Non-Sinusoidal Current -Phase Relations in Superconductor-Ferromagnet-Superconductor Josephson Junctions
Stoutimore, Micah John Atman
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https://hdl.handle.net/2142/80621
Description
Title
Non-Sinusoidal Current -Phase Relations in Superconductor-Ferromagnet-Superconductor Josephson Junctions
Author(s)
Stoutimore, Micah John Atman
Issue Date
2009
Doctoral Committee Chair(s)
Berzryadin, Alexey
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Condensed Matter
Language
eng
Abstract
We have directly measured the current-phase relation (CPR) of a superconductor-ferromagnet-superconductor (SFS) Josephson junction, and determined that it contains a positive second-harmonic term proportional to sin(2&phis;). The second-harmonic term becomes dominant near Tpi, the temperature at which the first-order term switches between the 0-state and the pi-state. In SFS junctions, the interaction of the correlated electron states from the superconductor with the ferromagnet causes the superconducting order parameter to oscillate so that the ground state can have a phase difference of zero, as in a conventional Josephson junction, or a phase difference of pi, depending on the barrier thickness. The oscillation length is a function of the ferromagnetic exchange energy, Eex, and temperature, so SFS junctions with Eex ≈ Tc (the superconducting transition temperature) that are fabricated near a 0-to-pi crossover thickness can also be modulated between the 0-state and pi-state as a function of temperature. We use the weak ferromagnetic alloy Cu47Ni53 as the barrier material with a thickness of 7nm, which is near the first thickness dependent 0-to-pi crossover point. At this crossover point, it has been predicted that an intrinsic second-order term would dominate the CPR due to the suppression of the first-order component. Alternative theories predict that a negative second-order term could arise, in a narrow temperature regime at the 0-to-pi crossover point, from a competition between the 0-state and pi-state due to inhomogeneities. Our direct measurement indicates a positive second-harmonic that is constant over the temperature range where it is accessible. Additional transport measurements are consistent with the direct measurement and indicate that the second-harmonic term persists over a temperature range of at least 1.5K. These results indicate an intrinsic second-harmonic in the CPR of our junctions.
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