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Vortex Induced Magnetoresistance Oscillations in Superconducting Nanowires

Steele, Benjamin David George (2019) Vortex Induced Magnetoresistance Oscillations in Superconducting Nanowires. PhD thesis, University of Leeds.

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Abstract

This thesis details work investigating the oscillatory magnetoresistance (OMR) effect observed in superconducting nanowires. The OMR effect is a repeated rise and fall of resistance, in the otherwise monotonic magnetic field induced superconducting transition. Two material systems are studied: Nb nanowires, grown by DC sputter deposition, are used as a typical, commonly used superconductor; In-SnTe nanowires, synthesised by vapour transport growth, are used as a candidate topological superconductor. In-SnTe nanowires were grown without Au nanoparticles as a catalyst because, unexpectedly, the Au was observed to contaminate the crystal growth. The topological surface states are investigated through the observation of weak anti-localisation. Fitting to a 2D weak anti-localisation effect yields a maximum of four surface conductance channels, which is expected for this topological crystalline insulator. This thesis find that oscillatory magnetoresistance is a misnomer, the resistance modulation is not in the form of an oscillatory function. The OMR effect observed is closely described by a set of Gaussian distributions, centred at a series of magnetic fields. Several methods are used to probe the OMR effect, such as: alternating current, constant direct current and pulsed current-voltage. The relationship between the OMR effect is tested with relation to the temperature, applied current amplitude, frequency of the alternating current and the orientation of the applied magnetic field. Of the presently known candidates to describe an OMR effect, the most likely candidate is found to be periodic vortex motion. Vortex motion dissipates energy, causing a finite resistance. In this model, rates of vortex travel are modulated periodically with respect to an applied magnetic field. There are two main factors that contribute: the rearrangement of the vortex lattice at regular intervals in magnetic field; and the modulation of vortex entry and exit energy barriers. These two mechanisms combine to regularly inhibit vortex motion, reducing the resistance, and allow vortex motion, increasing the resistance. Pinning is found to dominate the characteristics of the OMR effect. This effect is more apparent in comparisons between the two types of nanowire grown. The mechanism behind the OMR effect appears to be due to the probabilistic motion of vortices. These motion events can be assisted by thermal energy or current flow. A new MHz frequency cut off in the OMR is observed, which can be attributed to the vortex dynamics.

Item Type: Thesis (PhD)
Keywords: Oscillatory Magnetoresistance, Vortices, Topological, Superconductor, Insulator, Majorana
Academic Units: The University of Leeds > Faculty of Maths and Physical Sciences (Leeds) > School of Physics and Astronomy (Leeds)
Identification Number/EthosID: uk.bl.ethos.800500
Depositing User: Dr B D G Steele
Date Deposited: 06 Mar 2020 11:06
Last Modified: 11 Apr 2020 09:53
URI: http://etheses.whiterose.ac.uk/id/eprint/26235

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