Electronic and Magnetic Phenomena in Iron-based Superconductors

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Nontechnical description: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 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 mutual interplay between superconductivity and magnetism. They could also offer clues about other unconventional superconductors, such as the cuprates, and contribute to a more global understanding of the novel phenomenon of high temperature superconductivity. This highly interdisciplinary project allows graduate and undergraduate students, postdocs, and visitors 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 postdoc and graduate students. A female Ph.D. student participates in this project, hence, the project increases diversity within the field. The international collaborations with scientists in Romania and 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. She also addresses the professional development needs of science teachers in grades 6-10 by developing the physical science component of a new physical and life science graduate course. Finally, she provides middle school and high school physics teachers and their students workshops and lab tours. Technical description:This proposal addresses a major theme in condensed matter physics: unconventional superconductivity in Fe-based superconductors. The proposed research significantly enhances our fundamental understanding of charge conduction and magnetism of 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 novel electronic quantum states with real-space texture in iron pnictides/chalcogenites superconductors with either an antiferromagnetic Mott insulator or antiferromagnetic spin density wave parent compound; (2) reveal the microscopic coexistence of antiferromagnetism and superconductivity and its evolution with doping; (3) study quantum criticality in order to reveal possible quantum phase transitions induced by control parameters, elucidate the phase boundary between the paramagnetic and antiferromagnetic phases inside the superconducting state, and generate a phase diagram that provides direct evidence for a quantum critical line inside the superconducting phase; (4) study the symmetry of the superconducting gap in order to unambiguously distinguish between different pairing symmetries and reveal any universality in the doping dependence of the gap; (5) reveal the origin of the pseudogap region; (6) facilitate the training of highly qualified personnel through comprehensive and multifaceted research, improve STEM education through educator development at 6-10 grade levels, and perform outreach activities. The methods that are used in these studies are resistivity, magnetoresistivity, current-voltage, torque, and magnetization measurements. Understanding the intrinsic electronic, magnetic, and magnetotransport mechanisms in these complex materials may be a key component in understanding their unique and potentially useful physical properties. With iron pnictides/chalcogenites and cuprates to compare and contrast, the proposed research could contribute to finally uncovering the vital clues that theorists need to solve the mystery of high-temperature superconductivity. More broadly, the results from these basic investigations provide further understanding of the interplay between magnetism and superconductivity of other unconventional superconductors and insight into appropriate doping schemes to facilitate applications to electronic sensors and devices.