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We present a magnetotransport study of a disordered two-dimensional hole system in a strained Ge quantum well. As the magnetic field is increased, a clear transition from a low magnetic field insulator to the $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$ quantum Hall state at the lowest density range (controlled by a gate), and to the $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3$ state at higher densities, is observed. We find that these transitions are characterized by a new universality: At the critical point, the diagonal and Hall resistivities are equal, within experimental uncertainty. These results are in conflict with the ``floating'' scenario suggested by Khmel'nitzkii [JETP Lett. 38, 552 (1983)] and Laughlin [Phys. Rev. Lett. 52, 2304 (1984)].

Changes in surface roughness have been studied as a function of bulk compressive and tensile strains (biaxial in the plane of the sample surface) in thin films of compositionally uniform and dislocation-free ${\mathrm{Ge}}_{0.5}$${\mathrm{Si}}_{0.5}$. A pronounced surface roughness is observed only for films under compressive strains exceeding 1.4%. Molecular dynamics simulations show that this striking result has its origin in the strain-induced lowering of surface step free energies.

X-ray absorption measurements from H-passivated porous Si and from oxidized Si nanocrystals, combined with electron microscopy, ir absorption, \ensuremath{\alpha} recoil, and luminescence emission data, provide a consistent structural picture of the species responsible for the visible luminescence observed in these samples. The mass-weighted average structures in por-Si are particles, not wires, with dimensions significantly smaller than previously reported or proposed.