[en] We have studied, for the first time, the energy and the linewidth dispersion of a plasmon in a dense two-dimensional electron system in a metallic surface-state band on a silicon surface. As expected from the considerably high effective density and long Fermi wavelength of the system, the plasmon energy dispersion exhibited an excellent agreement with the nearly free-electron theory. However, in a small wave number region below the Landau edge, we have observed an anomalous linewidth dispersion which nearly free-electron theories do not predict

[en] A new method for chemical analysis of surfaces by total-reflection-angle X-ray spectroscopy in RHEED experiments (RHEED-TRAXS) has been developed. When the X-ray take-off angle is set to be the critical angle for total reflection of the characteristic X-ray emitted from the deposited atoms on surfaces, the detection efficiency for the deposit becomes drastically higher owing to the refraction effect of the X-ray. This enhancement of surface sensitivity is demonstrated with Ag on Si(111). The smallest detectable amount of Ag is about 0.01 monolayer or less. This sensitivity is comparable to or higher than that of AES. (author)

[en] Thanks to advances in in situ measurement techniques for electrical transport in ultra-high vacuum together with emergent materials such as Rashba-type surfaces, topological insulators, atomic-layer superconductors, and 2D materials like graphene, surface states and edge states on crystals provide intriguing topics, e.g. dissipation-less currents, spin-polarized electric current, and pure spin current. This is due to broken symmetry and strong spin–orbit and electron–phonon interactions. Here we review some examples of experimental techniques of multi-probe methods at macroscopic and microscopic scales, followed by transport phenomena revealed by them. These are opening a field in condensed matter physics driven by symmetry breaking at surfaces and atomic layers. (topical review)

No abstract available

[en] Several examples are known in which massive arrays of metal atomic chains are formed on semiconductor surfaces that show quasi-one-dimensional metallic electronic structures. In this review, Au chains on Si(557) and Si(553) surfaces, and In chains on Si(111) surfaces, are introduced and discussed with regard to the physical properties determined by experimental data from scanning tunneling microscopy (STM), angle-resolved photoemission spectroscopy (ARPES) and electrical conductivity measurements. They show quasi-one-dimensional Fermi surfaces and parabolic band dispersion along the chains. All of them are known from STM and ARPES to exhibit metal-insulator transitions by cooling and charge-density-wave formation due to Peierls instability of the metallic chains. The electrical conductivity, however, reveals the metal-insulator transition only on the less-defective surfaces (Si(553)-Au and Si(111)-In), but not on a more-defective surface (Si(557)-Au). The latter shows an insulating character over the whole temperature range. Compared with the electronic structure (Fermi surfaces and band dispersions), the transport property is more sensitive to the defects. With an increase in defect density, the conductivity only along the metal atomic chains was significantly reduced, showing that atomic-scale point defects decisively interrupt the electrical transport along the atomic chains and hide the intrinsic property of transport in quasi-one-dimensional systems.

[en] We performed in situ transport measurements of ultrathin Bi(111) films grown on Si(111) surface, with a four-tip scanning tunneling microscope using metal-coated carbon nanotube (CNT) tips. When the distance between the current injection tip (nonmagnetic Pt-coated CNT tip) and voltage tip (magnetic CoFe-coated CNT tip) was smaller than 1 μm, we found a violation of Green's reciprocity theorem which should hold with no spin transport. This was interpreted as a signal of the current-induced spin polarization (CISP) that occurs due to the Rashba spin-splitting surface-state bands of the Bi(111). The result was reasonably in accord with quantitative analyses based on the CISP theory of Rashba systems. (paper)

[en] A quasi-one-dimensional metallic Si(111)-(4x1)-In surface was investigated by a newly developed temperature-variable microscopic four-point probe method combined with in situ electron diffraction in ultrahigh vacuum. We have succeeded, for the first time, in detecting directly a surface metal-insulator transition around 130 K as a dramatic change of electrical conductivity through the surface states. An energy gap of ∼300 meV at the low-temperature phase, influences of defects and phase locking between the neighboring charge-density-wave chains were elucidated from the temperature dependence of conductivity

[en] Scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) were used to study the atomic and electronic structures of the Si(111)-√21 x √21-(Ag + Cs) surface (√21-Cs in short), which was induced by depositing caesium atoms on the Si(111)-√3 x √3-Ag surface at room temperature (RT). Compared with previously reported STM images of noble-metal induced √21 x √21 phases including the Si(111)-√21 x √21-(Ag+Ag) and Si(111)-√21 x √21-(Ag+Au) surfaces (√21-Ag and √21-Au, respectively), the √21-Cs surface displayed quite different features in STM images. The ARPES data of the √21-Cs surface revealed an intrinsic dispersive surface-state band, together with a non-dispersive one near the Fermi level, which was also different from those of the √21-Ag and √21-Au surfaces. These results strongly suggest different atomic arrangements between Cs- and noble-metal induced √21 x √21 phases. Unlike the √21-Ag and √21-Au phases, the framework of the initial Si(111)-√3 x √3-Ag substrate is totally broken at the √21-Cs phase. (author)

[en] We studied low-energy (∼1.5 keV) spin-polarized "4He"+ ion scattering on Bi(111) ultrathin films epitaxially grown on a Si(111) substrate. Even though Bi is a non-magnetic element, we observed that the scattered ion intensity differed between the incident He"+ ions with up and down spins, i.e., a spin asymmetric scattering. The spin asymmetry was not affected by the surface structure but depended on the scattering angle and the incident energy. These data indicate that the spin asymmetry originates from the spin-orbit coupling that acts transiently on the He"+ 1s electron spin in the binary collision and the resonant electron transfer between "4He"+ ions and Bi electronic orbitals possibly alters the behavior of the spin asymmetry compared to established models. (author)

[en] For in situ measurements of local electrical conductivity of well defined crystal surfaces in ultrahigh vacuum, we have developed microscopic four-point probes with a probe spacing of several micrometres, installed in a scanning-electron-microscope/electron-diffraction chamber. The probe is precisely positioned on targeted areas of the sample surface by using piezoactuators. This apparatus enables conductivity measurement with extremely high surface sensitivity, resulting in direct access to surface-state conductivity of the surface superstructures, and clarifying the influence of atomic steps upon conductivity. (author)