Molecular beam epitaxy (MBE) grown metal has been a renewed area of interest recently in order to achieve high quality metal films or nanostructures for plasmonics. Recently MBE grown silver films have been shown to possess optical constants closer to that of intrinsic silver leading to lower losses and thus allowing for higher quality plasmonics. MBE has also been used to grow silver nanocrystals and indium droplets, or islands, for plasmonics. These self-assembled nanostructures can be grown in close proximity to quantum confined structures such as InAs/GaAs quantum dots or InGaAs/GaAs quantum wells in a single process, without post-processing and fabrication, allowing for increased plasmonic enhancement due to the improved interface between the semiconductor and plasmonic structures.In this dissertation, widely tunable plasmonic resonances of indium islands will be discussed and plasmonic enhancement results will be presented and compared to those of nanoantennas constructed from standard fabrication processes. The coupling between near-surface quantum confined structures, both fabricated and self-assembled, will be compared to the coupling in typical dielectric cavities, such as photonic crystal nanobeams. Beyond the plasmonic possibilities of indium islands, indium becomes superconducting at 3.4 K. With the proximity effect allowing for electrons in materials in contact with a superconductor to occupy a superconducting like state, allowing for the possibility for a hybrid superconductor/semiconductor optical source. The observation of superconductivity in indium islands will be presented and considerations for a superconductor/semiconductor source will be discussed.

The study of topological superconductivity has been at the forefront of condensed matter physics for the past few years. Topological superconductors are predicted to have odd parity pairing and host so called Majorana fermions, which are not only of fundamental importance, but also proposed to be building blocks for fault-tolerant quantum computing. In this dissertation, we use point contact spectroscopy to study the pairing symmetry of candidate topological superconducting materials. We study proximity induced superconductivity in the topological insulator Bi2Se3 by a superconducting niobium tip, and propose a model to explain its features in point contact spectra. We further study the nature of the superconductivity in highly doped superconducting topological insulators, including CuxBi2Se3 and Sn1-xInxTe, using both a normal metal gold tip and a superconducting niobium tip. For CuxBi2Se3, we observe a robust zero-bias conductance peak (ZBCP) in the differential conductance spectra with the gold point contact, while with the niobium point contact we find the height of the peak exhibiting unusual non-monotonic temperature dependence. We argue that both observations cannot be explained by Andreev reflection within the standard Blonder-Tinkham-Klapwijk (BTK) model, but signify unconventional superconductivity in the material. For Sn1-xInxTe samples, we observe ZBCP in the differential conductance spectra with the gold point contact, while with the niobium point contact, the temperature dependence of ZBCP is monotonic as expected from conventional theory, leaving the nature of the superconductivity of Sn1-xInxTe still an open question.

Thesis (MSc)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: Two problems in the field of materials-based condensed matter physics, specifically in the field of superconductivity, are studied theoretically. In both problems, where each is of current exper- imental interest, an extension of Ginzburg-Landau theory is used to describe a physical system, with focus on the energy associated to the interface(s) occurring in the respective systems. The first physical system under consideration is that of a two-band superconductor. Using Ginzburg-Landau theory for two-band superconductors, the interface energy ¾s between normal and superconducting states coexisting at the thermodynamic critical magnetic field is determined. From the theoretical and numerical analysis of the interface energy, it is found that close to the transition temperature, where the Ginzburg-Landau theory is applicable, the two-band problem maps onto an effective single band problem. This finding puts into question the possibility of intermediate, so called type-1.5 superconductivity, in the regime where the Ginzburg-Landau theory applies. The second physical system is that of a system with competing superconductivity and anti- ferromagnetism. From Ginzburg-Landau theory for such competing systems in a thermodynamic critical magnetic field, it is shown that two possible interfaces can occur: an interface between a pure anti-ferromagnetic state and a pure superconducting state; and an interface between a state with coexisting superconductivity and anti-ferromagnetism and a pure anti-ferromagnetic state. The energy associated to both these interfaces is analysed theoretically and numerically from which the boundary between type-I and type-II superconductivity is obtained for certain specific cases. / AFRIKAANSE OPSOMMING: Twee probleme in die veld van materiaal-gebaseerde gekondenseerde materie fisika, spesifiek in die veld van supergeleiding, word teoreties bestudeer. In beide probleme, albei tans van eksper- imentele belang, word ’n fisiese sisteem beskryf deur ’n uitbreiding van enkel-band Ginzburg- Landau teorie, met fokus op die energie geassosieer met die koppelvlak(ke) wat in die onderskeie sisteme aangetref word. Die eerste fisiese sisteem wat beskou word is die van ’n twee-band supergeleier. Deur van Ginzburg-Landau teorie vir twee-band supergeleiers gebruik te maak, word die koppelvlak energie ¾s tussen die gelyktydig bestaande normaal- en supergeleidende toestand in die termodinamiese kritieke magneetveld bepaal. Deur beide teoretiese en numeriese analieses word bepaal dat na aan die oorgangstemperatuur, waar Ginzburg-Landau teorie geldig is, die twee-band probleem op ’n effektiewe een-band probleem afbeeld. Hierdie bevinding bevraagteken dus die moontlikheid van onkonvensionele, of sogenaamde tipe-1.5 supergeleiding, vir gevalle waar Ginzburg-Landau teorie geldig is. Die tweede fisiese siteem wat beskou word is ’n sisteem met kompeterende supergeleiding en anti-ferromagnetisme. Met behulp van Ginzburg-Landau teorie vir sulke sisteme in ’n termod- inamiese kritiese magneetveld word gewys dat daar twee moontlike koppelvlakke kan ontstaan: ’n koppelvlak tussen ’n uitsluitlik anti-ferromagnetiese toestand en ’n uitsluitlik supergeleidende toestand; sowel as ’n koppelvlak tussen ’n uitsluitlik anti-ferromagnetiese toestand en ’n toes- tand van beide supergeleiding en anti-ferromagnetisme. Die energie geassosieer met beide hierdie koppelvlakke word teoreties en numeries geanaliseer wat lei tot ’n beskrywing van die grenslyn tussen tipe-I en tipe-II supergeleiding in sekere spesifieke gevalle.

Thesis (PhD)--University of Stellenbosch, 2003. / ENGLISH ABSTRACT: The development of a tool kit for the design of superconducting programmable gate arrays (SPGAs) is discussed. A circuit optimizer using genetic algorithms is developed and evaluated. Techniques and a program are also developed for the generation of segmentized 3D models with which to calculate inductance in circuit structures through FastHenry. The ability to add random variations to the dimensions of the models is included. These tools are then used to design novel latching elements that allow the construction of reprogrammable Rapid Single Flux Quantum (RSFQ) circuits. A circular process is used, whereby layouts are converted back to circuit diagrams through element extraction, and reoptimized if necessary. Two programmable frequency dividers are then designed; one for testing the routing and switch structures and programming architecture of an SPGA, and another compact one for testing the latching elements and off-chip interface. The dissertation concludes with an overview of the circuits necessary for the implementation of a fully functional SPGA. / AFRIKAANSE OPSOMMING: Die ontwikkeling van ’n gereedskapstel vir die ontwerp van supergeleier FPGA’s (SPGA’s) word bespreek. Eerstens word ’n stroombaanoptimeerder, wat met genetiese algoritmes funksioneer, ontwikkel en geëvalueer. Daarna word tegnieke en ’n program ontwikkel om driedimensionele segmentmodelle te genereer waaruit FastHenry die induktansie van stroombaanstrukture kan bepaal. Die vermoë om toevalsveranderinge by die dimensies van die modelle te voeg, is ook ingesluit. Hierdie gereedskap word dan gebruik om nuwe grendelelemente te ontwerp waarmee herprogrammeerbare Rapid Single Flux Quantum (RSFQ) stroombane gebou kan word. ’n Sirkulêre proses word gevolg, waarvolgens uitlegte na stroombaandiagramme teruggeskakel kan word (deur elementonttrekkings) en, indien nodig, heroptimeer kan word. Twee programmeerbare frekwensiedelers word daarna ontwerp; een om die pulsvervoer- en skakelstrukture, asook programmeringsargitektuur van ’n SPGA te toets, en ’n ander, kompakter een om die grendelelemente en warmlogika koppelvlakke mee te toets. Die proefskrif sluit af met ’n oorsig oor die stroombane benodig vir die implementering van ’n volledig funksionele SPGA.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2006. / Integrated circuits are expensive to manufacture and it is important to verify the correct operation of a circuit before fabrication. Efficient, though accurate, parameter extraction of post-layout designs are required for estimation of circuit success rates. This thesis discusses electrical netlist and fast parameter extraction techniques suited for both intraand inter-gate connections. This includes the use of extraction windows and look-up tables (LUTs) for accurate inductance and capacitance estimation. These techniques can readily be implemented in automated layout software where fast parameter extraction is required for timing analysis and gate placement.

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

<p>The objective of this thesis is to evaluate a generic process for fabrication and characterization of the Superconductive coplanar waveguide (CPW) resonators. Superconductive CPW resonators with various lengths and shapes are designed to investigate their electrical and magnetic properties as well as resonance properties and sensitivities. In the first part of thesis, two different models are introduced in order to estimate the nonlinear kinetic inductance of a superconducting CPW resonator. The first model is based on Bean critical-state model and the second one is based on current dependence of London penetration depth. The existence of a shift in resonant frequency  of Superconductive CPW resonator caused by a non-linear kinetic inductance is also shown experimentally. Simulations were carried out to estimate the nonlinear kinetic inductance due to the self- induced magnetic field penetration.</p><p>The rest of the thesis is concerned with development of very smooth Aluminum (Al) thin films with RMS (Root Mean Square) roughness 1~nm and CAD (Computer Aid Design) of superconductive CPW resonators. Experimental investigation of a generic fabrication technique for superconductive CPW resonator is carried out. Many resonators are fabricated with different design parameters, such as centerline or gap width, film thickness and gap capacitors length. The fabrication process is described in detail. Electron Beam Lithography is used to fabricate Nb and Al CPW resonators which are coupled to outer conductors via gap capacitors. We have fabricated GHz frequency CPW resonators with quality factors, Q up 5X10^5.</p>

No description available.

No description available.

No description available.