Novel NMR methodologies using DC SQUIDs applied to fundamental condensed matter physics and biodiagnostics

The ability to probe the incredibly weak magnetism that results directly from nuclear spin is one of the wonders of physics, discovered in the middle of the last century. It relies on the fact that a nucleus will precess like a top in an applied magnetic field, at a rate that is species specific. In a MRI hospital magnet (typical field 1.5 T) the protons in the patient's body precess at around 60 MHz. This project relies on the use of Superconducting Quantum Interference Devices (SQUIDs), which operate as exquisitely sensitive flux to voltage transformers, to either measure still weaker signals from systems with low spin density or to perform spectroscopy and imaging in the Earth's magnetic field or lower. This work combines the technical challenge of building instruments, which open up a new domain for NMR, with new science. This new science ranges from fundamental low temperature physics to new applications for biological and medical diagnostics. Superfluidity is a state of matter that is characterised by the entire fluid being described by a macroscopic wavefunction. The lighter isotope of helium, helium-3, becomes superfluid when helium quasi-particles form pairs, at 0.94 mK for helium under its own vapour pressure. In 3He the pair diameter is around 70 nm at zero pressure. The aim of this project is to confine the superfluid in a cavity where the height is tuneable through the use of piezo-electric nanopositioning devices and comparable to the size of the pair. New phases and new physics are predicted to occur as the system is tuned into and through the two-dimensional limit. Predicted is an analogue of the quantum hall effect for transport of nuclear spin. But totally new, unexpected and exotic phenomena are likely to emerge as we enter uncharted territory. Nuclear magnetic resonance (NMR) experiments are sensitive to the phase of the superfluid and so provide the ideal probe with which to study the superfluid properties as a function of cavity height and temperature. The small size of these cavities, 10 -100 nm high, results in tiny signals so the high sensitivity of the SQUID, exploiting recent advances, is necessary for its detection. High resolution NMR spectroscopy and clinical magnetic resonance imaging, MRI, has revolutionised the study of chemical and biomolecular structure and the non-invasive diagnostic ability of the healthcare sector. One limiting factor to the use of these techniques is that the drive to improve performance has been focused on operation in ever higher magnetic fields. These high fields are obtained with sophisticated (expensive and large) superconducting magnets. This precludes the ability of these devices to be mobile, and results in a limited number of specialist facilities. This work aims to develop instruments for NMR and MRI that do not rely on a large superconducting magnet. The high, frequency independent, sensitivity of SQUIDs coupled with mechanisms to overcome the low intrinsic thermal polarisations in low fields means that operation in microtesla fields produced by simple magnets is possible. The long term aim is to couple SQUID based microtesla NMR/MRI instruments with cryogen free operation. More compact, cheaper, and mobile instruments, coupled to new imaging and spectroscopy techniques in the low field regime are expected to significantly extend the impact of the NMR method for analysis and a wide range of biodiagnostics. High spectral resolution, low field NMR spectroscopy offers the possibility of relating spin-lattice relaxation times, T1, to physical properties such as porosity. This could be exploited in the characterisation of bone porosity, used in the diagnosis of osteoporosis. The couplings between nuclei of different species result in a field independent frequency shift of the NMR signal that is sensitive to molecular conformations (the distance and angle between bonds) and has the potential to provide a sensitive detector of biomolecular interactions in vivo