Electronic and Magnetic Phenomena in Heavy-Fermion and Iron-Based Superconductors

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Non-Technical Abstract:Two major themes in condensed matter physics are quantum critical phenomena and unconventional superconductivity. A quantum phase transition takes place at zero temperature and describes a phase transition between competing ground states driven by an external parameter such as chemical composition, pressure, or magnetic field. The recent studies of unconventional superconductors show that superconductivity develops in proximity to a magnetically ordered phase. This raises the possibility of quantum phase transitions in these systems arising from competing types of such orders. The present studies of unconventional superconductors such as heavy fermions and iron-based superconductors mainly focus on the understanding of the normal state properties of these superconductors and on the mechanism of superconductivity. These studies enhance our fundamental understanding of the extent to which a quantum phase transition controls the finite temperature properties of these superconductors and promise insight into the interplay between magnetism and superconductivity of other unconventional superconductors. This new knowledge could provide a more global understanding of the phenomenon of unconventional SC. This highly interdisciplinary project allows graduate and undergraduate students to benefit from exposure to a diversity of experimental techniques, a variety of different physical systems and phenomena, and forefront topics in condensed matter physics. The diversity of the expertise gained by the participants in this research program is a substantial advantage in today's knowledge based, technology driven economy, being beneficial to a future career in industry, government, or academia. Professional mentoring is provided for the graduate students. The international collaborations with scientists in China contribute to the nation's infrastructure for research and education. The principal investigator develops teaching lab modules for a senior laboratory that verifies experimentally counterintuitive physical phenomena learned in Quantum Mechanics; e.g., how a single photon will interfere with itself, or how local realism can be violated. She also participates in the STEM education of K-12 students by contributing to several activities: Annual STEM Project Fair, Science Experience Internship, and Young Women's Summer Institute. In addition, she provides middle school and high school students and their physics teachers workshops and lab tours. Technical Abstract:This proposal addresses a major theme in condensed matter physics: quantum critical phenomena in heavy fermions and iron-based superconductors. The proposed research significantly enhances our fundamental understanding of charge conduction and magnetism of heavy fermions and iron pnictides/chalcogenites, addresses issues related with the interplay between magnetism and superconductivity, and contributes to a more global understanding of the novel phenomenon of high temperature superconductivity. The goals of this research are to: (1) study coexistence of antiferromagnetism and superconductivity; (2) reveal quantum phase transitions; (3) study nature of quantum critical points; (4) study pseudogap region and search for nematic states; (5) facilitate the training of highly qualified personnel through comprehensive and multifaceted research and improve K-12 STEM education. The methods that are used in these studies are resistivity, magnetoresistivity, current-voltage, torque, thermoelectric power, and magnetization measurements. The proposed research will significantly enhance our fundamental understanding of quantum critical phenomena, arising from competing types of orders, in iron-based and heavy-fermion superconductors. Specifically, it will address issues related with the interplay between magnetism and superconductivity, and reveal the nature of the ground state, for example a conventional metal or an ordered phase, which would have appeared had superconductivity not intervened. Fundamentally, it will reveal the extent to which quantum criticality controls the finite temperature properties of these systems. This research could also contribute to a more global understanding of the novel phenomenon of unconventional superconductivity. Understanding the intrinsic electronic, magnetic, and magnetotransport mechanisms in these complex materials is critical to understanding the nature of strongly correlated materials and revealing the origin of unconventional superconductivity.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.