Investigations of Crossed Andreev Reflection in Hybrid Superconductor-Ferromagnet Structures
Colci O'Hara, Madalina
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https://hdl.handle.net/2142/80606
Description
Title
Investigations of Crossed Andreev Reflection in Hybrid Superconductor-Ferromagnet Structures
Author(s)
Colci O'Hara, Madalina
Issue Date
2009
Doctoral Committee Chair(s)
Eckstein, James N.
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
Cooper pair splitting is predicted to occur in hybrid devices where a superconductor is coupled to two ferromagnetic wires placed at a distance less than the superconducting coherence length. This thesis searches for signatures of this process, called crossed Andreev reflection (CAR), in three device geometries. The first devices studied are lateral spin valves. In these structures, when electrons with energies less than the superconducting energy gap are injected from one ferromagnetic wire into the superconductor, nonlocal transport processes involving the second ferromagnetic wire are predicted to take place. We measure a negative nonlocal voltage in the antiparallel magnetization alignment of the ferromagnetic wires, which is the theoretically predicted signature of CAR. The second type of hybrid devices we measured consist of two superconducting electrodes connected by two ferromagnetic nanowires placed within a superconducting coherence length of each other, forming an S-FF-S junction. We find that below the critical temperature of the superconductor, the resistance versus temperature curves show re-entrant behavior, with the signal corresponding to antiparallel alignment of the magnetization of ferromagnetic wires distinctly larger than that of the parallel case. We discuss one possible explanation of this result in terms of Cooper pair splitting. We also report the first observation of multiple Andreev reflection peaks in the differential resistance of these devices. The third line of investigation briefly examines superconductor-ferromagnet dc SQUID-type devices to which we apply an external magnetic field to modulate the phase drop across the junctions. We do not observe coherent effects such as supercurrent or resistance oscillations, but suggest future directions of research.
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