Spin Fluctuations at Exposed Quantum Critical Points

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Non-technical AbstractQuantum materials promise to revolutionize future technologies. The study of quantum criticality plays a pivotal role in research of a poorly understood class of such materials called unconventional superconductors. Quantum criticality appears to be related to the occurrence of unconventional superconductivity. But important questions remain about the nature of this relationship, which often remains hidden because superconductivity "masks" the relevant quantum phenomena. This program focuses on using advanced measurements performed at ultra-low temperatures to fully characterize the relation between quantum criticality and superconductivity. The broader impact of this program involves the inclusion of high-school students, undergraduate students, graduate students, and postdoctoral scientists in interdisciplinary research and areas of scientific and technological significance, including collaborative and exchange programs with external institutions. An ongoing participation in the Graduate Resources Advancing Diversity with Maryland Astronomy and Physics (GRADMAP) program will involve undergraduates in research exposure programs designed to attract a broader audience to graduate studies. Researchers are also integrated into the UMD Physics department's prominent existing outreach programs, including Physics is Phun, Physics Lecture Demo, and the Summer Girl's Project. Ongoing participation in the NIST SURF (Summer Undergraduate Research Fellowships) Program will also continue to integrate facilities, mentorship and training between UMD and the NIST Center for Neutron Research.Technical Abstract:There are two closely related but disparate mechanisms for superconductivity in the vicinity of a quantum critical point: 1) an enhancement of the bosonic coupling strength due to the increase in fluctuations of the suppressed order parameter, and 2) an indirect enhancement of the pairing strength due to an increase in normal state entropy near a critical point that favors a lower-energy ground state. Both scenarios are quite important but are difficult to discern if superconductivity intervenes, but can be elucidated by carefully studying fluctuations in non-superconducting systems. This program focuses on the study of quantum criticality in systems that do not exhibit bulk-phase superconducting instabilities upon approach to their critical points. The approach is from three directions, utilizing three related systems that each entail uniquely different magnetic behavior: a) a benchmark electron-doped iron-pnictide material that, surprisingly, does not harbor bulk superconductivity upon suppression of antiferromagnetic order; b) an overdoped iron pnictide material that exhibits telltale signatures of quantum criticality, again without superconductivity; and c) a binary iron-pnictide material that harbors a metallic spin-density wave magnetic order that can be suppressed with chemical substitution toward a metallic ground state. The full characterization of critical behavior via measurements of both thermodynamic properties (specific heat, thermal transport, nuclear magnetic resonance) and dynamic susceptibilities using inelastic neutron scattering techniques, allows for an understanding of the failure to stabilize Cooper pairing in select systems, and hence helps elucidate the pairing mechanism in known families of high-temperature superconductors.