Electrical properties of superconducting and hybrid superconductor-ferromagnet nanowires

Open Access
Author:
Kumar, Nitesh
Graduate Program:
Physics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 23, 2009
Committee Members:
  • Moses Hung Wai Chan, Dissertation Advisor
  • Moses Hung Wai Chan, Committee Chair
  • Thomas E Mallouk, Committee Member
  • Jainendra Jain, Committee Member
  • Qiming Zhang, Committee Member
  • Jayanth R Banavar, Committee Member
Keywords:
  • superconductivity
  • nanowires
  • electrodeposition
  • ferromagnets
  • superconductor-ferromagnet
  • one-dimensional
Abstract:
Superconductivity in reduced dimensions has attracted a lot of attention in last few decades. According to Mermin-Wagner theorem, no long-range order can be possible in strictly one-dimensional and two-dimensional systems. Strong phase fluctuations are expected to destroy superconductivity in quasi one-dimensional superconducting nanowires. This results in dissipation or resistive behavior both near and below the superconducting transition temperature. Previous electrical transport measurements in thin superconducting nanowires have reported dissipative behavior due to thermal and quantum fluctuations. However, most of the experimental and theoretical works done so far has focused mainly on studying the nature of dissipation in superconducting nanowires. The overall purpose of this research was to investigate the interaction of dissipative superconducting nanowires with its environment and vice versa. One of the objectives of this work was to study the influence of bulk superconducting environment on the electrical properties of quasi one-dimensional superconducting nanowires. We have used electrochemical techniques to fabricate nanowires of various metals with high crystal quality, uniformity and diameters well below the superconducting coherence length. Electrical transport measurements on Zn nanowires indicate that bulk superconductor significantly increases the dissipation in nanowires, which can result in suppression of their superconducting characteristics. These results demonstrate that the superconducting properties of nanowires are highly influenced by their environment. In another scenario, we studied the interaction between superconducting and ferromagnetic environment in the case of hybrid superconductor-ferromagnet systems. The competition between the spin asymmetry properties of a ferromagnet and the correlations induced by superconductivity has made hybrid S-F systems an active area of research. Earlier works were done on layered two-dimensional S-F systems with granular and amorphous morphology. The second objective of this research was to explore the quasi one-dimensional hybrid S-F systems fabricated using high crystal quality nanowires. Spin polarized transport resulted in unusual magnetoresistance behavior in ferromagnetic nanowire attached to bulk superconductors. Contrary to the three-dimensional behavior, a long-range proximity effect was observed in ferromagnetic nanowires attached to superconducting electrodes. Electrical measurements were also performed on axially modulated nanowires fabricated by electrochemical deposition. Ferromagnetic properties of nanowires were observed to be strongly affected by the superconducting environment.