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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 have observed the electronic transitions within the ('4)T(,1g) ground state of Co('2+) in MgO by Raman spectroscopy. We identified the (GAMMA)(,6g) (--->) (GAMMA)(,8g)('1) transition at 305 cm('-1) and the (GAMMA)(,6g )(--->)(, )(GAMMA)(,8g)('2) transition at 930 cm('-1); the (GAMMA)(,6g )(--->)(, )(GAMMA)(,7g) transition is not seen and is lost in the two-phonon band. We also observed an impurity induced vibration mode at 280 cm('-1), in resonance with the acoustic phonon branches at the surface of the Brillouin zone. The position of the electronic levels cannot be explained by crystal field theory, with the spin-orbit interaction to second-order included. We have calculated the effect of a dynamical Jahn-Teller interaction and have shown, for the first time, the necessity to include the coupling to the T(,2g) mode, in addition to the coupling to the E(,g) mode, although Co('2+) in MgO is a weak Jahn-Teller system. / We also used Raman spectroscopy to observe the vibrational modes of (beta)-In(,2)Se(,2). We observed, for the first time, the layer to layer shear mode(E(,2g)('2)), at 19 cm('-1). The position of this peak and the others, at 42(E(,1g)('1)), 117(A(,1g)('1)), 179(E(,2g)('1)), 181(E(,1g)('2)) and 231 cm('-1)(A(,1g)('2)), follows the trend of the vibrational spectra of (beta)-Ga(,2)S(,2) and of (beta)-Ga(,2)Se(,2) {65}. Using the linear chain model, we calculated the shear force constants and deduced the position of the doublet (E(,2g)('1) and E(,1g)('2)) with very good agreement with the experimental values.