*Norio Yamaguchi, **Yasushi Sasajima, Kazuo Terashima and Toyonobu Yoshida

Department of Metallurgy and Materials Science, Graduate School and Faculty of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan
*norio@plasma.t.u-tokyo.ac.jp

**Faculty of Engineering, Ibaraki University, 4-12-1 Nakanarusawa, Hitachi-shi, Ibaraki 316-8511, Japan

The thermal plasma flash evaporation method has achieved high-rate deposition (>1mm/min) of YBa2Cu3O7-x (YBCO) epitaxial films [1]. This is because clusters generated in a thermal plasma are expected to be thermally activated and have some unique characteristics as deposition species [2]. Recently, scanning tunneling microscopy (STM) showed that such hot clusters easily deformed and rearranged into two-dimensional ones with an average height of approximately monolayer even on a room temperature substrate. This feature is thought to play an important role in hot cluster epitaxy. In this study, we simulate the deformation process of a high temperature cluster on a low temperature substrate using molecular dynamics (MD) in order to verify the deformation behavior and to evaluate the temperature of hot clusters.

For the first step, mono-atomic homogeneous system was simulated as follows; Spherical fcc stacked cluster consisting of 418 atoms corresponding to a 2nm-cluster, was placed on the 6 planes of fcc (111) substrate. The interacting potential was assumed to be the Morse function corresponding to the material with a melting point of 1680K. The temperature was evaluated from the mean kinetic energy of the atoms in the system. The substrate temperature Tsub and the cluster temperature Tcluster were varied from 300 to 1000K and from 1450 to 3000K, respectively. The calculation was performed up to 104 MD steps (2.5x10-11 s). In the case of Tcluster < 2100K, the degree of cluster deformation is little. Even at Tsub = 1000K, the final structure was 6 atomic-layer-height cap-like structure. On the other hand, clusters with Tcluster > 2400K deformed drastically into almost two-dimensional ones even at Tsub = 300K and the degree of deformation seemed to be independent on the substrate temperature. By comparing of these results with the STM images of the YBCO clusters on a graphite substrate, it is suggested that the temperature of hot clusters is at least above 2400K. Moreover, this simulation reconfirms the unique characteristics of the hot cluster.