Quantum and Thermal Creep of Skyrmions and Superconducting Vortices

Colorado School Of Mines, Golden CO

Investigators

Abstract

Non-technical Abstract: Vortices are topological excitations that appear in many different systems. In superconductors, vortices consist of supercurrents circulating around a non-superconducting core and are typically unwanted because their motion induces dissipation that often limits the performance of superconducting wires and devices in power, magnet, sensing, and computing applications. On the contrary, in certain magnetic materials, vortex-like excitations called skyrmions (winding configurations of magnetic moments) form which are predicted to be beneficial for use as information carriers in next-generation low-energy spintronic devices. Mitigating the deleterious effects of superconducting vortices and exploiting skyrmions in spintronic devices for magnetic memory and logic require a microscopic understanding of the complex interplay between vortices, material disorder, and thermal energy. In this work, the research team is investigating this interplay by comparing the rates of vortex and skyrmion motion in materials containing varying amounts of disorder. This research provides training for graduate and undergraduate students in low temperature measurement techniques, materials growth and microanalysis, and quantum materials physics, necessary skillsets in multiple industries including power, sensing, and computing. Additionally, the principal investigator is reaching out to the local community by organizing an annual Open House Community Day for which families in central Colorado will be invited to the Colorado School of Mines for lab tours, science demonstrations, and hands-on activities. Technical Abstract: The interaction of vortices with material disorder is a primary determinant of the electronic and magnetic properties of many systems. In type-II superconductors, vortices are magnetic flux lines that penetrate into the material upon exposure to magnetic fields. In chiral magnets and magnetic multilayers, vortex-like excitations called skyrmions (winding configurations of magnetic moments) can form due to antisymmetric, anisotropic exchange coupling between magnetic moments on lattice bonds. Though the origins of vortices in superconductors and skyrmions in magnetic systems are fundamentally different, striking similarities exist between their dynamics. For example, both can be modeled as particle-like excitations interacting with quench disorder, undergo disorder mediated collective interactions and exhibit glassines. Material disorder immobilizes vortices and skyrmions, whose motion can be induced by sufficiently high currents or thermal energy (thermal creep), or occur via quantum tunneling through disorder-defined energy barriers (quantum creep). Despite considerable previous research on superconductor vortex dynamics, serious gaps still exist in vortex physics. Creep rates are not predictable and no analytic expression exists that broadly captures the temperature and field dependence of creep. The objective of this work is to understand quantum creep of superconducting vortices and both quantum and thermal creep of skyrmions. To this end, the research team captures creep rates in many superconducting and magnetic materials in a range of temperatures and magnetic fields using magnetization and transport measurements. Subsequent comparisons of creep rates in disparate materials with varied disorder landscapes enables them to draw universal correlations between creep and fundamental material parameters. This research could fill a major gap in the understanding of how vortices overcome different energy barriers and enable efficacious design of defect landscapes in superconductors for many applications and magnetic devices for skyrmion-based spintronics. 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.