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Non-resonant microwave absorption (NRMA) in superconducting materials has become a new experimental technique to probe and understand superconducting materials. For example cuprate superconductors are well studied with this technique. At the same time the technique is also evolving. This technique (NRMA) has been used to study magnetic shielding effects/boundary current in Ba0.34K0.64Fe2As2 (BaK122) single crystals of iron pnictides superconducting sample measured at 9.4 GHz below TC (4.2K-32K). It has been observed that a small modulation field used in NRMA experiment yield the boundary current response. We have established that the boundary current response depends on both modulation amplitude and the temperature. At high modulation field amplitudes and temperatures close to Tc the boundary current response gets suppressed and flux modulated response dominates. At low temperatures far away from Tc, only the boundary current response dominates. / Physics / M. Sc. (Physics)

published_or_final_version / abstract / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

Quantum computing has been a new and challenging area of research since the concept was put forward in 1980s. A quantum computer is a computer that processes information encoded in systems that exhibit quantum properties and is proved in theory to be more powerful than classical computers. Various approaches to the implementation of the quantum computers have been studied over the decades, each of them having their own advantages and disadvantages in terms of the lifetime of the quantum information, processing time, and scalability of the implementation. Proposals for hybrid quantum processors are interesting because they benefit from the advantages of each comprising system, and thus providing a promising approach to a practical quantum computer. In this thesis, I demonstrate experimentally the principle of utilizing electron spin ensembles as a quantum memory for hybrid quantum processors. I demonstrate the storage and on-demand retrieval of multiple bits of quantum information into and from a single electron spin ensemble by applying magnetic field gradient pulses. I then study the coupling between an electron spin ensemble and a three-dimensional microwave cavity, in the aim of discussing the condition for the coherent information transfer between the excitations in solid-state matter and photons. As an alternative to the high power pulses in electron paramagnetic resonance (EPR), I study the possibility of controlling the electron spin states via adiabatic processes. I demonstrate the implementation of adiabatic geometric phase gates in electron spins and compare their performances to other phase gates achieved with microwave pulses in both simulation and experiment, verifying the robustness of the adiabatic gates against certain type of noises. Finally I present the simulation method developed for simulating the pulsed EPR experiments in this thesis, using a model more general than some currently-existing simulation packages.

Part I. The syntheses of a series of stable ferrocenylsilane compounds and their corresponding polyradical cations are reported. Electron spin properties of these molecules were investigated by cyclic voltammetry, ESR, and magnetic susceptibility measurements. All the compounds presented, showed significant electronic communication (>100 mV) between the redox centers by CV. Part II. Baeyer-Villiger oxidation of (1,9-dimethyl-PCU-8,11-dione) was performed using m-chloroperoxybenzoic acid in 1:2 molar ratios. The product obtained was the corresponding dilactone 113. The structure of the reaction products was established unequivocally via single crystal X-ray diffraction methods. The reaction of the 1,9-dimethyl-PCU-8,11-dione with 1:1 molar ratio of m-chloroperoxybenzoic acid produced again the dilactone 113, and not the expected monolactone 114. Ceric ammonium nitrate (CAN) promoted oxidation reaction of 1,9-dimethyl-PCU-8,11-dione afforded a mixture of dimethylated lactones, which indicated unique reaction mechanism pathways. These individual isomers, 115 and 116, have been isolated from these mixtures via column chromatography by using silica gel as adsorbent followed by fractional recrystallization of individual chromatography fractions. Structures of these pure products have been established unequivocally by application of single crystal X-ray crystallographic methods.

A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 2016 / The theoretical and experimental investigation of electron spin-resonance relaxation to deposit thermal energy into liquid gadolinium-based contrast agents for cancer hyperthermia treatment is presented. Previous works suggest that using protons in water are inadequate, with a thermal deposition rate of approximately 1 ◦C per two years. A novel component of this research relies on the use of gadolinium-chelated molecules, which are currently used as contrast agents in clinical MRI scans. The chelating agents, or ligands, investigated are Gadobenate (MultiHance R ), Gadopentetate (Magnevist R ), Gadoterate (DotaremR ) and Gadoteridol (ProHance R ). The gadolinium atom has seven unpaired electrons in its inner f shell orbital and as a result has a 660 times stronger paramagnetic response when placed in an external magnetic field. The research tests the hypothesis that by using an appropriate external homogeneous DC magnetic field, together with a radiofrequency excited resonator, that a measurable amount of thermal energy is deposited into a liquid gadolinium-based contrast agent. The aim of this research is to ultimately discover a new cancer hyperthermia treatment. The research theory suggests that a temperature rate of 13.4 ◦C · s−1 can be achieved using the gadolinium-based contrast agents under certain experimental conditions, and a maximum of 29.4 ◦C · s−1 under more optimal conditions. The temperature rates are calculated using parameter values commonly found in literature and practice. The simulation and design of the DC magnetic field coil system is discussed, together with the simulation results and design parameters of the radiofrequency loop-gap resonator. The experimental results and analysis indicate that the selected contrast agents have varied responses based on their chemical nature and that only two out of the four contrast agents, Dotarem and ProHance, show a measurable effect albeit sufficiently small that statistical techniques were necessary to distinguish the effect from background. A model fit to the data is performed in order to determine the spin-lattice relaxation time of the contrast agents under the specified experimental conditions. The model estimate is significantly smaller than the values found in literature under similar conditions, with a spin-lattice relaxation time τ1e of approximately 0.2 ps compared to the literature value of 0.1 ns. Although the observed electron spin resonance heating rate is in the milli-Watt range it is still notably larger (167 000 times) compared to the heating rate obtained using protons. The low temperature rates suggest that a more suitable agent or molecule with a larger spin-relaxation time be used, in order to achieve clinical useful temperature rates in the range of 14 ◦C · s−1. / MT2017

A survey of the optical and electron paramagnetic resonance (E.F.R.) absorption in a large number of diamonds from all major sources of production has revealed that perfect diamond is virtually non-existent. One or more of eleven different types of defect centres is found in each specimen. The presence or absence of nitrogen has long been known to give rise to the distinctive properties of Type I and Type II diamond.The present survey has shown that the form in which the nitrogen is present is significant. In most specimens the nitrogen is present in substitutional, non-paramagnetic platelet form, and these specimens were classified as Type Ia diamonds. A small group of transparent natural diamonds was found to contain dispersed paramagnetic nitrogen. The optical properties of these diamonds are unlike those of other diamond types and have hitherto not been reported. It is proposed that these diamonds be classified as Type lb.Three new systems of E.P.R. lines were found in Type Ib diamond. They are shown to be due to: (i) 13C atoms situated in different positions relative to the substitutional nitrogen, (ii) interaction of the small quadrupole moment of the nitrogen with the electric field gradient, and (iii) the presence of the l5N isotope. Synthetic diamonds are found to be exclusively of the Type Ib variety, whereas natural Type Ib diamond are rare exceptions. This is attributed to the growth history of the specimens. In order to investigate the defect centres associated with nitrogen in diamond, Type Ia and Ib diamond were irradiated with 0.78 MeV electrons. The effects observed were complicated and therefore led to a general investigation of irradiation damage, and the annealing of irradiation damage in diamond. In addition to the G.R.l and U.V. bands induced in all diamond by irradiation damage, another optical absorption feature, the N.D.l band, is found in all Type Ia diamond after irradiation and limited heating. It is suggested that the N.D.l centre arises from the combination of a carbon interstitial, and nitrogen in platelet form, and that the other primary product of irradiation damage, a vacancy, is responsible for both the G.R.l and U.V. bands. The N.D.l centre acts as an acceptor, the G.R.l centre as a donor. In the ionized state G.R.l is inactive in optical absorption; N.D.l is active. Electron transfer by thermal excitation results in the bleaching of G.R.l and the enhancement of N.J.l. Illumination with light in the N.D.l band causes electron transfer in the reverse direction, restoring band strengths to their former condition. A model is proposed which defines the energies within the forbidden gap of the ground and excited states of G.R.l and N.D.l. On heat treatment at temperatures of 500°C and above, the G.R.l band in all diamonds anneals out. The rate of annealing, however, is founa to be dependent on the nitrogen concentration. Thus in Type IIa diamond (which contains no nitrogen) G.R.l anneals very slowly, resulting in the formation of an absorption tail. In Type Ia diamond G.R.l anneals much faster (the actual rate depending on the nitrogen concentration), and two optical absorption bands, 5032A and H2, are formed. It is proposed that the vacancy in diamond becomes mobile at about 500° C, and that the G.R.l band in Type IIa diamond anneals because of the agglomeration of vacancies, which results in the formation of defects responsible for the absorption tail. In Type Ia diamond the nitrogen platelets are ideal sinks for vacancies, because the lattice on either side of a platelet is in compression. G.R.l therefore anneals more rapidly and 5032A centres are formed due to the combination of a vacancy and a nitrogen platelet. The nature of the H2 centre is much more obscure, but a possible explanation is that H2 centres are formed in addition, because the N.D.l centres (nitrogen platelets with embedded interstitials) also succeed in trapping vacancies. Vacancy/interstitial recombinations are prevented since these defects are pinned to different locations in the platelet region. In type Ib diamonds N. D.l centres were found to form at a lower temperature than in Type Ia diamond. It is suggested that the carbon interstitial in diamond is mobile, and combines with Substitutional nitrogen in isolated positions at temperatures below 250°C. In Type Ia specimens, where the nitrogen is segregated in platelets, this process only occurs at about 250°C, when the interstitial has enough kinetic energy to overcome the energy barrier preventing it from combining with nitrogen inside the platelet region where it will relieve strain. Because of the different substitutional nitrogen configuration, the energy levels of N.D.l centres in Type Ib diamond are such that electron transfer by thermal excitation from G.R.l to N.D.l occurs at room temperature. Most of the G.R.l centres are therefore permanently ionized and optically inactive. After heat treatment, a new band called the 6400A band is formed in irradiated Type Ib diamond. It is suggested that 6400A centres are formed by the combination of mobile vacancies with substitutional nitrogen in isolated positions. The 6400A band is therefore analogous to the 5032A band produced in Type Ia diamond. As expected no analogue of the H2 is formed in Type Ib diamond, as both an interstitial and a vacancy cannot co-exist in combination with a single isolated substitutional nitrogen atom.

Neste trabalho são apresentados o desenvolvimento e construção de um sistema de microondas para espectroscopia paramagnética eletrônica em baixa freqüência (bandas L e S). Basicamente o sistema é composto de um oscilador que opera na faixa de 1,0 a 2,0 GHZ, uma cavidade do tipo Bridged Loop-Gap, acopladores direcionais, circuladores, atenuadores, detetor, sistemas de controle automático de freqüência CAF e pré-amplificador. Todo o arranjo compõe uma ponte de microondas que é acoplada à unidade de processamento e registro do espectrômetro VARIAN (E-109) do Laboratório do grupo de biofísica do IFQSC. No sistema apresentado os estágios de amplificação de baixo ruído, circuito de CAF, pré-amplificador e sistema ressonante foram desenvolvidos e implantados localmente. A conveniência em se utilizar um espectrômetro de RPE em baixa freqüência é permitir o estudo de amostras que apresentam características espectrais vindas das interações hiperfinas e superhiperfinas parcialmente resolvidas em freqüências mais altas. Também é possível obter-se parâmetros de energia de campo zero de sistemas de spin com S &#62 1 se o sistema puder ser tratado por métodos aproximativos em banda X. Um exemplo utilizando o íon Cr3+ no garnet CGGG é mostrado neste trabalho / In this work a microwave bridge for electron paramagnetic resonance spectroscopy hás been developed. The system is suitable for operation at low frequency, basically L and S-band. It is composed of an oscilator with a 1.0 to 2.0 GHz bandwith, a Bridged loop-gap resonator, directional couplers, circulators, attenuators, reverse diode detector, AFC sub-system and a pré-amplifier. All these modules are assembled to a constitute a microwave bridge, which is coupled to the processing and register unit of the VARIAN E-109 EPR spectometer of the Biophysics Laboratory at IFQSC. The low-noise amplifier, AFC sub-system, pre-amplifier and the Bridged Loop-Gap resonator have been entirely developed in this work. The advantage in the use of the low frequency EPR spectometer is that it allows the study of samples which present paramagnetics structures that are obscured by g-strain at high frequency (X, Q band). These structures come out from hiperfine and superhiperfine poorly resolved in the high frequency range that could be conventienly resolved at low frequency. It is also possible, to obtain the zero field splitting parameters in spin systems with S &#62 1, if a perturbative approximation is useful, comparing the spectre in X and L band. One example of this new approad using Cr3+ as impurity in the CGGG garnet is presented.