The thermal conductivity of polycrystalline semiconductors with type-I clathrate hydrate crystal structure is reported. Ge clathrates (doped with Sr and/or Eu) exhibit lattice thermal conductivities typical of amorphous materials. Remarkably, this behavior occurs in spite of the well-defined crystalline structure and relatively high electron mobility (∼ 100 cm 2 /Vs). The dynamics of dopant ions and their interaction with the polyhedral cages of the structure are a likely source of the strong phonon scattering.

The intra-Laudau-band magnetoplasma spectrum of a periodically density-modulated twodimensional electron gas is shown to exhibit unusual oscillatory behavior. Such oscillations (periodic in \/B) are seen to arise from the commensurability of the spatial period of the modulation with the cyclotron-orbit size at the Fermi level.

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