http://www.dspace.cam.ac.uk:80/handle/1810/214775 Sun, 19 May 2013 23:32:36 GMT 2013-05-19T23:32:36Z http://www.dspace.cam.ac.uk:80/handle/1810/244393 Title: On-chip single photon sources based on quantum dots in photonic crystal structures Authors: Schwagmann, Andre Abstract: In order to harness the enormous potential of schemes in optical quantum information processing, readily scalable photonic circuits will be required. A major obstacle for this scalability is the monolithic integration of quantum light sources with the photonic circuit on a single chip. This dissertation presents the experimental demonstration of different in-plane single photon sources that allow for this integration with planar light circuits. To this end, the spontaneous recombination of excitons in single indium arsenide quantum dots was exploited to generate single photons. The emission into on-chip waveguides was achieved by the use of advanced two-dimensional photonic crystal structures. First, slow-light effects in a unidirectional photonic crystal waveguide were exploited to achieve on-demand single photon emission with a rate of up to 18.7 MHz, corresponding to a remarkable estimated internal device efficiency of up to 47%. Waveguide-coupled L3 defect cavities with record Q-factors of up to 5150 were then studied for improved Purcell enhancement of the emission, and in-plane single photon generation from such a device was demonstrated. Finally, an electrically tunable, integrable quantum light source with a total tuning range of 1.9 nm was demonstrated by exploiting the quantum-confined Stark effect in an electrical PIN diode. These results are the first demonstrations of in-plane single photon emission at optical wavelengths and mark an important cornerstone for the realisation of fully integrated quantum photonic circuits in optical quantum information science. Tue, 05 Feb 2013 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244393 2013-02-05T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244333 Title: Improved wave functions for quantum Monte Carlo Authors: Seth, Priyanka Abstract: Quantum Monte Carlo (QMC) methods can yield highly accurate energies for correlated quantum systems. QMC calculations based on many-body wave functions are considerably more accurate than density functional theory methods, and their accuracy rivals that of the most sophisticated quantum chemistry methods. This thesis is concerned with the development of improved wave function forms and their use in performing highly-accurate quantum Monte Carlo calculations. All-electron variational and diffusion Monte Carlo (VMC and DMC) calculations are performed for the first-row atoms and singly-positive ions. Over 98% of the correlation energy is retrieved at the VMC level and over 99% at the DMC level for all the atoms and ions. Their first ionization potentials are calculated within chemical accuracy. Scalar relativistic corrections to the energies, mass-polarization terms, and one- and two-electron expectation values are also evaluated. A form for the electron and intracule densities is presented and fits to this form are performed. Typical Jastrow factors used in quantum Monte Carlo calculations comprise electron-electron, electron-nucleus and electron-electron-nucleus terms. A general Jastrow factor capable of correlating an arbitrary of number of electrons and nuclei, and including anisotropy is outlined. Terms that depend on the relative orientation of electrons are also introduced and applied. This Jastrow factor is applied to electron gases, atoms and molecules and is found to give significant improvement at both VMC and DMC levels. Similar generalizations to backflow transformations will allow useful additional variational freedom in the wave function. In particular, the use of different backflow functions for different orbitals is expected to be important in systems where the orbitals are qualitatively different. The modifications to the code necessary to accommodate orbital-dependent backflow functions are described and some systems in which they are expected to be important are suggested. Tue, 05 Feb 2013 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244333 2013-02-05T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244270 Title: Bayesian methods for gravitational waves and neural networks Authors: Graff, Philip B. Abstract: Einstein’s general theory of relativity has withstood 100 years of testing and will soon be facing one of its toughest challenges. In a few years we expect to be entering the era of the first direct observations of gravitational waves. These are tiny perturbations of space-time that are generated by accelerating matter and affect the measured distances between two points. Observations of these using the laser interferometers, which are the most sensitive length-measuring devices in the world, will allow us to test models of interactions in the strong field regime of gravity and eventually general relativity itself. I apply the tools of Bayesian inference for the examination of gravitational wave data from the LIGO and Virgo detectors. This is used for signal detection and estimation of the source parameters. I quantify the ability of a network of ground-based detectors to localise a source position on the sky for electromagnetic follow-up. Bayesian criteria are also applied to separating real signals from glitches in the detectors. These same tools and lessons can also be applied to the type of data expected from planned space-based detectors. Using simulations from the Mock LISA Data Challenges, I analyse our ability to detect and characterise both burst and continuous signals. The two seemingly different signal types will be overlapping and confused with one another for a space-based detector; my analysis shows that we will be able to separate and identify many signals present. Data sets and astrophysical models are continuously increasing in complexity. This will create an additional computational burden for performing Bayesian inference and other types of data analysis. I investigate the application of the MOPED algorithm for faster parameter estimation and data compression. I find that its shortcomings make it a less favourable candidate for further implementation. The framework of an artificial neural network is a simple model for the structure of a brain which can “learn” functional relationships between sets of inputs and outputs. I describe an algorithm developed for the training of feed-forward networks on pre-calculated data sets. The trained networks can then be used for fast prediction of outputs for new sets of inputs. After demonstrating capabilities on toy data sets, I apply the ability of the network to classifying handwritten digits from the MNIST database and measuring ellipticities of galaxies in the Mapping Dark Matter challenge. The power of neural networks for learning and rapid prediction is also useful in Bayesian inference where the likelihood function is computationally expensive. The new BAMBI algorithm is detailed, in which our network training algorithm is combined with the nested sampling algorithm MULTINEST to provide rapid Bayesian inference. Using samples from the normal inference, a network is trained on the likelihood function and eventually used in its place. This is able to provide significant increase in the speed of Bayesian inference while returning identical results. The trained networks can then be used for extremely rapid follow-up analyses with different priors, obtaining orders of magnitude of speed increase. Learning how to apply the tools of Bayesian inference for the optimal recovery of gravitational wave signals will provide the most scientific information when the first detections are made. Complementary to this, the improvement of our analysis algorithms to provide the best results in less time will make analysis of larger and more complicated models and data sets practical. Mon, 08 Oct 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244270 2012-10-08T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244264 Title: Estimation of the Z->vv background to New Physics searches in ATLAS Authors: Sandoval, Tanya Abstract: This thesis describes a series of studies related to searches for new phenomena, beyond the Standard Model of particle physics, in high energy hadron collisions. In such searches, it becomes crucial to identify the Standard Model backgrounds in order to resolve a potential new signal. The thesis presents a method that uses photon events to determine one of such backgrounds, caused by the production of Z boson events. The studies performed to validate the method, both theoretically and experimentally, are presented and the method was shown to be successful as well as to provide reliable results. Theoretically, the method is found to be robust up to a ~10% uncertainty. Experimentally, the method is implemented to estimate the Z(vv) + jets background for the SUSY 0l + E_T^miss + jets search in the ATLAS experiment at the Large Hadron Collider, where this background is one of the most important components for the final sensitivity and is impossible to measure directly. The main experimental results presented are the latest from ATLAS at the time of writing, corresponding to the full dataset of proton-proton collisions delivered by the LHC in 2011 (4.7 fb^-1) at a centre of mass energy of 7 TeV. Given that this method has been mainstream since 2010, brief comparisons to the results from previous analyses that used smaller datasets with the same centre of mass energy are also given, as well as additional cross-checks that support the robustness and validity of the method. The results presented here have contributed to the determination of the world's best limits with respect to SUSY models, which currently exclude equal mass squarks and gluinos below 1.4 TeV. Tue, 05 Feb 2013 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244264 2013-02-05T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244105 Title: Plasmonic interactions in the quantum tunnelling regime Authors: Savage, Kevin John Abstract: Driven by exciting new research and applications, top-down and bottom-up fabrication techniques are producing ever more intricate, reproducible, plasmonic nano-architectures with gaps and junctions approaching the single nanometre and atomic scales. Such atomic-sized features promote the intersection of physics, chemistry and biology in plasmonics. Consequently, understanding light-matter interactions in such closely spaced, electromagnetically coupled, metallic nanosystems is of vital importance to a tremendous variety of current and future nanophotonic technologies. This thesis describes the first dynamically controlled, optically broadband, experimental investigations of light-driven plasmonic coupling between two metal nanostructures with sub-nanometre separation. A new experimental apparatus and nanosystem alignment technique was developed to enable the required sub-nanometre inter-nanoparticle geometry to be created and probed. Two conducting atomic force microscopy tips with nanoparticle functionalised apices are brought into nanoscale `tip-to-tip' axial alignment with dynamically-controlled spacing and ultra-wide optical access. Resonant electrical parametric mixing, created by oscillating the electromechanically coupled tips, is utilised to extract an electronic signal due to nanoscale changes in inter-tip position. Experimental results match theory confirming the viability of the technique. By functionalising the tip apices, this unique multi-functional observation platform allows the plasmonic response of nanoparticle dimers with sub-nanometre separations to be characterised. By simultaneously capturing both the electrical and optical properties of tip-mounted gold nanoparticles with controllable sub-nanometre separation, the first evidence for the quantum regime of optically driven tunnelling plasmonics is revealed in unprecedented detail. It is demonstrated that quantum mechanical effects are critically important at approximately the 0.3 nm scale where spatially non-local tunnelling plasmonics controls the optical response. All observed phenomena are in good agreement with a recently developed quantum-corrected model of plasmonic systems. The findings imply that tunnelling establishes a quantum limit for plasmonic field enhancement and confinement. Additionally, the work suggests the highly enhanced local density of photonic states in nanoscale cavities could enable coherent plasmon-exciton coupling. This thesis prompts new experimental and theoretical investigations into quantum-domain plasmonic systems, and impacts the future of nanoplasmonic device engineering, nanoscale photochemistry and plasmon-mediated electron tunnelling. Tue, 13 Nov 2012 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244105 2012-11-13T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243945 Title: Double parton scattering in proton-proton collisions Authors: Gaunt, Jonathan Abstract: Double hard parton-parton interactions are expected to occur frequently in proton-proton (p-p) collisions at the LHC. They can give rise to significant backgrounds to certain rare single scattering (SPS) signals, and are an interesting signal process in their own right. In this thesis, we discuss the theoretical description of the double parton scattering (DPS) cross section in the context of Quantum ChromoDynamics (QCD). After an overview of QCD and an introduction to DPS in Chapter 1, we describe in Chapter 2 a framework for calculating the p-p DPS cross section introduced by Snigirev et al., in which this cross section is expressed in terms of double PDFs D_p^{ij}(x_1,x_2,Q_A^2,Q_B^2) (dPDFs). We show that the equal-scale dPDFs are subject to momentum and number sum rule constraints, and use these in the construction of an explicit set of leading order (LO) equal-scale dPDFs (the 'GS09' dPDFs). The leptonic same-sign WW DPS signal obtained using GS09 dPDFs is compared with that obtained using simple factorised forms, and the prospects of observing this signal taking into account SPS backgrounds are analysed. We discuss two ways in which the dPDF framework for describing p-p DPS is deficient in Chapter 3. We discuss interference and correlated parton effects in flavour, spin, colour, and parton type, which are ignored by the dPDF framework. We then study DPS-type graphs in which the parton pairs from both protons have arisen from a perturbative 1->2 branching, derive an expression for the part of such graphs associated with the particles arising from the 1->2 branchings being almost on-shell, and use this to demonstrate that the treatment of these graphs by the the dPDF framework is unsatisfactory. In Chapter 4, we study DPS-type graphs in which the parton pair from only one proton has arisen from a perturbative 1->2 branching. We discover that such graphs contribute to the LO p-p DPS cross section, and that crosstalk between partons in the 'nonperturbatively generated' pair is allowed provided that it occurs at a lower scale than that of the perturbative 1->2 branching in the other proton. The result of this analysis is combined with that of the previous chapter to propose a formula for the LO total DPS cross section, and our proposal is compared with those from other authors. We finish in Chapter 5 with some conclusions and suggestions for further work. Mon, 08 Oct 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243945 2012-10-08T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243864 Title: Enhancing solar cells with plasmonic nanovoids Authors: Lal, Niraj Narsey Abstract: This thesis explores the use of plasmonic nanovoids for enhancing the efficiency of thin-film solar cells. Devices are fabricated inside plasmonically resonant nanostructures, demonstrating a new class of plasmonic photovoltaics. Novel cell geometries are developed for both organic and amorphous silicon solar cell materials. An external-quantum efficiency rig was set up to allow simultaneous microscope access and micrometer-precision probe-tip control for optoelectronic characterisation of photovoltaic devices. An experimental setup for angle-resolved reflectance was extended to allow broadband illumination from 380 - 1500nm across incident angles 0 - 70 degrees giving detailed access to the energy-momentum dispersion of optical modes within nanostructured materials. A four-fold enhancement of overall power conversion efficiency is observed in organic nanovoid solar cells compared to flat solar cells. The efficiency enhancement is shown to be primarily due to strong localised plasmon resonances of the nanovoid geometry, with close agreement observed between experiment and theoretical simulations. Ultrathin amorphous silicon solar cells are fabricated on both nanovoids and randomly textured silver substrates. Angle-resolved reflectance and computational simulations highlight the importance of the spacer layer separating the absorbing and plasmonic materials. A 20% enhancement of cell efficiency is observed for nanovoid solar cells compared to flat, but with careful optimisation of the spacer layer, randomly textured silver allows for an even greater enhancement of up to 50% by controlling the coupling to optical modes within the device. The differences between plasmonic enhancement for organic and amorphous silicon solar cells are discussed and the balance of surface plasmon absorption between a semiconductor and a metal is analytically derived for a broad range of solar cell materials, yielding clear design principles for plasmonic enhancement. These principles are used to outline future directions of research for plasmonic photovoltaics. Mon, 02 Jul 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243864 2012-07-02T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243618 Title: Sub-10-nanometre metallic gaps for use in molecular electronics Authors: Curtis, Kellye Suzanne Abstract: This thesis presents the development of a selective-etch fabrication process to create sub-10 nanometre metallic gaps and the subsequent use of the gaps to study the electronics of nanocrystals and molecules. A complete picture of the success of the process required both examination by scanning electron microscopy as well as probing the current response to an applied bias at low temperature. The empty gaps were fully characterised before self-assembling 7 nm CdSe nanocrystals onto the metal with the help of linker molecules. The I-V characteristics of the empty gaps showed a reduction of the tunnelling barrier height from the expected value (~5.1 eV, the work function of Au) when the results were fitted to the Simmons tunnelling model for a metal-insulator-metal system. Results indicate that after the barrier height is surpassed, a transition from direct to field-effect (Fowler-Nordheim) tunnelling occurs. After CdSe assembly, the collected I-V characteristics of the system at 77 K showed varied results. Many devices displayed conductance peaks at low voltages comparable to the results of the shadow evaporation process for 4.2 nm nanocrystals (also documented in this thesis). Several devices revealed switching between multiples of fundamental curves, suggesting conduction through multiples of nanocrystals. Mon, 02 Jul 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243618 2012-07-02T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/242281 Title: Transmission-mode imaging in the environmental scanning electron microscope (ESEM) Authors: Staniewicz, Lech Thomas Leif Abstract: Electron microscopy was first conducted in the 1930s with the advent of the TEM and later the STEM. In 1969, the first commercial SEM was released, with the possibility of retrofitting it to behave like a STEM following soon afterwards. In 1979, Danilatos and Robinson advanced electron microscopy by creating a new type of SEM which allowed a controlled quantity of gas into the sample chamber, termed ESEM. The most recent evolution in this line was the combination of ESEM and STEM in 2005, a procedure termed Wet STEM. The focus of this work is on investigating applications of this new technique, along with the contrast mechanisms involved in forming an image. To that end, a wide variety of samples will be imaged. Clay and paint suspensions (colloids) are used to test Wet STEM’s capacity to image submerged objects, as well as thin objects which are stacked together. Diblock copolymer films are used to test Wet STEM’s ability to distinguish chemically similar materials without staining, the physical effects of heavy metal staining and to demonstrate the necessity of gas for the purpose of charge neutralisation. Single cell biological samples are also investigated. Internal contrast in mammalian cells is visible without recourse to staining, but chemical fixation is required despite maintaining a high relative humidity. Bacteria are more resilient and as such are easier to image than animal cells, requiring no prior treatment. When exposed to low relative humidity, bacteria are found to collapse. The collapse pattern is observed to differ between wild-type and cytoskeletal-deficient bacteria of the same species and strain, so it is likely that dehydration-induced collapse offers information about the position and shape of the bacterial cytoskeleton. Tue, 07 Feb 2012 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/242281 2012-02-07T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/241720 Title: Solid-state photonic interfaces using semiconductor quantum dots Authors: Boyer de la Giroday, Antoine Abstract: New technologies based on the properties of quantum mechanics promise to revolutionise the way information is processed by outperforming what is possible using classical devices. Examples include massively parallel processing using quantum computers, verifiably secure communication using quantum cryptography, and measurement with sensitivity beyond classical limitation with quantum metrology. Realising the full potential of these technologies necessitates the ability to communicate quantum information over large distances, a key requirement for future quantum networks. However, developing practical implementations of long-distance quantum communication is challenging as it necessitates three major ingredients: light-matter interfaces, elementary quantum operations, and quantum memories. This thesis describes work that has been undertaken to address these requirements using semiconductor nanotechnology. We have first demonstrated that single InAs quantum dots embedded inside conventional diode structures constitute high-fidelity controllable interfaces between optical qubits and solid-state qubits. Indeed, the polarisation state of a photon was transferred into the spin state of an electron-hole pair and eventually restored through radiative recombination of the electron and the hole with a fidelity up to 95%. Moreover, spins were manipulated using subnanosecond modulation of a vertical electric field applied to the quantum dots. By controlling this electrical modulation, we demonstrated elementary phase-shift and spin-flip gate operations with near-unity fidelities. An electron-hole pair confi ned in a single quantum dot has a short radiative lifetime limiting therefore its use as an excitonic quantum memory. The solution we proposed was to use a quantum dot molecule to control the spatial separation of the electron and the hole and therefore prevent their recombination. Comprehensive studies of electric field eff ects upon the photoluminescence of quantum dot molecules lead to a clear understanding and a good control over their physical properties. Single photons were stored in individual quantum dot molecules up to 1μs and read out on a subnanosecond time scale. Moreover, the circular polarisation of individual photons was transferred into the spin state of electron-hole pairs with a fidelity above 90%, which does not degrade for storage times up to the 12.5 ns repetition period of the experiment. Our work on single quantum dots could be extended in the near future to allow for two-qubits quantum operations by con fining a second electron-hole pair to be electrically manipulated. Storage of a superposition of spin states in a quantum dot molecule should also be possible if the spin states are made degenerate, which is feasible using the electric fi eld dependence of the energy splitting between the spin states discussed in this thesis. We believe that combining both approaches will lead to the development of a controllable multi-qubit quantum memory for polarised light, a building block for long distance quantum communication based on semiconductor nanotechnology. Tue, 07 Feb 2012 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/241720 2012-02-07T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/241330 Title: Light-emitting diodes from polyfluorenes: characterisation and stability of performance Authors: Seeley, Alexander J. A. B. Abstract: This thesis deals with polymer light-emitting diodes (LEDs) containing materials from the polyfluorene family, and investigates their behaviour when employed in device structures. A study of poly(9,9’-dioctylfluorene-co-bis-N,N’-(4-butylphenyl)-bis-N,N’-phenyl-1,4-phenylenediamine) (PFB) by photothermal deflection spectroscopy (PDS) shows that the polymer undergoes a doping reaction with poly(styrene sulphonic acid). This is important because the two materials are found in intimate contact in LED structures. The conditions for reaction are investigated, and it is proposed that the reacted states are directly responsible for the drive-induced degradation of LEDs containing these two materials. LEDs are studied which contain various combinations of poly(9,9’-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB) and poly(9,9’-dioctylfluorene-co-benzothiadiazole) (F8BT), using pulsed electroluminescence. A strongly morphology-dependent spike-transient is observed in the electroluminescence at turn-on, and this is investigated by numerical modelling. Although not all features of the system can be well represented in the model, the spike transient is explicitly predicted without the need to impose any special conditions. The origin of this feature is elucidated by repeatedly running the model to a range of end-points and studying the time-evolution of space-charge distributions which result. Finally, F8BT devices are considered on their own, in order to study the evolution of device performance under low-intensity electrical excitation. A phenomenon is investigated in which the quantum efficiency is dramatically increased during the early stages of driving. Ionic motions are ruled out, and the observations are attributed to the trapping of charge in the vicinity of the anode, leading to enhanced hole injection. The reverse-bias behaviour of the effect, in which a further enhancement is seen, is also examined. The analogy is made with polymer LEDs in general which increase in performance following a period of reverse bias, and it is suggested that the causes may be related. Sat, 31 May 2003 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/241330 2003-05-31T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/241302 Title: Current-induced torque driven ferromagnetic resonance in magnetic microstructures Authors: Fang, Dong Abstract: This Dissertation explores the interaction between the magnetisation and an alternating current in a uniform ferromagnetic system. Diluted magnetic semiconductors (Ga,Mn)As and (Ga,Mn)(As,P) have been studied. Due to their strong spin-orbit coupling and well-understood band-structure, these materials are well-suited to this investigation. The combined effect of spinorbit coupling and exchange interaction permits the alternating current to induce an oscillating current-induced torque (CIT) on the magnetisation. In the frequency range close to the natural resonance frequency of the magnetic moments (gigahertz), CIT can excite precessional motion of the magnetisation, a process known as ferromagnetic resonance (FMR). CIT can be parameterised by an effective magnetic field. By analysing the lineshape of the measured FMR signals, the magnitude and orientation of this effective field have been accurately determined. Moreover, the current-induced fields in these ferromagnetic materials have been observed with symmetries of the Dresselhaus, and for the first time, Rashba spin-orbit coupling. A new class of device-scale FMR technique, named as CIT-FMR, has been established in this Dissertation, with the advantage of simple device structure (only a resistor is required) and scalability (measurements has been performed on devices sized from 4 μm down to 80 nm). This technique is not only limited to magnetic semiconductors, but can also be transferred to study other ferromagnetic systems such as ultrathin metal films. Finally, the CIT-FMR technique is employed to study the magnetic anisotropy in individual (Ga,Mn)As and (Ga,Mn)(As,P) micro-devices. Devices down to 80 nm in width have been measured in (Ga,Mn)(As,P), which show strong strain-relaxation-induced anisotropy, larger than any previously reported cases on (Ga,Mn)As. Furthermore, due to the tensile-strain on the (Ga,Mn)(As,P) epilayers, the anisotropy field due to patterning-induced strain-relaxation in these devices is observed to take the opposite direction compared to that in the compressively-strained (Ga,Mn)As samples. Tue, 15 Mar 2011 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/241302 2011-03-15T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/241264 Title: A trapped single ion inside a Bose-Einstein condensate Authors: Zipkes, Christoph Abstract: In recent years, improved control of the motional and internal quantum states of ultracold neutral atoms and ions has opened intriguing possibilities for quantum simulation and quantum computation. Many-body effects have been explored with hundreds of thousands of quantum-degenerate neutral atoms and coherent light-matter interfaces have been built. Systems of single or a few trapped ions have been used to demonstrate universal quantum computing algorithms and to detect variations of fundamental constants in precision atomic clocks. Now in our experiment we investigate how the two systems can be advantageously combined. We immerse a single trapped Yb+ ion in a Bose-Einstein condensate of Rb atoms. Our hybrid setup consists of a linear RF-Paul trap which is overlapped with a magnetic trap and an optical dipole trap for the neutral atoms. A first synergetic effect is the sympathetic cooling of the trapped ions to very low temperatures through collisions with the ultracold neutral gas and thus without applying laser light to the ions. We observe the dynamics of this effect by measuring the mean ion energy after having an initially hot ion immersed into the condensate for various interaction times, while at the same time monitoring the effects of the collisions on the condensate. The observed ion cooling effect calls for further research into the possibility of using such hybrid systems for the continuous cooling of quantum computers. To this end a good understanding of the fundamental interaction processes between the ion and the neutrals is essential. We investigate the energy dependent elastic scattering properties by measuring neutral atom losses and temperature increase from an ultracold thermal cloud of Rb. By comparison with a Monte-Carlo simulation we gain a deeper understanding of how the different parameters affect the collisional effects. Additionally, we observe charge exchange reactions at the single particle level and measure the energy-independent reaction rate constants. The reaction products are identified by in-trap mass spectrometry, revealing the branching ratio between radiative and non-radiative charge exchange processes. Mon, 10 Oct 2011 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/241264 2011-10-10T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/241040 Title: High-temperature superconductivity in a family of iron pnictide materials Authors: Gillett, Jack Abstract: The work in this thesis falls roughly into three parts, which I characterise loosely as a developmental stage, an exploratory stage, and an attempt to contribute to understanding of the field. In the developmental stage, I have worked to design a variety of methods to create high-quality samples of various Iron Pnictide superconductors, to dope them with various chemicals and to characterise the resulting crystalline samples. I discuss in depth the signature of good quality crystals and the various experiments that they have been used in by myself and my collaborators. These processes are ongoing and will hopefully continue to contribute to my research group’s capabilities. My exploratory work involves a detailed survey of one particular family, Sr(Fe1−xCox)2As2, as the level of Cobalt is varied, and the mapping of the phase diagram for the system. I have also made a comparison to the better-measured Barium analogue, and discuss the reasons for the differences in character between the two, most notably the lack of a splitting of the structural and magnetic transitions in the first species. I also discuss the effect of pressure, which can lead to superconductivity in lightly doped samples for very modest pressures; and annealing, which increases transition temperatures within samples, on a limited quantity of crystals. Finally, I attempt to contribute to the understanding of the field via a series of Resonant Ultrasound Spectroscopic experiments conducted by a collaborator on my crystals and analysed by me. I see distinct first-order transitions in the parent compounds, characterisable above the high-T structural transition within a Ginzburg-Landau pseudoproper ferroelastic scheme for a transition coupling weakly to strain but driven by another order parameter. My observations allow several statements about the symmetry of the order parameter and are suggestive of a non-magnetically driven structural transition. In the case of doped samples a much richer behavior is seen, with a broad transition and simultaneous relaxation of all elastic peaks and a broad temperature range of significant dispersion. The effect of the softening is seen far above TN and lends strong support to the familiy of models predicting such high-T fluctuations. Tue, 08 Nov 2011 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/241040 2011-11-08T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/241039 Title: Blind and pointed Sunyaev-Zel'dovich observations with the Arcminute Microkelvin Imager Authors: Shimwell, Timothy William Abstract: In this thesis I discuss my work on the Arcminute Microkelvin Imager (AMI). I focus on the detection of Sunyaev-Zel’dovich (SZ) signatures at 14-18GHz. Once the background science and operation of the instrument are described I proceed to present my contribution to the calibration of AMI, including: primary beam measurements; refinements to the known antenna geometry and flagging geostationary satellite interference. This is followed by an outline of the software that I have developed to subtract sources from visibilities, concatenate data from multiple observations, simulate data, and perform jack-knife tests to evaluate the magnitude of systematic errors. The Bayesian analysis that I use to obtain parameter estimates and to quantify the significance of putative SZ detections is described. I perform realistic simulations of clusters and use these to characterise the analysis. I then, for the first time, apply the analysis to data from the AMI blind cluster survey. I identify several previously unknown SZ decrements. Finally, I conduct pointed observations towards a high luminosity subsample of eight clusters from the Local Cluster Substructure Survey (LoCuSS). For each of these I provide probability distributions of parameters such as mass, radius, and temperature. I compare my results to those in the literature and find an overall agreement. Description: In this thesis I introduce the Arcminute Microkelvin Imager. I analyse observations towards known galaxy clusters and I present the initial results from a blind survey for galaxy clusters. Tue, 08 Nov 2011 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/241039 2011-11-08T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/241031 Title: A new model of spiral galaxies based on propagating star formation. Authors: Sleath, John Abstract: Many models exist in the literature of either star formation or galactic structure, but the former concentrate on small-scale details, whilst the latter, if they include star formation at all, adopt a very simple approach, for example by assuming a power law relationship between the rate of star formation and the gas density (a Schmidt Law). The new model described in this dissertation bridges the gap between these two extremes by adopting a simple, but not simplistic, approach to the detailed physics, allowing the effects of star formation on the broader scale to be investigated. 'Propagating star formation' considers the collapse of molecular clouds (and subsequent creation of new stars) to be triggered by the passage of a shock wave resulting from the supernovae explosions of members of the previous generation of stars. The approach taken is a stochastic one, i.e. we determine from the mass of a cloud the probability of star formation occurring, given that it has been shocked. Models using a similar approach have been described before, but the new model is unique in that it uses a particulate representation of the gas clouds and stellar associations. This permits us to simulate collisions between the particles as they orbit in a realistic galactic gravitational potential and more importantly, to impose a spiral density wave perturbation in a natural way. Such waves arise naturally in N-body simulations where the collective forces between particles are considered explicitly, but we are more interested in its effect on the star formation rate, and hence to make the code more manageable, impose the perturbation by hand. The model has been extremely successful; for example, predicting accurately, with no free parameters, the cluster formation rate for the Milky Way. A Schmidt Law arises as a natural consequence and with a power law index which is consistent with observational constraints. A wide range of galactic morphologies can be produced, including long-lived two-armed grand-design spirals, which have not resulted from any of the previous propagating star formation models. The spiral density wave orders the star formation, but does not simply result in the star formation tracing directly the potential minima - it is found that the pitch angles of the imposed and observed spiral patterns differ significantly. Moreover, the pitch angle of the observed pattern exhibits a maximum value equal to the maximum pitch angle observed in late-type spirals. To compare the results of this, and other, models of galactic structure with observed galaxies, we require some way of classifying the appearance of the data sets. There already exist a number of schemes, but they are all somewhat subjective, and a reliable, quantitative approach would form a valuable addition. I have investigated a number of schemes, namely Fourier transforms, minimal spanning tree edge-length spectra and multifractal dimensions, and considered their application to both simulated and observed data. The results of the analysis are encouraging, particularly for the multifractals, although it is not as yet possible to calculate a single, unique number which fully characterises the morphology. Mon, 01 Jan 1996 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/241031 1996-01-01T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/239412 Title: Hot spots in ammonium nitrate Authors: Taylor, Nicholas Abstract: Ammonium nitrate (AN) is commonly used as an explosive and as a fertilizer. In both roles it is provided as prills or pellets, approximately spherical and a few millimetres in diameter. The microstructures of several commercially-available AN compositions were investigated using environmental scanning electron microscopy (ESEM) and X-ray microtomography. Those intended for explosive use were found to be more porous than those intended for fertilizer use. The pores in explosive prills were also found to form a connected network. The elemental composition of pellets of mixed AN and dolomite was investigated using energy-dispersive X-ray spectroscopy (EDX); the dolomite additive was found to take the form of grains roughly 50 μm in size. The compaction behaviour of confined cylindrical beds of these prills and pellets was studied at strain rates between 4 × 10−4 s−1 and 200 s−1. Quasi-static experiments were performed using a screw-driven instrumented press, while higher-rate experiments used a drop weight, instrumented with a line laser and load cell. The resistance of a bed to compaction was found to depend on the microstructure of its prills in most cases. Denser prills offered greater resistance to compaction. The exception to this rule was a pellet, rather than prill, formulation. Beds were also found to offer more resistance to compaction at higher strain rates. The Kawakita compaction model was found to agree well with the experimental data. A commercial fertilizer, not containing any AN, was assessed for use as an inert mock for AN prills and pellets. Prills of a suitable size for this purpose were found using EDX to consist of P2O5, with a coating of unknown composition. They were supplied mixed with smaller K2CO3 and urea prills. The mixture was found to have comparable compaction behaviour to AN compositions, indicating that it was useful as a mock for those compositions. In a plate impact experiment on a single layer of P2O5 prills, very little light was observed. This indicated that these prills were sufficiently inert for these purposes. The light produced by shocked granular ammonium nitrate beds and single prill layers was investigated using high-speed framing photography, photodiodes and gated visible-light spectroscopy. Framing photography of prill layers suggested that reaction in prill beds was dominated by effects internal to prills. This was further supported by the similarity between photodiode recordings of prill beds and beds of inert prills containing a single reactive prills. Framing photography of drop weight experiments searching for a mechanism for initiation of reaction by interaction between prills found nothing. Decay of the light output of the beds suggested that in both granular and prill beds this light output was due to small regions heated to thousands of kelvin, which then cooled. Spectroscopic study confirmed this. These regions were found to reach a peak temperature of 6660 ± 20 K, well in excess of the approximately 2000 K predicted by a simple chemical model. Investigation of spectral lines observed during this study indicated that the exothermic reaction that led to heating of these emitting regions involved NO. Mon, 11 Jul 2011 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/239412 2011-07-11T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/239399 Title: Phenomenological aspects of new physics at high energy hadron colliders Authors: Papaefstathiou, Andreas Abstract: This thesis contains studies of phenomenological aspects of new physics at hadron colliders, such as the Large Hadron Collider (LHC). After a general introduction in chap- ter 1, in chapter 2 we outline the main features of the Standard Model (SM) of particle physics and the theoretical motivations for going beyond it. We subsequently provide brief descriptions of a few popular models that aim to solve the issues that arise within the SM. In chapter 3 we describe the general Monte Carlo method for evaluating multidimen- sional integrals and show how it can be used to construct a class of computational tools called Monte Carlo event generators. We describe the main generic features of event generators and how these are implemented in the HERWIG++ event generator. By applying resummation techniques, we provide, in chapter 4, analytical calcula- tions of two types of hadron collider observables. The first, global inclusive variables, are observables that make use of all measured particle momenta and can provide useful information on the scale of new physics. The second observable is the transverse energy of the QCD initial state radiation (ET ), associated with the either Drell-Yan gauge boson production or Higgs boson production. In both cases we provide comparisons to results obtained from Monte Carlo event generators. In chapter 5 we examine two well-motivated models for new physics: one of new heavy charged vector bosons (W prime), similar to the SM W gauge bosons, and a model moti- vated by strong dynamics electroweak symmetry breaking that contains new resonances, leptoquarks, that couple primarily to quarks and leptons of the third generation. In the prior model, we improve the current treatment of the W′ by considering interference ef- fects with the SM W and construct an event generator accurate to next-to-leading order which we use to conduct a phenomenological analysis. For the leptoquark model, starting from an effective Lagrangian for production and decay, we provide an implementation in the HERWIG++ event generator and use it to form a strategy for mass reconstruction. The thesis ends with some conclusions and suggestions for extensions of the work presented. Further details and useful formulæ are given in the appendices. Mon, 11 Jul 2011 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/239399 2011-07-11T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/239396 Title: Shock compression of water and solutions of ammonium nitrate Authors: Morley, Michael James Abstract: Modern mining explosives employ solutions of ammonium nitrate, where the solution is the oxidising component of a fuel/oxidiser mixture. This thesis is primarily concerned with the shock response of water and of aqueous solutions of ammonium nitrate. Of particular interest are the temperatures induced through shock compression. An experimental facility, using a single stage gas gun in the ‘plate impact’ configuration, is described, along with associated experimental diagnostics. Measurements of, and improvements to, the tilt at impact are reported. The problem of shock temperature is discussed, including a brief review of the relevant literature. It is demonstrated that direct measurement of shock temperature is a complex issue that is not yet fully understood, whereas determination of temperature from an equation of state is an established technique. In a series of experiments, plate impact techniques were utilised to determine the Hugoniot and, through shock/reload experiments, the equation of state of water and aqueous solutions of ammonium nitrate. In-situ manganin gauges were used to measure stresses in the liquids and, from the arrival times of the shock wave, determine the shock velocity. Linear shock velocity–particle velocity Hugoniots for the liquids were determined, up to particle velocities of 1km/s, with uncertainties on the intercept and slope of these Hugoniots of 5%. A Mie-Grüneisen equation of state was used to describe the shock/reload experiments. Approximate calculations of shock temperature are reported. Increasing ammonium nitrate concentrations resulted in greater calculated temperatures. It was demonstrated that the liquids investigated in this thesis show a temperature dependence of the Grüneisen parameter, Г, which cannot be accommodated in the model. The present work is believed to be the first demonstration of this effect in shock compressed liquids. The data presented provide constraints on future theoretical development of equations of state. Mon, 11 Jul 2011 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/239396 2011-07-11T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/239393 Title: Cell compliance: cytoskeletal origin and importance for cellular function. Authors: Lautenschlaeger, Franziska Abstract: Mechanical properties of cells, mainly defined by their cytoskeleton, are closely related to cell function and can be measured with a dual-beam laser trap (optical stretcher). Functional changes, which go hand in hand with changes of the cytoskeleton, also occur during differentiation of stem cells. This suggests monitoring differentiation by the changing compliance of the cells. During the course of my PhD I measured the compliance of three different types of stem cells before and after differentiation and was able to detect differences in some of the cell types. In order to relate rheological experiments to cell migration as a further example of functional change I investigated the migration behavior of cells that showed different compliance and found differences in migration. I was additionally able to show an altered migration behavior after I actively changed the mechanical behavior of one cell type using cytoskeletal drugs. These migration experiments have been carried out in 2D and 3D migration assays. Furthermore, the influence of the stiffness of the surrounding material on the migration behavior has been investigated. After relating functional changes to changes in compliance, I studied which mechanisms can be used to actually influence cell compliance and investigated the effect of cytoskeletal stabilizers or destabilizers as well as drugs acting on molecular motors. The effect of the surrounding temperature has been considered as well. Finally, I developed a new version of the optical stretcher measurement tool, which enables cell sorting and drug screening using a monolithic glass chip. With the results presented in this thesis I relate mechanical compliance to the cytoskeleton and specific cellular functions. I deliver insights how mechanical changes in cells can be used to identify and follow functional changes and how this knowledge can help to interfere with such functions, specifically in pathologies correlated to these functions. My modified optical stretcher would be developed to screen the effects of drugs on cell compliance and to sort cells with different mechanical properties. Such drug screening and cell sorting will offer diagnostic treatment options for various pathologies. Description: This thesis was released from embargo in July 2012 Mon, 11 Jul 2011 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/239393 2011-07-11T23:00:00Z