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Title: Design considerations for weak links made of boron doped diamond & simulations for the interaction between a dc SQUID and an integrated micromechanical doubly clamped cantilever
Author: Salman, Majdi
ISNI:       0000 0005 0285 5712
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2020
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As a part of a wide and term project, where quantum micro and nano-electromechanical (MEMS and NEMS) implemented in a superconducting quantum interference device (SQUID) made of boron doped diamond (BBD) to be explored, experimental and theoretical investigations, and solutions for related technical issues linked to these investigations, are presented in this thesis. Experimentally, current-voltage, I(V), characteristics and the differential resistance measurements have been performed for SIS tunnel junctions, and nanobridge devices made of BBD. On the basis of analyses of these measurements, temperature dependence of the critical current of a nanobridge device with bridge dimensions of L = 118 nm, and W = 109 nm, was attributed to the proximity effect described by Likharev’s theory of SNS weak-link junctions. Furthermore, temperature dependence of I(V) characteristics between 20 and 700 mK for another device with W=108 nm, and L=78 nm, shows resistive steps in the transition region of the I(V) curves around Ic. As phase slip events may arise due to vortex kinematics and the granularity of the superconducting films, the observed steps have been attributed to vortex kinematics, and granularity of BBD films from which the device have been fabricated. On the other side, for SIS junctions, fitting for I(V) characteristics measured for a typical SIS junction of a 6 nm vacuum gap, shows good agreement with the RSCJ model. An SIS junction with a wide vacuum gap of 76 nm, have been also investigated, where the measured temperature dependence of the I(V) characteristics of the junction, shows two transitions, the first transition, Ic1(T) was observed in the superconducting region, while the second one, Ic(T), occurs just before the normal state. The transition Ic(T) was attributed to Ambegaokar and Baratoff formula and BCS theory. Other measurements for I(V), and R(T) curves of superconducting strips of different strictures, have been performed. The measurements show resistive steps around transition regions of the I(V) curves, and corresponding spikes have been observed in the R(T) curves. Such behaviour is attributed to a collective effect that involves kinematic vortices, thermal fluctuations and/or quasiparticles diffusion (SBT model), and the granularity of BBD films from which the strips have been made. Technically, as superconducting devices are being influenced by a considerable amount of noise such as radio frequency (RF) noise, high performance RF filters were developed and fabricated. As results, the fabricated filters have shown that they are quite competitive to the earlier RF filters, efficient tools to attenuate the RF noise, and appropriate to be used for simultaneous measurements that take place in a cryogenic system where the temperatures down to few mK. In terms of theoretical aspect, simulations to quantitatively describe the interaction between a dc SQUID and an integrated doubly clamped cantilever were performed, where the unscaled dc SQUID equations coupled to the equations of motion of an integrated cantilever, have been numerically solved. In these simulations, an existing experimental configuration was selected to explore the motion of the integrated cantilever, and the voltage-displacement traces of a displacement detector were determined. Furthermore, the effect of the back-action between the SQUID and the doubly clamped cantilever have been analysed via the shift in the cantilever frequency, the line width, intensity, and shift in the position of the normal state. The simulations show how a sharp transition state drives the system into a nonlinear-like regime, and modulates the cantilever displacement amplitude, by tuning the bias current, and the external flux, which set the system in different regions of voltage-flux curve.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics