Symmetry Tests and Vortex Imaging in Unconventional Superconductors

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This individual investigator award will provide support to a professor for a project that will address the role of the phase and phase coherence in superconductor systems. One objective of the project will be to determine the symmetry of the order parameter in a series of exotic superconducting materials. Josephson interferometry experiments, instrumental in establishing the symmetry of the high temperature cuprates, will be applied to several other systems suspected to exhibit unconventional symmetry, including heavy fermion superconductors, which exhibit two distinct superconducting phases transitions, organic superconductors, believed to be d-wave, and certain ruthenate superconductors, thought to be p-wave with a complex order parameter that corresponds to a state with broken time-reversal symmetry. Supplementing these are measurements of the low temperature penetration depth in these materials, the most sensitive test of nodes in the superconducting order parameter. These experiments are crucial to identifying the pairing symmetry, thereby allowing characterization of the thermodynamic, electromagnetic, and transport properties of complex superconductors, and pointing the way to discovery of the microscopic mechanism responsible for the superconductivity. A second set of experiments will apply Scanning SQUID Microscopy to study the distribution and motion of magnetic vortices in superconducting films and crystals. Key problems to be addressed are the geometry of the vortex lattice and structure of the vortex core in unconventional superconductors, the pinning of vortices and flux creep at low temperatures, and the motion of vortices in patterned films including asymmetries in vortex flow in step and ratchet structures. In connection with this project, several significant advances of the Scanning SQUID Microscopy technique will be developed: enhancement of the spatial resolution into the submicron regime, implementation of imaging at ultralow temperatures in a dilution refrigerator, and development of schemes for moving and positioning individual vortices in superconductor systems that will enable direct tests of vortex interactions and pinning. Associated with this project will be the presentation of an established graduate level course in Superconductor Device Physics and the development of a new course in Nanoscale Devices and Probes. %%%This individual investigator award will provide support to a professor for a project that will address the role of the phase and phase coherence in superconductor systems. Superconducting materials are characterized by a quantity, the order parameter, which largely determines their electronic properties. More than any single quantity, it is the phase of this order parameter that is responsible for the fascinating properties of superconductors and leads to the unique potential and capabilities of superconductor electronic devices. Two of the most significant advances have been the identification of unconventional pairing in the cuprates superconductors that is characterized by a strong phase anisotropy (in contrast to the isotropic order parameter observed in ordinary superconductors), and the development of novel magnetic field detection instruments that make possible direct imaging of magnetic domains and vortices. This project will build on these advances in two directions: (1) Experiments will be carried out to determine the order parameter symmetry of several exotic superconducting materials that are suspected to be unconventional, including heavy fermion superconductors, organic superconductors, and ruthenate superconductors. Two complementary approaches will be used: phase-sensitive interferometry experiments which directly probe the anisotropy of the phase of the order parameter, the most definitive test of the pairing symmetry, and measurements of the low-temperature magnetic penetration depth, which is perhaps the most sensitive of the probe of the magnitude of the order parameter. (2) Scanning SQUID Microscopy will be used to study the distribution and motion of magnetic vortices in superconducting films and crystals. This instrument scans a sensitive dc SQUID detector over the surface of a sample to map out the magnetic field distribution with high magnetic field and spatial resolution. This project will provide training for a number of graduate research students in the field of superconducting phase electronics, an topic of current interest for quantum information processing and computing. Associated with this project will be the presentation of an established graduate level course in Superconductor Device Physics and the development of a new course in Nanoscale Devices and Probes. ***