なかのにっき

We investigate picosecond laser ablation of metals using a hybrid simulation scheme. Laser energy input into the electron system and heat conduction within it are modeled using a finite-difference scheme for solving the heat conduction equation. Atom motion in the near-surface part (72 nm) of the sample is modeled using molecular dynamics. Energy transfer between the electronic and atomic subsystems due to electron-phonon coupling is taken into account. For the special case of 0.5 ps UV laser irradiation of copper, we investigate the fluence dependence of the ablation yield, the temperature and pressure evolution in the target, and the ablation mechanism.

レーザーアブレーションの MD+熱拡散 FDM によるモデル化。 蒸発は レーザーで導入された stress によって物体が spallation するからとの議論。 "stress-confinment regime" らしいが、するとチャンクだらけになるわけか?

Recently, scanning tunneling microscopy results on the Brownian motion of two-dimensional vacancy islands on Ag(110) were presented. While the detachment of adatoms from the island and their diffusion on the terrace was permitted in the temperature range between 180 K and 220 K, the periphery diffusion of single adatoms was prohibited. The Brownian motion of the islands was found to follow a simple scaling law with terrace diffusion being the rate-limiting process. The scaling of the experimental results was confirmed by kinetic Monte Carlo (kMC) simulations. Here, we elaborate on the results and especially present a detailed derivation of the equation that relates the diffusivity of an anisotropic island to its size. Further details and results on the kMC simulations will be presented as well. Our results shed light on the recently discussed issue of attempt frequencies for surface diffusion. In this specific case of an anisotropic surface, we obtain prefactor ratios for exchange and hopping mechanisms.

Ag(110) 表面での vacancy island の移動の拡散係数 D と、island のサイズがスケール関係にあるという実験結果を受け、kinetic MC で再現を試みた論文。 このスケーリングパラメータは温度 independent らしいんだけど、 実験ではスケール性は確認されたものの、温度によってちょっと変わってるように見える。 anisotoropic な表面なので、kMC での hopping 移動と exchange 移動の確率を変えながら行ってみて island の aspect 比を実験と比較し、その比の範囲を求めたりしている。

The kinetics of diffusion-mediated smoothing of a TiO2 rutile (110) surface is studied over atomic length scales, by using a spot profile analysis of low-energy electron-diffraction data to characterize the morphology of the surface during thermal annealing. After the random removal of less than 1 ML of atoms by sputtering with an argon ion beam, the interface width and the distribution of terrace heights were found to stay nearly constant during annealing at 800 K, with terraces at just two heights making up 90% of the surface area over lateral distances of 400 Angstrom. Meanwhile, the coarsening of this nearly two-dimensional island structure is characterized by the growth of the average terrace width l from 20 to 60 Angstrom with annealing time, following l〜t^β with exponent β= 0.24±0.04. In addition, the stability of an 8% occupation of a third terrace height indicates negligible diffusion flux between layers during annealing. These results are compared with existing models for the microscopic dynamics involved.

スパッタダメージを与えた rutile TiO2 (110) の thermal smoothing を diffuse LEED で観察したもの。