A custom built thermal evaporator equipped with in situ electrical transport probes and an electromagnet, designed to investigate magnetic thin films and nanostructures, was constructed and calibrated. Magnetoresistance measurements were used to characterise a 20 nm thick film grown in 2 nm steps and measured in situ as a function of film thickness. It was found that the thin film had a smaller than expected anisotropic magnetoresistance (AMR) signal of 0.024%. It was suggested that an oxide formed at each 2nm thick layers during the growth phase altered the conductivity of the film and caused the measured AMR to be anomalously small. Lateral spin valves fabricated from a range of ferromagnetic and normal metal components were investigated. NiFe/Au/NiFe lateral spin valves were the most thoroughly investigated to determine the spin diffusion length in the Au, the spin polarisation of NiFe and the injection efficiency at the NiFe/Au interface. Lateral spin valves fabricated from NiFe/Al/NiFe and utilising tunnelling contacts were also investigated and a pure spin current detected. Other devices, including a non-local lateral spin valve dual spin injection structure, were fabricated and measured. Nanomachining using diamond coated silicon nitride atomic force microscope (AFM) tips was employed to modify nickel iron (NiFe) nanowires. The modifications to nanowires in this way subsequently altered the observed domain wall motion in the wires. AFM nanomachining was found mostly to increase the coercive field of the nanowires owing to the formation of a pinning site for domain walls. Magnetoresistance measurements were used to study the effect of machining nanowires of varying widths and thickness. Theoretical predictions regarding the change in coercive field due to machining were larger than those experimentally measured. Domain wall anisotropic magnetoresistance (DW AMR) was also studied as a function of width for two thicknesses of nanowire (10nm and 20nm). Deviation from existing theoretical models was observed consistently for both wire thicknesses. A dependence of the DW AMR on the proximity to the phase boundary between different domain wall types was observed for each thickness of nanowire studied.

In this thesis the transport of light through disordered, densely packed semiconductor nanowire mats is studied. It is found that the extremely high photonic strength of these samples leads to corrections to the traditional diffusion picture of light transport due to mesoscopic interference. Such effects are characterized by large intensity fluctuations and correlations, and it is found the transport is dominated by only a few independent transmission channels, close to the Anderson localisation regime. In addition to the strongly scattering nanowire samples, comparatively weakly scattering samples of ZnO are investigated, demonstrating mesoscopic effects in a less exotic, isotropic multiple scattering material. Control is obtained over the transmission by a combination of shaping the incident wavefront and harnessing the intrinsic nonlinearity of the semiconductor with ultrafast optical excitation. Through these techniques, a bright focus at an arbitrary point through the nanowires is created which can be modulated by up to 60% in a demonstration of a reconfigurable photonic switch.

The problem of design optimisation of thin film direct current Superconducting QUantum Interference Device (SQUID) magnetometers made of YBCO (YBa2Cu3O7-x) was considered. The inductances and effective areas were calculated using the software package 3D-MLSI. Resolution and reliability issues were first tested on simple superconducting systems, showing good agreement with analytical formulae and experimental results, and demonstrating that a remarkable precision can be obtained though at the expense of CPU time and memory. The software was then used to simulate a SQUID magnetometer fabricated in the Device Materials Group of the Department of Materials Science and Metallurgy, proving that 3D-MLSI can be used to predict the parameters of real systems with acceptable accuracy.

This thesis discusses experiments that have been performed to investigate ultrafast acoustoelectric effects in semiconductor devices. Current commonly employed techniques to generate ultrafast acoustic pulses and detect them with spectral resolution require a powerful pulsed laser system that is bulky, expensive and complicated. If the acoustic pulses could instead be generated and detected by electrical methods, picosecond acoustic techniques could become more readily available as a tool for other users. This thesis focusses on the electrical detection of acoustic pulses with spectral resolution. In many of the key experiments described in this thesis a picosecond strain pulse was generated optically on the opposite face of the sample to the semiconductor device of interest. The strain was generated either in a thin Al film thermally deposited on the sample surface, or directly in the GaAs substrate. Acoustic phonons generated by this method propagated across the substrate to the device. Transient voltages across the semiconductor device caused by the incident phonons were detected using a high frequency real-time oscilloscope. The first evidence of heterodyne mixing of coherent acoustic phonons with microwaves was obtained, for frequencies up to about 100 GHz. First, it was confirmed that Schottky diodes can produce a fast transient voltage in response to an incident acoustic wavepacket. The detection process occurs at the semiconductor-metal interface, and is due to the deformation potential. Bow-tie antenna fabricated directly onto the GaAs substrate proved to be ineffective at coupling microwaves from free space to the Schottky diode. A waveguide-coupled beam-lead Schottky diode provided by e2v had a sufficient response to the incident microwaves to proceed with the mixing experiments. The microwave local oscillator signal was mixed with a tunable narrow frequency band acoustic signal that was produced using a Fabry-Perot etalon external to the laser cavity. The intermediate frequency components were in the range of 1-12 GHz, which could be detected on the oscilloscope. Mixing was performed using both the fundamental frequency acoustic wave and the second harmonic generated in the sample. Semiconductor superlattices were also investigated as electrical detectors for ultrafast acoustic pulses. In this case, the transient voltage measured across the device contained an unexpected contribution in the form of a peak with a width of approximately 2 ns. This signal is too slow to be caused by a strain pulse and too fast for a heat pulse. It is proposed that this peak is caused by long-lived phonon modes from the centre of the mini-Brillouin zone being confined in the superlattice due to Bragg reflections. The peak caused by confined phonons and the two peaks caused by heat pulses also present in the detected signal were investigated for a range of experimental conditions. This allowed comparisons to be made to previous works. A similar superlattice structure had a very different response to the incident acoustic wavepacket. The polarity of the transient voltage detected was inverted and there was no evidence of an electronic response to the confined phonon modes, which would have been present in both samples. It is proposed that the barriers of the NU1727 superlattice sample are thicker than expected, and this strongly affects the electron transport through the structure. This thesis shows that semiconductor devices can be suitable for the electrical detection of ultrafast strain pulses. For this technique to reach its full potential, it is also necessary to be able to generate these strain pulses electrically. A step recovery diode has been considered for this purpose as part of the suggested future work.

Since 2004, graphene attracts intensive attention from scientists and engineers all over the world. During the last decades, the research relates to graphene and other 2 dimensional (2D) materials are rapidly increasing. Approximately, ten thousand journal papers have been published after the discovery of graphene in relative topics widely spread. On the other hand, the simple graphene properties research is nearly completed. Researchers turn their attention to other 2D materials or Van der Waals heterostructures. By increasing the liberty and knowledge of 2D materials, the Van der Waals heterostructures can start to build something on this 2D wander land. In this thesis the Van der Waals heterostructures is based on graphene and some other well known 2D materials such as hexagonal boron nitride (hBN) to study fundamental physics and possible applications in near future. In this thesis, three published papers which are related to Van der Waals heterostructures have been included. The electronic properties of encapsulated graphene on different 2D crystals have been investigated by the capacitance spectroscopy. Several 2D crystals have been tested as a substrate such as MoS2, WS2, mica, LiNbO3…etc. The quality of encapsulated device is correlates the interface self-cleaning. Follow with the fundamental physics study employed by a simple Van der Waals heterostructure. Graphene and hBN is lattice aligned within 2 degrees in difference and creates a new superlattice structure which just like moire pattern happens while two similar patterns overlapped. The basic electronic properties do not vary at near Dirac point. Away from the first generation Dirac point, the superlattice structure affects the band structure in higher carrier concentration. In this paper, aligned graphene-hBN capacitors have been demonstrated to discover more fine details of these many-body interactions in this superlattice structure. The final part is related to twist controlled graphene-graphene resonance tunneling transistors. A Van der Waals heterostructure is constructed by two aligned graphene stripes with a thin layer of hBN as a spacer. The electrons are tunneled from one stripe to another graphene stripe while a bias voltage applied. The resonance tunneling is occurred when two graphene flakes are aligned at certain bias voltage. In this paper, we contribute the resonance tunneling to momentum conservation of tunnelling electrons. Theory simulation is highly agreed with our experiment results.

Dilute magnetic semiconductors are an important family of materials that have many potential applications in spintronics; (Ga,Mn)As, (In,Ga,Mn)As and (Ga,Mn)N are of major interest. This thesis investigates dierent aspects of these, using the synchrotron radiation techniques of x-ray magnetic circular dichroism (XMCD) and x-ray magnetic linear dichroism (XMLD), supported by superconducting quantum interference device (SQUID) magnetometry and magnetotransport measurements. A large anisotropic XMLD signal is observed for the Mn L-edge in (Ga,Mn)As. In unannealed (Ga,Mn)As, an apparently reduced Mn magnetic moment is commonly observed. It is thought to be related to compensation of both carriers and magnetic moment, caused by interstitial Mn. This issue is investigated using combined data from XMCD, XMLD and SQUID magnetometry. The findings suggest that substitutional and interstitial Mn form `non-magnetic' pairs which do not have a preferred spin orientation. (Ga,Mn)N is studied by x-ray absorption and field-dependent XMCD at the Mn L-edge. Two distinct Mn congurations are identified: Mn2+ is prevalent towards the surface with nearly paramagnetic behaviour, while a weakly ferromagnetic Mn2+/Mn3+ mixed valence exists within the bulk. The weak ferromagnetism, often observed in (Ga,Mn)N, is attributed to coupling between the impurities by the double exchange mechanism. Finally, XMCD is used to measure the orbital polarization of As 4p states of (III,Mn)As materials. These states correspond to those of the holes involved in the itinerant exchange interaction in ferromagnetic semiconductors. The coupling between the localized d states of the magnetic impurities and the valence band p states of the host is demonstrated by an anisotropy in the orbital moment of these states. This is experimental confirmation of the origin of the magnetocrystalline anisotropy in dilute magnetic semiconductors.

This thesis presents the work done to characterise two superconducting materials. We study BiPd, a non-centrosymmetric superconductor which is theoretically expected to show signs of spin singlet and triplet mixing due to the strong spin-orbit scattering of its composing elements. We map the field-temperature superconducting phase diagram along two crystal directions using Small Angle Neutron Scattering (SANS), magnetisation and \(µ\)SR measurements and determine the microscopic parameters defining the superconducting state. We also uncover a rare behaviour displayed in low-\(k\) superconductors, the Intermediate Mixed State, which causes domains of vortex lattice with constant spacing to coexist with Meissner domains at low applied fields. Finally we show evidence that, unlike what was expected, the superconductivity in BiPd behaves conventionally. The second material studied is Nb3Sn, widely used to produce large magnetic fields in various devices such as MRI machines. We investigate the superconducting state of several polycrystalline samples with different tin concentrations, as recent evidence point towards a lack of change of the upper critical field with varying Sn doping, in contradiction with older measurements that see a drop in H\({c2}\) associated with the apparition of a structural (martensitic) crystalline transition. Using SANS, we show that these recent results were likely not measuring the bulk state of Nb3Sn and that we find large variation of H\({c2}\) with Sn concentration. We also present indications that the vortex lattice is influenced by non-local effects at large fields by measuring the change in the vortex lattice structure with field. Lastly, our measurements are consistent with a full single gap behaviour in Nb3Sn.

This thesis is concerned with the development of condition monitoring for future design of high temperature superconducting (HTS) power apparatus. In particular, the use of UHF sensing for detecting PD activity within HTS has been investigated. Obtained results indicate that fast current pulses during PD in LN2 radiate electromagnetic waves which can be captured by the UHF sensor. PD during a negative streamer in LN2 appears in the form of a series of pulses less than 1 μs apart. This sequence cannot be observed using conventional detection method due to its bandwidth limitation. Instead, a slowly damped pulse is recorded which shows the total amount of charge transferred during this period. A study into PD streamer development within LN2 has been undertaken that reveals the characteristics of pre-breakdown phenomena in LN2. For negative streamers, when the electric field exceeds a threshold value, field emission from the electrode becomes effective which leads to the formation of initial cavities. Breakdown occurs within these gaseous bubbles and results in the development of negative streamers. For positive streamers, the process is much less well-understood due to the lack of initial electrons. However, from the recorded current pulses and shadow graphs, the physical mechanism behind positive streamer development is likely to be a more direct process, such as field ionisation, compared with the step-wise expansion in the case of negative streamers. The mechanisms that cause damage to solid dielectrics immersed in LN2 have been investigated. Obtained results indicate that pre-breakdown streamers can cause significant damage to the solid insulation barrier. Damage is the result of charge bombardment and mechanical forces rather than thermal effects. Inhomogeneous materials, such as glass fibre reinforced plastic (GRP), tend to introduce surface defects which can create local trapping sites. The trapped charges when combined with those from streamers can create much larger PD events. Consequently, damage observed on GRP barriers is much more severe than that on PTFE barriers under similar experimental conditions. Thus, design of future HTS power apparatus must consider this degradation phenomenon in order to improve the reliability of the insulation system.

The major challenges in trapped-ion quantum computation are to scale up few-ion experiments to many qubits and to improve control techniques so that quantum logic gates can be carried out with higher fidelities. This thesis re- ports experimental progress in both of these areas. In the early part of the the- sis we describe the fabrication of a surface-electrode ion trap, the development of the apparatus and techniques required to operate it and the successful trap- ping of ⁴⁰Ca⁺ ions. Notably we developed methods to control the orientation of the principal axes and to minimise ion micromotion. We propose a repumping scheme that simplifies heating rate measurements for ions with low-lying D levels, and use it to characterise the electric field noise in the trap. Surface-electrode traps are important because they offer a route to dense integration of electronic and optical control elements using existing microfabrication technology. We explore this scaling route by testing a series of three traps that were microfabricated at Sandia National Laboratories. Investigations of micromotion and charging of the surface by laser beams were carried out and improvements to future traps are suggested. Using one of these traps we also investigated anomalous electrical noise from the electrode surfaces and discovered that it can be reduced by cleaning with a pulsed laser. A factor of two de- crease was observed; this represents the first in situ removal of this noise source, an important step towards higher gate fidelities. In the second half of the thesis we describe the design and construction of an experiment for the purpose of replacing laser-driven multi-qubit quantum logic gates with microwave-driven ones. We investigate magnetic-field-independent hyperfine qubits in ⁴⁰Ca⁺ as suitable qubits for this scheme. We make a design study of how best to integrate an ion trap with the microwave conductors required to implement the gate and propose a novel integrated resonant structure. The trap was fabricated and ions were successfully loaded. Single-qubit experiments show that the microwave fields above the trap are in excellent agreement with software simulations. There are good prospects for demonstrating a multi-qubit gate in the near future. We conclude by discussing the possibilities for larger-scale quantum computation by combining microfabricated traps and microwave control.

The aim of this thesis was to identify a candidate material for spin-triplet superconductivity with a two-component order parameter. This unconventional superconducting state is thought to allow for exotic quantum states such as Majorana fermions. A wide survey was taken into potential candidate materials, and UAu2 was chosen for in-depth investigation. This little-studied hexagonal heavy fermion compound's unusual resistivity behaviour, combined with a series of interesting features in magnetisation and heat capacity, make it an extremely interesting material to study. The phase diagram of UAu2 was determined with measurements of heat capacity, resistivity, magnetisation and magnetoresistance on the first single crystalline samples of this material. No superconductivity was detected. Instead, a range of magnetic phase transitions were observed, which were further investigated with muon-spin relaxation experiments and time-of- ight neutron powder diffraction. UAu2 was found to undergo a transition to an incommensurate antiferromagnetic state (q1 = (1=3; 1=3; δ)) below TN = 43:5 K, but then develops signatures of weak ferromagnetism below T = 20 K. The ferromagnetism coincides with a 2q magnetic structure, with a coexistence of q1 and q2=(1/3,1/3,0). The magnetic structures of both phases were found to be most likely amplitude-modulated, with moments aligned along the crystallographic c-axis. A transition to a ferromagnetic state was observed in magnetic fields applied parallel to the c-axis. TN was found to remain almost constant in applied magnetic fields up to 9 T, while hydrostatic pressures of up to 6 kbar weakly suppress the antiferromagnetic transition temperature. The field-induced transition was found to be strongly pressure-dependent, shifting to higher applied fields with increasing pressure. The residual resistivity of UAu2 samples prepared by both the Czochralski method and quenching from the melt is relatively large, which may inhibit Cooper pairing and hence may be the reason for the absence of superconductivity in the samples investigated. Solid-state electrotransport (SSE) equipment was developed, which can induce the motion of a crystal's constituents and thereby lead to vastly improved sample quality. Refinement of UAu2 samples with SSE could be a further step in the search for spin-triplet superconductivity in this material.