New types of quantum interference oscillations in transverse magnetoresistance of single crystals of ultrapure magnesium are reported. These oscillations occur as a function of the angle of rotation when the current is passed along the [112̄0] direction and the magnetic field H(→) is rotated by ≲ 1° from [101̄0] towards the [0001] symmetry axis. In order to characterize the oscillations, extensive qualitative data were taken for fields up to 24 kG and for temperatures in the range of 1.4 K-4.2 K. It is shown that these angle-dependent oscillations have the same origin as the field-dependent interferometer oscillations first reported by Stark and Friedberg. Both types of oscillations arise from the electron quantum states on the coupled orbit network, which is obtained for H(→)∥[1010]. It is shown that high sensitivity of the oscillations and the background magnetoresistance (for H within →1° from [101̄0] to field inhomogeneity and crystal strain yields strong evidence for a new regime of quantum transport. In this regime, quantum phase coherence of the electrons extends over distances of the order of 0.1 mm, and coherence determines not only the oscillation amplitudes but also the background. The "stacked mirror" model of Stark and Reifenberger is not applicable in such a regime of transport. A rudimentary model is presented that seems to be in qualitative agreement with the data. However, band structure calculations firmly establish that the large oscillations arise from the symmetry breaking of the two interferometer lobes as the field is rotated away from [101̄0].

We report here the results of a de Haas-van Alphen (dHvA) investigation of the coupled orbit system in magnesium. The data were taken for magnetic fields extending to 52 kG and temperatures down to 0.29° K. The experimental data are interpreted in the light of coupled orbit system theories of Pippard and of Falicov and Stachowiak; these are each reviewed in some detail. The data are found to disagree qualitatively with the predictions of Falicov and Stachowiak. Since this theory has been assumed, for more than a decade, to be equivalent to Pippard's theory, a detailed comparison of these was made. Full spectrum Fourier analysis of Pippard's band structure density of states shows that the two models disagree qualitatively and, therefore, that they are not equivalent. These experimental results, which do not appear to disagree with Pippard's theory, are interpreted to mean that it is fnally possible to obtain crystals of sufficient purity and perfection to make it necessary to use a band structure description of the delocalized electrons on the couple network. Evidence is presented for the existence of a new type of dHvA frequency corresponding to the ΓKM plane cross-section of the Brillouin zone. A proposed explanation for this dHvA frequency involves the field dependent modulation of the zero frequency component of the Fourier transform of the coupled orbit system density of states. Also included are discussions of crystal preparation and handling, cryogenic apparatus, analogue detection apparatus, digital data acquisition and processing hardware based on a microcomputer, and a new software system ideally suited to small computer research environments.

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