Naruo Sasaki

Indications of Proton-Dominated Cosmic-Ray Composition above 1.6 EeV

We report studies of ultrahigh-energy cosmic-ray composition via analysis of depth of air shower maximum (X(max)), for air shower events collected by the High-Resolution Flys Eye (HiRes) observatory. The HiRes data are consistent with a constant elongation rate d/d[log(E)] of 47.9+/-6.0(stat)+/-3.2(syst) g/cm2/decade for energies between 1.6 and 63 EeV, and are consistent with a predominantly protonic composition of cosmic rays when interpreted via the QGSJET01 and QGSJET-II high-energy hadronic interaction models. These measurements constrain models in which the galactic-to-extragalactic transition is the cause of the energy spectrum ankle at 4x10(18) eV.

Theory for the effect of the tip–surface interaction potential on atomic resolution in forced vibration system of noncontact AFM

Abstract We present a perturbation theory which enables us to understand the physics of the cantilever-forced vibration in noncontact atomic-force microscopy (nc-AFM). Analytical expressions of the resonance curve and frequency shift are given. This theory is applied to the model system with a van der Waals tip–surface interaction potential. Based on this case study, it is elucidated how the resonance frequency shift is analytically described by an integral of the tip–surface interaction force. Then nc-AFM image of Si(111) 7×7 surface is calculated by the present theory. It is examined that this theory works as an algorithm for nc-AFM image simulator.

Effect of Microscopic Nonconservative Process on Noncontact Atomic Force Microscopy

Effects of the tip-induced nonconservative processes of the tip or surface atom on the cantilever dynamics of noncontact atomic force microscopy (nc-AFM) are theoretically analyzed based on the time-averaging perturbation theory. The typical order of the magnitude of the Q value due to such nonconservative processes is estimated to be on the order of 104, which is comparable to the intrinsic dissipation of the cantilever free oscillation. The additional frequency shift due to the hysteresis loop of the force curve is estimated to be on the order of 10 Hz. This part of the frequency shift sets in like a step function when the tip turning point approaches the surface within a certain threshold height. This feature explains the experimental observation of the discontinuous frequency shift at chemical reactive sites.

THE RELATION BETWEEN RESONANCE CURVES AND TIP-SURFACE INTERACTION POTENTIAL IN NONCONTACT ATOMIC-FORCE MICROSCOPY

We present a perturbation theory which enables us to understand the physics of the cantilever-forced vibration in noncontact-mode atomic-force microscopy. Analytical expressions of the resonance curve and frequency shift are given. This theory is applied to the model system with a van der Waals tip-surface interaction potential. Based on this case study, it is elucidated how the resonance frequency shift is analytically described by an integral of the tip-surface interaction force over the traverse of the tip around the turning point. This quantity is rather sensitive to the interaction potential. We can see by this method how the dynamical instability occurs and significantly influences the observation of the resonance peak. Calculated resonance curves and frequency shifts agree fairly well with those obtained by a numerical integration of equation of motion outside the bistable region.

Visualization of nanoscale peeling of carbon nanotube on graphite

We have fabricated a manipulation system with a force detection, which uses a self-detective cantilever, in a chamber of a scanning electron microscope. This system can simultaneously manipulate a nanoparticle and detect a force needed to move it, a detective resolution of which is approximately 1nN. In this work, nanoscale peeling processes of a multiwalled carbon nanotube (MWCNT) on the graphite substrate have been studied. We have first experimentally obtained the vertical force-distance curve with the characteristic hysteresis loop which exhibits the multistable states between line contact and point contact of the MWCNT shape during the peeling processes.

Superlubricity of Fullerene Intercalated Graphite Composite

A novel superlubric system of fullerene intercalated graphite composite is reported. First, it is clarified that fullerene intercalated graphite films exhibit an ultralow average friction force and an excellent friction coefficient µ<0.001 smaller than µ<0.002 for MoS2 and µ\cong0.001 for graphite. Next, it is demonstrated that superlubricity can be controlled by changing the intercalant species. The C60 intercalated graphite film shows much less maximum static friction force than the C70 intercalated graphite film. Finally, we propose one of the simple guidelines on designing a practical superlubric system–reduction in the contact area between the intercalated fullerene and the graphite sheet to the pointlike contact. Our newly developed superlubric system will contribute to solving energy and environmental problems.

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.

Features of cantilever motion in dynamic-mode AFM

Abstract The dynamic motion of a cantilever in dynamic-mode atomic force microscopy is investigated theoretically with an elastic oscillator model of the cantilever interacting with the surface. For the tip–surface interaction, the Lennard–Jones and hard-wall models are assumed. The numerical integration of the equation of motion and the Poincare map method are used for the analysis of the tip motion. Various strange features of the tip motion are due to the extremely non-linear nature of the interaction, such as quasiperiodic oscillation and fractional resonance. In particular, the coexistence of the dynamic touching mode with the non-touching mode is revealed. The phase-frequency relation of the dynamic touching mode is obtained, and can be observed experimentally.

Simulation of Scan-Directional Dependence of Superlubricity of C60 Molecular Bearings and Graphite

The scan-directional dependence of the superlubricity of a C60 molecular bearing system (graphite/C60/graphite interface) is studied and compared with that of a graphite system (graphite/graphite/graphite interface) by molecular mechanics simulation. The mean lateral force reaches a maximum within a narrow region approximately in the [1010] direction. For other regions, has a nearly constant value of less than 1 pN. In particular, in the [1230] direction, reaches a minimum of nearly zero. It is clarified that reflects the following types of C60 motion: sliding above the carbon bond and a discrete slip to the neighboring AB-stacking position. The load dependence of also exhibits marked anisotropy. The orders of magnitude of the simulated friction coefficients are comparable to those obtained in our previous experiments.