Angular momentum transport in insulators: Magnons and other emergent excitations

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Non-technical:In a magnetic insulator, magnetism (“spin”) can be made to flow through the material without the heating that happens when flowing charge through an electrical conductor. This makes spin currents appealing for future low power technologies. In most magnetic insulators, spin travels via waves, each carrying a certain amount of magnetism, but in some materials the spin is thought to be carried in more complicated ways, so that it comes in packets of different amounts or arranged differently in time, as if packets of magnetism are tied to each other. The flow of spin can be driven and detected electrically using recently developed techniques. This project uses these methods to examine the flow of spin in several such materials as a function of temperature and other conditions, to try to test these ideas about non-wave-like spin motion. For example, fluctuations in the flow of spin can quantify the amount of magnetism carried per packet, in the same way that the fluctuating sound of rain gives information about the size of rain drops. The Principal Investigator is working with leading theorists in the interpretation of the data. Foundational knowledge of spin flow in these systems is essential for the full realization of their potential in future technologies, including quantum information processing. This project incorporates the research and communications training of two graduate students as well as undergraduate researchers recruited from Rice and nearby minority-serving institutions. These individuals are gaining valuable experience with quantum materials as well as written and oral communications skills, preparing them for the technological workforce. Results are spread to the scientific community via publications and conference presentations. The PI is working with Rice K12 teacher training programs, continuing outreach to the public via blogging, and developing/presenting a lifelong learning course about the physics of materials through the cooperation of Rice’s Glasscock School for Continuing Studies. Technical:Angular momentum transport via the spin degree of freedom is an alternative channel for the flow of information and energy in future technologies. Of particular interest is the propagation of spin through magnetic insulators, with the potential for ultralow dissipation in the absence of Ohmic charge flow. Recent methods based on the spin Hall effect have enabled the measurement of spin transport in a variety of magnetically ordered systems via magnons, the quantized spin waves of the electrons in the lattice. The intellectual merit of this project is the addressing of fundamental open questions, including: How is spin transported in materials that host exotic emergent spin-carrying excitations rather than magnons? Can spin transport be controlled through coupling to electric polarization in multiferroics? What are the fundamental limitations of noise in spin transport? Measurements will compare injection- and thermally-driven spin transport in classical spin liquids, a classical spin ice, a quantum spin ice, a candidate fermionic quantum spin liquid, and a multiferroic. Noise techniques are used to observe and quantify spin shot noise, the predicted (but not yet observed) fundamental fluctuations in driven angular momentum transport due to the discrete nature of spin-carrying excitations. The Principal Investigator is working with leading theorist collaborators in the interpretation of the data. Spin propagation is of interest for application in information technology, and quantum spin liquids are potentially relevant for quantum information processing. Foundational knowledge of spin propagation in these systems is essential for the full realization of their potential. This project incorporates the research and communications training of two graduate students as well as undergraduate researchers recruited from Rice and nearby minority-serving institutions. These individuals are gaining valuable experience with quantum materials as well as written and oral communications skills, preparing them for the technological workforce. Results are spread to the scientific community via publications and conference presentations. The PI is working with Rice K12 teacher training programs, continuing outreach to the public via blogging, and developing/presenting a lifelong learning course about the physics of materials through the cooperation of Rice’s Glasscock School for Continuing Studies.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.