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MSE 503 Graduate Seminar on 2-20-18

MATERIALS SEMINAR Department of Materials Science & Engineering
Tuesday February 20, 2018
2:00 – 3:00PM ~ JDT 500
This seminar will be followed by Dr. Eley’s Vision for Future Research in MSE 3-3:30
Please join us for refreshments at 3:30

Resistance is Not Futile: Pinning Down Elusive Vortices in Superconductors

Speaker: Dr. Serena Eley
Condensed Matter & Magnet Science
Los Alamos National Laboratory, Los Alamos, NM

Though superconductors are revered for their ability to carry dissipation-free supercurrents below a material-dependent critical current (Jc), the seemingly inconspicuous action of vortices can introduce dissipation even for currents well below Jc. In type II superconductors (including high-Tc cuprates, iron-based superconductors, and MgB2) immersed in high enough magnetic fields, vortices are formed by the penetration of magnetic flux and can be moved by current-induced forces and thermal energy. Vortex motion can be disruptive: it limits the current-carrying capacity in wires, can cause losses in microwave circuits, and, more generally, can induce phase transitions. Understanding vortex dynamics is a formidable challenge because of the complex interplay between moving vortices, material disorder (defining pinning sites) that can counteract vortex motion, and thermal energy that can cause vortices to escape from these pinning sites. In particular, we cannot precisely predict the rate of thermally activated vortex motion (creep) in a given sample, and tuning the creep rate by modifying the microstructure is typically achieved by means of trial and error. Furthermore, common techniques to enhance Jc by adjusting the disorder landscape (e.g., irradiation or incorporation of non-superconducting inclusions) are often accompanied by unfavorable increases in the creep rate. In this talk, I will discuss the importance of minimizing creep and my efforts to better understand vortex creep. I will cover results from studies of a wide variety of materials, including iron-based superconductors, films with different defect structures (e.g., varying densities of point defects, nanoparticles, columnar defects), and commercially available YBa_2 Cu_3 O_(7-δ) tapes. I will end by discussing a recent result in which we propose the existence of a universal minimum realizable creep rate that depends on material parameters. This limitation is of both fundamental and technological significance: it provides new clues about the interplay between material parameters and vortex dynamics and about how to engineer materials with slow creep. This hard constraint, applicable at low temperatures and fields, has two important implications: first, the creep problem in high-Tc superconductors cannot be fully eliminated and there is a limit to how much it can be ameliorated; and second, we can predict that any yet-to-be-discovered high-Tc superconductors will have fast creep.