Kazumasa Shinjo

Dynamics of friction: superlubric state

Abstract Dynamics in friction is studied from an atomistic point of view. Friction is formulated as a problem of whether or not a given kinetic energy for the translational motion dissipates into the kinetic energies for the internal motions during sliding. From the study of the Frenkel-Kontorova model with kinetic energy terms, it is found that two different regimes appear in the parameter space specifying the model: the superlubricity and the friction regimes. The friction exactly vanishes in the superlubric regime and appears in the friction regime. The conditions for the superlubricity to occur are described. It is emphasized that a high dimensionality in the friction system is a key to understanding the physics of superlubricity. For high dimensional systems, superlubricity is a generic phenomenon, appearing for a wide class of (strong or weak) adhesion such as the metallic bonding and the van der Waals interaction. The results are discussed in comparison with those obtained by assuming the case where the upper surface slides quasi-statically against the lower surface.

Superlubricity and frictional anisotropy

Abstract Superlubricity is the state in which two contacting surfaces slide with no resistance. This paper explains how the existence of superlubricity is confirmed by measurement of the friction of mica surfaces. It is shown that the frictional forces of mica are anisotropic with the commensurability of the contacting surfaces and the frictional anisotropy is consistent with the theoretical conclusion on the effects of the commensurability. Also, the usefulness of the frictional anisotropy is considered in relation to technological aspects of superlubricity. It is shown that friction can be controlled by adjusting the lattice misfit between elastically contacting surfaces.

Simultaneous alignment of multiple optical axes in a multistage optical system using Hamiltonian algorithm

We achieve a fast alignment of optical axes that must be precisely positioned with multiple degrees of freedom during coupling between optical devices using the Hamiltonian algorithm. The Hamil- tonian algorithm utilizes the mixing properties of the dynamical system and can efficiently search for the global minimum of the objective func- tion in an optimization problem. Simulations show the effects of the mix- ing for a quick search for the global minimum, avoidance of trapping at the local minima, and robustness against disturbance. Our experiments show that using this algorithm, optical axes with multiple degrees of free- dom can be aligned efficiently and several fibers can be aligned simul- taneously. Therefore, it is confirmed that the time for completing align- ment of many optical axes is greatly reduced.

Theoretical studies of underlayer chemisorption II. Electronic structure of Ti6X(X=C, O, F),Ti9and Ti9N clusters

Abstract The discrete variational (DV)-Xα cluster method is applied to perform a comparative study of the underlayer chemisorption of X(X=C, N, O, F) atoms at the Ti(0001) metal surface. The electronic structures of Ti 6 X(X=C, O, F) clusters where an X atoms sits at the body-centre of a Ti 6 cluster are calculated, and compared with that of a Ti 6 N cluster. A specific energy level below the Fermi level very sensitive to the chemisorption, found in the case of the N atom, is also found in all our clusters with C, O or F: the level is elevated above the Fermi level as a result of the chemisorption. The magnitudes of the elevations are much scattered: it is largest for a carbon atom and smallest for a fluorine atom. Changes of the electronic structure of the Ti 6 X (C, O) systems where the X atom moves from the outside to the inside of the Ti 6 cluster are examined in detail in comparison with that of the Ti 6 N cluster. It is found that, in the course of this movement of the X atom, the contraction of the Ti-Ti distance on the first layer is largest for an oxygen atom and smallest for a carbon atom, and elongation of the distance between the first and the second layer is largest for a carbon atom and smallest for an oxygen atom. Finally, the cluster size dependence of our results is studied by using larger clusters such as Ti 9 and Ti 9 N, and the validity of using small clusters is confirmed.

Charge Transfer Instability in Optical Processes of Adsorbates

Dynamical calculations of the optical absorption, photoelectron emission and luminescence spectra of the system consisting of an adsorbate and a metal substrate are made with a particular reference to the effects of the critical line, across which the valence charge of the adsorbate is expected to change discontinuously to a static approximation. It is assumed that the system is described by a simple spinless Hamiltonian, and a time-dependent selfconsistent mean-field approximation is applied. The calculated absorption and photoelectron emission spectra show a broadening when the initial effective valence level of the adsorbate crosses the critical line. The broadening may be ascribed to a charge-transfer instability of the system. The results are favorably compared with those of recent experiments. It is pointed out that the relative positions of the neutral and the charge-transferred excited states of the adsorbate may be known by observing the luminescence from the adsorbate.

Effects of Dipole-Dipole Interaction on Multivalency of Adatoms: a Model of Valence Transition

Effects of the dipole-dipole interaction on the multivalency of adatoms on the metal surfaces are studied. For the system with two adatoms, it is shown that the transition line exists, across which the valency of two adatoms in the ground state changes discontinuously. The result is extended to the system with many adatoms, and a model is proposed of the valence transition. The coverage dependence of the work function is also discussed.

Luminescence from adsorbates

Abstract A dynamical calculation of the line shapes of luminescence from adsorbates on metal surfaces has been performed by taking into account effects of the image-force stabilization in addition to those of the electron-transfer between the adsorbate and the substrate. An abrupt change of the lineshape is found in a mean-field approximation, when the valence level of the adsorbate stabilized by a bare core-hole goes up across the critical line which was defined in the static theory of multivalency of adsorbates.