Takashi Uchihashi

Resistive phase transition of the superconducting Si(111)-(7×37×3×

Recently, superconductivity was found on semiconductor surface reconstructions induced by metal adatoms, promising a new field of research where superconductors can be studied from the atomic level. Here we measure the electron transport properties of the Si(111)-(7×3)-In surface near the resistive phase transition and analyze the data in terms of theories of two-dimensional (2D) superconductors. In the normal state, the sheet resistances (2D resistivities) R□ of the samples decrease significantly between 20 and 5 K, suggesting the importance of the electron-electron scattering in electron transport phenomena. The decrease in R□ is progressively accelerated just above the transition temperature (Tc) due to the direct (Aslamazov-Larkin term) and the indirect (Maki-Thompson term) superconducting fluctuation effects. A minute but finite resistance tail is found below Tc down to the lowest temperature of 1.8 K, which may be ascribed to a dissipation due to free vortex flow. The present study lays the ground for a future research aiming to find new superconductors in this class of materials.

Electron conduction through quasi-one-dimensional indium wires on silicon

Electron conduction through quasi-one-dimensional (1D) indium atomic wires on silicon (the Si(111)-4×1-In reconstruction) is clarified with the help of local structural analysis using scanning tunneling microscopy. The reconstruction has a conductance per square as high as 100 μS, with global conduction despite numerous surface steps. A complete growth of indium wires up to both the surface steps and the lithographically printed electrodes is essential for the macroscopic transport. The system exhibits a metal–insulator transition at 130 K, consistent with a recent ultraviolet photoemission study [H. W. Yeom, S. Takeda, E. Rotenberg, I. Matsuda, K. Horikoshi, J. Schaefer, C. M. Lee, S. D. Kevan, T. Ohta, T. Nagao, and S. Hasegawa, Phys. Rev. Lett. 82, 4898 (1999)].

Fabrication and lateral electronic transport measurements of gold nanowires

A technique for fabrication of gold nanowires on a Si(111) surface in ultrahigh vacuum and their electronic transport properties are presented. Gold wires with widths as small as 4 nm are produced by using a gold-coated piezoresistive cantilever in atomic force microscope contact mode. This technique allows patterns to be written at will. In situ electronic transport measurements of a gold wire as long as 7 μm and 4 nm wide show unambiguous metallic behavior. This fabrication method could become pivotal within the next generation of nanoscale microprocessors.

Imaging Josephson vortices on the surface superconductor Si(111)-(√7×√3)-In using a scanning tunneling microscope.

We have studied the superconducting Si(111)-(√7×√3)-In surface using a ³He-based low-temperature scanning tunneling microscope. Zero-bias conductance images taken over a large surface area reveal that vortices are trapped at atomic steps after magnetic fields are applied. The crossover behavior from Pearl to Josephson vortices is clearly identified from their elongated shapes along the steps and significant recovery of superconductivity within the cores. Our numerical calculations combined with experiments clarify that these characteristic features are determined by the relative strength of the interterrace Josephson coupling at the atomic step.

Current-Driven Supramolecular Motor with In Situ Surface Chiral Directionality Switching

Surface-supported molecular motors are nanomechanical devices of particular interest in terms of future nanoscale applications. However, the molecular motors realized so far consist of covalently bonded groups that cannot be reconfigured without undergoing a chemical reaction. Here we demonstrate that a platinum-porphyrin-based supramolecularly assembled dimer supported on a Au(111) surface can be rotated with high directionality using the tunneling current of a scanning tunneling microscope (STM). Rotational direction of this molecular motor is determined solely by the surface chirality of the dimer, and most importantly, the chirality can be inverted in situ through a process involving an intradimer rearrangement. Our result opens the way for the construction of complex molecular machines on a surface to mimic at a smaller scale versatile biological supramolecular motors.

Nanostencil-Fabricated Electrodes for Electron Transport Measurements of Atomically Thin Nanowires in Ultrahigh Vacuum

Nanostenciling is an ideal technique for fabricating atomically clean nanostructures because of its resist-free process, i.e., vacuum deposition through a free-standing shadow mask. We have applied this method to grow metal electrodes separated by 200 to 500 nm on a substrate in ultrahigh vacuum. Without breaking vacuum, these fine electrodes can be electrically connected to outer instruments; this allows in situ electron transport measurements of atomically thin nanowires grown within the electrode gap. Two types of markers also produced via the shadow mask navigate an operator towards the electrode gap, enabling its scanning tunneling microscope (STM) observations. It is demonstrated that the conductance of erbium disilicide nanowires identified with an STM can be measured using the stenciled electrodes.

Highly ordered cobalt-phthalocyanine chains on fractional atomic steps: one-dimensionality and electron hybridization.

Precisely controlled fabrication of low-dimensional molecular structures with tailored morphologies and electronic properties is at the heart of the nanotechnology research. Especially, the formation of one-dimensional (1D) structures has been strongly desired due to their expected high performance for information processing in electronic/magnetic devices. So far, however, they have been obtained by tough and slow methods such as manipulation of individual molecules, which are totally unsuited for mass production. Here we show that highly ordered cobalt-phthalocyanine chains can be self-assembled on a metal surface using fractional atomic steps as a template. We also demonstrate that the substrate surface electrons, which can be confined by cobalt-phthalocyanine molecules, can propagate along the step arrays and can hybridize with the molecular orbitals. These findings provide a significant step toward readily realization of 1D charge/spin transport, which can be mediated either directly by the molecules or by the surface electrons.

Substrate Dependent Low-Temperature Growth of Thin Ag Films: Study on Si(111)?In Surfaces

The growth behaviors of thin Ag films using different substrate surfaces [Si(111) 7×7, Si(111) √3×√3–In, Si(111) √31×√31–In, and Si(111) 4×1–In] are modified. In the experiments, the two-step growth process consisting of low-temperature deposition of a metal followed by a mild annealing is employed to form Ag films. Scanning tunneling microscopy measurements reveal the formation of three-dimensional islands for Si(111) √31×√31–In and layer-by-layer growths on all the other surfaces. Ag growth on the Si(111) 4×1–In is highly anisotropic due to the indium chain arrays on the substrate. Two kinds of critical thickness, which is the minimum film thickness required to form a stable layer, are defined according to a previous work [C.-S. Jiang et al.: Surf. Sci. 518 (2002) 63]. Among the three surfaces that exhibit layer-by-layer growths, Si(111) 7×7 has the largest critical thickness, followed by Si(111) 4×1–In and Si(111) √3×√3–In in descending order. The results are discussed both in terms of geometric effects of the surface atomic structures and within the framework of the electronic growth model.

Disorder-induced suppression of superconductivity in theSi(111)−(7×3)-In surface: Scanning tunneling microscopy study

The critical effect of disorder on the two-dimensional (2D) surface superconductor Si(111)-(

Resistive phase transition of the superconducting Si(111)-(7×3)-In surface.

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