Heterointerface Effect on Two-Step Nucleation Mechanism of Bi Particles.

The nucleation and crystallization of Bi particles on two matrices, crystalline bismuth sulfide (c-Bi2S3) and amorphized bismuth titanium oxide (a-Bi12TiO20), were studied by using in situ transmission electron microscopy (TEM) analysis. The atomic structures of the Bi particles were monitored by acquiring high-resolution TEM images in real time. The Bi particles were grown on c-Bi2S3 and a-Bi12TiO20 via a two-step nucleation mechanism; dense liquid clusters were clearly observed at the initial stage of nucleation, and the coalescence of clusters was frequently observed during the growth. However, the nucleation and crystallization behaviors of Bi particles were governed by the matrix; in particular, the evolution of their morphology and atomic structure was confined on c-Bi2S3 but free from matrix effects on a-Bi12TiO20. The matrix effect on the two-step nucleation mechanism was demonstrated from a thermodynamic point of view.

Switching the charge state of individual surface atoms at Si(111)-√3 × âˆš3:B surfaces.

We show that each surface atom of heavily boron-doped, (111)-oriented silicon with a √3 × âˆš3 reconstruction has electrically switchable two charge states due to the strong electron-lattice coupling at this surface. The structural and electronic properties of the two charge states as well as their energetics are uncovered by employing scanning tunneling microscopy measurements and density functional theory calculations, which reveals that one of the two is a two-electron bound state or surface bipolaron. We also execute the single-atom bit operations on individual surface atoms by controlling their charge states while demonstrating implementation of the atomic scale memory at a silicon surface with an unprecedented recording density.

Chalcogen-height dependent magnetic interactions and magnetic order switching in FeSexTe1-x.

Magnetic properties of iron chalcogenide superconducting materials are investigated using density-functional calculations. We find that the stability of magnetic phases is very sensitive to the height of chalcogen species from the Fe plane: while FeTe with optimized Te height has the double-stripe (pi, 0) magnetic ordering, the single-stripe (pi, pi) ordering becomes the ground state when Te is lowered below a critical height by, e.g., Se doping. This behavior is understood by opposite Te-height dependences of the superexchange interaction and a longer range magnetic interaction mediated by itinerant electrons. We also demonstrate a linear temperature dependence of the macroscopic magnetic susceptibility in the single-stripe phase in contrast with the constant behavior in the double-stripe phase. Our findings provide a comprehensive and unified view on the magnetism in FeSexTe1-x and iron pnictide superconductors.

Defects responsible for the hole gas in Ge/Si core-shell nanowires.

The origin of the ballistic hole gas recently observed in Ge/Si core-shell nanowires has not been clearly resolved yet, although it is thought to be the result of the band offset at the radial interface. Here we perform spin-polarized density-functional calculations to investigate the defect levels of surface dangling bonds and Au impurities in the Si shell. Without any doping strategy, we find that Si dangling bond and substitutional Au defects behave as charge traps, generating hole carriers in the Ge core, while their defect levels are very deep in one-component Si nanowires. The defect levels lie to within 10 meV from or below the valence band edge for nanowires with diameters larger than 33 A and the Ge fractions above 30%. As carriers are spatially separated from charge traps, scattering is greatly suppressed, leading to the ballistic conduction, in good agreement with experiments.

Formation of dopant-pair defects and doping efficiency in B- and P-doped silicon nanowires.

First-principles calculations are performed to investigate the stability of dopant-related defects and the dependence of doping efficiency on wire diameter and orientation in hydrogen-passivated silicon nanowires doped with B and P dopants. As the diameter decreases below a critical value, it is energetically more favorable for donor atoms to form donor-pair defects, which consist of two donors separated at the nearest-neighbor distance. While donor-pair defects are unstable in bulk Si, the stability of these defects is greatly enhanced because of the confinement effect in nanostructures, which leads to the increase of band gap and thereby the shallow level of a substitutional donor. As donor-pair defects are electrically inactive defects, the doping efficiency is expected to be suppressed in small-diameter wires, regardless of the presence of surface or interface dangling-bond defects which were previously proposed to be the compensating defects. In the case of B dopants, the formation of pair defects is unfavorable against shallow acceptor levels, in contrast to n-type dopants, without affecting the doping efficiency.