Imaging the evolution of d states at a strontium titanate surface.

Oxide electronics is a promising alternative to the conventional silicon-based semiconductor technology, owing to the rich functionalities of oxide thin films and heterostructures. In contrast to the silicon surface, however, the electronic structure of the SrTiO3 surface, the most important substrate for oxide thin films growth, is not yet completely understood. Here we report on the electronic states of a reconstructed (001) surface of SrTiO3 determined in real space, with scanning tunneling microscopy/spectroscopy and density functional theory calculations. We found a remarkable energy dependence of the spectroscopic image: Theoretical analysis reveals that symmetry breaking at the surface lifts the degeneracy in the t2g state (dxy, dyz, and dzx) of Ti 3d orbitals, whose anisotropic spatial distribution leads to a sharp transition in the spectroscopic image as a function of energy. The knowledge obtained here could be used to gain further insights into emergent phenomena at the surfaces and interfaces with SrTiO3.

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.

Beyond the helix pitch: direct visualization of native DNA in aqueous solution.

The DNA double helix was first elucidated by J.D. Watson and F.H.C. Crick over a half century ago. However, no one could actually "see" the well-known structure ever. Among all real-space observation methods, only atomic force microscopy (AFM) enables us to visualize the biologically active structure of natural DNA in water. However, conventional AFM measurements often caused the structural deformation of DNA because of the strong interaction forces acting on DNA. Moreover, large contact area between the AFM probe and DNA hindered us from imaging sub-molecular-scale features smaller than helical periodicity of DNA. Here, we show the direct observation of native plasmid DNA in water using an ultra-low-noise AFM with the highly sensitive force detection method (frequency modulation AFM: FM-AFM). Our micrographs of DNA vividly exhibited not only overall structure of the B-form double helix in water but also local structures which deviate from the crystallographic structures of DNA without any damage. Moreover, the interaction force area in the FM-AFM was small enough to clearly discern individual functional groups within DNA. The technique was also applied to explore the synthesized DNA nanostructures toward the current nanobiotechnology. This work will be essential for considering the structure-function relationship of biomolecular systems in vivo and for in situ analysis of DNA-based nanodevices.

Gate-tunable large negative tunnel magnetoresistance in Ni-C60-Ni single molecule transistors.

We have fabricated single C(60) molecule transistors with ferromagnetic Ni leads (FM-SMTs) by using an electrical break junction method and investigated their magnetotransport. The FM-SMTs exhibited clear gate-dependent hysteretic tunnel magnetoresistance (TMR) and the TMR values reached as high as -80%. The polarity of the TMR was found to be always negative over the entire bias range studied here. Density functional theory calculations show that hybridization between the Ni substrate states and the C(60) molecular orbitals generates an antiferromagnetic configuration in the local density of states near the Fermi level, which gives a reasonable explanation for the observed negative TMR.

Spontaneous structural distortion and quasi-one-dimensional quantum confinement in a single-phase compound.

Oxide heterointerfaces often trigger unusual electronic properties that are absent in respective bulks. Here, direct evidence is offered for spontaneously assembled local structural distortions in a single-phase bulk, which confine electrons to within an atomic layer with notable orbital reconstruction and coupling, close the forbidden band, induce a ferromagnetic ordering, and give rise to a strongly anisotropic, spin-polarized quasi-one-dimensional electron gas.

Theoretical simulation of Kelvin probe force microscopy for Si surfaces by taking account of chemical forces.

A new method of theoretical simulation for Kelvin probe force microscopy (KPFM) imaging on semiconductor or metal samples is proposed. The method is based on a partitioned real space (PR) density functional based tight binding (DFTB) calculation of the electronic states to determine the multi-pole electro-static force, which is augmented with the chemical force obtained by a perturbation treatment of the orbital hybridization. With the PR-DFTB method, the change of the total energy is calculated together with the induced charge distribution in the tip and the sample by their approach under an applied bias voltage, and the KPFM images, namely the patterns of local contact potential difference (LCPD) distribution, are obtained with the minimum condition of the interaction force. However, since the interaction force is due to electro-static multi-poles, the spatial resolution of the KPFM images obtained by PR-DFTB is limited to the nano-scale range and an atom-scale resolution cannot be attained. By introducing an additional chemical force, i.e., the force due to the orbital hybridization, we succeeded in reproducing atom-scale resolution of KPFM images. Case studies are performed for clean and impurity embedded Si surfaces with Si tip models.

Submolecular-scale imaging of α-helices and C-terminal domains of tubulins by frequency modulation atomic force microscopy in liquid.

In this study, we directly imaged subnanometer-scale structures of tubulins by performing frequency modulation atomic force microscopy (FM-AFM) in liquid. Individual α-helices at the surface of a tubulin protofilament were imaged as periodic corrugations with a spacing of 0.53 nm, which corresponds to the common pitch of an α-helix backbone (0.54 nm). The identification of individual α-helices allowed us to determine the orientation of the deposited tubulin protofilament. As a result, C-terminal domains of tubulins were identified as protrusions with a height of 0.4 nm from the surface of the tubulin. The imaging mechanism for the observed subnanometer-scale contrasts is discussed in relation to the possible structures of the C-terminal domains. Because the C-terminal domains are chemically modified to regulate the interactions between tubulins and other biomolecules (e.g., motor proteins and microtubule-associated proteins), detailed structural information on individual C-terminal domains is valuable for understanding such regulation mechanisms. The results obtained in this study demonstrate that FM-AFM is capable of visualizing the structural variation of tubulins with subnanometer resolution. This is an important first step toward using FM-AFM to analyze the functions of tubulins.

Theoretical simulation of scanning probe microscopy.

Methods of theoretical simulation of scanning probe microscopy, including scanning tunneling microscopy (STM), atomic force microscopy(AFM) and Kelvin prove force microscopy (KPFM) have been reviewed with recent topics as case studies. For the case of the STM simulation, the importance of the tip electronic states is emphasized and some advanced formalism is presented based on the non-equilibrium Green's function theory beyond Bardeen's perturbation theory. For the simulation of AFM, we show examples of 3D-force map for AFM in water, and theoretical analyses for a nano-mechanical experiment on a protein molecule. An attempt to simulate KPFM images based on the electrostatic multi-pole interaction between a tip and a sample is also introduced.

Dimensionality-driven insulator-metal transition in A-site excess non-stoichiometric perovskites.

Coaxing correlated materials to the proximity of the insulator-metal transition region, where electronic wavefunctions transform from localized to itinerant, is currently the subject of intensive research because of the hopes it raises for technological applications and also for its fundamental scientific significance. In general, this tuning is achieved by either chemical doping to introduce charge carriers, or external stimuli to lower the ratio of Coulomb repulsion to bandwidth. In this study, we combine experiment and theory to show that the transition from well-localized insulating states to metallicity in a Ruddlesden-Popper series, La(0.5)Sr(n+1-0.5)Ti(n)O(3n+1), is driven by intercalating an intrinsically insulating SrTiO(3) unit, in structural terms, by dimensionality n. This unconventional strategy, which can be understood upon a complex interplay between electron-phonon coupling and electron correlations, opens up a new avenue to obtain metallicity or even superconductivity in oxide superlattices that are normally expected to be insulators.

[Shielding evaluation of lead-free board for diagnostic X-rays].

For physical foundation data used in the shielding calculation of structural facilities such as a radiation room, there are air kerma transmissions concerning the thickness of shielding objects, and half value layers and tenth value layers concerning a greatly attenuated wide X-ray beam. Accordingly, we evaluated the above-mentioned items with a lead-free board, which is mixed sulfuric acid calcium and barium sulfate with equiponderance for the amount of sulfuric acid calcium included in the usual plasterboard. Permeability in NCRP Report 147 is expressed by 3 parameters, α, ß and γ, and shielding objects x. It showed that it corresponds to the measurement point and permeability curve with parameters, α, ß and γ obtained by nonlinear regression analysis. Furthermore, we calculated the half value layer and tenth value layer concerning the greatly attenuated wide X-ray beam. The evaluated lead-free board, used in this examination, is useful as the shielding material for the diagnosis X-ray and, moreover, the partition wall materials are hard enough, with a board that is even heavier than the usual plaster board. Besides, the use of lead-free materials is friendly to the general environment.

[A questionnaire about radiation safety management of the draining-water system at nuclear medicine facilities].

We conducted a questionnaire survey about radiation-safety management condition in Japanese nuclear medicine facilities to make materials of proposition for more reasonable management of medical radioactive waste. We distributed a questionnaire to institutions equipped with Nuclear Medicine facilities. Of 1,125 institutions, 642 institutes (52.8%) returned effective answers. The questionnaire covered the following areas: 1) scale of an institution, 2) presence of enforcement of radiotherapy, 3) system of a tank, 4) size and number of each tank, 5) a form of draining-water system, 6) a displacement in a radioactive rays management area, 7) a measurement method of the concentration of medical radioactive waste in draining water system, 8) planned and used quantity of radioisotopes for medical examination and treatment, 9) an average displacement of hospital for one month. In most institutions, a ratio of dose limitation of radioisotope in draining-water system was less than 1.0, defined as an upper limitation in ordinance. In 499 hospitals without facilities of hospitalization for unsealed radioisotope therapy, 473 hospitals reported that sum of ratios of dose limits in a draining-water system was less than 1.0. It was calculated by used dose of radioisotope and monthly displacement from hospital, on the premise that all used radioisotope entered in the general draining-water system. When a drainage including radioactivity from a controlled area join with that from other area before it flows out of a institution, it may be diluted and its radioactive concentration should be less than its upper limitation defined in the rule. Especially, in all institutions with a monthly displacement of more than 25,000 m3, the sum of ratio of the concentration of each radionuclide to the concentration limit dose calculated by used dose of radioisotope, indicated less than 1.0.

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 Green's 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.

Interplay between nonlinearity, scan speed, damping, and electronics in frequency modulation atomic-force microscopy.

Numerical simulations of the frequency modulation atomic force microscope, including the whole dynamical regulation by the electronics, show that the cantilever dynamics is conditionally stable and that there is a direct link between the frequency shift and the conservative tip-sample interaction. However, a soft coupling between the electronics and the nonlinearity of the interaction may significantly affect the damping. A resonance between the scan speed and the response time of the system can provide a simple explanation for the spatial shift and contrast inversion between topographical and damping images, and for the extreme sensitivity of the damping to a tip change.

Efficient method for simulating quantum electron dynamics under the time-dependent Kohn-Sham equation.

A numerical scheme for solving the time evolution of wave functions under the time-dependent Kohn-Sham (TDKS) equation has been developed. Since the effective Hamiltonian depends on the wave functions, the wave functions and the effective Hamiltonian should evolve consistently with each other. For this purpose, a self-consistent loop is required at every time step for solving the time evolution numerically, which is computationally expensive. However, in this paper, we develop a different approach, expressing a formal solution of the TDKS equation, and prove that it is possible to solve the TDKS equation efficiently and accurately by means of a simple numerical scheme without the use of any self-consistent loops.

Visualization of thermally fluctuating surface structure in noncontact atomic-force microscopy and tip effects on fluctuation: theoretical study of Si(111)-(square root[3] x square root[3])-Ag surface.

We investigated noncontact atomic-force microscopy (NC-AFM) images of a thermally fluctuating surface structure together with tip effects based on the first-principles electronic state calculation. As an example the Si(111)-(square root[3] x square root[3])-Ag (square root[3]-Ag) surface is studied. We have succeeded in theoretically visualizing the thermal fluctuation of the square root[3]-Ag surface at room temperature, and in reproducing the observed NC-AFM image for the first time. Further, the pinning effect of the thermal fluctuation of the square root[3]-Ag surface by the tip is clarified, which shows a novel ability of NC-AFM to modify the surface structure.