ABSTRACT

NONTECHNICAL SUMMARY

This CAREER award supports theoretical research and education in the development of methods to stabilize spin liquids. Real materials at low temperatures still offer unexplored territory where the laws of physics and the emerging fundamental particles can differ significantly from the original building blocks of electrons and atoms. Spin liquids form one such exotic class of systems. They originate in insulating materials, which can be thought of as electrons frozen in place, with only their magnetic moments (spins) able to flip. At very low temperatures, spin-liquid physics emerges, which enables these materials to carry heat and spin currents, as if they were metals. An intuitive way to think about that situation is to imagine electrons that have split in half, leaving their charge half stuck to the lattice, while the magnetic half has become mobile. These fractional particles, as they are called, could possibly be exploited to enable quantum computation. Spin-liquid systems are a challenge to theorists, and require new ways of thinking. Developing and testing these new methods requires a continuous conversation between theoretical and experimental research. Spin liquids are rare both experimentally and theoretically, especially in models that can be connected to real materials. This project supports theoretical research in examining how to stabilize these spin liquids in a wide variety of models that can be realized in materials. The research will lead to the development of a new toolbox that would make it easier to study these exotic systems both numerically and experimentally. Moreover, these ideas could be extended to develop new ways to exploit related phenomena in other systems, which include high-temperature superconductors.

This award also supports outreach and educational efforts that are closely integrated with the research. The educational component aims towards facilitating the conversation between condensed matter experimentalists and theorists by developing a course module of condensed matter theory designed primarily for experimental graduate students. The module will be implemented at Iowa State University, and it will be made available online as a wiki course. The outreach component addresses the large number of physics undergraduates whose career aspirations lie outside academia, and who are currently under-served by many physics departments. The principal investigator will develop an extensive and publicly available web resource for undergraduate students, that clarifies the range of opportunities opened up by a physics degree, and teaches students how to prepare and apply for these careers. Increased awareness of the wide variety of available career options will help in increasing the number and diversity of physics majors.

TECHNICAL SUMMARY

This CAREER award supports theoretical research and education in the development of methods to stabilize spin liquids. Spin liquids are exotic magnetic phases that break no symmetries, fluctuate strongly at zero temperature, and have fractional mobile collective excitations that carry only the spin of the electron. These spinons can have strange particle statistics making them of interest for quantum computation. However, spin liquids are rarely found in nature, which is a major impediment to theoretically understanding and eventually exploiting their diverse behavior. Highly frustrated magnetic lattices are promising places to look for spin liquids, however, there is competition from neighboring ordered phases, making spin liquid regions narrow, or even nonexistent. This project will develop methods to stabilize spin liquids by weakly coupling extra degrees of freedom to frustrated lattices in order to exploit their fluctuating nature. The initial subprojects tackle magnetic degrees of freedom, where extra spins are added to a frustrated lattice in ways that favor the spin liquid. These begin with a honeycomb lattice coupled to spins in the center of the hexagons, and then explore how to most effectively couple in extra spins, and even how to use this coupling to select different spin liquids. Other degrees of freedom are: orbital, where increased orbital degeneracy can be stabilized by strong spin-orbit coupling, leading to increased quantum fluctuations; structural, where low-energy local phonons can couple strongly to the spins by modifying the superexchange paths, again increasing the fluctuations; or optical, where gapless spinons interacting with a periodic laser pulse can acquire a gap. Model Hamiltonians involving each of these degrees of freedom will be studied both analytically and with large-scale numerical calculations to determine how the phase boundaries change with the coupling, and how to engineer maximally stable spin liquids. Promising approaches will be used to propose new candidate materials. This research program will open up access to a diverse set of spin liquids in which to develop and test new theoretical ideas, as well as improve our understanding of spin liquids by understanding their fluctuations. Understanding spin liquids is fundamentally important, as they are at the vanguard of new strongly correlated physics and are deeply intertwined with unconventional superconductivity and heavy fermions.

This award also supports outreach and educational efforts that are closely integrated with the research. The educational component aims towards facilitating the conversation between condensed matter experimentalists and theorists by developing a course module of condensed matter theory designed primarily for experimental graduate students. The module will be implemented at Iowa State University, and it will be made available online as a wiki course. The outreach component addresses the large number of physics undergraduates whose career aspirations lie outside academia, and who are currently under-served by many physics departments. The principal investigator will develop an extensive and publicly available web resource for undergraduate students, that clarifies the range of opportunities opened up by a physics degree, and teaches students how to prepare and apply for these careers. Increased awareness of the wide variety of available career options will help in increasing the number and diversity of physics majors.

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