Inexpensive Microscopy for Introductory Laboratory Courses

We present an inexpensive apparatus for bright field and fluorescence microscopy with video capture, suitable for introductory laboratory courses. Experiments on Brownian motion and the Boltzmann distribution of suspended particles in a gravitational field are described. The Boltzmann constant is measured in three ways, and the results fall within 15% of the accepted value.Physic

Development of Non-Destructive Testing by Eddy Currents for Highly Demanding Engineering Applications

Defect detection with Non-Destructive Testing (NDT) is essential in accidents prevention, requiring R&TD to generate new scientific and procedural knowledge for new products with high safety requirements. A current challenge lies in the detection of surface and sub-surface micro defects with NDT by Eddy Currents (EC). The main objective of this work was the development of applied research, technological innovation and experimental validation of EC customized systems for three highly demanding inspection scenarios: micro defects in tubular geometries; brazed joints for the automotive industry; and high-speed moving composite materials. This objective implied starting from the scientific fundamentals of NDT by EC to design and simulate EC probes and the prototypes developed were tested in industrial environment, reaching a TRL ≈ 5. Another objective, of a more scientific and disruptive nature, was to test a new technique for the creation of EC in the materials to be inspect, named Magnetic Permeability Pattern Substrate (MPPS). This technique consists on the development of substrates/films with patterns of different magnetic permeabilities rather than the use of excitation bobbin coils or filaments of complex geometry. The experimental results demonstrated that the prototypes developed for the three industrial applications studied outperformed the state of the art, allowing the detection of target defects with a very good signal-to-noise ratio: in tubular geometries defects with depth of 0.5 mm and thickness of 0.2 mm in any scanning position; in the laser brazed weld beads pores with 0.13 mm diameter and internal artificial defects 1 mm from the weld surface; in composite materials defects under 1 mm at speeds up to 4 m/s and 3 mm lift-off. The numerical simulations assisted the probe design, allowing to describe and characterize electrical and magnetic phenomena. The new MPPS concept for the introduction of EC was validated numerically and experimentally

Lorentz force sigmometry novel technique to measure the electrical conductivity of solid and fluid metals

Lorentz force sigmometry, "LOFOS," is a novel technique with three different configuration setups for measuring different physical properties of molten metals such as electrical conductivity, viscosity, and density. However, this thesis focuses on measuring the electrical conductivity of the solid and molten metals using the so-called mobile LOFOS setup. The interaction of an electrically conducting fluid with an externally applied magnetic field leads to a force that acts upon the magnetic field generating system and drags it along the flow direction[Thess:2007]. This force linearly depends on the electrical conductivity of the conducting fluid and can be measured using force sensors. The aim of this study was to test and successfully prove that LOFOS can measure the electrical conductivity of solid and molten metals. Several experiments were performed to achieve this goal, starting with experiments using three solid cylindrical bars made of copper, aluminum, and brass 300 mm in length and 10 mm in diameter. The first series of experiments were carried out with known electrically conductive metals, aluminum and copper, in order to compute the calibration factor of the device. We then used the same calibration factor to estimate the unknown electrical conductivity of a brass bar [Alkhalil: 2015]. The setup of LOFOS for solid measurements required some technical changes as compared to the one for fluid measurements. This modification was necessary to extend the interaction time between the solid bar and the external magnetic field generated by a Halbach cylinder magnet. The second series of experiments were with molten metals. The first series with known physical properties alloy having the composition of Ga67In20.5Sn12.5. This is a eutectic alloy at room temperature, and its melting temperature is Tm = 10.5 °C whereas the second series was carried out with high-temperature molten tin at Tm = 232 °C. For fluid measurements, we fabricated a special quartz conical vessel able to withstand temperatures ranging from room temperature up to 1000 °C. The nozzle had a diameter of 8 mm, and it allowed the flow of molten metal across the magnet system during some seconds, which is reasonable for measuring the Lorentz force with good accuracy. In order to protect the LOFOS force measurement system from high temperatures, we added an external air compressor pump to the LOFOS setup. Both experiments with solid and molten metals prove that Lorentz force sigmometry is able to measure the electrical conductivity of solid and molten metals. The uncertainty of solid measurements is in total up to 5%, while for molten metals measurements is less than 10% .Lorentzkraft Sigmometrie „LOFOS“ ist eine neuartige Technik zur Messung unterschiedlicher physikalischer Eigenschaften von Flüssigmetall, wie der elektrischen Leitfähigkeit, Viskosität oder Dichte. Der Fokus dieser Arbeit liegt auf der Messung der elektrischen Leitfähigkeit von festen und flüssigen Metallen mit dem sogenannten mobilen LOFOS. Diese Technik basiert auf den Grundsätzen der Magnetohydrodynamik: Durch die Bewegung eines elektrischen Leiters in einem von außen angelegtem Magnetfeld werden Wirbelströme innerhalb des Leiters induziert. Nach dem Ampereschen Gesetz erzeugen diese Wirbelströme wiederrum ein sekundäres Magnetfeld. Durch Wechselwirkung zwischen den Wirbelströmen und dem magnetischen Gesamtfeld entstehen Lorentzkräfte, die den Fluss abschwächen. Nach Newtons drittem Gesetz (actio und reactio) wirken die Lorentzkräfte auch auf den Magneten mit gleichem Betrag, aber in entgegengesetzter Richtung [Thess:2007]. Diese Kraft kann gemessen werden und ist proportional zu der elektrischen Leitfähigkeit des flüssigen oder festen Leiters. Diese Tatsache wird für die kontaktlose Geschwindigkeitsmessung „Lorentz force velocimetry“ und für die Erkennung von Defekten in Festkörpern „Lorentz force eddy "current testing“ genutzt. Ziel dieser Arbeit ist es zu testen und zu zeigen, dass LOFOS erfolgreich die elektrische Leitfähigkeit von festen und flüssigen Metallen messen kann. Mehrere Experimente wurden durchgeführt um dieses zu zeigen, beginnend mit festen Zylindern aus Kupfer, Aluminium und Messing, die eine Länge von 300 mm und einem Durchmesser von 10 mm haben. Die ersten Experimente wurden an Metallen mit bekannter elektrischer Leitfähigkeit durchgeführt (Aluminium und Kupfer), um den Kalibrierungsfaktor zu ermitteln. Dieser Kalibrierungsfaktor wurde dann benutzt um die Leitfähigkeit eines Zylinders aus Messing zu messen [Alkhalil: 2015]. Die Anordnung von LOFOS für die Messung an Festkörpern bedarf einiger technischer Veränderungen im Vergleich zu der Messung an Fluiden. Diese Veränderungen sind nötig um die Zeit der Wechselwirkung zwischen Festkörper und externen magnetischen Feld, welches durch einen Halbach Zylinder Magneten erzeugt wird, zu verlängern. Die zweite Reihe an Experimenten erfolgte mit Flüssigmetall. Zu Beginn wurde die Legierung Ga67In20.5Sn12.5 verwendet, deren physikalische Eigenschaften bekannt sind. Diese eutektische Legierung hat einen Schmelzpunkt von 10.5 °C und ist daher flüssig bei Raumtemperatur, wohingegen die zweiten Versuche mit flüssigem Zinn durchgeführt wurden, welches einen Schmelzpunkt von 232 °C hat. Für die Strömungsmessungen fertigten wir einen speziellen kegelförmigen Behälter aus Quarz an, welcher Temperaturen von Raumtemperatur bis zu 1000 °C aushält. Die Düse hat einen Durchmesser von 8 mm und ermöglicht den Durchfluss von flüssigem Metall durch das Magnetsystem in Dt=5s, was notwendig für die Messung der Lorentzkraft mit hoher Genauigkeit ist. Um das LOFOS Lorentzkraft Messsystem vor hohen Temperaturen zu schützen, haben wir eine externe Luftkompressor Pumpe hinzugefügt. Beide Experimente bestätigen, dass Lorentzkraft Sigmometrie die elektrische Leitfähigkeit von festen und flüssigen Metall ermitteln kann. Der Fehler für die Messungen an Festkörpern beträgt bis zu 5%, für die Messungen an flüssigem Metall bis zu 10%

Fermi Surfaces and Where to Find Them: Quantum Oscillations in Kondo Insulators and High-Temperature Superconductors

The Fermi surface is a geometric concept that codifies the momenta of all electrons at the Fermi level. It is these electrons that underpin most physical properties of metals, and have made quantum oscillations, the experimental manifestation of the Fermi surface, a hallmark signature of metals. In this thesis we have studied systems that are distinctly non-Fermi liquid, and discovered that, contrary to this canon, quantum oscillations occur even in the absence of a Fermi liquid. We present the high magnetic field studies of two classes of correlated electron systems. We survey the striking quantum oscillations in the magnetisation of SmB₆ and YbB₁₂, two strongly-correlated Kondo insulators. We also present magnetic and transport measurements of underdoped YBa₂Cu₃O₆₊ₓ, a high-temperature superconductor, that prompt us to reinterpret the quantum oscillations previously associated with the non-superconducting normal state. The surprising observation of quantum oscillations in the magnetisation of Kondo insulating SmB₆, but unaccompanied by oscillations in the electrical resistance, has attracted much attention. Here, we detail magnetic torque measurements that establish the intrinsic, bulk nature of the quantum oscillations, and reveal a moderate angular dependence of the oscillation frequencies, characteristic of a bulk, three-dimensional Fermi surface. We identify a finite linear specific heat coefficient down to the lowest temperatures, a distinguishing feature between metals and insulators. We demonstrate that the measured finite linear specific heat coefficient is in good agreement with the density of states at the Fermi level estimated from quantum oscillations. The unconventional nature of the ground state of SmB₆ is further evidenced by a non-zero thermal conductivity that is enhanced in a magnetic field. Through an extensive suite of characterisation techniques we confirm the high purity of our single crystals, with material properties consistent with an impurity concentration of less than 0.05%, and therefore further establishing the intrinsic character of the observed quantum oscillations. In the search for other non-Fermi liquids that are host to a Fermi surface, we identify YbB₁₂ as the second Kondo insulator that exhibits intrinsic, bulk quantum oscillations. We present a detailed study of the de Haas-van Alphen oscillations, corresponding to a heavy semimetal Fermi surface. Our results show many similarities with the ground state of SmB₆, including the large absolute size of the quantum oscillations and a finite linear specific heat coefficient, but also some key differences, namely the heavy effective masses and the proximity to a magnetic-field-induced or applied-pressure-induced insulator-metal transition. The observation of quantum oscillations in underdoped YBa₂Cu₃O₆₊ₓ refocused efforts to understand the pseudogap ground state of cuprate superconductors. Distinct from the large hole orbits of the Fermi liquid-like overdoped regime, the pseudogap regime was found to be characterised by a small electron pocket and the absence of antinodal states. A proposal associated the quantum oscillations with a conventional metallic state that emerges at a magnetic field of ∼20 T, however magnetic and thermal measurements have been at odds with the destruction of the superconducting order parameter at such modest magnetic fields. We employ high-magnetic fields to explore the region characterised by quantum oscillations, in search for the origin of the missing antinodal states in underdoped YBa₂Cu₃O₆₊ₓ, and the true extent of superconductivity. We find that the measured quantum oscillations display a signature sawtooth waveform, that rule out vestigial residual density of states, and instead point towards a complete gapping of the antinodal regions. We present current-dependent transport measurements performed in DC magnetic fields, down to millikelvin temperatures, that reveal the high-field superconducting state to be characterised by non-ohmic signatures associated with a quantum vortex matter state. In contrast to previous proposals, the quantum oscillations are found to occur well within this gapped vortex phase, as established by their co-existence with zero resistivity and hysteretic torque magnetisation, that are found to persist to magnetic fields beyond 45 T.EPSRC studentshi

Magnetotransport properties of rare earth element modified carbon nanotubes

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Doctor of Philosophy. University of the Witwatersrand JohannesburgFunctionalization and filling of carbon nanotubes has been tailored over years to modify the exceptional properties of the 1-dimensional (1D) conductor for magnetic properties based applications. Hence such a system exploits the spin and charge property of the electron, analogous to a quantum conductor coupled to magnetic impurities which poses an interesting scenario for the study of Kondo physics and related phenomena. A study of the low temperature electronic transport and magnetic properties of carbon nanotubes modified with gadolinium derivatives is presented in this thesis. The methods of modification used are chemical functionalization and capillary filling. The presence of gadolinium in the nanostructures extends the functionality of the nanotubes from conventional electronics to spintronics. Filled and functionalized multiwalled carbon nanotubes are characterized as well as filled double walled carbon nanotubes. This system gives a chance to study the interaction of a ballistic conductor with magnetic impurities. Multiwalled carbon nanotubes functionalized with a gadolinium based supramolecular complex show enhanced magnetic properties and unexpected electronic behaviour that has not been observed in this material before. A newly developed synthesis technique has been employed for the synthesis and it is found that the functionalization method of the nanocomposite enhances the strength of magnetic interaction leading to a large effective moment of 15.79 μB and non-superparamagnetic behaviour unlike what has been previously reported. Saturating resistance at low temperatures is fitted with the numerical renormalization group formula verifying the Kondo effect for magnetic impurities on a metallic electron system. Magnetoresistance shows that devices fabricated from aligned gadolinium functionalized MWNTs exhibit spin-valve switching behaviour of up to 8%. The electronic transport properties of MWNTs filled with GdCl3 nanomagnets clearly shows the co-existence of Kondo correlation and cotunelling within the superparamagnetic limit. The Fermi liquid description of the Kondo effect and the interpolation scheme are fitted to the resistance-temperature dependence yielding the onset of the Kondo scattering temperature and a Kondo temperature for this nanocomposite, respectively. Cotunneling of conduction electrons inhibiting a Kondo type interaction has been verified from the exponential decay of the intensity of the fano shaped non zero bias anomalous conductance peaks which also show strong resonant features observed only in GdCl3 filled MWNT devices. Hence these features are explained in terms of magnetic coherence and spin-flip effects along with the competition between the Kondo effect and co-tunneling. The properties of doublewalled carbon naotubes filled with GdCl3 are also presented. They show superparamagnetic behaviour and zero bias anomalies similar to what was observed in Gd filled MWNTs. This work is the first on such lanthanide modified CNT hybrid bundle devices. The study raises a new possibility of tailoring magnetic interactions for spintronic applications in carbon nanotube systems. It highlights the possibility of enhancing magnetic interactions in carbon systems through chemical modification. Furthermore, the study demonstrates the rich physics that might be useful for developing spin-based quantum computing elements based on 1D channels.MT 201

Measurements of Magnetic Field Penetration of Materials for Superconducting Radiofrequency Cavities

Superconducting Radio Frequency (SRF) cavities used in particle accelerators are typically formed from or coated with superconducting materials. Currently high purity niobium is the material of choice for SRF cavities which have been optimized to operate near their theoretical field limits. This brings about the need for significant R&D efforts to develop next generation superconducting materials which could outperform Nb and keep up with the demands of new accelerator facilities. To achieve high quality factors and accelerating gradients, the cavity material should be able to remain in the superconducting Meissner state under high RF magnetic field without penetration of quantized magnetic vortices through the cavity wall. Therefore, the magnetic field at which vortices penetrate in a superconductor is one of the key parameters of merit of SRF cavities. Techniques to measure the onset of magnetic field penetration on thin film samples need to be developed to mitigate the issues with the conventional magnetometry measurements which are strongly influenced by the film orientation and shape and edge effects. The applied magnetic field is also needed to be parallel to the one side of the superconductor to resemble the magnetic field profile at the surface of the SRF cavities operating at fundamental accelerating mode. In this work we report the development of an experimental setup called Magnetic Field Penetration (MFP) magnetometer to measure the field of full flux penetration through bulk, thin films and multilayered superconductors. Our system combines a small superconducting solenoid which can generate the magnetic field up to 500 mT at the sample surface and three Hall probes to detect the full flux penetration through the planner superconductor with 2 inch diameter. This setup was used to study alternative materials which could potentially outperform niobium, as well as SIS multilayer coatings on niobium

Unconventional Fermi surface in insulating SmB6 and superconducting YBa2Cu3O6+x probed by high magnetic fields

Fermi surface, the locus in momentum space of gapless low-energy excitations, is a concept of fundamental importance in solid state physics. Electronic properties of a material are determined by the long-lived low-energy excitations near the Fermi surface. Conventionally, Fermi surface is understood as a property exclusive to a metallic state, contoured by electronic bands crossed by the Fermi level, although there has been a continuing effort in searching for Fermi surface outside the conventional description. In this thesis, techniques developed to prepare high-quality single crystals of SmB$_6$ and YBa$_2$Cu$_3$O$_{6+x}$ (abbreviated as YBCO$_{6+x}$ hereinafter) are described. By utilising measurement techniques of exceptional sensitivity and exploring a wide range of temperatures, magnetic fields, and electrical currents, we found signatures of unconventional Fermi surfaces beyond the traditional description in these strongly correlated electronic systems. SmB$_6$ is a classic example of Kondo insulators whose insulating behaviour arises due to strong correlation between the itinerant $d$-electrons and localised $f$-electrons. The peculiar resistivity plateau onsets below 4 K has been a decades-long puzzle whose origin has been recently proposed as the manifestation of topological conducting surface states. We found that the insulating behaviour in electrical transport is robust against magnetic fields up to 45 T, while prominent quantum oscillations in magnetisation are observed above 10 T. Angular dependence of the quantum oscillations revealed a three-dimensional characteristics with an absolute amplitude consistent with a bulk origin, and temperature dependence showed a surprising departure from the conventional Lifshitz-Kosevich formalism. Complementary thermodynamic measurements showed results consistent with a Fermi surface originating from neutral itinerant low-energy excitations at low temperatures. Theoretical proposals of the unconventional ground state uncovered by our measurements in SmB$_6$ are discussed. YBCO$_{6+x}$ is a high-temperature superconductor with a maximum $T_{\rm c}$ of 93.5 K and the cleanest member in the family of copper-oxide, or {\it cuprate}, superconductors. The correct description of electronic ground state in the enigmatic pseudogap regime, where the antinodal density of states are suppressed below a characteristic temperature $T^*$ above $T_{\rm c}$, has been a subject of active debates. While the quantum oscillations observed in underdoped YBCO$_{6+x}$ have been predominately interpreted as a property of the normal state where the superconducting parameter is completely suppressed at $\approx$ 23 T, we made the discovery that YBCO$_{6.55}$ exhibits zero resistivity up to 45 T when a low electrical current is used, consistent with the observation of a hysteresis loop in magnetisation. Quantum oscillations in the underdoped YBCO$_{6+x}$ are thus seen to coexist with $d$-wave superconductivity. Characteristics of the quantum oscillations are consistent with an isolated Fermi pocket reconstructed by a charge density wave order parameter and unaccompanied by significant background density of states, suggesting the antinodal density of states is completely gapped out by a strong order parameter involving pairing correlations, potentially in addition to the other order parameters. Transport measurements performed over a wide doping range show signatures consistent with pairing correlations that persist up to the pseudogap temperature $T^*$. The surprising observation of quantum oscillations in insulating SmB$_6$ and superconducting YBCO$_{6+x}$ demonstrates a possible new paradigm of a Fermi surface without a conventional Fermi liquid. A new theoretical framework outside the realm of Fermi liquid theory may be needed to discuss the physics in these strongly correlated materials with enticing electronic properties.Ministry of Education, Taiwa

Electrodeposition of multilayered nanostructures for giant magnetoresistance and thermoelectric applications

The electrodeposition of novel materials such as multilayer nanotubes for giant magneto resistance (GMR) applications and bismuth telluride nanotubes for thermoelectric applications are presented in this dissertation. The motivation for the multilayer electrodeposition is the investigation of giant magnetoresistance (GMR), a change in the material resistance in an applied magnetic field as a consequence of antiferromagnetic coupling. The nanowire high aspect ratio geometry allows the measurement of GMR with the current applied perpendicular to the plane (CPP) of the multilayers, which has been theoretically identified as being larger than the GMR in the (CIP) configuration (current in plane of multilayers). The current perpendicular to the plane giant magneto-resistance (CPP)-(GMR) effect makes multilayered nanowires of huge interest as magnetic sensor materials. Electrodeposition is the most efficient method for fabricating magnetic nanowires. In addition to the cost-effectiveness, electrodeposition is one of the few methods that can overcome the geometrical restrictions of inserting metals into very deep nanometric recesses, making it the favored method for nanowire and nanotube fabrication. In this dissertation, the quaternary CoNiFeCu alloy system was investigated in order to electrodeposit multilayered nanowires/nanotubes for GMR effect. Electrodeposited multilayer CoNiFeCu/Cu nanowires and nanotubes were fabricated by pulsed applied electric potential and their giant magnetoresistance (GMR) behavior characterized. The effect of electrolyte concentration on the GMR was investigated. The FeSO4, CoSO4 and NiSO4 concentrations were varied to optimize the GMR and the saturation field of the multilayered nanowires. Nanolayer thicknesses were controlled and varied for commercially viable GMR results. Furthermore, the influence of electrolyte temperature on nanotube formation and the resulting GMR was explored. Micro fluidic magnetic nanoparticles sensors based on CoNiFeCu/Cu GMR nanowires were fabricated for the first time. The test results show that the sensors are highly sensitive to small nanoparticle concentrations. Employing the potentiostatic electrodeposition, nanotubes of bismuth-telluride (Bi2Te3) were obtained. The electrolyte concentration was varied and affected the nanotube formation and the resulting Seebeck coefficients