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GaAs nanowires (NWs) grown by molecular-beam epitaxy may contain segments of both the zincblende (ZB) and wurtzite (WZ) phases. Depending on the growth conditions, we find that optical emission of such NWs occurs either predominantly above or below the band gap energy of ZB GaAs (E ZB g ). This result is consistent with the assumption that the band gap energy of wurtzite GaAs (E WZ g ) is larger than E ZB g and that GaAs NWs with alternating ZB and WZ segments along the wire axis establish a type II band alignment, where electrons captured within the ZB segments recombine with holes of the neighboring WZ segments. Thus, the corresponding transition energy depends on the degree of confinement of the electrons, and transition energies exceeding E ZB g are possible for very thin ZB segments. At low temperatures, the incorporation of carbon acceptors plays a major role in determining the spectral profile as these can effectively bind holes in the ZB segments. From cathodoluminescence measurements of single GaAs NWs performed at room temperature, we deduce a lower bound of 55 meV for the difference E WZ g − E ZB g .

We present experimental evidence of the equilibrium coexistence between crystalline phases in heteroepitaxial films of MnAs on GaAs. The phases, which can coexist in the bulk system only at one temperature point, coexist in the epitaxial film over a wide temperature interval. An apparent contradiction with the Gibbs phase rule is resolved by the presence of strain in the film.

We investigate the low-temperature transport properties in Cu x Bi 2−x Se 3 films prepared by a hot-wall-epitaxy growth of Bi 2 Se 3 layers on Cu-deposited substrates. We observe a positive magnetoresistance due to the weak antilocalization effect and a classical magnetoresistance that exhibits a power-law dependence on the magnetic field. The resistance increases logarithmically with lowering temperature regardless of the strength of the magnetic field. The electron-electron interaction effect is thus evidenced to be strong. While the magnitude of the weak antilocalization effect is in reasonable agreement with theory, the correction to the conductivity due to the electron-electron interaction effect is unaccountably larger than the theoretical prediction. The discrepancy may indicate that the contribution from the bulk state is as large as that from the surface states, at least, for the interaction effect.

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