Orbital magnetic susceptibility of zigzag carbon nanobelts: a tight-binding study.

The magnetic properties of a circular graphene nanoribbon (carbon belt) in a magnetic field parallel to its central axis is studied using a tight-binding model. Orbital magnetic susceptibility is calculated using an analytical expression of the energy eigenvalues as a function of the magnetic flux density for any size, and its temperature dependence is considered. In the absence of electron hopping parallel to the magnetic field, the orbital magnetic susceptibility diverges at absolute zero if the chemical potential is zero and the number of atoms is a multiple of four. As the temperature increases, the magnitude of susceptibility decreases according to the power law, whose exponent depends on the size. In the presence of electron hopping parallel to the magnetic field, the divergence of the susceptibility near absolute zero disappears, and the sign changes with the transfer integral parallel to the magnetic field and the temperature.

Molecular dynamics simulations of Lennard-Jones systems confined between suspended nanoscale graphene sheets.

The distribution of particles interacting with Lennard-Jones potentials and confined between parallel graphene sheets is investigated by molecular dynamics simulations. For small separation distances, the particles are densely localized in the central region between the graphene sheets. However, two high-density layers appear as the separation distance increases. The particle distribution also depends on the temperature, tensile force of the graphene sheets, and the initial configuration, and various configurations are observed for large separation. For example, an argon cluster initially located between the graphene sheets changes shape, and a bridge between the parallel walls is formed at low temperature. In contrast to the Lennard-Jones system sandwiched between rigid walls, the flexibility of the graphene sheets strongly affects the distribution of particles in the direction perpendicular to the graphene sheets.

Mass spectrometric analysis of the dissociation of argon cluster ions in collision with several kinds of metal.

RATIONALE: Collisions of clusters with solids have become important, especially in the fields of thin film growth or surface processing such as etching or topography smoothing. However, it is not clear how much of the theory or model used in macroscopic collisions is appropriate for the consideration of microscopic collisions. METHODS: We considered a cluster ion consisting of thousands of argon atoms as a continuum and examined the possibility that classical mechanics could analyze its collision with metals. A mass spectrometric analysis of the dissociated ions of argon cluster ions (Ar(+)1500) in collision with five different metals was performed. RESULTS: In the mass spectra at an incident kinetic energy per atom of less than 10 eV, no monatomic argon ions (Ar(+)) were observed regardless of the prominence of Ar2(+) or Ar3(+). The branching ratio for the ion yield Ar2(+)/∑Arn(+) (n ≥ 2), representing the dissociation rate, was found to be significantly different for each metal. The relationship between the branching ratio and the impulsive stress caused by the collision of the cluster ion with metal was investigated. The impulsive stress was calculated based on the Young's modulus and density of the clusters and metal, under the assumption that the collision was initially elastic. As a result, the magnitude correlation in the branching ratio corresponded well with that in the impulsive stress. CONCLUSIONS: This result is important in that it indicates that collision of nano-sized clusters with solids at low energies can be modeled using elastic theory. Furthermore, the result suggests a new method for evaluating a physical property of a material such as its Young's modulus.

Thermal fluctuations and stability of a particle levitated by a repulsive Casimir force in a liquid.

We study the vertical Brownian motion of a gold particle levitated by a repulsive Casimir force to a silica plate immersed in bromobenzene. The time evolution of the particle distribution starting from an equilibrium position, where the Casimir force and gravitational force are balanced, is considered by solving the Langevin equation using the Monte Carlo method. When the gold particle is very close to the silica plate, the Casimir force changes from repulsive to attractive, and the particle eventually sticks to the surface. The escape rate from a metastable position is calculated by solving the Fokker-Plank equation; it agrees with the value obtained by Kramers' escape theory. The duration of levitation increases as the particle radius increases up to around 2.3 µm. As an example, we show that a 1-µm-diameter gold particle can be levitated for a significantly long time by the repulsive Casimir force at room temperature.

Nonequilibrium phase transition of contact processes with the Kauffman NK model.

We consider a multistate contact process (CP) in which new particles are created with probabilities that depend on the fitness of the parent particle and with mutations that occur at the time of creation. The fitness is determined by the Kauffman NK model. Using Monte Carlo simulations, we show that such an evolutional CP exhibits critical behaviors that differ from the basic CP. In addition, we present numerical results suggesting that the fitness averaged over surviving particles exhibits a maximum value at the critical point.

Soft-sputtering of insulin films in argon-cluster secondary ion mass spectrometry.

In secondary ion mass spectrometry (SIMS) of organic substances, the dissociation of the sample molecules is crucial. We have developed SIMS equipment capable of bombardment, where the primary ions are argon cluster ions with kinetic energy per atom controllable down to 1 eV. We previously reported the detection of intact ions of insulin and cytochrome C using this equipment. In this paper, we present a detailed characterization of the emission of secondary ions from insulin, focusing on the difference in secondary ion yield between intact ions and fragment ions by varying the incident angle of the cluster ions. The emission intensity of the intact ions was changed drastically due to the exposed dosage and incident angle of the cluster ions in contrast to the fragment ions. We discuss these results based on the manner in which the argon-cluster ions collide with the organic solid.

Enhanced surface sensitivity in secondary ion mass spectrometric analysis of organic thin films using size-selected Ar gas-cluster ion projectiles.

A size-selected argon (Ar) gas-cluster ion beam (GCIB) was applied to the secondary ion mass spectrometry (SIMS) of a 1,4-didodecylbenzene (DDB) thin film. The samples were also analyzed by SIMS using an atomic Ar(+) ion projectile and X-ray photoelectron spectroscopy (XPS). Compared with those in the atomic-Ar(+) SIMS spectrum, the fragment species, including siloxane contaminants present on the sample surface, were enhanced several hundred times in the Ar gas-cluster SIMS spectrum. XPS spectra during beam irradiation indicate that the Ar GCIB sputters contaminants on the surface more effectively than the atomic Ar(+) ion beam. These results indicate that a large gas-cluster projectile can sputter a much shallower volume of organic material than small projectiles, resulting in an extremely surface-sensitive analysis of organic thin films.

Actuation of a suspended nano-graphene sheet by impact with an argon cluster.

Using a molecular dynamics simulation, we examine the actuation of nanodrums consisting of a single graphene sheet. The membrane of the nanodrum, which contains 190 carbon atoms, is bent by collision with a cluster consisting of 10 argon atoms. The choice of an appropriate cluster velocity enables nanometre deformation of the membrane in sub-picosecond time without rupturing the graphene sheet. Theoretical results predict that, if an adsorbed molecule exists on the graphene sheet, the quick deformation due to the impact with the cluster can break the weak bonding between the adsorbed molecule and the graphene sheet and release the molecule from the surface; this suggests that this system has attractive potential applications for purposes of molecular ejection.

One-dimensional three-state quantum walk.

We study a generalized Hadamard walk in one dimension with three inner states. The particle governed by the three-state quantum walk moves, in superposition, both to the left and to the right according to the inner state. In addition to these two degrees of freedom, it is allowed to stay at the same position. We calculate rigorously the wave function of the particle starting from the origin for any initial qubit state and show the spatial distribution of probability of finding the particle. In contrast with the Hadamard walk with two inner states on a line, the probability of finding the particle at the origin does not converge to zero even after infinite time steps except special initial states. This implies that the particle is trapped near the origin after a long time with high probability.