Investigations of quantum phase slips in superconducting nanocrystalline diamond devices

Megmmi, Yehya 2024. Investigations of quantum phase slips in superconducting nanocrystalline diamond devices. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

This thesis investigates the fluctuation phenomena in microbridges, nanobridges, strips, and rings made from Boron-Doped Nanocrystalline Diamond (BDNcD) using fluctuation spectroscopy. These devices, designed with varying widths and geometries, aim to explore dimensional transitions in the fluctuation conductivity of Cooper pairs, transitioning from three-dimensional to quasi-zero-dimensional regimes. Temperature-dependent resistance measurements are performed across a range of excitation currents from 0.005 microamperes to 1 microampere and temperature ranges spanning from 300 kelvin to 400 millikelvin. All devices exhibited behaviour consistent with the Lerner-Varlamov-Vinokur (LVV) model, which was used to interpret the fluctuation phenomena observed in BDNcD near the superconducting transition temperature. Additionally, this study examines quantum and thermal phase slips within BDNcD. These phase slips are explained using the Langer-Ambegaokar-McCumber-Halperin (LAMH) theory for thermally activated phase slips and Giordano's model for quantum tunnelling. The results highlight the critical role of phase slips in the behaviour of BDNcD and suggest significant implications for its use in superconducting quantum interference devices (SQUIDs) and other quantum technologies. Several types of Josephson junction devices, including nanobridges, gaps, and point contact junctions were examined under varying temperature, AC modulation current, and magnetic field. The differential resistance peaks were found to decrease as the AC modulation current amplitude increased, which was attributed to the closing of the superconducting gap under the influence of the gate voltage. The temperature dependence of both the superconducting gap and the critical currents aligned well with the BCS theory and the Kulik-Omelyanchuk 1 model, respectively. However, the response of the critical current peaks to changes in the magnetic field did not follow the expected Fraunhofer diffraction pattern, which is typically observed in such junctions. This deviation was attributed to the granularity of the material. The findings regarding the occurrence of quantum phase slips have significant implications for SQUIDs. Specifically, the suppression of SQUID oscillations in BDNcD can be caused by quantum phase slips.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Physics and Astronomy
Subjects:	Q Science > QC Physics
Uncontrolled Keywords:	Boron-Doped Nanocrystalline Diamond SQUID, fluctuation spectroscopy, superconducting transition, Cooper pairs, quantum phase slips, thermally activated phase slips, Josephson junctions, SQUID, quantum tunnelling.
Funders:	Self funded
Date of First Compliant Deposit:	10 March 2025
Last Modified:	10 Mar 2025 15:52
URI:	https://orca.cardiff.ac.uk/id/eprint/176757

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