Quantum transport in superconducting, ferromagnetic and normal nanowires
Open Access
- Author:
- Singh, Meenakshi
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 08, 2012
- Committee Members:
- Moses Hung Wai Chan, Dissertation Advisor/Co-Advisor
Nitin Samarth, Committee Member
Chaoxing Liu, Committee Member
Thomas E Mallouk, Committee Member
Qiming Zhang, Committee Member - Keywords:
- superconductivity
transport
confined geometry
nanowires
anti-proximity effect
proximity effect
ferromagnetic nanowires - Abstract:
- Electrons in superconductors condense to form pairs known as Cooper pairs [1]. The characteristic decay length of the wavefunction of this pair (or the “size” of a Cooper pair) is the superconducting coherence length ξ. When any of the 3 physical dimensions of a superconducting system become comparable to this characteristic length the effect of fluctuations become important and superconductivity is predicted to be destroyed [2]. In 1D systems (nanowires with diameter < ξ) the limit at which superconductivity is quenched and the mechanism by which it is quenched is an active field of study. Fluctuations and changes in boundary conditions lead to many novel phenomena in 1D nanowires. Experimental exploration of these novel effects forms the basis of this dissertation. Metallic 1D nanowires have been fabricated using template-based electrodeposition and evaporation. Availability of nanowires of different morphologies helps in performing comparative experiments to isolate effects due to disorder from true 1D physics. The electronic transport properties of superconducting, ferromagnetic and normal nanowires with superconducting and normal electrodes have been studied in various measurement geometries. Experiments on aluminum nanowires studying the counterintuitive anti-proximity effect (APE) [3] have been performed. The results of these experiments appear to bring a complete understanding of the phenomenon and have resolved a number of puzzles in the early experiments. In addition, measurements of a single resistance reading found switching from the superconducting to the normal state close to Tc of the wire and at low temperatures in the APE regime. The switching at low temperature is triggered by individual quantum phase slips. These results indicate that the low temperature APE regime offers a clean platform for the observation of individual quantum phase slips, a goal eluded in numerous experiments. Systematic studies on crystalline and granular ferromagnetic cobalt and nickel nanowires sandwiched between superconducting electrodes have been performed. A very long-range proximity effect (~ 600 nm) was found. This range is two orders of magnitude larger than that measured for bulk superconductor-ferromagnet systems. The superconducting transition was foreshadowed by a large peak in resistance dubbed the ‘critical peak’. Possible explanations of these counterintuitive effects have been discussed. In earlier experiments on crystalline gold nanowires contacted with superconducting tungsten electrodes, a mini-gap state along with magnetoresistance oscillations indicating individual vortex trapping were found [4]. The experiment has been repeated here with different electrodes and different nanowire morphologies. The mini-gap state persists in these samples demonstrating it is a robust state independent of nanowire and electrode morphology.