New Zealand has one of the highest asthma prevalence rates among developed countries and previous research attributes this partly to poor socioeconomic conditions and to insufficient home heating in particular. Retrospective empirical studies from overseas suggest that home heating is associated with asthma rates. However, the evidence to date is not conclusive. In this thesis, I present a theoretical framework and empirically investigate the link between home heating and asthma hospital admissions in New Zealand using panel data techniques and controlling for endogeneity. The hypothesis that higher electricity prices (via less adequate heating) increase asthma admissions is tested and receives strong empirical support across a number of model specifications and datasets used.

No description available.

Understanding the physics taking place in coupled one-dimensional systemsis one of the many challenges of modern day condensed matter physics and nanoelectronics. While experimental studies in coupled quantum wires have recently confirmed some of the most striking predictions of Luttinger liquid theory such as spin-charge separation and charge partitioning, much remains to be done prior a complete understanding of one-dimensional physical phenomenons is achieved, especially in the field of one-dimensional Coulomb drag. In this thesis, I report our experimental study of one-dimensional Coulomb drag between quantum wires coupled at the nanoscale. The quantum wires are coupled in a vertical geometry, allowing the wires to be separated by a hard barrier only 15 nm wide and providing us with the possibility to study Coulomb drag in a regime never achieved previously. Our study of the 1D subband dependency of Coulomb drag shows an oscillation of the drag resistance (RD) with 1D subband occupancy. Peaks in the drag signal are observed concomitant with the opening of 1D subbands in either wire, regardless of 1D subband alignment between the wires, and a novel high electronic density re-entrant negative regime for RD is observed. These findings are not fully understood within the current theoretical momentum-transfer models for Coulomb drag between quantum wires. However, some of the predictions of a charge-fluctuation induced model for 1D Coulomb drag in mesoscopic circuits are consistent with our observations and raise questions as to whether mesoscopic physics play an important role in one-dimensional Coulomb drag. The temperature dependence of the drag resistance is also presented in the one-dimensional regime where both wires have no more than a single 1D subband occupied. As the temperature is reduced below the Fermi temperature TF , a decrease in RD and a subsequent upturn is observed in three different devices at T* ∼ 1.6 K, flagging a regime where RD increases with decreasing T (verified down to ∼ 75 mK). This upturn in the drag resistance andthe diverging drag resistance at the lowest temperatures is consistent with expectations from Tomonaga-Luttinger liquid models of 1D quantum wires ,potentially validates models including forward scattering corrections and is a strong sign that interaction effects and momentum-transfer play an important role in one-dimensional Coulomb drag. A crucial step for the future of electronic nano-devices is the development of doped shallow two-dimensional electron gases (2DEGs). In an effort towards this goal, we have also studied scattering mechanisms in shallow 2DEGs in parallel to our Coulomb drag experiment. In this endeavor, we achieved the fabrication of 2DEGs as shallow as 60 nm deep with a mobility in excess of 1×10^5 cm^2/ V · s and determined that scattering of intentional remote charged impurities is the dominant scattering mechanism in samples 130 nm deep and shallower. / La compréhension de la physique prenant place dans les sysèmes unidimensionels couplés est un des nombreux défis auxquels la physique de la matière condensée moderne et la nano-électronique sont confrontées. En dépit du fait que certaines études portant sur des fils quantiques couplés aient confirmé certaines des prédictions les plus fascinantes de la théorie des liquides de Luttinger tels que la séparation des spins et des charges ainsi que la partition des charges, beaucoup reste à faire avant qu'une compréhension complète des phénomènes prenant naissance dans les systèmes unidimensionels ne soit atteinte, surtout en ce qui a trait à la traînée de Coulomb unidimensionnelle. Dans cette thèse, nous rapportons l'étude expérimentale de la traînée de Coulomb unidimensionnelle entre des fils quantiques couplés à l'échelle nanométrique. Les fils quantiques sont couplés dans une géométrie verticale permettant aux fils d'être séparés par une barrière large de seulement 15 nm, nous donnant ainsi l'occasion d'étudier la traînée de Coulomb dans un régime jamais exploré auparavant. Les résultats de notre étude de la dépendance de la traînée de Coulomb avec le niveau d'occupation des sous-bandes unidimensionnelles des fils quantiques montrent une oscillation de la résistance de la traînée de Coulomb (RD) en fonction du nombre de sous-couches occupées dans les fils quantiques. Des maximums dans RD sont observés simultanément à l'ouverture de sous-bandes unidimensionnelles dans l'un ou l'autre des fils et un nouveau régime de RD négatif et résurgent est observé à haute densité. Ces observations ne sont pas complètement expliquées par les modèles actuels expliquant la traînée de Coulomb par un transfer de quantité de mouvement. Toutefois, certaines prédictions des modèles expliquant l'émergence de la traînée de Coulomb unidimensionnelle par un échange de fluctuations sont en accord avec nos résultats et soulèvent des doutes à savoir si la physique mésoscopique joue un role dans l'émergence de la traînée de Coulomb unidimensionnelle. La dépendence en température de la résistance de la traînée de Coulomb est également présentée dans le régime unidimentionel où les fils ont au plus une seule sous-bande de populée. Alors que la température est abaissée sous la température de Fermi, une diminution de RD est observée, suivie d'un renversement de cette tendence. Ce renversement est observé dans trois dispositifs distincts à une température T* ∼ 1.6 K et marque une transition vers un régime où RD augmente alors que la température diminue (mesuré jusqu'à ∼75 mK). La présence de ce renversement et d'une divergence de RD à basse température est en accord avec les prédictions de la théorie des liquides de Tomonoga-Luttinger pour des fils quantiques unidimentionels, confirment potentiellement les modèles incluant des corrections pour des faibles valeurs de transfer de quantité de mouvement et suggèrent fortement que les interactions et le transfer de quantité de mouvement sont importants dans l'émergence de la traînée de Coulomb unidimensionnelle. Une étape cruciale pour le futur développement de nano-dispositifs électroniques est la création de puits quantiques peu profonds et dopés. Dans l'espoir d'atteindre cet objectif, nous avons étudié les mécanismes de diffusion dans des puits quantiques peu profonds en parallèle à notre étude de la traînée de Coulomb unidimensionnelle. Au cours de cet effort, nous sommes parvenus à fabriquer des puits quantiques situés seulement 60 nm sous la surface avec une mobilité supérieure à 1 × 10^5 cm^2/ V · s et nous avons déterminé que ladiffusion par les impuretés ionisées volontairement insérées dans la structure est le mode de diffusion dominant dans les puits quantiques profonds de moins de 130 nm.

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A current-switched superconducting field-cycling NMR spectrometer has been designed and built for studying the role of quantum tunnelling in molecular dynamics. The instrument is designed for work in the solid state with sample temperatures extending from 4K up to 300K. The maximum field-switching rate is 10Ts-1. Among the samples studied in this thesis is the nuclear spin-relaxation and proton tunnelling. Concerted double proton transfer in the hydrogen bonds of carboxylic acid dimers is well established as the model system for translational quantum tunnelling. The model system has been chosen to illustrate the smooth quantum-to-classical transition and at all temperatures the proton transfer is characterised by a single correlation time. Quadrupolar interactions introduce an additional relaxation to the proton spin polarisation. The enhanced relaxation of the proton spin appears as a dip in the proton magnetisation curve. This technique is employed to measure the quadrupolar transition frequency of 14N and 35Cl and determine the structure of heroin hydrochloride. The introduction of a second spin species has a significant effect on the spin-lattice relaxation. Compared with homonuclear systems, the spectral density acquires additional components characterised by the sum and difference Larmor frequencies of the two nuclei. Further, instead of a single relaxation time, there are four elements of a relaxation matrix. Therefore, the magnetisation recovery becomes bi-exponential and the initial polarisation state of the second nucleus strongly affects the magnetisation recovery of the nucleus which is being observed. We shall report on the results of spin-lattice relaxation investigations on 1H-13C, 1H-19F systems. The role of heteronuclear interactions in spin-lattice relaxation and the newly developed methodology of field-cycling relaxometry will be discussed. This represents the first 13C field-cycling NMR experiment and the first to measure the field dependence of the off-diagonal element of the relaxation matrix.

The technique of functional magnetic resonance imaging is rapidly moving from one of technical interest to wide clinical application. However, there are a number of questions regarding the method that need resolution. Some of these are investigated in this thesis. High resolutionf MRI is demonstrated at 3.0 T, using an interleaved echo planar imaging technique to keep image distortion low. The optimum echo time to use in fMRI experiments is investigated using a multiple gradient echo sequence to obtain six images, each with a different echo time, from a single free induction decay. The same data are used to construct T2* maps during functional stimulation. Various techniques for correcting the N/2 ghost are tested for use in fMRI experiments, and a method for removing the image artefact caused by external r. f. interference in a non-linearly sampled matrix is presented. The steps in the analysis of fMRI data are detailed, and two new non-directed analysis techniques, particularly for data from single events, as opposed to epoch based paradigms, are proposed. The theory behind software that has been written for fMRI data analysis is also given. Finally, some of the results from an fMRI study into the initiation of movement are presented, illustrating the power of single event experiments in the separation of cognitive processes.

The aim of this work was to implement a thorough method for quantifying the errors introduced to frame-based neurosurgical stereotactic procedures by the use of MRI. Chang & Fitzpatrick's reversed gradient distortion correction method was used, in combination with a phantom, to measure these errors. Spatial distortion in MR images of between 1 mm and 2 mm was measured. Further analysis showed that this typically introduced an additional error in the coordinate of the actual treatment point of 0.7 mm. The implications of this are discussed. The main source of distortion in the MR images used for stereotaxis was found to be the head ring. A comparison between imaging sequences and MR scanners revealed that the spatial distortion depends mainly on the bandwidth per pixel of the sequence rather than other differences in the imaging sequences. By comparison with a phase map distortion correction technique, the imaging parameters required to allow successful distortion correction with the reversed gradient method were identified. The most important was the use of full Fourier spin echo acquisitions. The reversed gradient correction method was applied to two contemporary EPI techniques. Considerable improvement was seen in the production of ADC maps after the images had been corrected for distortion. The method also was shown to be valid in application to BOLD fMRI data.

Interest in coherent remote sensing systems has stimulated investigations in the properties laser propagation through extended atmospheric turbulence. This thesis investigates the statistics of phase, and phase related, observables using analytical and computational techniques, together with experimental results. The phase screen technique is used to simulate perturbations to the refractive index of a medium through which the radiation propagates. Several different turbulence models (Gaussian correlated noise, Kolmogorov turbulence, Tatarski and Von Karman spectral models) are investigated, and their relative merits for describing experimental conditions and descriptive statistical measures are compared and contrasted. The phase power spectrum is crucial to an understanding of the practical operation of a coherent imaging system, and later part of the thesis is devoted to the investigation of a LIDAR system in particular. Several turbulence regimes are investigated, from an analytical treatment of a weakly turbulent, extended atmosphere, to large 3D computations designed to simulate experimental arrangements. The 3D simulation technique presented herein has been developed to allow for the investigation of temporal statistics. New power law behaviours are found to appear in temporal frequency spectra which differ from the -8/3 power law form that has been accepted in much of the literature. Strongly turbulent regimes result in a -2 power law while the use of a Gaussian beam profile in an extended medium gives a -11/3 power law under weak turbulence conditions. Please note: Pagination in electronic reproduction differs from print original. The print version is the version of record.

This thesis addresses the problem of radiofrequency probe design for Ultra High Frequency Magnetic Resonance Imaging (7T). The signal-to-noise ratio available in Magnetic Resonance Imaging (MRI) is determined by the static magnetic field strength, causing a continued drive toward higher fields to enable faster image acquisition at finer spatial resolution. The resonant frequency increases linearly with static field strength. At 7T the proton resonant frequency is 300MHz, with a wavelength of approximately 13cm in tissue. As this is smaller than the dimensions of the human head, the phase of the radiofrequency (RF) signal varies considerably across the sample, producing field cancellation due to interference. A full wave electromagnetic simulator, using the Transmission Line Matrix (TLM) method, was developed to investigate RF probes at high frequency. A Birdcage probe operating at 64, 128 and 300MHz (corresponding to 1.5, 3 and 7T) was simulated, loaded with an anatomically detailed human head model. A half-wave microstrip was investigated for use as a high frequency probe element. Magnetic and electric fields produced by a single microstrip were simulated, and the strip dimensions varied to investigate the effect on field penetration into the head and Specific Absorption Rate (SAR). A transmit-receive array probe using four microstrip elements was then developed. Bloch simulations were run, using TLM generated magnetic fields, to investigate imaging at short wavelength. Parallel receive probes are demonstrated to offer considerable advantage over volume probes, as signals from receive elements can be combined without interference. There is no transmit equivalent to parallel reception; simultaneous excitation of independent probe elements causes interference in exactly the same manner as a volume probe. A new imaging sequence was developed using a Burst-like encoding to allow sequential excitation of probe elements, without interference, which can be recalled in a single readout. An improvement in image homogeneity was demonstrated, and SENSE acceleration of the new imaging sequence is shown. The sequence was implemented at 3T using a purpose built four element microstrip probe. An RF multiplexer was also built to enable transmit element switching during the imaging sequence. It was demonstrated that images due to different RF excitations, acquired in a single EPI readout, can be separated.

This thesis describes the development and application of quantitative echo planar magnetic resonance imaging techniques to the study of human placental development in normal and compromised pregnancies. Initially, a method of rapidly and accurately measuring the transverse relaxation time is proposed using a multi-echo measurement sequence. The method is described, validated on CUS04 phantoms and applied in the study of the human placenta and gastric dilution. It is shown that the inversion provided by sinc pulsesis insufficient to generate an accurate measurement but using adiabatic refocusing pulses yields a measurement that is comparable with a single spin echo. Subsequently, a rapid magnetisation transfer method is presented that allows the quantification of the relative size of the bound proton pool. An experimental pulse sequence is proposed, along with a theoretical model, that permits the investigation of the bound proton pool's transition towards the steady state. The sequence and model are validated using agar gel phantoms and shown to agree well with literature values. When applied in the study of the human placenta, it is shown that there is no significant variation in the fitted value of the bound proton pool size with increasing gestational age or in compromised pregnancies. Finally, several methods of measuring the oxygenation level of blood within the human placenta are investigated. The signal intensities of cardiac gated T~• and T~ weighted images acquired at various points in the maternal cardiac cycle are explored but no significant variation is shown through the cycle. A pulsed gradient spin echo sequence that utilises anti-symmetric sensitising gradients is validated and then applied in the human placenta. Oxygenation measurements with this technique are shown to be unfeasible but the potential of the sequence to monitor blood flow in the placenta is demonstrated.