Shin Yaginuma

Electronic Structure of Ultrathin Bismuth Films with A7 and Black-Phosphorus-like Structures

Using scanning tunneling spectroscopy and first-principles calculations, we have studied the electronic structure of two different ultrathin bismuth films on a Si(111)-7×7 substrate; a hexagonal film (HEX film) having a bulklike (A7-like) structure and a film having a black-phosphorus-like structure (BP film). The HEX film is metallic because of spin–orbit (SO)-split surface-state bands lying inside the projected bulk band gap near the Fermi level. Another SO-split surface state is also observed inside the SO gap. The BP film exhibits a significant reduction in metallicity in contrast to the HEX film. This is related to the formation of a very stable paired-layer structure, the mechanism of which is similar to that of the stabilization of semiconducting bulk black P. However, unlike bulk black P, a certain extent of metallicity still remains. This slight metallicity can be associated with buckling and strain in the BP film, which is analogous to the fact that shear angle distortion in bulk Bi is responsib...

Sound-Wave-Like Collective Electronic Excitations in Au Atom Chains

Sound-wave-like collective excitation in Au atom chains on the Si(557) surface is investigated. Electron scattering spectroscopy using a highly collimated slow electron beam has detected a characteristic low-energy one-dimensional (1D) plasmon (wire plasmon) that is confined in the atom chains. Theoretical analysis adopting a quantum-mechanical scheme beyond the free-electron model indicates a significant dynamic exchange-correlation effect due to strong 1D confinement. By cooling to below 100 K, we have detected for the first time a significant change in the plasmon dispersion in a tiny momentum and energy region, which definitely reflects a gap opening due to a metal-to-insulator transition of the atom chains.

The excitation of one-dimensional plasmons in Si and Au–Si complex atom wires

An atom-scale quantum wire array at the Au adsorbed Si(111) surface is studied by electron energy loss spectroscopy. Clear one-dimensional metallicity is verified by the observation of low-energy plasmonic excitation which exhibits a strong anisotropic dispersion. Our theoretical analysis using a quantum-mechanical nonlocal response theory shows that the plasmons are most probably supported in conductive channels made of Si honeycomb wires rather than those made of Au-Si complex wires.

Edge States of Bi Nanoribbons on Bi Substrates: First-Principles Density Functional Study

By using fully relativistic first-principles calculations, we study edge states of the Bi(001) nanoribbons. We find that freestanding zigzag bismuth nanoribbons (ZBNRs) have two spin degenerate bands around the Fermi energy, whose wave functions are localized at the edges. The wave functions are sharply localized at the edges at the zone boundary and become delocalized as the wave number decreases. In the case of the ZBNR on Bi substrates, the inversion symmetry is broken. As a result, the spin degenerate bands split and thus the density of states near the Fermi level has broad distributions; therefore, the electronic structures are expected to be stabilized. Because of the edge state near the Fermi energy, conduction along the edge lines is expected. However, the topological insulator predicted in the case of the freestanding ZBNR is not achieved in the case of the ZBNR on Bi substrates.

Comparative Study of Atomic and Electronic Structures of P and Bi Nanofilms

We perform first-principles calculations on P and Bi nanofilms and clarify the atomic and electronic structures of these films. The energy gap of the two-atomic-layer film of P has a band gap of 1.2 eV, and as the thickness becomes large, the band gap becomes close to that of the bulk, which is a semiconductor. The most stable structure of the two-atomic-layer Bi film has an energy gap of 0.2 eV and two atoms on the surface are buckled. The nonbuckled structure is metallic and is metastable and its energy is slightly (12 meV) higher than that of the buckled structure. We discuss why the P films are nonbucked and the stable Bi films are buckled.

Phase-operation for conduction electron by atomic-scale scattering via single point-defect

In order to propose a phase-operation technique for conduction electrons in solid, we have investigated, using scanning tunneling microscopy, an atomic-scale electron-scattering phenomenon on a 2D subband state formed in Si. Particularly, we have noticed a single surface point-defect around which a standing-wave pattern created, and a dispersion of scattering phase-shifts by the defect-potential against electron-energy has been measured. The behavior is well-explained with appropriate scattering parameters: the potential height and radius. This result experimentally proves that the atomic-scale potential scattering via the point defect enables phase-operation for conduction electrons.

Resonance enhancement of difference-frequency generation through localized surface plasmon excitation

We report the experimental observation of difference-frequency generation in gold nanoparticles under localized surface plasmon excitation. A zero-delay peak is detected in the differential transmission signal for a gold nanoparticle film with a MgF2 overlayer, showing that the energy transfer from pump light to probe light through the difference-frequency generation is resonantly enhanced by the excitation. This peak of differential transmission decreases in strength with higher probe fluences. Both the enhancement and the power dependence of the difference-frequency generation are explained by modeling the localized surface plasmons as a nonlinear Lorentz resonator.

Thickness dependent phase transition of Bi films quench condensed on semiconducting surfaces

Using electron diffraction methods, we have studied the amorphous to crystalline phase transition of quench condensed bismuth films on three semiconducting surfaces: Si(111)-β--Bi, Si(111)-7 × 7, and Si(111)--In. Using reflection high energy electron diffraction (RHEED), we monitored the film growth in real-time, and observed a thickness dependent amorphous to crystalline phase transition. For the same growth conditions, we found a substrate dependent crystallisation thickness: 4 ML on Si(111)-β--Bi, and 8 ML on both the Si(111)-7 × 7 and the Si(111)--In surfaces. We interpret these results in terms of the free energies of crystalline Bi thin films, and hypothesise that the differing thickness results from a lowering of the free energy of crystalline Bi films on the Si(111)–β--Bi surface, with respect to the Si(111)-7 × 7 and the Si(111)–-In surfaces.

Softening versus hardening transition in surface bilayer bonding of bismuth nanofilm

We have found that the surface bilayer of Bi(001) undergoes an unusual reversible transition at 350 K, well below the bulk melting temperature. Surface Debye temperature abruptly decreases without an anharmonic signature and the vertical-to-lateral ratio of surface thermal expansion increases in a critical manner, as revealed by high-resolution diffraction measurement. Because of the metallic-covalent bilayer structure of semimetal Bi, the most feasible explanation for the result is that bond softening occurs between the first and second bilayers at 350 K. This softening transition leads to the simultaneous hardening of the topmost intrabilayer bonds.

Controlling molecular condensation/diffusion of copper phthalocyanine by local electric field induced with scanning tunneling microscope tip

We have discovered the condensation/diffusion phenomena of copper phthalocyanine (CuPc) molecules controlled with a pulsed electric field induced by the scanning tunneling microscope tip. This behavior is not explained by the conventional induced dipole model. In order to understand the mechanism, we have measured the electronic structure of the molecule by tunneling spectroscopy and also performed theoretical calculations on molecular orbitals. These data clearly indicate that the molecule is positively charged owing to charge transfer to the substrate, and that hydrogen bonding exists between CuPc molecules, which makes the molecular island stable.