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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Multiferroics, materials in which both magnetic and electric fields can induce each other, resulting in a magnetoelectric response, have been attracting increasing attention, although the induced magnetic susceptibility and dielectric constant are usually small and have typically been reported for low temperatures. The magnetoelectric response usually depends on d-electrons of transition metals. Here we report that in [(GeTe)2(Sb2Te3)l]m superlattice films (where l and m are integers) with topological phase transition, strong magnetoelectric response may be induced at temperatures above room temperature when the external fields are applied normal to the film surface. By ab initio computer simulations, it is revealed that the multiferroic properties are induced due to the breaking of spatial inversion symmetry when the p-electrons of Ge atoms change their bonding geometry from octahedral to tetrahedral. Finally, we demonstrate the existence in such structures of spin memory, which paves the way for a future hybrid device combining nonvolatile phase-change memory and magnetic spin memory.

Weak antilocalization and electron-electron interaction effects are investigated in Sb 2 Te 3 layers. We accomplish smooth top and bottom surfaces for the layer using molecular-beam epitaxy, as revealed by the Kiessig oscillations in the x-ray reflectivity. The two helical surface states of the layer are found to contribute identically to the weak antilocalization effect. They are left intact in spite of low mobility and high concentration of unintentionally doped holes. The magnitude of the electron-electron interaction effect is consistent with the indication that both of the surface states survive in the layer. The robustness of the surface states demonstrates superiority of Sb 2 Te 3 over Bi 2 Se 3 and Bi 2 Te 3 . We also show that the phase-change property of Sb 2 Te 3 provides controllability to switch the existence of the surface states.

GaAs–MnAs core-shell structures are grown by molecular-beam epitaxy using wurtzite GaAs nanowires on GaAs(111)B. The nanowire structures curve due to the strain at the heterointerface when the substrate is not rotated during the growth, evidencing the diffusion length in the MnAs overgrowth being less than the perimeter of the columns. The MnAs growth is thus demonstrated to take place by direct deposition on the sidewall. The MnAs envelope is m-plane-oriented with the c-axis along the nanowire axis. The magnetic easy axis hence lies in the surface plane of the substrate, which is confirmed by magnetization measurements and magnetic-force microscopy.