A Novel Gap Spectroscopy Tool and Discovery of New Nodal Superconductors

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Non-Technical AbstractBecause of their high transition temperatures and exotic new correlated electronic states, high temperature superconductors show a great potential for transformative applications, such as quantum information processing. The objective of this project is to discover new classes of unconventional superconducting materials and to examine them for evidence of unconventional superconductivity using a new experimental technique that the principal investigator discovered. The project trains graduate students with backgrounds in physics and engineering to discover new materials and phenomena at the forefront of condensed matter physics research. The students also participate in international research, carrying out part of their Ph.D. research at the Center for Functional Nanostructures at the Karlsruhe Institute of Technology in Germany. The US and German laboratories complement each other, thus enhancing the educational experience for the graduate students involved. The principal Investigator has a history of broadening the participation of underrepresented groups and is involved in outreach activities to recruit more underrepresented students. This project is establishing a new experimental technique of interest to condensed matter researchers, creating new partnerships, collaborations, and facilities that impact the landscape of the superconducting materials and applications fields. Technical AbstractThis award from the Condensed Matter Physics Program in the Division of Materials Research supports the University of Maryland with a project focused on the quest to discover new superconducting materials with new properties, and to further refine the anisotropic nonlinear Meissner effect spectroscopy method into a broadly useful technique that contributes significant results to the ongoing debates about the pairing mechanism in unconventional superconductors. The principal investigator's discovery of the anisotropic nonlinear Meissner effect photoresponse helps to speed up the discovery of superconducting materials with nontrivial and exotic properties. The method relies on the anisotropic nonlinear Meissner effect, and the presence of Andreev bound states, to create images of the nonlinear electrodynamic response of the superconductor that illustrate the superconducting gap nodal directions. The method is sensitive to both bulk properties (through the superfluid anisotropic nonlinear response) and surface properties (through measurement of Andreev bound states associated with the unconventional order parameter). A superconducting thin film is patterned into a compact self-resonant spiral structure, excited near resonance in the radio-frequency range, and scanned with a focused laser beam perturbation. At low temperatures, direction-dependent nonlinearities in the reactive and resistive properties of the resonator create photoresponse that maps out the directions of nodes, or of bound states associated with these nodes, on the Fermi surface of the superconductor. The method was demonstrated on a known nodal superconductor, so the next step is to discover new nodal superconductors by extending the method to work on single crystals and un-patterned thin films. The co-PI grows the new superconducting materials and the PI performs the experiments both in-house and in collaboration with a group in Karlsruhe, Germany, to discover new nodal superconductors. The new materials discovered should have potential for transformative applications, including new forms of superconducting electronics, or sensitive single-photon detectors. The results are disseminated through publications, talks and posters at international conferences, a research web site, a tutorial web site explaining how to set up a nonlinear Meissner effect experiment with minimal equipment, and continuation of the outreach activities by the PI, co-PI and students.