Flexible Transmission Lines and Asymmetrically Counter-Poised Monopoles

In the pursuit of miniaturised antennas and flexible transmission lines, several techniques have been explored in the open literature to address physical aspects of size, weight, flexibility and electrical reconfigurability, all whilst sustaining a reasonable degree of electrical operation. In the first part of this thesis, these themes are further explored with transmission lines, through the experimentation of transmission lines that use standard, readily available materials of a conformal nature, that would appreciably suit our context of operation. Two distinct types of transmission lines are designed, simulated, investigated and reported on. In the second half of this thesis, we explore asymmetrical antenna design in relation to antenna miniaturisation, aiming at creating a design method for this type of antennas. The first transmission line examined in the first part of the thesis, is what we now refer to as a Wire-Over-Ground-Plane transmission line. Structurally, this is standard gauge wire, placed over a polyimide sheet beneath which exists ground plane. A model of this transmission line is created and thereafter numerically simulated. A rough prototype of this design was created to validate operation. Thereafter, proposed is the design with realistic simulations of a branch-line coupler and modified Marchand balun, using this technology. The second transmission line created, is a stripline transmission line constructed primarily from fleece and a conductive textile. Once more, this structure is numerically simulated to validate operation prior to construction. Thereafter a number of samples are created to explore the physical robustness of the connection, through the application of mechanical stress and strain of varied transmission line constructions. In the second part of the thesis three antennas were created taking an asymmetrical approach to antenna realisation. The first Counter-Poised monopole antenna is experimentally realised by replacing one of the dipole arms with a coil of equivalent inductance. Decent performance here led to a second antenna design that builds on the first asymmetrical design, implementing a planar integrated balun into the feed-structure and developing a planar counter-poise fabricated on a circuit board. The third antenna design, builds further on the second design by adding a frequency reconfigurability feature through the addition of active circuit elements to the counter-poise. As we worked from developing the experimental antenna to realising the frequency reconfigurable variant, we have sought to understand the principle of operation and establish a design method which has allowed for the development of an advanced reconfigurable variant. The counter-poise is the fulcrum of all three asymmetrical antennas designed here, so a thorough grasp on its design and operation is required to ensure adequate antenna operation. In summation, this thesis develops ideas and realisations of flexible transmission lines using standard off-the-shelf components as well as conductive textiles and clothing. Further, asymmetrical antenna design techniques are explored leading to antenna miniaturisation. Thereafter, design methods are developed to aid in planar fixed-frequency implementation, whereupon a more advanced frequency reconfigurable variant is created.Thesis (MPhil) -- University of Adelaide, School of Electrical and Electronic Engineering, 202

InAs-Al Hybrid Devices Passing the Topological Gap Protocol

We present measurements and simulations of semiconductor-superconductor heterostructure devices that are consistent with the observation of topological superconductivity and Majorana zero modes. The devices are fabricated from high-mobility two-dimensional electron gases in which quasi-one-dimensional wires are defined by electrostatic gates. These devices enable measurements of local and non-local transport properties and have been optimized via extensive simulations for robustness against non-uniformity and disorder. Our main result is that several devices, fabricated according to the design's engineering specifications, have passed the topological gap protocol defined in Pikulin {\it et al.}\ [arXiv:2103.12217]. This protocol is a stringent test composed of a sequence of three-terminal local and non-local transport measurements performed while varying the magnetic field, semiconductor electron density, and junction transparencies. Passing the protocol indicates a high probability of detection of a topological phase hosting Majorana zero modes. Our experimental results are consistent with a quantum phase transition into a topological superconducting phase that extends over several hundred millitesla in magnetic field and several millivolts in gate voltage, corresponding to approximately one hundred micro-electron-volts in Zeeman energy and chemical potential in the semiconducting wire. These regions feature a closing and re-opening of the bulk gap, with simultaneous zero-bias conductance peaks at {\it both} ends of the devices that withstand changes in the junction transparencies. The measured maximum topological gaps in our devices are 20-$30\,\mu$eV. This demonstration is a prerequisite for experiments involving fusion and braiding of Majorana zero modes.Comment: Fixed typos. Fig. 3 is now readable by Adobe Reade