UBC Theses and Dissertations
Muonium in some insulating oxides and diamond Spencer, David Philip
Muonium can be regarded as an isotopic analogue of the hydrogen atom, with a positive muon replacing the proton. This thesis is concerned with searches for muonium in weak transverse and zero magnetic fields in a variety of non-magnetic oxide insulators with few nuclear moments, and in diamond, a semiconductor. The technique of Muon Spin Rotation (µSR) was used, which can be thought of in analogy with NMR and EPR. It relies on measuring the direction of the muon spin with time via the unique signature of the parity violating decay of the muon, in which its decay positron is emitted preferentially along the muon spin direction. This can provide a sensitive measure of the interaction of the muon spin with its environment. Muonium has been searched for successfully in ɑ-quartz, fused quartz., hexagonal and fused germanium dioxide, magnesium oxide, and diamond; and unsuccessfully in tetragonal germanium dioxide, rutile, strontium titanate, zircon, and beryl. All the samples save the quartzes showed a large "missing fraction"; that is, not all the initial muon polarization could be accounted for. This indicates that muonium was formed but rapidly depolarized. The missing fractions were found to be very strongly temperature dependent in strontium titanate but only weakly in rutile. At room temperature, the observed muonium fractions were: ɑ-quartz and fused quartz (65±5)%, hexagonal germanium dioxide <2%, fused germanium dioxide (10±4)%, magnesium oxide (30±10)%, and diamond (33± 4)%. It is noteworthy that the fractions are very different in quartz and germanium dioxide, despite their structural analogy. The observation of muonium in diamond represents the first time that muonium has been seen in a room temperature semiconductor. A spin-Hamiltonian new to µSR was found in low-temperature ɑ-quartz and hexagonal germanium dioxide, in which the hyperfine interaction is completely anisotropic. In low-temperature ɑ-quartz in zero magnetic field this interaction gives rise to three signals whose frequencies are orientation independent, but whose amplitudes are strongly orientation dependent By studying the orientation dependence it was determined that muonium occupies a site which is identical with that seen for hydrogen in EPR studies. This was the first time that muonium was shown to behave just like an isotope of hydrogen in a solid. In hexagonal GeO₂, eight frequencies were observed at 6K, in marked contrast to the observation of only three frequencies in structurally analogous ɑ-quartz. The interpretation of this is that there are three or four different sites, the exact nature of which has not been determined.