Katsunori Tagami

STS observations of Landau levels at graphite surfaces.

Scanning tunneling spectroscopy (STS) measurements were made on surfaces of two different kinds of graphite samples, Kish graphite and highly oriented pyrolytic graphite (HOPG), at very low temperatures and in high magnetic fields. We observed a series of peaks in the tunnel spectra associated with Landau quantization of the quasi-two-dimensional electrons and holes. A comparison with the calculated local density of states at the surface layers allows us to identify Kish graphite as bulk graphite and HOPG as graphite with a finite thickness of 40 layers. This explains the qualitative difference between the two graphites reported in the recent transport measurements which suggested the quantum-Hall effect in HOPG. This work demonstrates how powerful the combined approach between the high quality STS measurement and the first-principles calculation is in material science.

Visualization of hydration layers on muscovite mica in aqueous solution by frequency-modulation atomic force microscopy

A three-dimensional interaction force mapping experiment was carried out on a muscovite mica surface in an aqueous solution using a high-resolution and low-thermal drift frequency-modulation atomic force microscope. By collecting oscillatory frequency shift versus distance curves at the mica∕solution interface, complicated hydration structures on the mica surface were visualized. Reconstructed two-dimensional frequency shift maps showed dot-like or honeycomb-like patterns at different tip-sample distances with a separation of 0.2 nm with each other, which agree well to the water molecule density maps predicted by a statistical-mechanical theory. Moreover, site-specific force versus distance curves showed a good agreement with theoretically calculated site-specific force curves by a molecular dynamics simulation. It is found that the first and second hydration layers give honeycomb-like and dot-like patterns in the two-dimensional frequency shift images, respectively, corresponding to the lateral distribution function in each layer.

Electronic transport of benzothiophene-based chiral molecular solenoids studied by theoretical simulations

By the density-functional-derived tight-binding method, the electronic transport properties of two types of benzothiophene-based molecular wires, i.e., the linear and helical molecular wires have been investigated. In the molecular bridge system where these molecules are connected to the gold electrodes by S–Au bonds, the transmission peaks are found to lie at the energies somewhat lower than 0.5 eV below the Fermi level for both cases. Thus the conductances of both types of wires for the bias voltage less than 1.0 V are not so large without doping. Upon iodine doping, however, the new transmission peaks are found to appear around the Fermi level, particularly in the case of helical wires. It means that the conductances of the helical wires are expected to be improved dramatically by the chemical doping. Therefore, the doped helical molecular wires are predicted to work as molecular solenoids even under lower bias voltages. Next, the applicability of the current-induced magnetic field generated in such a ...

Quantum transport through multiterminal phenalenyl molecular bridges

The quantum transport properties of multiterminal molecular bridge systems are theoretically studied with the Greens functions method based on an empirical tight-binding model. As an illustrated example, we adopt a phenalenyl molecule which has a nonbonding singly occupied molecular orbital (SOMO). For a comparative study, first the two-terminal molecular bridges, then the three- and four-terminal molecular bridges are calculated. For the two-terminal case, we find that the transmission spectra significantly depend on the terminal sites connected to the leads. For example, the transmission spectrum has a peak at E=0.0 (SOMO level) as long as both the source and drain are connected to the α sites, but otherwise a dip structure appears at this energy. As a general trend, even when the third and fourth terminals are connected, the transmission spectra do not change considerably from the corresponding spectra of the two-terminal cases. However, some attractive aspects, such as the disappearance of the dip at the SOMO level and a shift in the location of the large loop current, are newly found.

Theoretical Predictions of Electronic Transport Properties of Differently Conjugated Porphyrin Molecular Wires

Based on the self-consistent tight-binding calculations and the Keldysh Greens functions technique, we predicted the transport properties of three types of differently conjugated porphyrin molecular wires, i.e., the tape-porphyrin, the butadiyne-linked porphyrin, and the edge-fused porphyrin wires. We found that among the three types of molecular wires the tape-porphyrin wires are the most conductive because of their extremely small HOMO–LUMO energy gaps. The other two types of wires are found to form semiconductive devices. For all the types of porphyrin wires the current is found to bypass the zinc atom sites because the zinc-related energy levels are away from the Fermi level. Then we examined the transport properties of their junctions where the butadiyne-bridged porphyrin molecule is inserted in the infinite tape-porphyrin wire. Their transport properties are found to be easily tuned from metallic to semiconductive by increasing the number of butadiyne-linked-porphyrin building blocks.

Spintronic Transport through Polyphenoxyl Radical Molecules.

The coherent quantum transport properties through the spin-polarized polyphenoxyl radical molecule have been investigated, using the density-functional-derived tight-binding model and the Greens functions method. The majority and minority spin components exhibit considerably different transmission spectra in the vicinity of the Fermi level. Namely, each spin component carries a different amount of current when the bias voltage is applied between the two electrodes that sandwich the polyradical molecule. Therefore, if the magnetization axis of the polyradical is fixed by the external magnetic field, and if the spin flip does not occur during the transmission, the assumed molecular bridge is expected to work as a spin filter or a spin valve. Furthermore, as long as the bias voltage is weak, the total spin current is observed to be larger than the current through its reduced molecular form. It indicates that the adsorption of some chemical species on the radical sites can be sensed by the change in conductance of the molecular bridge.

Theory of Quantum Conductance of Atomic and Molecular Bridges

Recent topics on theoretical analyses and prediction of the nano-scale structures connected between the electrodes are presented with discussions on several fundamental issues of these systems. Theoretical approaches used are the first-principles recursion transfer matrix method implemented with non-local pseudo-potentials, as well as non-equilibrium Greens function technique with tight binding bases. The former method is utilized for the analyses of atom wires and field emission process. As for the atom wire bridges, a very large effect of the terminal impurity and localized bias field distribution is elucidated. On the other hand, the latter method is mainly used for the analyses of the molecular bridges. The significant terminal effect is also found for the molecular bridge systems, if the transport is caused by the resonant tunneling. In addition, the effect of the conformational change inside the molecule is found to be sufficiently large for the STM tunneling current. Next, the remarkable feature o...

Simulated Noncontact Atomic Force Microscopy Images of Si(001) Surface with Silicon Tip

We simulated the noncontact atomic force microscopy (nc-AFM) images of Si(001) surfaces using the Si tip based on the tight-binding model. We find that only up dimer atoms are observed slightly outside the dimer sites. This outward shift is explained based on two points. One point is that the dangling bonds on the up dimer atoms, which interact with the tip apex, are tilted outward. The other point is that the space between the adjacent dimer rows looks slightly bright on the c(4 ×2) phase, since the tip located above the midpoint of the two dimer rows is subjected to attractive forces from the up dimer atoms on both sides.

Simulated nc-AFM images of Si(0 0 1) surface with nanotube tip

We predicted the non-contact atomic force microscopy (nc-AFM) images of Si(0 0 1) surface using the nanotube tip from the theoretical calculations based on the tight-binding model. The images are found to depend highly on the tip shape and orientation, and the ghost atoms are frequently observed. These abnormal images are due to the effect of the multi-atom apex, which is analogous to the STM.

A Tight-Binding Study of Chemical Interaction of Nanotube Tip with Si(001) Surface

The features of the chemical interaction between the nanotube tip and the Si(001) surface are investigated based on a self-consistent tight-binding model. When the tip oscillates on top of the up dimer atom, it feels discontinuous changes in the normal force, which are accompanied by drastic transformation of the the dimer structure. These events are explained by switching to the other branches on the potential energy surface. In addition, as long as the tip does not come close to the surface, it is found to become negatively (positively) charged on top of up (down) dimer atom, which is caused by the its interaction to the local density of states of the surface dimer near the fermi level.