Although the pairing mechanism in MgB2 is thought to be phonon mediated, there are still many experimental results that lack appropriate explanation. For example, there is no consensus about the magnitude of the energy gap, its temperature dependence, and whether it has only one-gap or not. Many techniques have been used to investigate this, like Raman spectroscopy, farinfrared transmission, specific heat, high-resolution photoemission and tunneling. Most tunneling data on MgB2 are obtained from mechanical junctions. Measurements of energy gap by these junctions have many disadvantages like the instability to temperature and field changes. On the other hand, sandwich-like planar junctions offer a stable and reliable measurement for temperature dependence of the energy gap, where any variation in the tunneling spectra can be interpreted as a direct result from the sample under study. To the best of our knowledge, we report the first energy gap temperature- and magnetic field-dependence of MgB2/Pb planar junctions. Study of the temperature-dependence shows that the small gap value (reported by many groups and explained as a result of surface degradation) is a real bulk property of MgB2. Moreover, our data is in favor of the two-gap model rather than the onegap, multi-gap, or single anisotropic gap models. The study of magnetic field effect on the junctions gave an estimation of the upper critical field of about 5.6 T. The dependence of energy gap on the field has been studied as well. Our junctions show stability against temperature changes, but "collapsed" when the magnetic field (applied normal to the junction barrier) is higher than 3.2 T. The irreversible structural change switched the tunnling mechanism from quisiparticle tunneling into Josephson tunneling. Josephson I-V curves at different temperatures have been studied and the characteristic voltages are calculated. The estimated MgB2 energy gap from supercurrent tunneling in weak link junctions agrees very well with that from quasiparticle tunneling. Reported properties on polycrystalline, single crystal and thin film MgB2 samples are widely varied, depending on the details of preparation procedure. MgB2 single crystals are synthesized mainly by heat treatment at high temperature and pressure. Single crystals prepared by this way have the disadvantages of Mg deficiency and shape irregularity. On the other hand, improving the coupling of grain boundaries in polycrystalline MgB2 (has the lowest normal state resistivity in comparison to many other practical superconductors) will be of practical interest. Consequently, we have been motivated to look for a new heat treatment to prepare high quality polycrystalline and single crystal MgB2 in the same process. The importance of our new method is its simplicity in preparing single crystals (neither high pressure cells nor very high sintering temperatures are required to prepare single crystals) and the quality of the obtained single crystal and polycrystalline MgB2. This method gives high quality and dense polycrystalline MgB2 with very low normal state resistivity (σ(40 ) = 0.28 cm). Single crystals have an average diagonal of 50 m and 10 m thickness with a unique shape that resembles the hexagonal crystal structure. Furthermore, preparing both forms in same process gives a great opportunity to study inconsistencies in their properties. On the other hand, magnesium diboride thin films have also been prepared by magnetron sputtering under new preparation conditions. The prepared thin films have a transition temperature of about 35.2 K and they are promising in fabricating tunnel junctions.

This thesis concerns itself with the scientific study of the recently discovered organic magnetoresistance (OMAR) whose underlying mechanism is currently not known with certainty. As an introduction, we briefly review the major findings from prior work done by my colleagues. They found that OMAR can be as large as ~10% magnetoresistance at 10 mT magnetic fields at room temperature. Both OMAR and other kinds of magnetic field effect data in organics can be fitted using the empirical laws B^2/(B^2+B_0^2) or B^2/(|B|+B_0)^2, dependent on material. The fitting parameter B_0 is a measure of the characteristic magnetic field strength of OMAR. We explore the dependence of B_0 on material parameters to clarify the origin of OMAR. Various pi-conjugated semiconductor OMAR devices were studied to explore the possibility that hyperfine interaction causes OMAR. For a quantitative analysis of the experiments, we developed a theoretical fitting formula to relate B_0 to the hyperfine coupling strength. In addition, organic materials with different spin-orbit coupling strengths were also measured. Fluorescence and phosphorescence spectroscopies were used to estimate the spin-orbit coupling strength from the measured spectra. For analyzing our measurements, we developed a fitting formula from the time-dependent Schrodinger equation that takes into account the combined effect of hyperfine and spin-orbit coupling on spin-dynamics. We found that in the case of strong spin-orbit coupling, it dominates the behavior, resulting in magnetic field effect traces that are much wider than those in ordinary organics. However, a small cone remains at zero field with a width equal to the hyperfine coupling strength. We find qualitative agreement between the experimental results and the model. We also investigated the question whether OMAR is related to an excitonic effect, or is primarily a transport effect. We measured the magnetic field effects on current, photocurrent and electroluminescence to address this question. By varying the injection efficiency of the minority carriers, we show that OMAR most likely is not an excitonic effect. Our results provide strong evidence in support of the claim that OMAR is caused by spin-dynamics. However, further study is required to study the mechanism connecting spin-dynamics and conductivity.

Magnetic field effects (MFE) on electrochemical systems have been of interest to researchers for the past 60 years. MFEs on mass transport, such as magnetohydrodynamics and magnetic field gradients effects are reported, but MFEs on electron transfer kinetics have been rarely investigated. Magnetic modification of electrodes enhances electron transfer kinetics under conditions of high concentrations and low physical diffusion conditions, as shown by Leddy and coworkers. Magnetic microparticles embedded in an ion exchange polymer (e.g., Nafion) applied to electrode surfaces. Rates of electron transfer reactions to diffusing redox probes and to adsorbates are markedly enhanced. This work reports MFEs on hydrogen evolution on illuminated p-Si; MFEs on hydrogen evolution on noncatalytic electrodes; a model for MFEs on homogeneous self-exchange reactions; and a convolution based voltammetric method for film modified electrodes. First, a MFE on the photoelectrochemical hydrogen evolution reaction (HER) at p-Si semiconductors is demonstrated. The HER is an adsorbate reaction. Magnetic modification reduces the energetic cost of the HER by 400 - 500 mV as compared to Nafion modified electrodes and by 1200 mV as compared to unmodified p-Si. Magnetically modified p-Si achieves 6.2 % energy conversion efficiency. Second, from HER on noncatalytic electrodes, the MFE on photoelectrochemical cells arises from improved heterogeneous electron transfer kinetics. On glassy carbon electrodes, magnetic modification improves heterogeneous electron transfer rate constant, k₀,for HER 80,000 fold. Third, self exchange reaction rates are investigated under magnetic modification for various temperatures, outersphere redox probes, and magnetic particles. Arrhenius analyses of the rate constants collected from the experiments show a 30 - 40 % decrease in activation energy at magnetically modified electrodes. A kinetic model is established based on transition state theory. The model includes pre-polarization and electron nuclear spin polarization steps and characterizes a majority of the experimental results. Lastly, a convolution technique for modified with uniform films electrodes is developed and coded in Matlab (mathematical software) for simple and straightforward analysis of Nafion modified electrodes.

Organic semiconductors (OSCs) have already been shown to have great potential to play an important role in the future of clean energy generation (organic solar cells) and provide energy efficient lighting (organic light-emitting diodes, OLED). Prior research has found that the light-emission efficiency of OLED is severely limited by the magnetic state (technically the spin-configuration) of the light-emission process. In this thesis, we work on the processes using external magnetic fields that can overcome these magnetic limitations. A major focus of this research is to enhance the performance of OLED, while at the same time to unravel the scientific mechanisms by which magnetic fields act on OSCs devices. Thermally activated delayed fluorescence (TADF) is a next-generation OLED emission technology which enables nearly 100% light-emission efficiency without using heavy precious metals. TADF characteristics depend on the probability of reverse intersystem crossing (RISC) from the triplet excited states (T1) to singlet excited states (S1). The conversion (T1 to S1) process depends strongly on spin dynamics, thus we predict a dramatic magnetic field effects (MFEs) in such TADF OLED devices. In subsequent experiments we observed that changes in TADF devices due to various forms of electrical stress can lead to enormous increases in magnetic field effects (MFEs) on the current (> 1400%) and electroluminescence (> 4000%). Our work provides a flexible and inexpensive pathway towards magnetic functionality and field sensitivity in current organic devices. Such OLED pave the way for novel magnetic sensitive OSCs devices with integrated optical, electronic and magnetic characteristics. Organic magnetoresistance (OMAR) has been observed to alter the current and efficiency of OLED without any ferromagnetic components. Here we utilizes slight alterations to the device properties, the addition of a radical-doped functional layer, in which the spin-relaxing effects of localized nuclear spins and electronic spins interfere, to address the assumption about the importance of the hyperfine interaction and to attempt to differentiate between the different models for OMAR. A feature where the magnitude of OMAR exhibits a plateau over a wide range of doping fraction was observed at all temperatures investigated. This phenomenon is well explained by a theory in which a single dopant spin strongly interacts, by exchange, with one of the bottleneck sites. A similar can be used to explain the efficiency increases observed in organic solar cells for certain doping fractions.

It has been found that non-magnetic organic semiconductors can show some magnetic responses in low magnetic field (<100 >mT). When applying magnetic field, the electroluminescence, electrical current, photocurrent, and photoluminescence could change with magnetic field, which are called magnetic field effects. Magnetic field effects are generated through spin-dependent process affected by the internal magnetic interaction. In nonmagnetic materials, hyperfine interaction has been supposed to dominantly affect the spin-dependent process recently. But the conclusion was made in weak spin-orbital coupling organic semiconductor. The hyperfine interaction might not be the main reason responsible for magnetic field effects in strong spin-orbital coupling materials. Therefore, the study of magnetic field effects in strong spin-orbital coupling organic semiconductor is important to get a whole view of the origin of the magnetic field effects in nonmagnetic organic semiconductors. This dissertation will clarify the generation mechanism of magnetic field effect in nonmagnetic organic semiconductors and further explore how the strong spin-orbital coupling affecting the magnetic field effect. It has been found the intermolecular excited states are important inter-median for magnetic field effects. The change of intersystem crossing at intermolecular excited states will change the singlet/triplet ratio and further generate magnetic field effects through different recombination and dissociation properties of singlet and triplet intermolecular excited states. Both the energy transfer effect coupled spin orbital coupling and energy transfer effect free spin orbital-coupling are discussed in the dissertation. The tuning of the magnetic field effect by adjusting the spin-orbital coupling is also established through distance effect and interface effect. It has been found that changing inter-molecular spin-orbital coupling is a critical factor to generate magnetic field effects in organic semiconductors. And the sensitivity of different magnetic field effects to strong spin-orbital coupling strength is depending on the final product. The internal magnetic interaction can be hyperfine interaction, spin orbital coupling and spin-spin interaction between electrons. The hyperfine interaction and spin orbital coupling are important in nonmagnetic organic semiconductors. But the electron spin-spin interaction is important in magnetic organic semiconductors. The magnetocurrent for magnetic and nonmagnetic organic semiconductors at different temperature has been compared.

No description available.

Superconducting Nanowire Single Photon Detectors (SNSPDs) are a promising technology for detecting single photon emissions with high efficiency and low noise. This detector class has numerous applications in quantum optics, communication, and sensing. One typical design for these devices is the hairpin structure, in which a superconducting nanowire is patterned into a meandering shape. The combination of academic research and interest from the industry is boosting the development of hairpin SNSPD devices to achieve high detection efficiency while maintaining fast response time and low jitter, requiring optimization of the device geometry, materials properties, and sophisticated readout electronics. This thesis qualitatively enquires about different device geometries, varying turn designs and features. Moreover, proposing a promising experimental setup with the potential of being scaled up to simultaneously test numerous devices with a varying magnetic field, driving the hairpin SNSPDs to their detection limit, and enabling further quantitative studies to deepen the understanding of the underlying mechanisms currently hindering the SNSPDs. Analyzing the acquired data draws results regarding the critical current and dark counts trends. Furthermore, at low magnetic field strength, the enquired devices are found to have their critical current enhanced. Moreover, comparisons are drawn among similar design structures. Furthermore, a discussion on manufacturing defects detrimental to the SNSPD performance is initiated. Finally, further studies on this topic adopting the presented method are encouraged to acquire additional quantitative results to be compared with theoretical models describing the thin superconducting structures. / Superconducting Nanowire Single Photon Detectors (SNSPD) är en lovande teknik för att detektera utsläpp av enstaka fotoner med hög effektivitet och lågt brus. Denna detektorkategori har många tillämpningar inom kvantoptik, kommunikation och avkänning. En typisk konstruktion för dessa anordningar är hairpin-strukturen, där en supraledande nanotråd är mönstrad i en slingrande form. Kombinationen av akademisk forskning och intresse från industrin ökar utvecklingen av hairpin-SNSPD-enheter för att uppnå hög detektionseffektivitet med bibehållen snabb responstid och låg jitter, vilket kräver optimering av enhetens geometri, materialegenskaper och sofistikerad avläsningselektronik. I denna avhandling undersöks kvalitativt olika anordningsgeometrier, varierande vridningsdesign och funktioner. Dessutom föreslås en lovande experimentell uppställning med potential att skalas upp för att samtidigt testa många anordningar med ett varierande magnetfält, vilket driver hairpin-SNSPD:erna till sin detektionsgräns och möjliggör ytterligare kvantitativa studier för att fördjupa förståelsen av de underliggande mekanismer som för närvarande hindrar SNSPD:erna. Analysen av de insamlade uppgifterna ger resultat när det gäller den kritiska strömmen och trenderna för mörkertalet. Dessutom visar sig de undersökta enheterna ha en ökad kritisk strömstyrka vid låg magnetfältsstyrka. Dessutom görs jämförelser mellan liknande konstruktionsstrukturer. Dessutom inleds en diskussion om tillverkningsfel som är skadliga för SNSPD:s prestanda. Slutligen uppmuntras ytterligare studier i detta ämne med den presenterade metoden för att få ytterligare kvantitativa resultat som kan jämföras med teoretiska modeller som beskriver tunna supraledande strukturer.

This thesis describes the development and application of condensed-phase cavity-based spectroscopic techniques - namely cavity ring-down spectroscopy (CRDS); cavity enhanced absorption spectroscopy (CEAS); broadband cavity enhanced absorption spectroscopy (BBCEAS) and evanescent wave (EW) variants of all three. The recently-developed cavity technique of EW-broadband cavity enhanced absorption spectroscopy (EW-BBCEAS) has been used—in combination with a supercontinuum source (SC) and a sensitive, fast readout CCD detector—to record of the full visible spectrum (400–700 nm) of a silica-liquid interfacial layer (with an effective thickness ca. 1 µm), at rapid acquisition rates (> 600 Hz) that are sufficient to follow fast kinetics in the condensed phase, in real time. The sensitivity achieved (A_min= 3.9 x 10^-5) is comparable with previous EW-CRDS and EW-CEAS studies, but the spectral region accessed in this broadband variant is much larger. The study of liquid|air interfaces using EW cavity-based techniques is also illustrated for the first time. The first application of BBCEAS to the analysis of microfluidic samples, flowing through a microfluidic chip, is illustrated. Proof-of-principle experiments are presented, demonstrating the technique’s ability to provide full visible broadband spectral measurements of flowing microfluidic droplets, with both high detection sensitivity (α_min < 10^-2 cm^-1) and excellent spatial and temporal resolution: an SC light source and sensitive, fast readout CCD allowed measurement repetition rates of 273 Hz, whilst probing a very small sample volume (ca. 90 nL). A significant portion of this thesis is devoted to demonstrating the powerful capabilities of CEAS, CRDS and BBCEAS in monitoring radical recombination reactions and associated magnetic field effects (MFEs) in solution. The efficacy of CEAS as a high-sensitivity MFE detection method has been established in a proof-of-principle study, using narrow band CEAS in combination with phase-sensitive detection: MFE-induced absorbance changes of ca. 10^-6 could be detected using the modulated CEAS technique and the data are shown to be superior to those obtained using conventional transient absorption (TA) methods typically employed for MFE measurements. The powerful capabilities of CRDS in monitoring radical recombination reactions and associated MFEs are also demonstrated. In particular, a pump-probe CRDS variant allows not only high sensitivity (A_min on the order 10^-6), but also sub-microsecond time-resolution. Combined, these features represent significant advantages over TA. Finally, SC-BBCEAS is used to measure full visible spectra of photoinduced reactions and their MFEs. The applicability of this approach to in vitro MFE studies of Drosophila cryptochrome is demonstrated—the results mark the first in vitro observation of a magnetic field response in an animal cryptochrome, a key result supporting the hypothesis that cryptochromes are involved in the magnetic sense in animals.

Magnetic fields influence the rate and/or yield of chemical reactions that proceed via spin correlated radical pair intermediates. The field of spin chemistry centres around the study of such magnetic field effects (MFEs). This thesis is particularly concerned with the effects of the weak magnetic fields B₀ ~ 1mT relevant in the ongoing debates on the mechanism by which animals sense the geomagnetic field and on the putative health effects of environmental electromagnetic fields. Relatively few previous studies have dealt with such weak magnetic fields. This thesis presents several new theoretical tools and applies them to interpret experimental measurements. Chapter 1 surveys the development and theory of spin chemistry. Chapter 2 introduces the use of Tikhonov and Maximum Entropy Regularisation methods as a new means of analysing MARY field effect data. These are applied to recover details of the diffusive motion of reacting pyrene and N,N-dimethylaniline radicals. Chapter 3 gives a fresh derivation and appraisal of an approximate, semiclassical approach to MFEs. Monte Carlo calculations allow the elucidation of several "rules of thumb" for interpreting MFE data. Chapter 4 discusses recent optically-detected zero-field EPR measurements, adapting the gamma-COMPUTE algorithm from solid state NMR for their interpretation. Chapter 5 explores the role of RF polarisation in producing MFEs. The breakdown in weak fields of the familiar rotating frame approximation is analysed. Chapter 6 reviews current knowledge and landmark experiments in the area of animal magnetoreception. The origins of the sensitivity of European robins Erithacus rubecula to the Earth’s magnetic field are given particular attention. In Chapter 7, Schulten and Ritz’s hypothesis that avian magnetoreception is founded on a radical pair mechanism (RPM) reaction is appraised through calculations in model systems. Chapter 8 introduces quantitative methods of analysing anisotropic magnetic field effects using spherical harmonics. Chapter 9 considers recent observations that European robins may sometimes be disoriented by minuscule RF fields. These are shown to be consistent with magnetoreception via a radical pair with no (effective) magnetic nuclei in one of the radicals.