challenging to study. This research activity is aimed at developing new, innovative theoretical and computational methods to study the electronic properties of the heavy fermion materials. In addition, the PI will combine existing methods of quantum chemistry with the state-of-the-art computational techniques to capture the salient features of these materials, in particular the interplay between magnetic and various interesting quantum mechanical properties such as the ability of materials to conduct electricity without any loss of power, called superconductivity.
This award will allow the PI to integrate educational and outreach activities with the fundamental research. The educational component will include participation in a continuing education course for in-service teachers, with the ultimate goal of helping enrich the middle- and high-school curricula with modern physics content, centered on the concepts of magnetism and superconductivity that are central to this research. This award will also support the education of a graduate student and a postdoctoral research associate at the frontiers of condensed matter theory. It will partially support a broad outreach effort aimed at showcasing the role of electromagnetic forces in modern physics. It will reach out to the general public and schoolchildren with the goal of popularizing science and increasing public awareness of its importance.
This CAREER award supports theoretical research in the so-called strange metal (non-Fermi liquid) behavior in heavy fermion materials, in particular close to quantum phase transition from a magnet to an ordinary metal. Recent experimental advances have identified a number of materials in which the quantum criticality cannot be understood in terms of the established Hertz-Millis theory. In addition, new quantum phases often emerge in the vicinity of a quantum transition, such as unconventional superconductivity whose mechanism is not completely understood. These problems will be studied in the present research program, whose objectives are fourfold:
(1) To analyze the nature of the strange metal state in heavy fermion materials by using a combination of quantum chemistry and existing state-of-the-art techniques to capture strong electron interactions.
(2) To develop novel methods to treat strong electron correlations, in particular in proximity to quantum criticality. One promising approach that the PI will develop is the large-N slave-fermion approach using SU(N) Schwinger boson representation for localized moments. This approach has been demonstrated to perform well for a single Kondo impurity and the ultimate goal is to make it work for a dense Kondo lattice, which describes the low-energy sector of heavy-fermion materials.
(3) To address new quantum phenomena emerging from this proximity, in particular the unconventional superconductivity and its interplay with magnetism. The PI will employ the variational cluser approximation, which is particularly well suited to study ordered phases. The large-N slave-fermion approach, to be developed in (2) will also be used. The results of various methodologies will be compared to experiment.
(4) Qualitatively, the heavy fermion systems can be understood in terms of the so-called Doniach phase diagram, describing the suppression of magnetic Ruderman-Kittel-Kasuya-Yosida interaction between localized moments by the Kondo screening. At the same time, it has long been known that magnetic frustrations can also destroy long-range magnetic order. Recently, several researchers, including the PI, have proposed to combine these two mechanisms into a generalized phase diagram, pertinent to a number of recently discovered heavy fermion materials. The PI will study the interplay between magnetic frustrations and the Kondo effect using a variety of approaches, including the aforementioned large-N slave-fermion method.
This award will allow the PI to integrate educational and outreach activities with the fundamental research. The educational component will include participation in a continuing education course for in-service teachers, with the ultimate goal of helping enrich the middle- and high-school curricula with modern physics content, centered on the concepts of magnetism and superconductivity that are central to this research. This award will also support the education of a graduate student and a postdoctoral research associate at the frontiers of condensed matter theory. It will partially support a broad outreach effort aimed at showcasing the role of electromagnetic forces in modern physics. This will reach out to the general public and schoolchildren with the goal of popularizing science and increasing public awareness of its importance.
