Shinichiro Hatta

Large Rashba spin splitting of a metallic surface-state band on a semiconductor surface

The generation of spin-polarized electrons at room temperature is an essential step in developing semiconductor spintronic applications. To this end, we studied the electronic states of a Ge(111) surface, covered with a lead monolayer at a fractional coverage of 4/3, by angle-resolved photoelectron spectroscopy (ARPES), spin-resolved ARPES and first-principles electronic structure calculation. We demonstrate that a metallic surface-state band with a dominant Pb 6p character exhibits a large Rashba spin splitting of 200 meV and an effective mass of 0.028 me at the Fermi level. This finding provides a material basis for the novel field of spin transport/accumulation on semiconductor surfaces. Charge density analysis of the surface state indicated that large spin splitting was induced by asymmetric charge distribution in close proximity to the nuclei of Pb atoms.

H-atom relay reactions in real space

Hydrogen bonds are the path through which protons and hydrogen atoms can be transferred between molecules. The relay mechanism, in which H-atom transfer occurs in a sequential fashion along hydrogen bonds, plays an essential role in many functional compounds. Here we use the scanning tunnelling microscope to construct and operate a test-bed for real-space observation of H-atom relay reactions at a single-molecule level. We demonstrate that the transfer of H-atoms along hydrogen-bonded chains assembled on a Cu(110) surface is controllable and reversible, and is triggered by excitation of molecular vibrations induced by inelastic tunnelling electrons. The experimental findings are rationalized by ab initio calculations for adsorption geometry, active vibrational modes and reaction pathway, to reach a detailed microscopic picture of the elementary processes.

Diamagnetic Properties of High-Tc Bi-Sr-Ca-Cu-O Superconducting Thin Films

The diamagnetic properties of superconductive Bi-Sr-Ca-Cu oxide thin films were studied, by using an rf SQUID susceptometer. The specimen showing diamagnetization up to about 100 K has also indicated the zero-resistance temperature of 102 K. The critical current density is estimated from the diamagnetization shielding effect to be 3.3×106 A/cm2 at 4.2 K and 7×104 A/cm2 at 77 K.

Preparation and Properties of Superconducting Bi-Sr-Ca-Cu-O Thin Films

Thin films of the superconducting Bi-Sr-Ca-Cu-O system have been prepared on MgO and SrTiO3 substrates by rf-magnetron sputtering and subsequent heat treatment. The films had preferred orientation and exhibited superconducting transition with zero- ρ temperature of around 70 K. Large resistivity drops at 110 K were observed only when the films were deposited at relatively high substrate temperatures (>600°C) and heat-treated.

Creation of Strong Pinning Sites by X-Ray-Irradiation for Gd1Ba2Cu3O7-x Superconducting Thin-Films

A large enhancement of critical current density with the small rate of flux creep was realized by x‐ray irradiation before the oxygen annealing for Gd1Ba2Cu3O7−x superconducting thin films. The significantly increased magnetization showed both the temperature independence and the small magnetic relaxation. The activation energy estimated by the flux creep model increased from 0.1 to 0.25 eV with the x‐ray irradiation treatment.

Possibility of High Tc Superconducting Thin Films in Magnetic Application

Magnetic properties of BiSrCaCuO and other superconducting oxide films in a technical field region were surveyed from a viewpoint of magnetic application. Permeability, hysteresis loops, magnetic anisotropy and high frequency response were measured in the superconducting region with discussion. These results seem to be well explained by the Bean model in which a number of magnetic fluxoids thread the superconductor. These films will be helpful as soft magnetic materials in a very high frequency region.

Controlling single-molecule junction conductance by molecular interactions

For the rational design of single-molecular electronic devices, it is essential to understand environmental effects on the electronic properties of a working molecule. Here we investigate the impact of molecular interactions on the single-molecule conductance by accurately positioning individual molecules on the electrode. To achieve reproducible and precise conductivity measurements, we utilize relatively weak π-bonding between a phenoxy molecule and a STM-tip to form and cleave one contact to the molecule. The anchoring to the other electrode is kept stable using a chalcogen atom with strong bonding to a Cu(110) substrate. These non-destructive measurements permit us to investigate the variation in single-molecule conductance under different but controlled environmental conditions. Combined with density functional theory calculations, we clarify the role of the electrostatic field in the environmental effect that influences the molecular level alignment.

Role of hydrogen bonding in the catalytic reduction of nitric oxide

Heterogeneous catalysis is inherently complex, and this makes it difficult to trace the reaction and clarify the mechanism. In this study, we investigated the reduction of nitric oxide (NO) by water on Cu(110) in a well-defined environment. Scanning tunnelling microscopy was used to control and image the reaction, and to characterize the product and the intermediate. A one-to-one reaction yields a characteristic NO–water complex, in which water induces partial filling of the empty 2π* orbital of NO, leading to N–O bond weakening. Subsequent reaction of the complex with another water molecule induces further weakening of the N–O bond, leading to bond rupture. We reveal that hydrogen-bond coupling induces back-donation and thus plays a crucial role in N–O bond cleavage; this provides a fundamental insight into the catalytic reduction of NO under ambient conditions.

Imaging sequential dehydrogenation of methanol on Cu(110) with a scanning tunneling microscope.

Adsorption of methanol and its dehydrogenation on Cu(110) were studied by using a scanning tunneling microscope (STM). Upon adsorption at 12 K, methanol preferentially forms clusters on the surface. The STM could induce dehydrogenation of methanol sequentially to methoxy and formaldehyde. This enabled us to study the binding structures of these products in a single-molecule limit. Methoxy was imaged as a pair of protrusion and depression along the [001] direction. This feature is fully consistent with the previous result that it adsorbs on the short-bridge site with the C-O axis tilted along the [001] direction. The axis was induced to flip back and forth by vibrational excitations with the STM. Two configurations were observed for formaldehyde, whose structures were proposed based on their characteristic images and motions.

Structure determination of Bi/Ge(111)- by dynamical low-energy electron diffraction analysis and scanning tunneling microscopy

We have determined the atomic structure of the Bi/Ge(111)-[Formula: see text] surface by dynamical low-energy electron diffraction (LEED) analysis and scanning tunneling microscopy (STM). The optimized atomic structure consists of Bi atoms which are adsorbed near the T(1) sites of the bulk-truncated Ge(111) surface and form triangular trimer units centered at the T(4) sites. The atomically resolved STM image was consistent with the LEED result. The structural parameters agree well with those optimized by a first-principles calculation which supports the interpretation of the electronic band splitting on this surface in terms of the giant Rashba effect.